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BELVEDERE TIBURON LIBRARY
3 1111 02149 0469
STEPHEN JAY GOULD
THE STRUCTURE OF
$39,95
STEPHEN JAY GOULD THE STRUCTURE OF EVOLUTIONARY THEORY
The world’s most revered and eloquent interpret¬ er of evolutionary ideas offers here a work of explanatory force unprecedented in our time—a landmark publication, both for its historical sweep and for its scientific vision. With characteristic attention to detail, Stephen Jay Gould first describes the content and discuss¬ es the history and origins of the three core com¬ mitments of classical Darwinism: that natural selection works on organisms, not genes or species; that it is almost exclusively the mecha¬ nism of adaptive evolutionary change; and that these changes are incremental, not drastic. Next, he examines the three critiques that currently challenge this classic Darwinian edifice: that selection operates on multiple levels, from the gene to the group; that evolution proceeds by a variety of mechanisms, not just natural selection; and that causes operating at broader scales, including catastrophes, have figured prominently in the course of evolution. Then, in a stunning tour de force that will likely stimulate discussion and debate for decades, Gould proposes his own system for integrating these classical commit¬ ments and contemporary critiques into a new structure of evolutionary thought. In 2001 the Library of Congress named Stephen Jay Gould one of America’s eighty-three Living Legends—people who embody the “quintessentially American ideal of individual creativity, con¬ viction, dedication, and exuberance.” Each of these qualities finds full expression in this peerless work, the likes of which the scientific world has not seen—and may not see again—for well over a century.
BEL-TIB BOOKS 576.8 Gould 2002 Gould, Stephen Jay The structure of evolutionary theory
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THE STRUCTURE OF EVOLUTIONARY THEORY
STEPHEN JAY GOULD
The Structure Of Evolutionary Theory
THE BELKNAP PRESS OF HARVARD UNIVERSITY PRESS CAMBRIDGE, MASSACHUSETTS AND LONDON, ENGLAND 2002
Copyright © 2002 by the President and Fellows of Harvard College All rights reserved Printed in the United States of America
Library of Congress Cataloging-in-Publication Data Gould, Stephen Jay. The structure of evolutionary theory / Stephen Jay Gould, p.
cm.
Includes bibliographical references (p. ) ISBN 0-674-00613-5 (alk. paper) 1. Evolution (Biology) QH366.2 .G663 576.8—dc21
2. Punctuated equilibrium (Evolution)
2002 2001043556
I. Title.
For Niles Eldredge and Elisabeth Vrba May we always be the Three Musketeers Prevailing with panache From our manic and scrappy inception at Dijon To our nonsatanic and happy reception at Doomsday
All For One and One For All
Contents
Chapter 1: Defining and Revising the Structure of Evolutionary Theory
1
Part I, Chapters 2-7 The History of Darwinian Logic and Debate
91
Segue to Part II
585
Part II, Chapters 8-12 Towards a Revised and Expanded Evolutionary Theory
593
Bibliography
1344
Illustration Credits
1388
Index
1393
vii
*
\
Expanded Contents
Chapter 1: Defining and Revising the Structure of Evoiutionary Theory
1
■ Theories Need Both Essences and Histories
1
■ The Structure of Evolutionary Theory: Revising the Three Central
■
Features of Darwinian Logic
12
Apologia Pro Vita Sua
24
A Time to Keep
24
A Personal Odyssey
33 48
" Epitomes for a Long Development Levels of Potential Originality
48
An Abstract of One Long Argument
S3
Part I: The History of Darwinian Logic and Debate Chapter 2: The Essence of Darwinism and the Basis of Modern Orthodoxy: An Exegesis of the Origin of Species
93
■ A Revolution in the Small
93
■ Darwin as a Historical Methodologist
97
One Long Argument
97
The Problem of History
99
103
A Pourfold Continuum of Methods for the Inference of History ■ Darwin as a Philosophical Revolutionary
116 116
The Causes of Nature’s Harmony Darwin and William Paley
116
Darwin and Adam Smith
121 12S
The Lirst Theme: The Organism as the Agent of Selection
ix
Contents
The Second Theme:,Natural Selection as a Creative Force
137 141
The Requirements for Variation
141
Copious *
143
Small >
4
Undirected
144
Gradualism
146
The Adaptationist Program
155
The Third Theme: The Uniformitarian Need to Extrapolate: Environment as Enabler of Change
159
Judgments of Importance
163
Chapter 3: Seeds of Hierarchy
170
Lamarck and the Birth of Modern Evolutionism in Two-Factor Theories
170
The Myths of Lamarck
170
Lamarck as a Source
174
Lamarck’s Two-Factor Theory: Sources for the Two Parts
175
The First Set: Environment and Adaptation
176
The Second Set: Progress and Taxonomy
179
Distinctness of the Two Sets
181
Lamarck’s Two-Factor Theory: The Hierarchy of Progress and Deviation
175
Antinomies of the Two-Factor Theory
189
An Interlude on Darwin’s Reaction
192
No Allmacbt without Hierarchy: Weissman on Germinal Selection
197
The Allmacht of Selection
197
Weismann’s Argument on Lamarck and the Allmacbt of Selection
201
The Problem of Degeneration and Weismann’s Impetus for Germinal Selection
203
Some Antecedents to Hierarchy in German Evolutionary Thought 208 Haeckel’s Descriptive Hierarchy in Levels of Organization
208
Roux’s Theory of Intracorporeal Struggle
210
Germinal Selection as a Helpmate to Personal Selection
214
Germinal Selection as a Full Theory of Hierarchy
219
Hints of Hierarchy in Supraorganismal Selection: Darwin on the Principle of Divergence
224
Divergence and the Completion of Darwin’s System
224
The Genesis of Divergence
232
Contents
Divergence as a Consequence of Natural Selection
234
The Failure of Darwin’s Argument and the Need for Species Selection
236
The Calculus of Individual Success
238
The Causes of Trends
240
Species Selection Based on Propensity for Extinction
246
Postscript: Solution to the Problem of the “Delicate Arrangement” 248 ■ Coda
249
Chapter 4: Internalism and Laws of Form: Pre-Darwinian Alternatives to Functionalism
251
■ Prologue: Darwin’s Fateful Decision
251
■ Two Ways to Glorify God in Nature
260
William Paley and British Functionalism: Praising God in the Details of Design
262
Fouis Agassiz and Continental Formalism: Praising God in the Grandeur of Taxonomic Order
271
An Epilog on the Dichotomy
278
■ Unity of Plan as the Strongest Version of Formalism: The Pre-Darwinian Debate
281
Mebr Licbt on Goethe’s Feaf
281
Geoffroy and Cuvier
291
Cuvier and Conditions of Existence
291
Geoffroy’s Formalist Vision
298
The Debate of 1830: Foreplay and Aftermath
304
Richard Owen and English Formalism: The Archetype of Vertebrates
312
No Formalism Please, We’re British
312
The Vertebrate Archetype: Constraint and Nonadaptation
316
Owen and Darwin
326
■ Darwin’s Strong but Fimited Interest in Structural Constraint
330
Darwin’s Debt to Both Poles of the Dichotomy
330
Darwin on Correlation of Parts
332
The “Quite Subordinate Position” of Constraint to Selection
339
Chapter 5: The Fruitful Facets of Gabon’s Polyhedron: Channels and Saltations in Post-Darwinian Formalism ■ Gabon’s Polyhedron
342 342
xi
4
xii
Contents
■ Orthogenesis as a Theory of Channels and One-Way Streets:
351
the Marginalization of Darwinism Misconceptions and Relative Frequencies
351
*
Theodor Eimer and the Obnmacbt of Selection
355
Alpheus Hyatt: An Orthogenetic Hard Line from the World of Mollusks
365
C.O. Whitman: An Orthogenetic Dove in Darwin’s World of Pigeons •
383
Saltation as a Theory of Internal Impetus: A Second Formalist Strategy for Pushing Darwinism to a Causal Periphery
396
William Bateson: The Documentation of Inherent Discontinuity
396
Hugo de Vries: A Most Reluctant Non-Darwinian
415
Dousing the Great Party of 1909
415
The (Not So Contradictory) Sources of the Mutation Theory
418
The Mutation Theory: Origin and Central Tenets
425
Darwinism and the Mutation Theory
439
Confusing Rhetoric and the Personal Factor
439
The Logic of Darwinism and Its Different Place in de Vries’ System
443
De Vries on Macroevolution
446
Richard Goldschmidt’s Appropriate Role as a Formalist Embodiment of All that Pure Darwinism Must Oppose
Chapter 6: Pattern and Progress on the Geological Stage ■ Darwin and the Fruits of Biotic Competition
451 467 467
A Geological License for Progress
467
The Predominance of Biotic Competition and Its Sequelae
470
■ Uniformity on the Geological Stage
479
Lyell’s Victory in Fact and Rhetoric
479
Catastrophism as Good Science: Cuvier’s Essay
484
Darwin’s Geological Need and Kelvin’s Odious Spectre
492
A Question of Time (Too Little Geology)
496
A Question of Direction (Too Much Geology)
497
Chapter 7: The Modern Synthesis as a Limited Consensus
503
■ Why Synthesis?
503
■ Synthesis as Restriction
505
The Initial Goal of Rejecting Old Alternatives
505
Contents
R. A. Fisher and the Darwinian Core
508
J. B. S. Haldane and the Initial Pluralism of the Synthesis
514
J. S. Huxley: Pluralism of the Type
516
■ Synthesis as Hardening The Later Goal of Exalting Selection’s Power
518 518
Increasing Emphasis on Selection and Adaptation between the First (1937) and Last (1951) Edition of Dobzhansky’s Genetics
and the Origin of Species
524
The Shift in G. G. Simpson’s Explanation of “Quantum Evolution” from Drift and Nonadaptation (1944) to the Embodiment of Strict Adaptation (1953)
528
Mayr at the Inception (1942) and Codification (1963): Shifting from the “Genetic Consistency” to the “Adaptationist” Paradigm 531 Why Hardening? • Hardening on the Other Two Legs of the Darwinian Tripod
541 543
Levels of Selection
544
Extrapolation into Geological Time
556
■ From Overstressed Doubt to Overextended Certainty
566
A Tale of Two Centennials
566
All Quiet on the Textbook Front
576
Adaptation and Natural Selection
577
Reduction and Trivialization of Macroevolution
579
Segue to Part II
585
Part II: Towards a Revised and Expanded Evolutionary Theory Chapter 8: Species as Individuals in the Hierarchical Theory of Selection " The Evolutionary Definition of Individuality An Individualistic Prolegomenon
595 595 595
The Meaning of Individuality and the Expansion of the Darwinian Research Program
597
Criteria for Vernacular Individuality
602
Criteria for Evolutionary Individuality
608
■ The Evolutionary Definition of Selective Agency and the Fallacy of Selfish Genes
613
xiii
4
xiv
Contents
A Fruitful Error of Logic
613
Hierarchical vs. Genic Selectionism
614
The Distinction of Replicators and Interactors as a *
Framework for Discussion
615
Faithful Replication as the Central Criterion for the GeneCentered View of Evolution
616
Sieves, Plurifiers, and the Nature of Selection: The Rejection of Replication as a Criterion of Agency
619
Interaction as the Proper Criterion for Identifying Units of Selection
622
The Internal Incoherence of Gene Selectionism
625
Bookkeeping and Causality: The Fundamental Error of Gene Selectionism
632
Gambits of Reform and Retreat by Gene Selectionists
637
■ Logical and Empirical Foundations for the Theory of Hierarchical Selection Logical Validation and Empirical Challenges R. A. Fisher and the Compelling Logic of Species Selection
644 644 644
The Classical Arguments against Efficacy of Higher-Level Selection
646
Overcoming These Classical Arguments, in Practice for Interdemic Selection, but in Principle for Species Selection Emergence and the Proper Criterion for Species Selection Differential Proliferation or Downward Effect?
648 652 652
Shall Emergent Characters or Emergent Fitnesses Define the Operation of Species Selection? Hierarchy and the Sixfold Way
656 673
A Literary Prologue for the Two Major Properties of Hierarchies
673
Redressing the Tyranny of the Organism: Comments on Characteristic Features and Differences among Six Primary Levels The Gene-Individual
681 683
Motoo Kimura and the “Neutral Theory of Molecular Evolution”
684
True Genic Selection
689
The Cell-Individual
695
The Organism-Individual
700
The Deme-Individual
701
The Species-Individual
703
Contents Species as Individuals
703
Species as Interactors
704
Species Selection as Potent
709
The Clade-Individual ■ The Grand Analogy: A Speciational Basis for Macroevolution
712 714
Presentation of the Chart for Macroevolutionary Distinctiveness
714
The Particulars of Macroevolutionary Explanation
716
The Structural Basis
716
Criteria for Individuality
720
Contrasting Modalities of Change: The Basic Categories
721
Ontogenetic Drive: The Analogy of Lamarckism and Anagenesis
722
Reproductive Drive: Directional Speciation as an Important and Irreducible Macroevolutionary Mode Separate from Species Selection
724
Species Selection, Wright’s Rule, and the Power of Interaction with Directional Speciation
731
Species Level Drifts as More Powerful than the Analogous Phenomena in Microevolution
735
The Scaling of External and Internal Environments
738
Summary Comments on the Strengths of Species Selection and its Interaction with Other Macroevolutionary Causes of Change
741
Chapter 9: Punctuated Equilibrium and the Validation of Macroevolutionary Theory ■ What Every Paleontologist Knows
745 745
An Introductory Example
745
Testimonials to Common Knowledge
749
Darwinian Solutions and Paradoxes
755
The Paradox of Insulation from Disproof
758
The Paradox of Stymied Practice
761
■ The Primary Claims of Punctuated Equilibrium
765
Data and Definitions
765
Microevolutionary Links
774
Macroevolutionary Implications
781
Tempo and the Significance of Stasis
782
Mode and the Speciational Foundation of Macroevolution
783
xv
*
xin
Contents
■ The Scientific Debate on Punctuated Equilibrium: Critiques
784
and Responses Critiques Based on the Definability of Paleontological Species
784
*
Empirical Affirmation
784
Reasons for a Potential Systematic Underestimation of Biospecies by Paleospecies
789
Reasons for a Potential Systematic Overestimation of Biospecies by Paleospecies
792
Reasons Why an Observed Punctuational Pattern Might Not Represent Speciation
793
Critiques Based on Denying Events of Speciation as the Primary Locus of Change
796
Critiques Based on Supposed Failures of Empirical Results to Affirm Predictions of Punctuated Equilibrium Claims for Empirical Refutation by Cases
802 802
Phenotypes
802
Genotypes
810
Empirical Tests of Conformity with Models ■ Sources of Data for Testing Punctuated Equilibrium Preamble
812 822 822
The Equilibrium in Punctuated Equilibrium: Quantitatively Documented Patterns of Stasis in Unbranched Segments of Lineages
824
The Punctuations of Punctuated Equilibrium: Tempo and Mode in the Origin of Paleospecies
839
The Inference of Cladogenesis by the Criterion of Ancestral Survival
840
The “Dissection” of Punctuations to Infer Both Existence and Modality
850
Time
851
Geography
852
Morphometric Mode
852
Proper and Adequate Tests of Relative Frequencies: The Strong Empirical Validation of Punctuated Equilibrium
854
The Indispensability of Data on Relative Frequencies
854
Relative Frequencies for Higher Taxa in Entire Biotas
856
Relative Frequencies for Entire Clades
866
Causal Clues from Differential Patterns of Relative Frequencies
870
Contents
■ The Broader Implications of Punctuated Equilibrium for Evolutionary Theory and General Notions of Change
874
What Changes May Punctuated Equilibrium Instigate in Our Views about Evolutionary Mechanisms and the History of Life? The Explanation and Broader Meaning of Stasis
874 874
Frequency
875
Generality
876
Causality
877
Punctuation, the Origin of New Macroevolutionary Individuals, and Resulting Implications for Evolutionary Theory
885
Trends
886
The Speciational Reformulation of Macroevolution
893
Life Itself
897
General Rules
901
Particular Cases
905
Horses as the Exemplar of “Life’s Little Joke”
905
Rethinking Human Evolution
908
Ecological and Higher-Level Extensions
916
Punctuation All the Way Up and Down? The Generalization and Broader Utility of Punctuated Equilibrium (in More Than a Metaphorical Sense) at Other Levels of Evolution, and for Other Disciplines In and Outside the Natural Sciences
922
General Models for Punctuated Equilibrium
922
Punctuational Change at Other Levels and Scales of Evolution
928
A Preliminary Note on Homology and Analogy in the Conceptual Realm
928
Punctuation Below the Species Level
931
Punctuation Above the Species Level
936
Stasis Analogs: Trending and Non-Trending in the Geological History of Clades
936
Punctuational Analogs in Lineages: The Pace of Morphological Innovation
939
Punctuational Analogs in Faunas and Ecosystems
946
Punctuational Models in Other Disciplines: Towards a General Theory of Change
952
Principles for a Choice of Examples
952
Examples from the History of Human Artifacts and Cultures
952
Examples from Human Institutions and Theories about the Natural World
957
xvii
Contents
962
Two Concluding Examples, a General Statement, and a Coda Appendix: A Largely Sociological (and Fully Partisan) History of the Impact and Critique of Punctuated Equilibrium
972
The Entrance of Punctuated Equilibrium fnto Common Language and General Culture
> •
972
An Episodic History of Punctuated Equilibrium
979
Early Stages and Future Contexts
979
Creationist Misappropriation of Punctuated Equilibrium
986
Punctuated Equilibrium in Journalism and Textbooks
990
The Personal Aspect of Professional Reaction
999
The Case Ad Hominem against Punctuated Equilibrium
1000
An Interlude on Sources of Error
1010
The Wages of Jealousy
1014
The Descent to Nastiness
1014
The Most Unkindest Cut of All
1019
The Wisdom of Agassiz’s and von Baer’s Threefold History of Scientific Ideas
1021
A Coda on the Kindness and Generosity of Most Colleagues
1022
Chapter 10: The Integration of Constraint and Adaptation (Structure and Function) in Ontogeny and Phylogeny: Historical Constraints and the Evolution of Development
1025
Constraint as a Positive Concept
1025
Two Kinds of Positivity
1025
An Etymological Introduction
1025
The First (Empirical) Positive Meaning of Channeling
1027
The Second (Definitional) Positive Meaning of Causes outside Accepted Mechanisms
1032
Heterochrony and Allometry as the Locus Classicus of the First Positive (Empirical) Meaning. Channeled Directionality by Constraint.
1037
The Two Structural Themes of Internally Set Channels and Ease of Transformation as Potentially Synergistic with Functional Causality by Natural Selection: Increasing Shell Stability in the
Grypbaea Heterochronocline
1040
Ontogenetically Channeled Allometric Constraint as a Primary Basis of Expressed Evolutionary Variation: The Full Geographic and Morphological Range of Cerion uva
1045
Contents
The Aptive Triangle and the Second Positive Meaning: Constraint as a Theory-Bound Term for Patterns and Directions Not Built Exclusively (Or Sometimes Even at All) by Natural Selection 1051 The Model of the Aptive Triangle
1051
Distinguishing and Sharpening the Two Great Questions
1053
The Structural Vertex
1053
The Historical Vertex
1055
An Epitome for the Theory-Bound Nature of Constraint Terminology
1057
■ Deep Homology and Pervasive Parallelism: Historical Constraint as the Primary Gatekeeper and Guardian of Morphospace
1061
A Historical and Conceptual Analysis of the Underappreciated Importance of Parallelism for Evolutionary Theory
1061
A Context for Excitement
1061
A Terminological Excursus on the Meaning of Parallelism
1069
The Nine Fateful Little Words of E. Ray Lankester
1069
The Terminological Origin and Debate about the Meaning and Utility of Parallelism 1076 A Symphony in Four Movements on the Role of Historical Constraint in Evolution: Towards the Harmonious Rebalancing of Form and Function in Evolutionary Theory 1089 Movement One, Statement: Deep Homology across Phyla: Mayr’s Functional Certainty and Geoffroy’s Structural Vindication 1089 Deep Homology, Archetypal Theories, and Historical Constraint
1089
Mehr Licbt (More Light) on Goethe’s Angiosperm Archetype
1092
Hoxology and Geoffroy’s First Archetypal Theory of Segmental Homology
1095
An Epitome and Capsule History of Hoxology
1095
Vertebrate Homologs in Structure and Action
1101
Segmental Homologies of Arthropods and Vertebrates: Geoffroy’s Vindication
1106
Rediscovering the Vertebrate Rhombomeres
1107
More Extensive Homologies throughout the Developing Somites
1109
Some Caveats and Tentative Conclusions
1112
Geoffrey’s Second Archetypal Theory of Dorso-Ventral Inversion in the Common Bilaterian Groundplan
1117
xix
4
xx
Contents Movement Two, Elaboration: Parallelism of Underlying Generators: Deep Homology Builds Positive Channels of
1122
Constraint Parallelism All the Way Down: Shining a Light and Feeding the Walk
v>
1122
Parallelism in the Large: Pax-6 and the Homology of Developmental Pathways in Homoplastic Eyes of Several Phyla
1123
Data and Discovery
1123
Theoretical Issues
1127
A Question of Priority
1130
Parallelism in the Small: The Origin of Crustacean Feeding Organs
Pharaonic Bricks and Corinthian Columns
1132 1134
Movement Three, Scherzo: Does Evolutionary Change Often Proceed by Saltation Down Channels of Historical Constraint?
1142
Movement Four, Recapitulation and Summary: Early Establishment of Rules and the Inhomogenous Population of Morphospace: Dobzhansky’s Landscape as Primarily Structural and Historical, Not Functional and Immediate
1147
Bilaterian History as Top-Down by Tinkering of an Initial Set of Rules, Not Bottom-Up by Adding Increments of Complexity
1147
Setting of Historical Constraints in the Cambrian Explosion
1155
Channeling the Subsequent Directions of Bilaterian History from the Inside
1161
An Epilog on Dobzhansky’s Landscape and the Dominant Role of Historical Constraint in the Clumped Population of Morphospace
1173
Chapter 11: The Integration of Constraint and Adaptation (Structure and Function) in Ontogeny and Phylogeny: Structural Constraints, Spandrels, and the Centrality of Exaptation in Macroevolution " The Timeless Physics of Evolved Function
1179 1179
Structuralism’s Odd Man Outside
1179
D’Arcy Thompson’s Science of Form
1182
The Structure of an Argument
1182
The Tactic and Application of an Argument
1189
The Admitted Limitation and Ultimate Failure of an Argument
1196
Contents Odd Man In (D’Arcy Thompson’s Structuralist Critique of Darwinism) and Odd Man Out (His Disparagement of Historicism)
1200
An Epilog to an Argument
1207
Order for Free and Realms of Relevance for Thompsonian Structuralism ■ Exapting the Rich and Inevitable Spandrels of History Nietzsche’s Most Important Proposition of Historical Method
1208 1214 1214
Exaptation and the Principle of Quirky Functional Shift: The Restricted Darwinian Version as the Ground of Contingency How Darwin Resolved Mivart’s Challenge of Incipient Stages
1218 1218
The Two Great Historical and Structural Implications of Quirky Functional Shift
1224
How Exaptation Completes and Rationalizes the Terminology of Evolutionary Change by Functional Shifting
1229
Key Criteria and Examples of Exaptation
1234
The Complete Version, Replete with Spandrels: Exaptation and the Terminology of Nonadaptative Origin
1246
The More Radical Category of Exapted Features with Truly Nonadaptive Origins as Structural Constraints
1246
Defining and Defending Spandrels: A Revisit to San Marco
1249
Three Major Reasons for the Centrality of Spandrels, and Therefore of Nonadaptation, in Evolutionary Theory
1258
■ The Exaptive Pool: The Proper Conceptual Formula and Ground of Evolvability
1270
Resolving the Paradox of Evolvability and Defining the Exaptive Pool
1270
The Taxonomy of the Exaptive Pool
1277
Franklins and Miltons, or Inherent Potentials vs. Available Things
1277
Choosing a Fundamentum Divisionis for a Taxonomy: An Apparently Arcane and Linguistic Matter That Actually Embodies a Central Scientific Decision
1280
Cross-Level Effects as Miltonic Spandrels, Not Franklinian Potentials: The Nub of Integration and Radical Importance
1286
A Closing Comment to Resolve the Macroevolutionary Paradox that Constraint Ensures Flexibility Whereas Selection Crafts Restriction
1294
xxi
k
xxii
Contents Chapter 12: Tiers of Time and Trials of Extrapolationism, With an Epilog on the Interaction of General Theory and Contingent History
1296
■ Failure of Extrapolationism in the Non-Isotropy of Time and Geology
' *
1296
The Specter of Catastrophic Mass Extinction: Darwin to Chicxulub
1296
The Paradox of the First Tier: Towards a General Theory of Tiers of Time
1320
■ An Epilog on Theory and History in Creating the Grandeur of This View of Life
1332
THE STRUCTURE OF EVOLUTIONARY THEORY
fc
CHAPTER ONE
Defining and Revising the Structure of Evolutionary Theory
Theories Need Both Essences and Histories In a famous passage added to later editions of the Origin of Species, Charles Darwin (1872, p. 134) generalized his opening statement on the apparent ab¬ surdity of evolving a complex eye through a long series of gradual steps by re¬ minding his readers that they should always treat “obvious” truths with skepticism. In so doing, Darwin also challenged the celebrated definition of science as “organized common sense,” as championed by his dear friend Thomas Henry Huxley. Darwin wrote: “When it was first said that the sun stood still and world turned round, the common sense of mankind declared the doctrine false; but the old saying of Vox populi, vox Dei [the voice of the people is the voice of God], as every philosopher knows, cannot be trusted in science.” Despite his firm residence within England’s higher social classes, Darwin took a fully egalitarian approach towards sources of expertise, knowing full well that the most dependable data on behavior and breeding of domesticated and cultivated organisms would be obtained from active farmers and hus¬ bandmen, not from lords of their manors or authors of theoretical treatises. As Ghiselin (1969) so cogently stated, Darwin maintained an uncompromis¬ ingly “aristocratic” set of values towards judgment of his work—that is, he cared not a whit for the outpourings of vox populi, but fretted endlessly and fearfully about the opinions of a very few key people blessed with the rare mix of intelligence, zeal, and attentive practice that we call expertise (a demo¬ cratic human property, respecting only the requisite mental skills and emo¬ tional toughness, and bearing no intrinsic correlation to class, profession or any other fortuity of social circumstance). Darwin ranked Hugh Falconer, the Scottish surgeon, paleontologist, and Indian tea grower, within this most discriminating of all his social groups, a panel that included Hooker, Huxley and Lyell as the most prominent mem¬ bers. Thus, when Falconer wrote his important 1863 paper on American fos¬ sil elephants (see Chapter 9, pages 745-749, for full discussion of this inci¬ dent), Darwin flooded himself with anticipatory fear, but then rejoiced in his critic’s generally favorable reception of evolution, as embodied in the closing 1
2
THE STRUCTURE OF EVOLUTIONARY THEORY sentence of Falconer’s key section: “Darwin has, beyond all his cotemporaries [sic], given an impulse to the philosophical investigation of the most back¬ ward and obscure branch of the Biological Sciences of his day; he has laid the foundations of a great edifice; but he need ngt be surprised if, in the progress of erection, the superstructure is altered by his successors, like the Duomo of Milan, from the roman to a different style of architecture.” In a letter to Falconer on October 1, 1862 (in F. Darwin, 1903, volume 1, p. 206), Darwin explicitly addressed this passage in Falconer’s text. (Darwin had received an advance copy of the manuscript, along with Falconer’s re¬ quest for review and criticism—hence Darwin’s reply, in 1862, to a text not printed until the following year): “To return to your concluding sentence: far from being surprised, I look at it as absolutely certain that very much in the
Origin will be proved rubbish; but I expect and hope that the framework will stand.” The statement that God (or the Devil, in some versions) dwells in the de¬ tails must rank among the most widely cited intellectual witticisms of our time. As with many clever epigrams that spark the reaction “I wish I’d said that!”, attribution of authorship tends to drift towards appropriate famous sources. (Virtually any nifty evolutionary saying eventually migrates to T. H. Huxley, just as vernacular commentary about modern America moves to¬ wards Mr. Berra.) The apostle of modernism in architecture, Ludwig Mies van der Rohe, may, or may not, have said that “God dwells in the details,” but the plethora of tiny and subtle choices that distinguish the elegance of his great buildings from the utter drabness of superficially similar glass boxes throughout the world surely validates his candidacy for an optimal linkage of word and deed. Architecture may assert a more concrete claim, but nothing beats the ex¬ traordinary subtlety of language as a medium for expressing the importance of apparently trivial details. The architectural metaphors of Milan’s cathe¬ dral, used by both Falconer and Darwin, may strike us as effectively identical at first read. Falconer says that the foundations will persist as Darwin’s leg¬ acy, but that the superstructure will probably be reconstructed in a quite dif¬ ferent style. Darwin responds by acknowledging Falconer’s conjecture that the theory of natural selection will undergo substantial change; indeed, in his characteristically diffident way, Darwin even professes himself “absolutely certain” that much of the Origin's content will be exposed as “rubbish.” But he then states not only a hope, but also an expectation, that the “framework” will stand. We might easily read this correspondence too casually as a polite dialogue between friends, airing a few unimportant disagreements amidst a commit¬ ment to mutual support. But I think that this exchange between Falconer and Darwin includes a far more “edgy” quality beneath its diplomacy. Consider the different predictions that flow from the disparate metaphors chosen by each author for the Duomo of Milan—Falconer’s “foundation” vs. Darwin’s “framework.” After all, a foundation is an invisible system of support, sunk into the ground, and intended as protection against sinking or toppling of the
Defining and Revising the Structure of Evolutionary Theory overlying public structure. A framework, on the other hand, defines the basic form and outline of the public structure itself. Thus, the two men conjure up very different pictures in their crystal balls. Falconer expects that the underly¬ ing evolutionary principle of descent with modification will persist as a fac¬ tual foundation for forthcoming theories devised to explain the genealogical tree of life. Darwin counters that the theory of natural selection will persist as a basic explanation of evolution, even though many details, and even some subsidiary generalities, cited within the Origin will later be rejected as false, or even illogical. I stress this distinction, so verbally and disarmingly trivial at a first and superficial skim through Falconer’s and Darwin’s words, but so incisive and portentous as contrasting predictions about the history of evolutionary the¬ ory, because my own position—closer to Falconer than to Darwin, but in ac¬ cord with Darwin on one key point—led me to write this book, while also supplying the organizing principle for the “one long argument” of its entirety. I do believe that the Darwinian framework, and not just the foundation, per¬ sists in the emerging structure of a more adequate evolutionary theory. But I also hold, with Falconer, that substantial changes, introduced during the last half of the 20th century, have built a structure so expanded beyond the origi¬ nal Darwinian core, and so enlarged by new principles of macroevolutionary explanation, that the full exposition, while remaining within the domain of Darwinian logic, must be construed as basically different from the canonical theory of natural selection, rather than simply extended. A closer study of the material basis for Falconer and Darwin’s metaphors— the Duomo (or Cathedral) of Milan—might help to clarify this important dis¬ tinction. As with so many buildings of such size, expense, and centrality (both geographically and spiritually), the construction of the Duomo occupied sev¬ eral centuries and included an amalgam of radically changing styles and pur¬ poses. Construction began at the chevet, or eastern end, of the cathedral in the late 14th century. The tall windows of the chevet, with their glorious flamboyant tracery, strike me as the finest achievement of the entire structure, and as the greatest artistic expression of this highly ornamented latest Gothic style. (The term “flamboyant” literally refers to the flame-shaped element so extensively used in the tracery, but the word then came to mean “richly deco¬ rated” and “showy,” initially as an apt description of the overall style, but then extended to the more general meaning used today.) Coming now to the main point, construction then slowed considerably, and the main western facade and entrance way (Fig. 1-1) dates from the late 16th century, when stylistic preferences had changed drastically from the points, curves and traceries of Gothic to the orthogonal, low-angled or gently rounded lintels and pediments of classical Baroque preferences. Thus, the first two tiers of the main (western) entrance to the Duomo display a style that, in one sense, could not be more formally discordant with Gothic elements of de¬ sign, but that somehow became integrated into an interesting coherence. (The third tier of the western facade, built much later, returned to a “retro” Gothic style, thus suggesting a metaphorical reversal of phylogenetic conventions, as
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4
THE STRUCTURE OF EVOLUTIONARY THEORY
1-1. The west facade (main entrance) of Milan Cathedral, built in baroque style in the 16th century, with a retro-gothic third tier added later.
up leads to older—in style if not in actual time of emplacement!) Finally, in a distinctive and controversial icing upon the entire structure (Fig. 1-2), the “wedding cake,” or row-upon-row of Gothic pinnacles festooning the tops of all walls and arches with their purely ornamental forms, did not crown the edifice until the beginning of the 19th century, when Napoleon conquered the city and ordered their construction to complete the Duomo after so many centuries of work. (These pinnacle forests may amuse or disgust architectural purists, but no one can deny their unintended role in making the Duomo so uniquely and immediately recognizable as the icon of the city.) How, then, shall we state the most appropriate contrast between the
Duomo of Milan and the building of evolutionary theory since Darwin’s Ori¬ gin in 1859? If we grant continuity to the intellectual edifice (as implied by
Defining and Revising the Structure of Evolutionary Theory comparison with a discrete building that continually grew but did not change its location or basic function), then how shall we conceive “the structure of evolutionary theory” (chosen, in large measure, as the title for this book be¬ cause I wanted to address, at least in practical terms, this central question in the history and content of science)? Shall we accept Darwin’s triumphalist stance and hold that the framework remains basically fixed, with all visually substantial change analogous to the non-structural, and literally superficial, icing of topmost pinnacles? Or shall we embrace Falconer’s richer and more critical, but still fully positive, concept of a structure that has changed in radi-
1-2. The “wedding cake” pinnacles that festoon the top of Milan Cathedral, and that were not built until the first years of the 19th century after Napoleon con¬ quered the city.
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6
THE STRUCTURE OF EVOEUTIONARY THEORY cal ways by incorporating entirely different styles into crucial parts of the building (even the front entrance!), while still managing to integrate all the differences into a coherent and functional whole, encompassing more and more territory in its continuing enlargement? Darwin’s version remains Gothic, and basically unchanged beyond the visual equivalent of lip service. Falconer’s version retains the Gothic base as a positive constraint and director, but then branches out into novel forms that mesh with the base but convert the growing structure into a new entity, largely defined by the outlines of its history. (Note that no one has suggested the third alternative, often the fate of cathedrals—destruction, either total or partial, followed by a new building of contrary or oppositional form, erected over a different foundation.) In order to enter such a discourse about “the structure of evolutionary the¬ ory” at all, we must accept the validity, or at least the intellectual coherence and potential definability, of some key postulates and assumptions that are often not spelled out at all (especially by scientists supposedly engaged in the work), and are, moreover, not always granted this form of intelligibility by philosophers and social critics who do engage such questions explicitly. Most importantly, I must be able to describe a construct like “evolutionary theory” as a genuine “thing”—an entity with discrete boundaries and a definable his¬ tory—especially if I want to “cash out,” as more than a confusingly poetic image, an analogy to the indubitable bricks and mortar of a cathedral. In particular, and to formulate the general problem in terms of the specific example needed to justify the existence of this book, can “Darwinism” or “Darwinian theory” be treated as an entity with defining properties of “ana¬ tomical form” that permit us to specify a beginning and, most crucially for the analysis I wish to pursue, to judge the subsequent history of Darwinism with enough rigor to evaluate successes, failures and, especially, the degree and character of alterations? This book asserts, as its key premise and one long argument, that such an understanding of modern evolutionary theory places the subject in a particularly “happy” intellectual status—with the cen¬ tral core of Darwinian logic sufficiently intact to maintain continuity as the centerpiece of the entire field, but with enough important changes (to all ma¬ jor branches extending from this core) to alter the structure of evolutionary theory into something truly different by expansion, addition, and redefini¬ tion. In short, “The structure of evolutionary theory” combines enough sta¬ bility for coherence with enough change to keep any keen mind in a perpetual mode of search and challenge. The distinction between Falconer’s and Darwin’s predictions, a key ingredi¬ ent in my analysis, rests upon our ability to define the central features of Darwinism (its autapomorphies, if you will), so that we may then discern whether the extent of alteration in our modern understanding of evolutionary mechanisms and causes remains within the central logic of this Darwinian foundation, or has now changed so profoundly that, by any fair criterion in vernacular understanding of language, or by any more formal account of de¬ parture from original premises, our current explanatory theory must be de-
Defining and Revising the Structure of Evolutionary Theory scribed as a different kind of mental “thing.” How, in short, can such an in¬ tellectual entity be defined? And what degree of change can be tolerated or accommodated within the structure of such an entity before we must alter the name and declare the entity invalid or overthrown? Or do such questions just represent a fool’s errand from the start, because intellectual positions can’t be reified into sufficient equivalents of buildings or organisms to bear the weight of such an inquiry? As arrogant as I may be in general, I am not sufficiently doltish or vainglo¬ rious to imagine that I can meaningfully address the deep philosophical ques¬ tions embedded within this general inquiry of our intellectual ages—that is, fruitful modes of analysis for the history of human thought. I shall therefore take refuge in an escape route that has traditionally been granted to scien¬ tists: the liberty to act as a practical philistine. Instead of suggesting a princi¬ pled and general solution, I shall ask whether I can specify an operational way to define “Darwinism” (and other intellectual entities) in a manner spe¬ cific enough to win shared agreement and understanding among readers, but broad enough to avoid the doctrinal quarrels about membership and alle¬ giance that always seem to arise when we define intellectual commitments as pledges of fealty to lists of dogmata (not to mention initiation rites, secret handshakes and membership cards—in short, the intellectual paraphernalia that led Karl Marx to make his famous comment to a French journalist: “je ne suis pas marxiste”). As a working proposal, and as so often in this book (and in human affairs in general), a “Goldilocks solution” embodies the blessedly practical kind of approach that permits contentious and self-serving human beings (God love us) to break intellectual bread together in pursuit of common goals rather than personal triumph. (For this reason, I have always preferred, as guides to human action, messy hypothetical imperatives like the Golden Rule, based on negotiation, compromise and general respect, to the Kantian categorical im¬ peratives of absolute righteousness, in whose name we so often murder and maim until we decide that we had followed the wrong instantiation of the right generality.) We must, in short and in this case, steer between the “too lit¬ tle” of refusing to grant any kind of “essence,” or hard anatomy of defining concepts, to a theory like Darwinism; and the “too much” of an identification so burdened with a long checklist of exigent criteria that we will either spend all our time debating the status of particular items (and never addressing the heart or central meaning of the theory), or we will waste our efforts, and poi¬ son our communities, with arguments about credentials and anathemata, ap¬ plied to individual applicants for membership. In his brilliant attempt to write a “living” history and philosophy of science about the contemporary restructuring of taxonomic theory by phenetic and cladistic approaches, Hull (1988) presents the most cogent argument I have ever read for “too little” on Goldilocks’s continuum, as embodied in his de¬ fense of theories as “conceptual lineages” (1988, pp. 15-18). I enthusiasti¬ cally support Hull’s decision to treat theories as “things,” or individuals in the crucial sense of coherent historical entities—and in opposition to the stan-
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THE STRUCTURE OF EVOLUTIONARY THEORY dard tactic, in conventional scholarship on the “history of ideas,” of tracing the chronology of expression for entirely abstract concepts defined only by formal similarity of content, and not at all by ties of historical continuity, or even of mutual awareness among defenders*across centuries and varied cul¬ tures. (For example, Hull points out that such a conventional history of the >
“chain of being” would treat this notion as an invariant and disembodied Platonic archetype, independently “borrowed” from the eternal storehouse of potential models for natural reality, and then altered by scholars to fit local contexts across millennia and cultures.) But I believe that Hull’s laudable desire to recast the history of ideas as a narrative of entities in historical continuity, rather than as a disconnected chronology of tidbits admitted into a class only by sufficient formal similarity with an abstract ideological archetype, then leads him to an undervaluation of actual content. Hull exemplifies his basic approach (1988, p. 17): “A con¬ sistent application of what Mayr has termed ‘population thinking’ requires that species be treated as lineages, spatiotemporally localized particulars, in¬ dividuals. Hence, if conceptual change is to be viewed from an evolutionary perspective, concepts must be treated in the same way. In order to count as the ‘same concept,’ two term-tokens must be part of the same conceptual lin¬ eage. Population thinking must be applied to thinking itself.” So far, so good. But Hull now extends this good argument for the necessity of historical connectivity into a claim for sufficiency as well—thus springing a logical trap that leads him to debase, or even to ignore, the “morphology” (or idea content) of these conceptual lineages. He states that he wishes to “organize term-tokens into lineages, not into classes of similar term-types” (pp. 16-17). I can accept the necessity of such historical continuity, but nei¬ ther I nor most scholars (including practicing scientists) will then follow Hull in his explicit and active rejection of similarity in content as an equally neces¬ sary criterion for continuing to apply the same name—Darwinian theory, for example—to a conceptual lineage. At an extreme that generates a reductio ad absurdum for rejecting Hull’s conclusion, but that Hull bravely owns as a logical entailment of his own prior decision, a pure criterion of continuity, imbued with no constraint of content, forces one to apply the same name to any conceptual lineage that has remained consciously intact and genealogically unbroken through several generations (of passage from teachers to students, for example), even if the current “morphology” of concepts directly inverts and contradicts the central arguments of the original theory. “A proposition can evolve into its contra¬ dictory,” Hull allows (1988, p. 18). Thus, on this account, if the living intel¬ lectual descendants of Darwin, as defined by an unbroken chain of teaching, now believed that each species had been independently created within six days of 24 hours, this theory of biological order would legitimately bear the name of “Darwinism.” And I guess that I may call myself kosher, even though I and all members of my household, by conscious choice and with great ideo¬ logical fervor, eat cheeseburgers for lunch every day—because we made this
Defining and Revising the Structure of Evolutionary Theory dietary decision in a macromutational shift of content, but with no genealogi¬ cal break in continuity, from ten previous generations of strict observers of kashrut. The objections that most of us would raise to Hull’s interesting proposition include both intellectual and moral components. Certain kinds of systems are, and should be, defined purely by genealogy and not at all by content. I am my father’s son no matter how we interact. But such genealogical defini¬ tions, as validated by historical continuity, simply cannot adequately charac¬ terize a broad range of human groupings properly designated by similarity in content. When Cain mocked God’s inquiry about Abel’s whereabouts by ex¬ claiming “Am I my brother’s keeper” (Genesis 4:9), he illustrated the appro¬ priateness of either genealogy by historical connection or fealty by moral re¬ sponsibility as the proper criterion for “brotherhood” in different kinds of categories. Cain could not deny his genealogical status as brother in one sense, but he derided a conceptual meaning, generally accorded higher moral worth as a consequence of choice rather than necessity of birth, in disclaim¬ ing any responsibility as keeper. As a sign that we have generally privileged the conceptual meaning, and that Cain’s story still haunts us, we need only re¬ member Claudius’s lament that his murder of his own brother (and Hamlet’s father) “hath the primal eldest curse upon’t.” Ordinary language, elementary logic, and a general sense of fairness all combine to favor such preeminence for a strong component of conceptual continuity in maintaining a name or label for a theory. Thus, if I wish to call myself a Darwinian in any just or generally accepted sense of such a claim, I do not qualify merely by documenting my residence within an unbroken lin¬ eage of teachers and students who have transmitted a set of changing ideas organized around a common core, and who have continued to study, aug¬ ment and improve the theory that bears such a longstanding and honorable label. I must also understand the content of this label myself, and I must agree with a set of basic precepts defining the broad ideas of a view of natural real¬ ity that I have freely chosen to embrace as my own. In calling myself a Dar¬ winian I accept these minimal obligations (from which I remain always and entirely free to extract myself should my opinions or judgments change); but I do not become a Darwinian by the mere default of accidental location within a familial or educational lineage. Thus, if we agree that a purely historical, entirely content-free definition of allegiance to a theory represents “too little” commitment to qualify, and that we must buttress any genealogical criterion with a formal, logical, or anatom¬ ical definition framed in terms of a theory’s intellectual content, then what kind or level of agreement shall we require as a criterion of allegiance for in¬ clusion? We now must face the opposite side of Goldilocks’s dilemma—for once we advocate criteria of content, we do not wish to impose such strin¬ gency and uniformity that membership becomes more like a sworn obedience to an unchanging religious creed than a freely chosen decision based on per¬ sonal judgment and perception of intellectual merits. My allegiance to Dar-
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THE STRUCTURE OF EVOLUTIONARY THEORY winian theory, and my willingness to call myself a Darwinian biologist, must not depend on my subscription to all 95 articles that Martin Luther nailed to the Wittenburg church door in 1517; or to all 80 items in the Syllabus of Er¬
rors that Pio Nono (Pope Pius IX) proclaimed in 1864, including the “fal¬ lacy,” so definitionally uncongenial to science, that “the Roman Pontiff can and should reconcile himself to and agree with progress, liberalism and mod¬ ern civilization”; or to all 39 articles of the Church of England, adopted by Queen Elizabeth in 1571 as a replacement for Archbishop Thomas Cranmer’s 42 articles of 1553. Goldilocks’s “just right” position between these extremes will strike nearly all cooperatively minded intellectuals, committed to the operationality and advance of their disciplines, as eminently sensible: shared content, not only historical continuity, must define the structure of a scientific theory; but this shared content should be expressed as a minimal list of the few defining at¬
tributes of the theory’s central logic—in other words, only the absolutely es¬ sential statements, absent which the theory would either collapse into fallacy or operate so differently that the mechanism would have to be granted an¬ other name. Now such a minimal list of such maximal centrality and importance bears a description in ordinary language—but its proper designation requires that evolutionary biologists utter a word rigorously expunged from our profes¬ sional consciousness since day one of our preparatory course work: the con¬ cept that dare not speak its name—essence, essence, essence (say the word a few times out loud until the fear evaporates and the laughter recedes). It’s high time that we repressed our aversion to this good and honorable word. Theories have essences. (So, by the way, and in a more restrictive and nuanced sense, do organisms—in their limitation and channeling by con¬ straints of structure and history, expressed as Bauplane of higher taxa. My critique of the second theme of Darwinian central logic, Chapters 4-5 and 10-11, will treat this subject in depth. Moreover, my partial defense of or¬ ganic essences, expressed as support for structuralist versions of evolutionary causality as potential partners with the more conventional Darwinian func¬ tionalism that understandably denies intelligibility to any notion of an es¬ sence, also underlies the double entendre of this book’s title, which honors the intellectual structure of evolutionary theory within Darwinian traditions and their alternatives, and which also urges support for a limited version of
structuralist theory, in opposition to certain strict Darwinian verities.) Our unthinking rejection of essences can be muted, or even reversed into propensity for a sympathetic hearing, when we understand that an invocation of this word need not call down the full apparatus of an entirely abstract and eternal Platonic eidos—a reading of “essence” admittedly outside the logic of evolutionary theory, and historical modes of analysis in general. But the solu¬ tion to a meaningful notion of essence in biology lies within an important epi¬ sode in the history of emerging evolutionary views, a subject treated exten¬ sively in Chapter 4 of this book, with Goethe, Etienne Geoffroy St. Hilaire, and Richard Owen as chief protagonists.
Defining and Revising the Structure of Evolutionary Theory After all, the notion of a general anatomical blueprint that contains all par¬ ticular incarnations by acting as a fundamental building block (Goethe’s leaf or Geoffroy’s vertebra) moved long ago from conceptualization as a disem¬ bodied and nonmaterial archetype employed by a creator, to an actual struc¬ ture (or inherited developmental pathway) present in a flesh and blood ances¬ tor—a material basis for channeling, often in highly positive ways, the future history of diversity within particular phyletic lineages. This switch from ar¬ chetype to ancestor permitted us to reformulate the idea of “essence” as broad and fruitful commonalities that unite a set of particulars into the most meaningful relationships of common causal structure and genesis. Our active use of this good word should not be hampered by a shyness and disquietude lacking any validity beyond the vestiges of suspicions originally set by battles won so long ago that no one can remember the original reasons for anathe¬ matization. Gracious (and confident) victors should always seek to revive the valid and important aspects of defeated but honorable systems. And the tran¬ scendental morphologists did understand the importance of designating a small but overarching set of defining architectural properties as legitimate es¬ sences of systems, both anatomical and conceptual. Hull correctly defines theories as historical entities, properly subject to all the principles of narrative explanation—and I shall so treat Darwinian logic and its substantial improvements and changes throughout this book. But the¬ ories of range and power also feature inherent “essences,” implicit in their logical structure, and operationally definable as minimal sets of proposi¬ tions so crucial to the basic function of a system that their falsification must undermine the entire structure, and also so necessary as an ensemble of mutual implication that all essential components must work in concert to set the theory’s mechanism into smooth operation as a generator and explana¬ tion of nature’s order. In staking out this middle Goldilockean ground be¬ tween (1) the “too little” of Hull’s genealogical continuity without commit¬ ment to a shared content of intellectual morphology and (2) the “too much” of long lists of ideological fealty, superficially imbibed or memorized, and then invoked to define membership in ossified cults rather than thoughtful al¬ legiance to developing theories, I will argue that a Darwinian essence can be minimally (and properly) defined by three central principles constituting a tri¬ pod of necessary support, and specifying the fundamental meaning of a pow¬ erful system that Darwin famously described as the “grandeur in this view of life.” I shall then show that this formulation of Darwinian minimal commit¬ ments proves its mettle on the most vital ground of maximal utility. For not only do these three commitments build, in their ensemble, the full frame of a comprehensive evolutionary worldview, but they have also defined the chief objections and alternatives motivating all the most interesting debate within evolutionary theory during its initial codification in the 19th century. More¬ over, and continuing in our own time, these three themes continue to specify the major weaknesses, the places in need of expansion or shoring up, and the locus of unresolved issues that make evolutionary biology such a central and
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THE STRUCTURE OF EVOEUTIONARY THEORY exciting subject within the ever changing and ever expanding world of mod¬ ern science.
The Structure of Evolutionary Theory: Revising the Three Central Features of Darwinian Logic In the opening sentence of the Origin's final chapter (1859, p. 459), Darwin famously wrote that “this whole volume is one long argument.” The pres¬ ent book, on “the structure of evolutionary theory,” despite its extravagant length, is also a brief for an explicit interpretation that may be portrayed as a single extended argument. Although I feel that our best current formulation of evolutionary theory includes modes of reasoning and a set of mechanisms substantially at variance with strict Darwinian natural selection, the logical structure of the Darwinian foundation remains remarkably intact—a fasci¬ nating historical observation in itself, and a stunning tribute to the intel¬ lectual power of our profession’s founder. Thus, and not only to indulge my personal propensities for historical analysis, I believe that the best way to ex¬ emplify our modern understanding lies in an extensive analysis of Darwin’s basic logical commitments, the reasons for his choices, and the subsequent manner in which these aspects of “the structure of evolutionary theory” have established and motivated all our major debates and substantial changes since Darwin’s original publication in 1859. I regard such analysis not as an antiquarian indulgence, but as an optimal path to proper understanding of our current commitments, and the underlying reasons for our decisions about them. As a primary theme for this one long argument, I claim that an “essence” of Darwinian logic can be defined by the practical strategy defended in the first section of this chapter: by specifying a set of minimal commitments, or broad statements so essential to the central logic of the enterprise that disproof of any item will effectively destroy the theory, whereas a substantial change to any item will convert the theory into something still recognizable as within the Bauplan of descent from its forebear, but as something sufficiently differ¬ ent to identify, if I may use the obvious taxonomic metaphor, as a new subclade within the monophyletic group. Using this premise, the long argu¬ ment of this book then proceeds according to three sequential claims that set the structure and order of my subsequent chapters: 1. Darwin himself formulated his central argument under these three basic premises. He understood their necessity within his system, and the difficulty that he would experience in convincing his contemporaries about such unfa¬ miliar and radical notions. He therefore presented careful and explicit de¬ fenses of all three propositions in the Origin. I devote the first substantive chapter (number 2) to an exegesis of the Origin of Species as an embodiment of Darwin’s defense for this central logic. 2. As evolutionary theory experienced its growing pains and pursued its founding arguments in the late 19th and early 20th centuries (and also in
Defining and Revising the Structure of Evolutionary Theory its pre-Darwinian struggles with more inchoate formulations before 1859), these three principles of central logic defined the themes of deepest and most persistent debate—as, in a sense, they must because they constitute the most interesting intellectual questions that any theory for causes of descent with modification must address. The historical chapters of this book’s first half then treat the history of evolutionary theory as responses to the three central issues of Darwinian logic (Chapters 3-7). 3. As the strict Darwinism of the Modern Synthesis prevailed and “hard¬ ened,” culminating in the overconfidences of the centennial celebrations of 1959, a new wave of discoveries and theoretical reformulations began to challenge aspects of the three central principles anew—thus leading to an¬ other fascinating round of development in basic evolutionary theory, extend¬ ing throughout the last three decades of the 20th century and continuing to¬ day. But this second round has been pursued in an entirely different and more fruitful manner than the 19th century debates. The earlier questioning of Darwin’s three central principles tried to disprove natural selection by offer¬ ing alternative theories based on confutations of the three items of central logic. The modern versions accept the validity of the central logic as a foun¬ dation, and introduce their critiques as helpful auxiliaries or additions that enrich, or substantially alter, the original Darwinian formulation, but that leave the kernel of natural selection intact. Thus, the modern reformula¬ tions are helpful rather than destructive. For this reason, I regard our modern understanding of evolutionary theory as closer to Falconer’s metaphor, than to Darwin’s, for the Duomo of Milan—a structure with a firm foundation and a fascinatingly different superstructure. (Chapters 8-12, the second half of this book on modern developments in evolutionary theory, treat this third theme.) Thus, one might say, this book cycles through the three central themes of Darwinian logic at three scales—by brief mention of a framework in this chapter, by full exegesis of Darwin’s presentation in Chapter 2, and by lengthy analysis of the major differences and effects in historical (Part 1) and modern critiques (Part 2) of these three themes in the rest of the volume. The basic formulation, or bare-bones mechanics, of natural selection is a disarmingly simple argument, based on three undeniable facts (overproduc¬ tion of offspring, variation, and heritability)* and one syllogistic inference (natural selection, or the claim that organisms enjoying differential reproduc¬ tive success will, on average, be those variants that are fortuitously better adapted to changing local environments, and that these variants will then pass their favored traits to offspring by inheritance). As Huxley famously, and ruefully, remarked (in self-reproach for failing to devise the theory him¬ self), this argument must be deemed elementary (and had often been formu*Two of these three ranked as “folk wisdom” in Darwin’s day and needed no further justification—variation and inheritance (the mechanism of inheritance remained unknown, but its factuality could scarcely be doubted). Only the principle that all organisms produce more offspring than can possibly survive—superfecundity, in Darwin’s lovely term—ran counter to popular assumptions about nature’s benevolence, and required Darwin’s specific defense in the Origin.
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THE STRUCTURE OF EVOLUTIONARY THEORY lated before, but in negative contexts, and with no appreciation of its power—see p. 137), and can only specify the guts of the operating machine, not the three principles that established the range and power of Darwin’s rev¬ olution in human thought. Rather, these three larger principles, in defining the Darwinian essence, take the guts of the machine, and declare its simple operation sufficient to generate the entire history of life in a philosophical manner that could not have been more contrary to all previous, and cher¬ ished, assumptions of Western life and science. The three principles that elevated natural selection from the guts of a working machine to a radical explanation of the mechanism of life’s history can best be exemplified under the general categories of agency, efficacy, and scope. I treat them in this specific order because the logic of Darwin’s own de¬ velopment so proceeds (as I shall illustrate in Chapter 2), for the most radical claim comes first, with assertions of complete power and full range of appli¬ cability then following. Agency. The abstract mechanism requires a locus of action in a hierar¬ chical world, and Darwin insisted that the apparently intentional “benevo¬ lence” of nature (as embodied in the good design of organisms and the har¬ mony of ecosystems) flowed entirely as side-consequences of this single causal locus, the most “reductionistic” account available to the biology of Darwin’s time. Darwin insisted upon a virtually exceptionless, single-level theory, with organisms acting as the locus of selection, and all “higher” order emerging, by the analog of Adam Smith’s invisible hand, from the (unconscious) “strug¬ gles” of organisms for their own personal advantages as expressed in dif¬ ferential reproductive success. One can hardly imagine a more radical refor¬ mulation of a domain that had unhesitatingly been viewed as the primary manifestation for action of higher power in nature—and Darwin’s brave and single-minded insistence on the exclusivity of the organismic level, although rarely appreciated by his contemporaries, ranks as the most radical and most distinctive feature of his theory. Efficacy. Any reasonably honest and intelligent biologist could easily understand that Darwin had identified a vera causa (or true cause) in natural selection. Thus, the debate in his time (and, to some extent, in ours as well) never centered upon the existence of natural selection as a genuine causal force in nature. Virtually all anti-Darwinian biologists accepted the reality and action of natural selection, but branded Darwin’s force as a minor and negative mechanism, capable only of the headsman’s or executioner’s role of removing the unfit, once the fit had arisen by some other route, as yet uniden¬ tified. This other route, they believed, would provide the centerpiece of a “real” evolutionary theory, capable of explaining the origin of novelties. Dar¬ win insisted that his admittedly weak and negative force of natural selection could, nonetheless, under certain assumptions (later proved valid) about the nature of variation, act as the positive mechanism of evolutionary novelty— that is, could “create the fit” as well as eliminate the unfit—by slowly accu¬ mulating the positive effects of favorable variations through innumerable generations.
Defining and Revising the Structure of Evolutionary Theory Scope. Even the most favorably minded of contemporaries often admit¬
ted that Darwin had developed a theory capable of building up small changes (of an admittedly and locally “positive” nature as adaptations to changing environments) within a “basic type”—the equivalent, for example, of making dogs from wolves or developing edible corn from teosinte. But these critics could not grasp how such a genuine microevolutionary process could be ex¬ tended to produce the full panoply of taxonomic diversity and apparent “progress” in complexification of morphology through geological time. Dar¬ win insisted on full sufficiency in extrapolation, arguing that his micro¬ evolutionary mechanism, extended through the immensity of geological time, would be fully capable of generating the entire pageant of life’s history, both in anatomical complexity and taxonomic diversity—and that no further causal principles would be required. Because primates are visual animals, complex arguments are best portrayed or epitomized in pictorial form. The search for an optimal icon to play such a role is therefore no trivial matter (although scholars rarely grant this issue the serious attention so richly merited)—especially since the dangers of confu¬ sion, misplaced metaphor, and replacement of rigor with misleading “intu¬ ition” stand so high. I knew from the beginning of this work that I needed a suitable image for conveying the central logic of Darwinian theory. As one of my humanistic conceits, I hoped to find a historically important scientific im¬ age, drawn for a different reason, that might fortuitously capture the argu¬ ment in pictorial form. But I had no expectation of success, and assumed that I would need to commission an expressly designed figure drawn to a long list of specifications. The specific form of the image—its central metaphorical content, if you will—plays an important role in channeling or misdirecting our thoughts, and therefore also requires careful consideration. In the text of this book, I speak most often of a “tripod” since central Darwinian logic embodies three major propositions that I have always visualized as supports—perhaps be¬ cause I have never been utterly confident about this entire project, and I needed some pictorial encouragement to keep me going for twenty years. (And I much prefer tripods, which can hold up elegant objects, to buttresses, which may fly as they preserve great Gothic buildings, but which more often shore up crumbling edifices. Moreover, the image of a tripod suits my major claim particularly well—for I have argued, just above, that we should define the “essence” of a theory by an absolutely minimal set of truly necessary propositions. No structure, either of human building or of abstract form, captures this principle better than a tripod, based on its absolute minimum of three points for fully stable support in the dimensional world of our physical experience.) But organic images have always appealed more strongly, and I preferred a biological icon. If the minimal logic can be represented by a tripod pointing downward, then the same topology can be inverted into a structure growing upward. Darwin’s own favorite image of the tree of life immediately sug¬ gested itself, and I long assumed that I would eventually settle on a botanical
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THE STRUCTURE OF EVOEUTIONARY THEORY icon. But I also remembered Darwin’s first choice for an organic metaphor or picture of branching to capture his developing views about descent with mod¬ ification and the causes of life’s diversity—the “coral of life” of his “B Note¬ book” on transmutation, kept during the 18^0’s as he became an evolutionist and struggled towards the theory of natural selection (see Barrett et al., 1987). As I began to write this summary chapter, I therefore aimlessly searched through images of Cnidaria from my collection of antiquarian books in pale¬ ontology. I claim no general significance whatsoever for my good fortune, but after a lifetime of failure in similar quirky quests, I was simply stunned to find a preexisting image—not altered one iota from its original form, I promise you, to suit my metaphorical purposes—that so stunningly embodied my needs, not only for a general form (an easy task), but down to the smallest de¬ tails of placement and potential excision of branches (the feature that I had no right or expectation to discover and then to exapt from so different an original intent). The following figure comes from the 1747 Latin version of one of the semi¬ nal works in the history of paleontology—the 1670 Italian treatise of the Sicilian savant and painter Agostino Scilla, La vana speculazione disingannata dal senso (“Vain speculation undeceived by the senses”—Scilla’s de¬ fense, at the outset of “the scientific revolution” of Newton’s generation, for empirical methods in the study of nature, and specifically, in this treatise, for a scientific paleontology and the need to recognize fossils as remains of an¬ cient organisms, not as independent products of the mineral kingdom). This work, famous not only for an incisive text, but also for its beautiful plates (see Fig. 1-3), engraved by an author known primarily as an artist of substan¬ tial eminence, includes this figure, labeled Coralium articulatum quod copiosissimum in rupibus et collibus Messanae reperitur (“Articulated coral, found in great abundance in the cliffs and hills of Messina”). This model, and its organic features, work uncommonly well as a meta¬ phor for the Goldilockean position of definition by a barest minimum of truly fundamental postulates. For Scilla’s coral, with its branching structure (see Fig. 1-4)—particularly as expressed in the lessening consequences of excising branches at ever higher levels nearer the top (the analogs of disconfirming theoretical features of ever more specialized and less fundamental import)— so beautifully captures the nature and operation of the intellectual structure that I defended above for specifying the essences of theories. The uncanny ap¬ propriateness of Scilla’s coral lies in the fortuity that this particular specimen (accurately drawn from nature by Scilla, I assume, and not altered to as¬ sert any general point) just happens to include exactly the same number of branches (three) as my Darwinian essential structure. (They terminate at the same upper level, so I could even turn the specimen over into a tolerably unwobbly tripod!) Moreover, since this particular genus of corals grows in discrete segments, the joining points correspond ideally with my metaphor of chopping planes for excising parts of structures at various levels of impor¬ tance in an intellectual entity. But, most incredibly, the segmental junctions of
Defining and Revising the Structure of Evolutionary Theory
1-3. The famous frontispiece from Scilla’s treatise of 1670 defending the organic nature of fossils. The solid young man, representing the truth of sensory experi¬ ence, shows a fossil sea urchin in his right hand to a wraithlike figure represent¬ ing the former style of speculative thinking. With his left hand, the solid figure points to other fossils found in Sicily. The text proclaims: “Vain speculation un¬ deceived by the senses.”
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THE STRUCTURE OF EVOEUTIONARY THEORY this particular specimen just happen to occupy the exact places that I needed a priori to make my central point about lower choppings that destroy theo¬ ries, middle choppings that change theories in a Falconerian way (major al¬ terations in structure upon a preserved foundation), and upper choppings that change theories in the lesser manner of Darwin’s Milanese metaphor (smaller excisions that leave the framework intact as well). The central trunk (the theory of natural selection) cannot be severed, or the creature (the theory) dies. (The roots, if you will, represent sources of evi¬ dence; any one may be excised, if recognized as incorrect by later study, so long as enough remain to anchor the structure). This central trunk then di¬ vides into a limited number of major branches. These basic struts—the three
R2
/ K2
1-4. Agostino Scilla was also a celebrated painter as well as a scien¬ tist. The plates of his 1670 trea¬ tise are therefore particularly well done. This figure, representing a fossil coral that Scilla found near Messina, fortuitously (and without ^ any alteration whatsoever), pre¬ sents a detailed picture of the basic logic of Darwinian theory as recog¬ nized in this book. See text for details.,
Defining and Revising the Structure of Evolutionary Theory branches of the Darwinian essence in this particular picture—are also so es¬ sential that any severing of a complete branch either kills, or so seriously compromises, the entire theory that a new name and basic structure becomes essential. We now reach the interesting point where excisions and regraftings pre¬ serve the essential nature of an intellectual structure, but with two quite dif¬ ferent levels of change and revision, as characterized by Falconer’s and Dar¬ win’s competing metaphors for the Duomo of Milan. I would argue that a severing low on any one of the three major branches corresponds with a revi¬ sion profound enough to validate the more interesting Falconerian version of major revision upon a conserved foundation. (The Falconerian model is, in this sense, a Goldilockean solution itself, between the “too much” of full de¬ struction and the “too little” of minor cosmetic revision.) On the other hand, the severing of a subbranch of one of the three branches symbolizes a less portentous change, closer to Darwinian models for the Milanese Duomo— an alteration of important visual elements, but without change in the basic framework. My fascination with the current state of evolutionary theory, at least as I read current developments in both logic and empirics, lies in its close con¬ formity to the Falconerian model—with enough continuity to make the past history of the field so informative (and so persistently, even emotionally, com¬ pelling), but with enough deep difference and intellectual fascination to stim¬ ulate anyone with a thirst for the intriguing mode of novelty that jars previ¬ ous certainty, but does not throw a field into the total anarchy of complete rebuilding (not a bad thing either, but far from the actual circumstance in this case). To summarize my views on the utility of such a model for the essence of Darwinian logic, I will designate three levels of potential cuts or excisions to this organic (and logical) coral of the structure of evolutionary theory, as originally formulated by Darwin in the Origin of Species, and as revised in a Falconerian way in recent decades. The most inclusive and most fundamental K-cuts (killing cuts) sever at least one of the three central principles of Dar¬ winian logic and thereby destroy the theory tout court. The second level of Rcuts (revision cuts) removes enough of the original form on one of the three central branches to ensure that the new (and stronger or more arborescent) branch, in regrowing from the cut, will build a theory with an intact Darwin¬ ian foundation, but with a general form sufficiently expanded, revised or re¬ constructed to present an interestingly different structure of general explana¬ tion—the Falconerian model for the Duomo of Milan. Finally, the third level of S-cuts (subsidiary cuts) affects only a subbranch of one of the three major branches, and therefore reformulates the general theory in interesting ways, while leaving the basic structure of explanation intact—the Darwinian model for the Duomo of Milan. I wrote this book because I believe that all three pillars, branches, or tri¬ pod legs, representing the three fundamental principles of Darwinian central logic, have been subjected to fascinating R-cuts that have given us at least the
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THE STRUCTURE OF EVOLUTIONARY THEORY firm outlines—for the revised structure of evolutionary explanation remains a work vigorously in progress, as only befits the nature of its subject, after all!—of a far richer and fascinatingly different theory with a retained Dar¬ winian core rooted in the principles of natural selection. In short, we live in the midst of a Falconerian remodelling of our growing and multiform, yet co¬ herently grounded, intellectual mansion. I will not, in this chapter, detail the nature of the K-cuts that failed (thus preserving the central logic of Darwinism), the R-cuts that have succeeded in changing the structure of evolutionary theory in such interesting ways, and the S-cuts that have refurbished major rooms in particular wings of the edi¬ fice—for these specifications set the subject matter of all following chapters. But to provide a better opening sense of this book’s argument—and to clarify the nature of the three central claims of Darwinian logic—I shall at least dis¬ tinguish, for each branch, the K-cuts that never prevailed (and therefore did not fell the structure) from the R-cuts that have affected each branch, and have therefore provoked our current process of building an enriched struc¬ ture for evolutionary theory. Returning to Scilla’s coral (Fig. 1-4), consider the central branch as the first leg of the tripod (agency, or the claim for organismal selection as the causal locus of the basic mechanism), the left branch as the second leg (efficacy, or the claim that selection acts as the primary creative force in building evo¬ lutionary novelties), and the right branch as the third leg (scope, or the claim that these microevolutionary modes and processes can, by extrapola¬ tion through the vastness of geological time, explain the full panoply of life’s changes in form and diversity). The cut labeled K1 on Figure 1-4 would have severed the entire coral by disproving natural selection as an evolutionary force at all. The cut labeled K2 would have fully severed the second branch, leaving natural selection as a legitimate cause, but denying it any creative role, and thereby dethroning Darwinism as a major principle in explaining life’s history. (We shall see, in Chapters 3-6, that such a denial of creativity underlay the most common anti-Darwmian argument in the first generations of debate.) The cut labeled K3 would have fully severed the third branch, allowing that natural selection might craft some minor changes legitimately called “creative” in a local sense, but denying that Darwin’s mechanism could then be extended to explain the panoply of macroevolutionary processes, or the actual pageant of life’s his¬ tory. The success of any one of these K-cuts would have destroyed Darwinian theory, plain and simple. None of them succeeded, and the foundation of Darwinian central logic remains intact and strong. In striking, and most positive, contrast, I believe that higher R-cuts—leav¬ ing the base of each major branch intact, but requiring a substantial regrowth and regrafting of an enlarged structure upon the retained foundation—have been successfully wielded against all three branches of Darwinian logic, as the structure of evolutionary theory developed in the last third of the 20th cen¬ tury (following too rigid a calcification of the original structure, a good ad¬ umbration of the coral metaphor!, in the hardening of the Modern Synthesis
Defining and Revising the Structure of Evolutionary Theory that culminated in the Darwinian centennial celebrations of 1959). On the first branch of agency, the cut labeled R1 (see Fig. 1-4) expanded Darwin’s unilevel theory of organismal selection into a hierarchical model of selection acting simultaneously on several legitimate levels of Darwinian individuality (genes, cell-lineages, organisms, demes, species, and clades). I shall show in Chapters 3, 8, and 9 how the logic of this pronounced expansion builds a the¬ ory fascinatingly different from, and not just a smooth extension of, Darwin’s single level mechanism of agency—my reason for portraying the hierarchical model as a deeply interesting R-cut rather than a more superficial S-cut. On the second branch of efficacy, the cut labeled R2 accepts the validity of Darwin’s argument for creativity (by leaving the base of the branch intact), but introduces a sufficient weight of formalist thinking—via renewed appre¬ ciation for the enormous importance of structural, historical, and develop¬ mental constraint in channeling the pathways of evolution, often in highly positive ways—that the pure functionalism of a strictly Darwinian (and ex¬ ternalist) approach to adaptation no longer suffices to explain the channeling of phyletic directions, and the clumping and inhomogeneous population of organic morphospace. The strict Darwinian form of explanation has thereby been greatly changed and enriched, but in no way defeated. I shall discuss the historical aspect of this branch in Chapters 4 and 5, and modern reformula¬ tions of this R2 cut in Chapters 10 and 11. On the final branch of scope, the cut labeled R3 accepts the Darwinian contention that microevolutionary modes and principles can build grand pat¬ terns by cumulation through geological immensity, but rejects the argument that such extrapolations can render the entire panoply of phenomena in life’s history without adding explicitly macroevolutionary modes for distinctive expression of these processes at higher tiers of time—as in the explanation of cladal trends by species sorting under punctuated equilibrium, rather than by extended adaptive anagenesis of purely organismal selection, and in the ne¬ cessity of titrating adaptive microevolutionary accumulation with occasional resetting of rules and patterns by catastrophically triggered mass extinctions at time’s highest tier. Chapters 6 and 12 discuss historical and modern cri¬ tiques of Darwinian extrapolationism. For now, I will say little about the even higher and more superficial S-cuts of subbranches, but I will at least indicate how I construe this category by stating a hypothetical example for each branch: an SI cut, for example, might accept the selective basis of evolutionary change in a purely mechanical sense, but then deny full force to Darwin’s deliciously radical philosophical claim that all apparent “higher level” harmony arises consequentially, through the invisible hand of lower levels acting for personal reproductive success. One might, in principle, propose such a revision by arguing that a higher force, operating by an overarching principle of order, “employs” natural selection as its mechanical agent. (I speak only hypothetically here, for no such defendable scientific hypothesis now exists, although the concept certainly remains intelligible. Explicitly theological versions don’t count as science, whatever their kind or form of potential validity.) An S2 cut might be assayed by a
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THE STRUCTURE OF EVOLUTIONARY THEORY developmental saltationist who accepted the selectionist basis of adaptive change but felt that, at a sufficient relative frequency to be counted as impor¬ tant, the initial steps of such changes may be larger than the pure continuationism of Darwinian selection can admit. And an S3 cut might accept the full validity of microevolutionary extrapolationism, but deny the subsidiary de¬ fense of progress that Darwin grafted onto this apparatus (see Chapter 6) with ecological arguments about plenitude and the priority of biotic over abiotic competition. As a paleontologist and part-time historian of science by profession, my reading of these important R-cuts arose from a macroevolutionary perspec¬ tive framed largely in terms of longstanding difficulties faced by Darwinism in extending its successes for explaining small changes in palpable time into equally adequate causal accounts for broader patterns and processes in geo¬ logical history. I have, in this effort, also benefited from my personal study of Darwin’s life and times, and especially the late 19th century debates on mech¬ anisms of evolution (as promulgated largely by professionals who could nei¬ ther fully understand nor accept the radical philosophical commitments un¬ derlying Darwin’s view). This historical study allowed me to grasp the continuity in basic themes from Darwin’s own formulation, through these foundational debates, right down to the major theoretical struggles of our own time. An appreciation of this continuity allowed me to discern and de¬ fine the distinctively Darwinian view of life. But I recognize only too well that every strength comes paired with weak¬ nesses. In my case, a paleontological focus leads me into relative ignorance for an equally important locus of reform in the structure of Darwinism—in¬ creasing knowledge of the nature of genomes and the mechanics of develop¬ ment. (I try to cover the outlines of important theoretical critiques from this “opposite” realm of the smallest, but the relative weightings in my text reflect my own varying competencies far more than the merits of the cases. For ex¬ ample, although I do discuss, and perhaps even adequately outline, the im¬ portance of Kimura and King’s neutralist theory in questioning previous as¬ sumptions of adaptationist hegemony, I surely do not give an appropriate volume of attention to this enormously important subject.) Nonetheless, I hope that I have managed to present an adequate account of the coordinating themes that grant such interest and coherence to modern reformulations of the structure of evolutionary theory. Such thematic consis¬ tency in revision becomes possible largely because Darwin himself, in his characteristically brilliant way, tied the diverse threads of his initiating argu¬ ment into an overall view with a similarly tight structure—thus granting clear definition to his own commitments, and also permitting their revision in the form of an equally coherent “package.11 I would argue, moreover, and with¬ out wishing to become extravagantly hagiographical (for I wrote this book, after all, primarily to discuss a critique and revision of strict Darwinism), that our modern sense of limitations in the canonical version arises from decisions that Darwin made for tough and correct reasons in the context of his initiat¬ ing times—reasons that made his account the first operational theory of evo-
Defining and Revising the Structure of Evolutionary Theory
lution in modern science. In particular, as Chapter 2 will discuss in detail, Darwin converted evolution from untestable speculation to doable science by breaking through the old paradox (as embedded most prominently in Lamarck’s system) of contrasting a palpable force of small-scale change that could do little in extension, with a basically nonoperational (and orthogonal) mechanism of large-scale change putatively responsible for all the interesting patterns of life’s history, but imperceptible and untestable from the uniformitarian study of modern organisms. By claiming that the small-scale mechanics of modern change could, by ex¬ tension, explain all of evolution, Darwin opened the entire held to empirical study. And yet, as Hegel and so many other students of change have noted, progress in human (and other) affairs tends to spiral upwards in cycles of pro¬ posal (thesis), then countered by opposition (antithesis), and finally leading to a new formulation combining the best aspects of both competitors (synthe¬ sis). Darwin’s thesis established evolution as a science, but his essential com¬
mitments, as expressed in the three legs of his necessary logical tripod (or the three branches of his conceptual tree or coral, as in the alternate metaphor of Fig. 1-4), eventually proved too narrow and confining, thus requiring an an¬ tithesis of extension and reformulation on each branch, and leading—or so this book maintains as a central thesis of its own—to a still newer and richer synthesis expressing our best current understanding of the structure of evolu¬ tionary theory. In fact, and to repeat my summary in this different form, one might encapsulate the long argument of this book in such a Hegelian format. PreDarwinian concepts of evolution remained speculative and essentially non¬ operational, largely because (see Chapter 3) they fell into the disabling para¬ dox of contrasting an effectively unknowable large-scale force of cosmic progress against an orthogonal, palpable and testable small-scale force that could generate local adaptation and diversity, but that couldn’t, in principle, explain the macroevolutionary pattern of life. Then Darwin, in his thesis (also an antithesis to these earlier sterile constructions), brilliantly argued that the putative large-scale force did not exist, and that all evolution could be explained by upward extrapolation from the small-scale force, now prop¬ erly understood as natural selection. In a first stage of debate during the late 19th and early 20th centuries (Chapters 3-6), most critiques of Darwinism— one might designate them as a first round of ultimately destructive antithe¬ ses—simply denied sufficient agency, efficacy and range to natural selection, and reasserted the old claim of duality, with selection relegated to triviality, and some truly contrary force sought as the explanation for major features of evolution. Strict Darwinism eventually fended off these destructive critiques, reasserted itself in the triumphant, and initially (and generously) pluralistic form of the Modern Synthesis, but eventually calcified into a “hardened” ver¬ sion (Chapter 7). Then, in a strikingly different, and ultimately fruitful, second round of an¬ titheses, a renewed debate about central theoretical issues arose during the last three decades of the 20th century, and reshaped the field by recognizing
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THE STRUCTURE OF EVOLUTIONARY THEORY that selection needed to be amplified, reformulated and invigorated by other, non-contrary (and, at most, orthogonal) causes, not rejected as wrong, or scorned as trivial (Chapters 8-12). The one long argument of this book holds that a synthesis (still much in progress) has^now sufficiently coagulated from this debate to designate our best current understanding of the structure of evolutionary theory as something rich and new, with a firmly retained basis in Darwinian logic—in other words, and following the organizing and opening metaphor of this chapter, as a validation of Falconer’s, rather than Darwin’s, concept of the historical growth and change of Milan’s cathedral. Ariel’s telling verse in Shakespeare’s The Tempest proclaims in dense meta¬ phor: Full fathom five thy father lies; Of his bones are coral made; Those are pearls that were his eyes: Nothing of him that doth fade But doth suffer a sea-change Into something rich and strange. With the exception of one possible (and originally unintended) modern reading of these images, this famous and haunting verse provides a beautiful description of both the priceless worth and intriguing modern transformation of Darwin’s original theory. (For the exception, several connotations of deep burial in the sea—full fathom five—might be viewed negatively, as in “deep sixing” or going to Davy Jones’s locker. But, for natural historians who read this book, and coming from an invertebrate paleontologist as author, the seafloor could not represent a more positive resting place or point of origin— and I intend to evoke only these upbeat images in citing Ariel’s lines.) Other¬ wise, Darwin’s original structure has only yielded greater treasure in cascad¬ ing implications and developments through the subsequent history of evolu¬ tionary thought—the conversion of the bones of an original outline into precious coral and pearls of current substance. Nothing of Darwin’s central logic has faded or fully capsized, but his theory has been transformed, along his original lines, into something far different, far richer, and far more ade¬ quate to guide our understanding of nature. The last three lines of Shakespeare’s verse also appear on the tombstone of the great poet Percy Bysshe Shelley (also the author of the preface to his wife’s novella, Frankenstein, which cites Erasmus Darwin in its first line of text). I believe that these words would suit, and honor, Charles Darwin just as well and just as rightly.
Apologia Pro Vita Sua A TIME TO KEEP The Preacher spoke ever so truly in writing his famous words (Ecclesiastes 3:1-7): “For every thing there is a season, and a time to every purpose ... A
Defining and Revising the Structure of Evolutionary Theory
time to break down, and a time to build up ... A time to rend, and a time to sew: a time to keep silence, and a time to speak.” Evolutionary theory now stands in the happier second state of these genuine dichotomies (in part be¬ cause the first state had been mined to the limited extent of its utility): we live in a time for building up, for sewing together, and for speaking out. Not all times are such good times, and not all scientists win the good for¬ tune to live within these times of motion. For theories grow as organisms do, with periods of Sturm und Drang, long latencies of youth or ossifications of age, and some happy times of optimally productive motion in between (an¬ other Goldilockean phenomenon). I recently studied the life and career of E. Ray Lankester (Gould, 1999a), clearly the most talented evolutionary mor¬ phologist of the generation just after Darwin. He did “good” work and had a “good” career (see Chapter 10, pages 1069-1076 for his best theoretical efforts), but he never transcended the ordinary. Perhaps the limitation lay largely within his own abilities. However, I rather suspect that he did possess both the temperamental gumption and the requisite intellectual might—but that the tools of major empirical advance just didn’t emerge in his generation, for he remained stuck in a relatively unproductive middle, as Darwin had seized the first-fruits from traditional data of natural history, and the second plucking required a resolution of genetic mechanisms. I felt a similar kind of frustration in 1977, after writing my first technical book, Ontogeny and Phylogeny (see Chapter 10, pages 1061-1063). I had spent the best years of a young career on a subject that I knew to be rele¬ vant (at a time when most of the profession had forgotten). But then defeat snatched my prize from the jaws of victory. I am proud of the book, and I do believe that it helped to focus interest on a subject that became doable soon thereafter. But I ran up against a wall right at the end—for the genetics of de¬ velopment clearly held the key to any rapprochement of embryology and evo¬ lution, and we knew effectively nothing about eukaryotic regulation. Indeed, as we could then only characterize structural genes by electrophoretic tech¬ niques, our major “arguments” for regulatory effects (if they even merited such a positive designation) invoked such negative evidence as the virtual identity in structural genes between chimps and humans, coupled with a fair visceral sense of extensive phenotypic disparity in anatomy and behavior— with the differences then attributed to regulatory genes that we could not, at the time, either study or even identify. By sheer good fortune (abetted in minuscule ways by my own pushes and those of my paleontological colleagues), the field moved fast and I lived long enough to witness a sea change (if I may cite Ariel yet again) towards potentiation on all three major intellectual and social substrates for con¬ verting a subject from great promise combined with even more frustrating inoperability, into a discipline bursting with new (and often utterly surpris¬ ing) data that led directly to testable hypotheses about basic issues in the structure of evolutionary theory. Empirics. During the last third of the 20th century, new techniques and conceptualizations opened up important sources of data that challenged or-
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THE STRUCTURE OF EVOEUTIONARY THEORY thodox formulations for all three branches of essential Darwinian logic. To cite just one relevant example for each branch, theoretical development and accumulating data on punctuated equilibrium allowed us to reconceptualize species as genuine Darwinian individuals, fully competent to participate in processes of selection at their own supraorganismic (and suprademal) level— and then to rethink macroevolutiori as the differential success of species rather than the extended anagenesis of organismal adaptation (see Chapter 9). This validation of the species-individual aided the transformation of what had begun as a particular argument about group (or interdemic) selection into a fully generalized hierarchical theory, with good cases then documented from the genic to the cladal level (see Chapter 8). On the second branch of full efficacy for natural selection as an externalist and functionalist process, the stunning discoveries of extensive deep homolo¬ gies across phyla separated by more than 500 million years (particularly the vertebrate homologs of arthropod Hox genes)—against explicit statements by architects of the Modern Synthesis (see p. 539) that such homologies could not exist in principle, in a world dominated by their conception of nat¬ ural selection—forced a rebalancing or leavening of Darwinian functionalism with previously neglected, or even vilified, formalist perspectives based on the role of historical and structural constraints in channeling directions of evolu¬ tionary change, and causing the great dumpings and inhomogeneities of morphospace—phenomena that had previously been attributed almost exclu¬ sively to functionalist forces of natural selection. On the third branch of extrapolation, the discovery and relatively quick validation, beginning in 1980, of a truly catastrophic trigger for at least one great mass extinction (the K-T event of 65 million years ago), fractured the uniformitarian consensus, embraced by a century of paleontological compla¬ cency, that all apparent faunal overturns could be “spread out” into sufficient time for explanation by ordinary causes under plausible intensifications that would not alter conventional modes of evolution and extinction. Moreover, as we shall see, these three apparently rather different kinds of data and their attendant critiques cohere into a revised general structure for evolutionary theory—thus marking our age as a time for building up and not only as a time for breaking down. Concepts. Following the Kantian dictum that percepts without con¬ cepts are blind, but concepts without percepts empty, these two categories interpenetrate as “pure” data suggest novel ideas (how can one not rethink the causes of mass extinction when evidence surfaces for a bolide, 7-10 km in diameter, and packing 104 the megatonnage of all the earth’s nuclear weapons combined), whereas “abstract” concepts then taxonomize the natural world in different ways, often “creating1' data that had never been granted enough previous intellectual space even to be conceived (as when punctuated equilib¬ rium made stasis a theoretically meaningful and interesting phenomenon, and not just an embarrassing failure to detect “evolution,” in its traditional definition of gradual change—and paleontologists then began active stud¬ ies of a subject that had previously been ignored as uninteresting, if conceptu-
Defining and Revising the Structure of Evolutionary Theory
alized at all). But, speaking parochially as a student of the fossil record, I can at least say that the conceptual revolution in macroevolutionary thinking re¬ vitalized the field of paleobiology (even creating the name as a subdisciplme of paleontological endeavor). Whatever the varied value of different individ¬ ual efforts in this burgeoning field, we may at least be confident that our pro¬ fession will no longer be humiliated as a synecdoche for ossified boredom among the natural sciences—as Nature did in 1969 when editorializing about the salutary value of plate tectonics in revitalizing the geological sciences: “Scientists in general might be excused for assuming that most geologists are paleontologists and most paleontologists have staked out a square mile as their life’s work. A revamping of the geologist’s image is badly needed” (Anonymous, 1969, p. 903). The intricate and multifaceted concepts that have nuanced and altered the central logic on all three branches of Darwinism’s essential postulates repre¬ sent ideas of broad ramification and often remarkably subtle complexity, as we vain scientists soon discovered in our fractured bubbles of burst pride— for we had been so accustomed to imagining that an evening in an armchair could conquer any merely conceptual issue, whereas we all acknowledge the substantial time and struggle that empirical problems, demanding collection and evaluation of data, often require. Yet, on these basic questions in formu¬ lating evolutionary theory, we often read and thought for months, and ended up more confused than when we began. The general solution to such procedural dilemmas lies in a social and intel¬ lectual activity that scientists do tend to understand and practice better than colleagues in most other academic disciplines—collaboration. Far more than most colleagues, I have tended to work alone in my professional life and pub¬ lication. But for each of the conceptually difficult and intellectually manifold issues of reevaluation for the central logic of the three essential Darwinian postulates, I desperately needed advice, different skills, and the give and take of argument, from colleagues who complemented my limited expertise with their equally centered specialties and aptitudes for other aspects of these large and various problems. Thus, on the first leg or branch of hierarchy theory, I worked with Niles Eldredge on punctuated equilibrium, and with Elisabeth Vrba on levels of selection and sorting. On the second leg of structuralist al¬ ternatives to adaptationist argument, I worked with Dick Tewontin on span¬ drels, Elisabeth Vrba on exaptation, David Woodruff on the functional and structural morphology of Cenon, and with “the gang of four1' (increased to five with the later inclusion of Jack Sepkoski)—Dave Raup, Tom Schopf, Dan Simberloff, and me—on trying to specify how many aspects of apparently or¬ dered phyletic patterns, heretofore confidently attributed to selection for little reason beyond the visual appearance of order itself, could plausibly be gener¬ ated within purely random systems. And on the third leg of extrapolationism, my earliest interests in the logic and justification of uniformitarianism in phi¬ losophy, and of Lyellian perspectives in the history of science, could not have developed without advice and substantial aid (but not collaborative publica¬ tion this time) with historians Martin Rudwick, Reijer Hooykaas, and Cecil
27
THE STRUCTURE OF EVOEUTIONARY THEORY Schneer, and with philosophers Nelson Goodman, Bonnie Hubbard, and George Geiger. (Geiger, my mentor at Antioch College, was the last student of John Dewey and played with Lou Gehrig on the Columbia University base¬ ball team, thus embodying both my professional and avocational interests.) In fact, and as a comment within the sociology of science, I would venture that future historians might judge the'numerous seminal (and published) col¬ laborations between evolutionary biologists and professional philosophers of science as the most unusual and informative operational aspect of the recon¬ struction of evolutionary theory in the late 20th century. Research scientists tend to be a philistine lot, with organismic biologists perhaps at the head of this particular pack (for we work with “big things” that we can see and un¬ derstand at our own scale. Thus, we suppose that we can afford to be more purely empirical in our reliance on “direct” observation, and less worried about admittedly conceptual problems of evaluating things too small or too fast to see). Most of us would scoff at the prospect of working with a profes¬ sional philosopher, regarding such an enterprise as, at best, a pleasant waste of time and, at worst, an admission that our own clarity of thought had be¬ come addled (or at least as a fear that our colleagues would so regard our in¬ terdisciplinary collaboration). And yet, the conceptual problems presented by theories based on causes operating at several levels simultaneously, of effects propagated up and down, of properties emerging (or not) at higher levels, of the interaction of random and deterministic processes, and of predictable and contingent influ¬ ence, have proven to be so complex, and so unfamiliar to people trained in the simpler models of causal flow that have served us well for centuries (see the next section on Zeitgeist), that we have had to reach out to colleagues ex¬ plicitly trained in rigorous thinking about such issues. Thus, we learned, to our humbling benefit, that conceptual muddles do not necessarily resolve themselves “automatically” just because a smart person—namely one of us, trained as a scientist—finally decides to apply some raw, naive brain power to the problem. Professional training in philosophy does provide a set of tools, modes and approaches, not to mention a feeling for common dangers and fal¬ lacies, that few scientists (or few “smart folks” of any untrained persuasion) are likely to possess by the simple good fortune of superior raw brainpower. (We might analogize this silly and vainglorious, although regrettably com¬ mon, belief to the more popular idea that great athletes should be able to ex¬ cel at anything physical by reason of their general bodily virtue—a myth and chimaera that dramatically exploded several years ago when Michael Jordan discovered that he could not learn to hit a curve ball, just because he excelled so preeminently in basketball, and possessed the world’s best athletic body in general—for he ended up barely hitting over 0.200 in a full season of minor league play. I do, however, honor and praise his persistence in staying the course and taking his lumps.) Indeed, I know of no other substantial conceptual advance in recent science so abetted by the active collaboration of working scientists and professional philosophers (thus obviating, for once, the perennial, and justified, complaint
Defining and Revising the Structure of Evolutionary Theory of philosophers of science that no scientists read their journals or even en¬ counter their analyses). Several key achievements in modern evolutionary theory, particularly the successful resolution of conceptual difficulties in for¬ mulating a workable theory of hierarchical selection (rooted in concepts like emergence and simultaneous selection at several levels that our minds, with their preferences for two-valued logics, don’t handle either automatically, or well at all), have appeared as joint publications of biologists and philoso¬ phers, including the books of Sober and Wilson, 1998, and Eldredge and Grene, 1992; and articles of Sober and Lewontin, 1982, and Mayo and Gilinsky, 1987. My own understanding of how to formulate an operational theory of hierarchical selection, and my “rescue” from a crucial conceptual error that had stymied my previous thinking (see Chapter 8, pages 656-673), emerged from joint work with Elisabeth Lloyd, a professional philosopher of science. I take great pride in our two joint articles (Lloyd and Gould, 1993; Gould and Lloyd, 1999), which, in my partisan judgment, resolve what may have been the last important impediment to the codification of a conceptually coherent and truly operational theory of hierarchical selection. Zeitgeist. Although major revisions to the structure of evolutionary theory emerged mainly from the conventional substrates of novel data and clearer concepts, we should not neglect the admittedly fuzzier, but by no means unimportant, input from a distinctive social context, or intellectual “spirit of the times” (a literal meaning of Zeitgeist) that, at the dawn of a calendrical millennium, has suffused our general academic culture with a set of loosely coherent themes and concerns far more congenial with the broad revisions here proposed within evolutionary theory than any previous set of guiding concepts or presuppositions had been. Needless to say, Zeitgeists are two edged swords of special sharpness—for either they encourage sheeplike conformity with transient ghosts of time (another literal meaning of Zeitgeist) that will soon fade into oblivion, or they open up new paths to insights that previous ages could not even have conceptualized. Any intellectual would therefore be a fool to argue that conformity with a Zeitgeist manifests any preferential correlation with scientific veracity ipso facto. Zeitgeists can only suggest or facilitate. Nonetheless, we would be equally foolish in our naive empiricism if we claimed that major advances in science must be entirely data driven, and that social contexts can only act as barriers to our vision of nature’s factuality. Both the social and scientific world were “ready” for evolution in the mid 19th century. People of equal intelligence could neither have formulated nor owned such a concept in Newton’s generation, even if some hypothetical Dar¬ win had then advanced such a claim (and probably ended up in Bedlam for his troubles). In Chapter 2,1 shall document not only this general readiness of Western science within the Zeitgeist of Darwin’s time, but also the specific so¬ cial impetus that Darwin gained from studying the distinctive theories (also a product of the earlier Enlightenment Zeitgeist, and not accessible before) of Adam Smith and the Scottish economists. Thus, and by analogy a century later, the altered Zeitgeist of our own time may also facilitate a fruitful recon-
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THE STRUCTURE OF EVOEUTIONARY THEORY sideration of major evolutionary concepts that still bear the originating stamp of a Victorian scientific context strongly committed to unidirectional, singlelevel and deterministic views of natural causality—subtly controlling con¬ cepts that many scientists would now label #s limiting and outmoded. Although the next few paragraphs will be the most vague and impressionis¬ tic (I trust) of the entire book, I venture these ill-formulated statements about Zeitgeist because I feel that something important lurks behind my inability to express these inchoate thoughts with precision. I argue above (page 14) that the key concerns of the three essential branches of Darwinian logic might be identified as agency, efficacy and scope of natural selection. In each of these domains, I believe, the revised structure of evolutionary theory, as presented in this book, might be characterized as expansion and revision according to a set of coordinated principles, all consonant with our altered Zeitgeist vs. the scientific spirit of Darwin’s own time. The modern revision seeks to replace Darwin’s unifocal theory of orgamsmic selection with a hierarchical account (leg one); his unidirectional theory of adaptational construction in the func¬ tionalist mode with a more balanced interaction of these external causes, treating internal (or structural) constraints primarily as positive channels, and not merely as limitations (leg two); and his unilevel theory of micro¬ evolutionary extrapolation with a model of distinctive but interacting modes of change, each characteristic for its tier of time. In short, a hierarchy of inter¬ acting levels, each important in a distinctive way, for Darwin’s single locus; an interaction of environmental outsides with organic insides for Darwin’s single direction of causal flow; and a set of distinctive temporal tiers for Dar¬ win’s attempt to situate all causality in the single microevolutionary world of our own palpable moments. I do sense a common underlying vision behind all these proposed reforms. Strict Darwinism, although triumphant within mid 20th century evolutionary theory, embodied several broad commitments (philosophical or metatheoretical, in the technical sense of these terms), more characteristic of 19th than of 20th century thought (and, obviously, not necessarily wrong, or even to be discounted, for this reason—as nothing can be more dangerous to the prog¬ ress of science than winds of fashion, and we do, after all, learn some things, and develop some fruitful approaches, with validity and staying power well beyond their time of origin and initial popularity). Some aspects of Darwin’s formulation broke philosophical ground in a sense quite consonant with our modern Zeitgeist of emphasis upon complexity and interaction—particularly, Darwin’s focus on the interplay of chance and necessity in sources of varia¬ tion vs. mode of selection. Indeed, Darwin paid the usual price for such inno¬ vation in the failure of nearly all his colleagues, even the most intellectually acute, to grasp such a radical underlying philosophy. But, in many command¬ ing respects, Darwinism follows the norms of favored scientific reasoning in his time. The logic of Darwin’s formulation rests upon several preferences in scien¬ tific reasoning more characteristic of his time than of ours—preferences that
Defining and Revising the Structure of Evolutionary Theory many scientists would now view as unduly restrictive in their designation of a privileged locus of causality, a single direction of causal flow, and a smooth continuity in resulting effects. Classical Darwinism follows standard reduc¬ tionist preferences in designating the lowest level then available—the organ¬ ism, for Darwin—as an effectively unique locus of causality (the first leg of agency). In this sense, the efforts of Williams and Dawkins (see Chapter 8) to reduce the privileged locus even further to the genic level (perforce unavail¬ able to Darwin) should be read as a furthering and intensification of Darwin’s intent—in other words, a basically conservative adumbration of Darwin’s own spirit and arguments, and not the radical conceptual revision that some have imagined. At this single level of causality, classical Darwinism then envisages a simi¬ larly privileged direction of causal flow, as information from the environment (broadly construed, of course, to include other organisms as well as physical surroundings) must impact the causal agent (organisms struggling for repro¬ ductive success) and be translated, by natural selection, into evolutionary change. The organism supplies raw material in the form of “random” varia¬ tion, but does not “push back” to direct the flow of its own alteration from inside. Darwinism, in this sense, is a functionalist theory, leading to local ad¬ aptation as the environment proposes and natural selection disposes. Finally, classical Darwinism completes a trio of privileged causal places and conse¬ quently directional flows by postulating strict continuity in results, as local selection scales smoothly through the immensity of geological time to engen¬ der life’s history by pure extrapolation of lowest-level modes and causes. By contrast, the common themes behind the reformulations defended in this book all follow from serious engagement with complexity, interaction, multiple levels of causation, multidirectional flows of influence, and pluralis¬ tic approaches to explanation in general-—a set of integrated approaches that strongly contribute to the Zeitgeist of our moment. To anticipate and make a preemptive strike against the obvious counterattack from Darwinian tradi¬ tionalists, these alternative themes do not substitute a “laid back, laissezfaire, anything goes” kind of sloppy tolerance for contradiction and fuzziness in argument against the genuine rigor of old-line Darwinism. The social and psychological contributions of a Zeitgeist to the origin of hypotheses bear no logical relationship to any subsequent scientific defense and validation of the same hypotheses. Moreover, on this subject of test and confirmation, I es¬ pouse a rigorously conventional and rather old-fashioned “realist” view that an objective factual world exists “out there,” and that science can access its ways and modes. Whatever the contribution of a Victorian Zeitgeist to Darwin’s thinking, or of a contemporary Zeitgeist to our revisions, the dif¬ ferences are testable and subject to validation or disproof by the usual arma¬ mentarium of scientific methods. That is, either Darwin is right and effec¬ tively all natural selection occurs at the orgamsmic level (despite the logical conceivability of other levels), or the hierarchical theory is right and several levels make interestingly different and vitally important simultaneous con-
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THE STRUCTURE OF EVOLUTIONARY THEORY tributions to the overall pattern of evolution. The same ordinary form of testability can be applied to any other contrast between strict Darwinism and the revised and expanded formulations defended in this book. As the most striking general contrast that plight be illuminated by reference to the different Zeitgeists of Darwin’s time and our own, modern revisions for each essential postulate of Darwinian logic substitute mechanics based on in¬ teraction for Darwin’s single locus of causality and directional flow of effects. Thus, for Darwin’s near exclusivity of organismic selection, we now propose a hierarchical theory with selection acting simultaneously on a rising set of levels, each characterized by distinctive, but equally well-defined, Darwinian individuals within a genealogical hierarchy of gene, cell-lineage, organism, deme, species, and clade. The results of evolution then emerge from complex, but eminently knowable, interactions among these potent levels, and do not simply flow out and up from a unique causal locus of organismal selection. A similar substitution of interaction for directional flow then pervades the second branch of selection’s efficacy, as Darwin’s functionalist formulation— with unidirectional flow from an external environment to an isotropic or¬ ganic substrate that supplies “random” raw material but imposes no direc¬ tional vector of its own to “push back” from internal sources of constraint— yields to a truly interactive theory of balance between the functionalist Dar¬ winian “outside” of natural selection generated by environmental pressures, and a formalist “inside” of strong, interesting and positive constraints gen¬ erated by specific past histones and timeless structural principles. Finally, on the third and last branch of selection’s range, the single and control¬ ling microevolutionary locus of Darwinian causality yields to a multileveled model of tiers of time, with a unified set of processes working in distinctive and characteristic ways at each scale, from allelic substitution in observable years to catastrophic decimation of global biotas. Thus, and in summary, for the unifocal and noninteractive Darwinian models of exclusively organismal selection, causal flow from an environmental outside to an organismal in¬ side, and a microevolutionary locus for mechanisms of change that smoothly extrapolate to all scales, we substitute a hierarchical selectionist theory of numerous interacting levels, a balanced and bidirectional flow of causal¬ ity between external selection and internal constraint (interaction of func¬ tionalist and structuralist perspectives), and causal interaction among tiers of time. Among the many consequences of these interactiomst reformulations, punctuational rather than continuationist models of change (with stronger structuralist components inevitably buttressing the punctuational versions) may emerge as the most prominent and most interesting. The Darwinian me¬ chanics of functionalism yield an expectation of continuously improving lo¬ cal adaptation, with longterm stability representing the achievement of an optimum. But interactionist and multi-leveled models of causality reconcep¬ tualize stasis as a balance, actively maintained among potentially competing forces at numerous levels, with change then regarded as exceptional rather than intrinsically ticking most of the time, and punctuational rather than
Defining and Revising the Structure of Evolutionary Theory
smoothly continuous when it does occur (representing the relatively quick transition that often accompanies a rebalancing of forces). To end this admittedly vague section with the punch of paradox (and even with a soundbite), I would simply note the almost delicious irony that the for¬ mulation of a hierarchical theory of selection—the central concept of this book, and invoking a non-vernacular meaning of hierarchy in the purely structural sense of rising levels of inclusivity—engenders, as its most impor¬ tant consequence, the destruction of a different and more familiar meaning of hierarchy: that is, the hierarchy of relative value and importance embod¬ ied in Darwin’s privileging of orgamsmic selection as the ultimate source of evolutionary change at all scales. Thus, a structural and descriptive hierarchy of equally effective causal levels undermines a more conventional hierarchy of relative importance rooted in Darwin’s exclusive emphasis on the micro¬ evolutionary mechanics of orgamsmal selection. And so, this structuralist view of nature’s order enriches the structure of evolutionary theory—carry¬ ing the difference between strict Darwinism and our current understanding through more than enough metatheoretical space to fashion a Falconerian, not merely a Darwinian, rebuilding and extension for our edifice of coherent explanation.
A PERSONAL ODYSSEY For reasons beyond mere self-indulgence or egotism, I believe that defenders of such general theories about large realms of nature owe their readers some explanation for the personal bases and ontogeny of their choices—for at this level of abstraction, no theory can claim derivation by simple logical or em¬ pirical necessity from observed results, and all commitments, however well defended among alternative possibilities, will also be influenced by authorial preferences of a more contingent nature that must then be narrated in order to be understood. Moreover, and in this particular case, the structure of this book includes a set of vigorously idiosyncratic features that, if not acknowl¬ edged and justified, might obscure the far more important raison d’etre for its composition: the presentation of a tight brief for substantial reformulation in the structure of evolutionary theory, with all threads of revision conceptually united into an argument of different thrust and form, but still sufficiently con¬ tinuous with its original Darwinian base to remain within the same intellec¬ tual lineage and logic. Two aspects of my idiosyncratic procedures require explicit commentary here because, at least as my intention, they should reinforce this book’s cen¬ tral argument for coherence (logical, historical and empirical) of the revised and general structure of evolutionary theory, and not further the opposite, al¬ beit customary, function of such “confessional” writing—namely, to slake authorial egos, fight old battles, and relate twice-told tales to one’s own ad¬ vantage (although I claim no immunity from these all too human foibles). This book will be published in the Spring of 2002, an auspicious and palindromic year just one step out of the starting gate for a new millennium.
Jd
4
THE STRUCTURE OF EVOLUTIONARY THEORY At the same time, and fortuitously, my 10th and last volume of monthly essays in Natural History Magazine, written without a single break from Janu¬ ary 1974 to January 2001, will also appear in print. In an eerie coincidence (with no meaning that I can discern), my first technical book, Ontogeny and Phytogeny, appeared exactly 25 years before, in 1977, also at the same time as my first book of Natural History essays, Ever Since Darwin. This odd and twofold simultaneous appearance, 25 years apart, of my best youthful efforts in the contrasting (but not really conceptually different) realms of technical and popular science, and then of my best shots from years of greater maturity in the same two realms, has forced me to think long and hard about the meaning of continuity, commitment and personal perspective. My popular volumes fall into the explicit and well recognized category of essays, a literary genre defined, ever since Montaigne’s initiating 16th century efforts, as the presentation of general material from an explicitly personal and opinionated point of view—although the best essays (literally meaning “attempts,’’ after all) tend to be forthright in their expression of opinions, generous (or at least fair) to other views, and honest in their effort to specify the basis of authorial preferences. On the other hand, technical treatises in science do not generally receive such a license for explicitly personal expres¬ sion. 1 believe that this convention m technical writing has been both harmful and more than a bit deceptive. Science, done perforce by ordinary human beings, expressing ordinary motives and foibles of the species, cannot be grasped as an enterprise without some acknowledgment of personal dimen¬ sions in preferences and decisions—for, although a final product may display logical coherence, other decisions, leading to other formulations of equally tight structure, could have been followed, and we do need to know why an author proceeded as he did if we wish to achieve our best understanding of his accomplishments, including the general worth of his conclusions. Logical coherence may remain formally separate from ontogenetic con¬ struction, or psychological origin, but a full understanding of form does re¬ quire some insight into intention and working procedure. Perhaps some pre¬ sentations of broad theories in the history of science—Newton’s Principia comes immediately to mind—remain virtually free of personal statement (sometimes making them, as in this case, virtually unreadable thereby). But most comprehensive works, in all fields of science, from Galileo’s Dialogo to Darwin’s Origin, gain stylistic strength and logical power by their suffu¬ sion with honorable statements about authorial intents, purposes, prejudices, and preferences. I cannot think of a single major book in natural history— from Buffon’s Histoire naturelle and Cuvier’s Ossemens fossiles to Simpson’s Tempo and Mode, and Mayr's Animal Species—that does not include such extensive personal information, either in explicit sections, or inserted by-theby throughout. (Even so abstract a presentation as R. A. Fisher’s Genetical Th eory of Natural Selection gains greatly in comprehension through its long and final, if in retrospect regrettable, section on the author’s idiosyncratic eugenical views about human improvement.) I have included personal discus¬ sion throughout this text, but let me also devote a few explicit pages to the
Defining and Revising the Structure of Evolutionary Theory two points that I regard as most crucial to understanding the general argu¬ ment through (or despite) conscious idiosyncrasies in my presentation.
Elistory Many technical treatises in science begin with a short section on previous his¬ tory of work in the field—usually written in the hagiographical mode to de¬ pict prior history as a march towards final truths revealed in the current vol¬ ume. Sometimes, authors get a bit carried away, and these historical sections expand into substantial parts of the final book. Lest anyone make the false in¬ ference that my full first half of history arose in this haphazard and initially unintended way, I hasten to assure readers that my final result was my inten¬ tion from the start. For several reasons, I always conceived this book as a smooth joining of two halves, roughly equal in length and importance. First, and ontogenetically, I had written my earlier technical book, Ontogeny and Pbylogeny, in this admittedly unusual manner—and I remain pleased with both the distinc¬ tiveness and the efficacy of the result. Second, I believe that the history of evo¬ lutionary thought, and probably of any other subject imbued with such im¬ portance to our lives and to our understanding of nature, constitutes an epic tale of fascinating, and mostly honorable, people engaged in a great struggle to comprehend something very deep and very difficult. Thus, such histories capture a bit of the best in us (also of the worst, but all human endeavors so conspire)—a bit, moreover, that cannot be expressed in any other way. We re¬ ally do need to honor the temporal substrate of our current understanding, not only as a guide to our continuing efforts, but also as a moral obligation to our forebears. But a third and practical reason trumps all others. Although I would not state such a claim as a generality for all scientific analyses, in this particular case I do not see how the structure of evolutionary theory can be resolved and the appropriate weights of relative importance assigned to the different com¬ ponents thereof, absent such a historical perspective. (Would it not be odd to claim, in any case, that the quintessential science for resolving the nature of life’s history can itself be understood as a pristine construction, a fully-formed conceptual entity drawn intact from some analog of Zeus’s brow, rather than an “organic” structure of ideas with its own ontogeny and history?) To give one example at the largest and at the smallest scales of my argu¬ ment, I don’t know how I could have properly defended my identification and explication of the threefold essence of Darwinian logic without documenting the history of theoretical debate in order to tease out the components that have always been most troubling, most central, and most directive. A pure description of the theory’s abstract logic simply will not suffice. To epitomize, I have identified these essential components on three basic grounds: that logic compels (Chapter 2), that history validates (Chapters 3-7), and that current debate reaffirms (Chapters 8-12). The middle term of this epitome unites the end members; I cannot present a coherent or compelling defense without this linkage. The three issues of agency, efficacy and scope build the Darwinian es-
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THE STRUCTURE OF EVOLUTIONARY THEORY sence both because the logical structure of the theory so dictates, and because the history and current utility of the theory so document. To complement this most general statement with just one example of the utility of historical analysis at the smaller scale of details, I offer the following case as the strongest argument for my central claim that Darwin’s brave at¬ tempt to construct a single-level, exclusively organismic theory of natural se¬ lection must fail in principle, and that all selectionists must eventually own a hierarchical model. What better evidence can we cite than the historical dem¬ onstration (see Chapters 3 and 5 for details) that each of the only three foun¬ dational thinkers who truly understood the logic of selectionism—August Weismann, Hugo de Vries, and Charles Darwin himself—tried mightily to make the single-level version work as a fully sufficient explanation for evolu¬ tion. And each failed, after intense intellectual struggle, and for fascinatingly different reasons documented later in the book—Darwin for explaining di¬ versity by reluctant resort to species selection; Weismann for a strongest ini¬ tial commitment to a single level, and an eventual recognition of full hierar¬ chy as the most important and distinctive conclusion of his later career (by his own judgment); and de Vries for reconciling his largely psychological fealty to Darwin as his intellectual hero, with his clearly non-Darwinian account of the origin of species and the explanation of trends (including an explicit coin¬ ing of the term “species selection” for explaining cladal patterns). One might cite various truisms telling us that people ignorant of history will be condemned to repeat its errors. But I would rather re-express this ac¬ curate and rueful observation in a more positive manner by illustrating the power of historical analysis to aid both our current understanding and the depth of our appreciation for the intellectual importance of our enterprise. Finally, and to loosen the rein on personal bravado that I usually try to hold at least somewhat in check, no scholar should impose a project of this length upon his colleagues unless he believes that some quirk of special skill or expe¬ rience permits him to proceed in a unique manner that may offer some insight to others. In my case, and only by history’s fortune of no immediate competi¬ tion in a small held, I may be able to combine two areas of professional com¬ petence not otherwise conjoined among current evolutionists. I am not a credentialed historian of science, but I believe that I have done sufficient work in this held (with sufficient understanding of the difference between the Whiggish dilettantism of most enthusiastic amateurs, and the rigorous meth¬ ods applied by serious scholars) to qualify as adequately knowledgeable. (At least I can read all the major works in their original languages, and I stay close to the “internalist” style of analysis that people who understand the logic and history of theories, but cannot claim truly professional expertise in the “externalist” factors of general social and historical context, can usefully pursue.) Meanwhile I am, for my sins, a lifelong and active professional pale¬ ontologist, a commitment that began at age hve as love at hrst sight with a di¬ nosaur skeleton. Many historians possess deeper knowledge and understanding of their im¬ mediate subject than I could ever hope to acquire, but none enjoy enough in-
Defining and Revising the Structure of Evolutionary Theory timacy with the world of science (knowing its norms in their bones, and its quirks and foibles in their daily experience) to link this expertise to contem¬ porary debates about causes of evolution. Many more scientists hold superb credentials as participants in current debates, but do not know the historical background. As I hope I demonstrated by practical utility in The Mismeasure of Man (Gould, 1981a), a small and particular—but I think quite impor¬ tant—intellectual space exists, almost entirely unoccupied, for people who can use historical knowledge to enlighten (not merely to footnote or to pret¬ tify) current scientific debates, and who can then apply a professional’s “feel” for the doing of science to grasp the technical complexities of past debates in a useful manner inaccessible to historians (who have therefore misinter¬ preted, in significant ways, some important incidents and trends in their sub¬ ject). I only hope that I have not been wrong in believing that my devotion of a lifetime’s enthusiasm to both pursuits might make my efforts useful, in a distinctive way, to my colleagues.
Theory I admire my friend Oliver Sacks extravagantly as a writer, and I could never hope to match him in general quality or human compassion. He once said something that touched me deeply, despite my continuing firm disagreement with his claim (while acknowledging the validity of the single statement rele¬ vant to the present context). Oliver said that he envied me because, although we had both staked out a large and generous subject for our writing (he on the human mind, me on evolution), I had enjoyed the privilege of devising and developing a general theory that allowed me to coordinate all my work into a coherent and distinctive body, whereas he had only written descrip¬ tively and aimlessly, albeit with some insight, because no similar central focus underlay his work. I replied that he had surely sold himself short, because he had been beguiled by conventional views about the nature and limits of what may legitimately be called a central scientific theory—and that he certainly held such an organizing concept in his attempt to reintroduce the venerable “case study method” of attention to irreducible peculiarities of individual pa¬ tients in the practice of cure and healing in medicine. Thus, I argued, he held a central theory about the importance of individuality and contingency in gen¬ eral medical theory, just as I and others had stressed the centrality of histori¬ cal contingency in any theoretical analysis and understanding of evolution and its actual results. Oliver saw the theory of punctuated equilibrium itself, which I developed with Niles Eldredge and discuss at inordinate length in Chapter 9, as my co¬ ordinating centerpiece, and I would not deny this statement. But punctuated equilibrium stands for a larger and coherent set of mostly iconoclastic con¬ cerns, and I must present some intellectual autobiography to explain the rea¬ sons and the comings together, as best I understand them myself—hence my rip-off of Cardinal Newman’s famous title for the best similar effort ever made, albeit in a maximally different domain. In his Apologia Pro Vita Sua (an apology for one’s own life), Newman intends the operative word as I do,
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THE STRUCTURE OE EVOEUTIONARY THEORY in its original and positive meaning, not in the currently more popular nega¬ tive sense—“something said or written in defense or justification of what ap¬ pears to others to be wrong or of what may be liable to disapprobation” (per Webster’s). As my first two scientific commitments, I fell in love with paleontology when I met Tyrannosaurus in the Museum of Natural History at age five, and with evolution at age 11, when I read G. G. Simpson’s The Meaning of Evolu¬ tion, with great excitement but minimal comprehension, after my parents, as members of a book club for folks with intellectual interests but little eco¬ nomic opportunity or formal credentials, forgot to send back the “we don’t want anything this month” card, and received the book they would never have ordered (but that I begged them to keep because I saw the little stick fig¬ ures of dinosaurs on the dust jacket). Thus, from day one, my developing pro¬ fessional interests united paleontology and evolution. For some reason still unclear to me, I always found the theory of how evolution works more fasci¬ nating than the realized pageant of its paleontological results, and my major interest therefore always focused upon principles of macroevolution/1' I did come to understand the vague feelings of dissatisfaction (despite Simpson’s attempt to resolve them in an orthodox way by incorporating paleontology within the Modern Synthesis) that some paleontologists have always felt with the Darwinian premise that microevolutionary mechanics could construct their entire show just by accumulating incremental results through geological immensity. As I began my professional preparation for a career in paleontology, this vague dissatisfaction coagulated into two operational foci of discontent. First (and with Niles Eldredge, for we worried this subject virtually to death as graduate students), I became deeply troubled by the Darwinian convention that attributed all non-gradualistic literal appearances to imperfections of the geological record. This traditional argument contained no logical holes, but the practical consequences struck me as unacceptable (especially at the out¬ set of a career, full of enthusiasm for empirical work, and trained in statis¬ tical techniques that would permit the discernment of small evolutionary
*As so much unnecessary rancor has been generated by simple verbal confusion among different meanings of this word, and not by meaningful conceptual disagreements, I should be clear that I intend only the purely descriptive definition when I write “macroevolu¬ tion”—that is, a designation of evolutionary phenomenology from the origin of species on up, in contrast with evolutionary change within populations of a single species. In so doing, I follow Goldschmidt’s own definitional preferences (1940) in the book that established his apostasy within the Modern Synthesis. Misunderstanding has arisen because, to some, the world “macroevolution" has implied a theoretical claim for distinct causes, particularly for nonstandard genetic mechanisms, that conflict with, or do not occur at, the microevo¬ lutionary level. But Goldschmidt—and I follow him here—urged a nonconfrontational definition that could stand as a neutral descriptor for a set of results that would then permit evolutionists to pose the tough question without prejudice: does macroevolutionary phe¬ nomenology demand unique macroevolutionary mechanics? Thus, in this book, “macro¬ evolution” is descriptive higher-level phenomenology, not pugnacious anti-Darwinian in¬ terpretation.
Defining and Revising the Structure of Evolutionary Theory changes). For, by the conventional rationale, the study of microevolution be¬ came virtually nonoperational in paleontology—as one almost never found this anticipated form of gradual change up geological sections, and one there¬ fore had to interpret the vastly predominant signal of stasis and geologically abrupt appearance as a sign of the record’s imperfection, and therefore as no empirical guide to the nature of evolution. Second, I became increasingly dis¬ turbed that, at the higher level of evolutionary trends within clades, the ma¬ jority of well documented examples (reduction of stipe number in graptolites, increasing symmetry of crinoidal cups, growing complexity of ammonoid su¬ tures, for example) had never been adequately explained in the terms de¬ manded by Darwinian convention—that is, as adaptive improvements of constituent organisms in anagenetic sequences. Most so-called explana¬ tions amounted to little more than what Lewontin and I, following Kipling, would later call “just-so stories,” or plausible claims without tested evidence, whereas other prominent trends couldn’t even generate a plausible story in adaptationist terms at all. As Eldredge and I devised punctuated equilibrium, I did use the theory to resolve these two puzzles to my satisfaction, and each resolution, when finally generalized and further developed, led to my two major critiques of the first two branches of the essential triad of Darwinian central logic—so Oliver Sacks’s identification of punctuated equilibrium as central to my theoretical world holds, although more as a starting point than as a coordinating focus. By accepting the geologically abrupt appearance and subsequent extended stasis of species as a fair description of an evolutionary reality, and not only as a sign of the poverty of paleontological data, we soon recognized that spe¬ cies met all criteria for definition and operation as genuine Darwinian indi¬ viduals in the higher-level domain of macroevolution—and this insight (by complex routes discussed in Chapter 9) led us to concepts of species selection in particular and, eventually, to the full hierarchical model of selection as an interesting theoretical challenge and contrast to Darwinian convictions about the exclusivity of organismal selection. In this way, punctuated equilibrium led to the reformulation proposed herein for the first branch of essential Dar¬ winian logic. Meanwhile, in trying to understand the nature of stasis, we initially fo¬ cused (largely in error, I now believe) upon internal constraints, as vaguely represented by various concepts of “homeostasis,” and as exemplified in the model of Gabon’s polyhedron (see Chapter 4). These thoughts led me to extend my doubts about adaptation and the sufficiency of functionalist mechanisms in general—especially in conjunction with my old worries about paleontological failures to explain cladal trends along traditional adapta¬ tionist lines. Thus, these aspects of punctuated equilibrium strongly contrib¬ uted to my developing critiques of adaptationism and purely functional me¬ chanics on the second branch of essential Darwinian logic (although other arguments struck me as even more important, as discussed below). Nonetheless, and despite the centrality of punctuated equilibrium in devel¬ oping a broader critique of conventional Darwinism, my sources extended
39
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THE STRUCTURE OF EVOTUTIONARY THEORY outward into a diverse and quirky network of concerns that seemed, to me and at first, isolated and uncoordinated, and that only later congealed into a coherent critique. For this curious, almost paradoxical, reason, I have be¬ come even more convinced that the elements of my overall critique hang to¬ gether, for I never sensed the connections when I initially identified the com¬ ponents as, individually, the most challenging and intriguing items I had encountered in my study of evolution. When one accumulates a set of things only for their independent appeals, with no inkling that any common intellec¬ tual ground underlies the apparent miscellany, then one can only gain con¬ fidence in the “reality” of a conceptual basis discerned only later for the cohe¬ sion. I would never argue that this critique of strict Darwinism gains any higher probability of truth value for initially infecting me in such an uncoor¬ dinated and mindless way. But I would assert that a genuinely coherent and general alternative formulation must exist “out there” in the philosophical universe of intellectual possibilities—whatever its empirical validity—if its isolated components could coagulate, and be discerned and selected, so un¬ consciously. If I may make a somewhat far-fetched analogy to my favorite Victorian novel, Daniel Deronda (the last effort of Darwin’s friend George Eliot), the hero of this story, a Jew raised in a Christian family with no knowledge of his ethnic origins, becomes, as an adult, drawn to a set of apparently inde¬ pendent activities with no coordinating theme beyond their relationship, en¬ tirely unknown to Deronda at the time of his initial fascination, to Jewish history and customs. Eventually, he recognizes the unifying theme behind such apparent diversity, and learns the truth of his own genetic background. (I forgive Eliot for this basically silly fable of genealogical determinism be¬ cause her philosemitic motives, however naive and a bit condescending, shine forth so clearly in the surrounding antisemitic darkness of her times.) But I do feel, to complete the analogy, rather like a modern, if only culturally or psy¬ chologically predisposed, Deronda who gathered the elements of a coherent critique solely because he loved each item individually—and only later sensed an underlying unity, which therefore cannot be chimaerical, but may claim some logical existence prior to any conscious formulation on my part. In fact, the case for an external and objective coherence of this alternative view of evolution seems even stronger to me because I gathered the indepen¬ dent items not only in ignorance of their coordination, but also at a time when I held a conscious and conventional view of Darwinian evolution that would have actively denied their critical unity and meaning. I fledged in sci¬ ence as a firm adaptationist, utterly beguiled by the absolutist beauty (no doubt, my own simplistic reading of a more subtle, albeit truly hardened, Modern Synthesis) of asserting, a la Cain and other ecological geneticists of the British school, that all aspects of organismal phenotypes, even the most trivial nuances, could be fully explained as adaptations built by natural selec¬ tion. I remember two incidents of juvenilia with profound embarrassment to¬ day: First, an undergraduate evening bull session with the smartest physics
Defining and Revising the Structure of Evolutionary Theory major at Antioch College, as his skepticism evoked my stronger insistence that our science matched his in reductiomstic rigor because “we” now knew for certain that natural selection built everything for optimal advantage, thus making evolution as quantifiable and predictive as classical physics. Second, as a somewhat more sophisticated, but still beguiled, assistant professor, 1 re¬ member my profound feeling of sadness and disappointment, nearly amount¬ ing to an emotional sense of betrayal, upon learning that an anthropological colleague favored drift as the probable reason for apparently trivial genetic differences among isolated groups of Papua-New Guinea peoples. I remem¬ ber remonstrating with him as follows: Of course your argument conforms to logic and empirical possibility, and I admit that we have no proof either way. But your results are also consistent with selection—and our panselectionist paradigm has forged a theory of such beauty and elegant simplicity that one should never favor exceptions for their mere plausibility, but only for docu¬ mented necessity. (I recall this discussion with special force because my emo¬ tional feelings were so strong, and my disappointment in his “unnecessary apostasy” so keen, even though I knew that neither of us had the empirical “goods.”) Finally, if I could, in a species of Devil’s bargain, wipe any of my publications off the face of the earth and out of all memory, I would gladly nominate my unfortunately rather popular review article on “Evolutionary paleontology and the science of form” (Gould, 1970a)—a ringing paean to selectionist absolutism, buttressed by the literary barbarism that a “quantifunctional” paleontology, combining the best of biometric and mechanical analyses, could prove panadaptationism even for fossils that could not be run through the hoops of actual experiments. Against this orthodox background—or, rather, within it and quite uncon¬ sciously for many years—I worked piecemeal, producing a set of separate and continually accreting revisionary items along each of the branches of Darwin¬ ian central logic, until I realized that a “Platonic” something “up there” in ideological space could coordinate all these critiques and fascinations into a revised general theory with a retained Darwinian base. The first branch of levels in selection proceeded rather directly and linearly because the generality flowed so clearly from punctuated equilibrium itself, once Eldredge and I finally worked through the implications and extensions of our own formulations (Eldredge and Gould, 1972). Steve Stanley (1975) and Elisabeth Vrba (1980) helped to show us what we had missed in rami¬ fications leading from the phenomenology of stasis and geologically abrupt appearance, to recognizing species as genuine Darwinian individuals, to des¬ ignating species as, therefore and potentially, the basic individuals of macro¬ evolution (comparable with the role of the organism in microevolution), to the validity of species selection, and eventually to the full hierarchical model and its profound departure from the exclusively organismal accounts of con¬ ventional Darwinism (or the even more reduced and equally monistic genic versions of Williams and Dawkins)—see Vrba and Gould, 1986. Finally, by adopting the interactor rather than the replicator approach to defining selec¬ tion, and by recognizing emergent fitness, rather than emergent characters, as
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4
42
THE STRUCTURE OF EVOLUTIONARY THEORY the proper criterion for identifying higher-level selection (Lloyd and Gould, 1993; Gould and Lloyd, 1999), I think that we finally reached, by a circuitous route around many stumbling blocks of my previous stupidity, a consistent and truly operational theory of hierarchical selection (see Chapter 8). I must also confess to some preconditioning beyond punctuated equilib¬ rium. I had admired Wynne-Edwards’s pluck (1962) from the start, even though I agreed with Williams’s (1966) trenchant criticisms of his particular defenses for group selection, rooted in the ability of populations to regulate their own numbers in the interests of group advantage. Still, I felt, for no rea¬ son beyond vague intuition, that group selection made logical sense and might well find other domains and formulations of greater validity—a feeling that has now been cashed out by modern reformulations of evolutionary the¬ ory (see especially Wilson and Sober, 1998, and Chapters 8 and 9 herein). My odyssey on the second branch of balancing internal constraint with external adaptation in understanding the patterning and creative population of novel places in evolutionary morphospace followed a much more com¬ plex, meandering and diverse set of pathways. As an undergraduate, I loved D’Arcy Thompson’s Growth and Form (1917; see Gould, 1971b, for my first “literary” paper), and wrote a senior thesis on his theory of morphology. But I thought that I admired the book only for its incomparable prose, and I at¬ tacked the anti-Darwinian (and structuralist) components of his theory un¬ mercifully. I then took up allometry for my first empirical studies, somehow fascinated by structural constraint and correlation of growth, but thinking all the while that my task must center on a restoration of adaptationist themes to this “holdout” bastion of formalist thought—particularly the achievement of biomechanical optima consistent with the Galilean principle of decreasing surface/volume ratios with increasing size in isometric forms. I remain proud of my first review article, dedicated to this subject (Gould, 1966), written when I was still a graduate student, but I am now embarrassed by the fervor of my adaptationist convictions. I emphasized allometric analysis, now in a directly multivariate reformula¬ tion, in my first set of empirical studies on the Bermudian pulmonate snail Toecilozonites (see especially Gould, 1969—the published version of my Ph.D. dissertation). And yet, of all the long and largely adaptationist treatises in this series, and for some reason that I could not identify at the time, the conclusion that I reached with most satisfaction, and that I somehow re¬ garded as most theoretically innovative (without knowing why), resided in a short, and otherwise insignificant, article that I wrote for a specialized pale¬ ontological journal on a case of convergence produced by structural necessity, given modes of coiling and allometry in this genus, rather than by selectionist honing (for some cases rested upon ecophenotypic expression, others on paedomorphosis, and still others on gradual change that could be read as conventionally adaptive): “Precise but fortuitous convergence in Pleistocene land snails” (Gould, 1971c). Five disparate reasons underlie my more explicit recognition, during the 1970’s and early 1980’s, of the importance and theoretical interest (and icon-
Defining and Revising the Structure of Evolutionary Theory oclasm versus Darwinian traditions) of nonadaptationist themes rooted in structural and historical constraint. First, I stood under the dome of San Marco during a meeting in Venice and then wrote a notorious paper with Dick Lewontin on the subject of spandrels, or nonadaptive sequelae of prior structural decisions (Gould and Lewontin, 1979—see Chapter 11, pp. 12461258). Second, I recognized, with Elisabeth Vrba, that the lexicon of evolu¬ tionary biology possessed no term for the evidently important phenomenon of structures coopted for utility from different sources of origin (including nonadaptive spandrels), and not directly built as adaptations for their cur¬ rent function. We therefore devised the term “exaptation” (Gould and Vrba, 1982) and explored its implications for structuralist revisions to pure Dar¬ winian functionalism. Third, I worked with a group of paleontological col¬ leagues (Raup et al, 1973; Raup and Gould, 1974; Gould et al., 1977) to develop more rigorous criteria for identifying the signals that required selec¬ tionist, rather than stochastic, explanation of apparent order in phyletic pat¬ terns. This work left me humbled by the insight that our brains seek pattern, while our cultures favor particular kinds of stories for explaining these pat¬ terns—thus imposing a powerful bias for ascribing conventional determinis¬ tic causes, particularly adaptationist scenarios in our Darwinian traditions, to patterns well within the range of expected outcomes in purely stochastic sys¬ tems. This work sobered me against such a priori preferences for adaptation¬ ist solutions, so often based upon plausible stories about results, rather than rigorous documentation of mechanisms. Fourth, and most importantly, I read the great European structuralist liter¬ atures in writing my book on Ontogeny and Phylogeny (Gould, 1977b). I don’t see how anyone could read, from Goethe and Geoffroy down through Severtzov, Remane and Riedl, without developing some appreciation for the plausibility, or at least for the sheer intellectual power, of morphological ex¬ planations outside the domain of Darwinian functionalism—although my resulting book, for the last time in my career, stuck closely to selectionist or¬ thodoxy, while describing these alternatives in an accurate and sympathetic manner. Fifth, my growing unhappiness with the speculative character of many adaptationist scenarios increased when, starting in the mid 1970’s, the growing vernacular (and some of the technical) literature on sociobiology touted conclusions that struck me as implausible, and that also (in some cases) ran counter to my political and social beliefs as well. Personal distaste, needless to say, bears no necessary relationship to scien¬ tific validity. After all, what could be more unpleasant, but also more fac¬ tually undeniable, than personal mortality? But when distasteful conclusions gain popularity by appealing to supposedly scientific support, and when this “support” rests upon little more than favored speculation in an orthodox mode of increasingly dubious status, then popular misuse can legitimately sharpen a scientist’s sense of unhappiness with the flawed theoretical basis be¬ hind a particular misuse. In any case, I trust that this compendium of reasons will dispel Cain’s (1979) hurtful assertion that Lewontin, I, and other evolu¬ tionists who questioned early forms of sociobiology by developing a general
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THE STRUCTURE OF EVOLUTIONARY THEORY critique of adaptationism, had acted cynically, and even anti-scientifically, in opposing biological theories that we knew to be true because we disliked their political implications for explaining human behavior. My own growing doubts about adaptationism arose from several roots, mostly paleontologi¬ cal, with any displeasure about sociobiology serving as a late and minor spur to further examination and synthesis. I then tried to apply my general critique of pure Darwinian functionalism, and my conviction that important and positive constraints could be actively identified by quantitative morphometric study (and not merely passively in¬ ferred from failures of adaptationist scenarios) in my work on “covariance sets” in the growth, variation, and evolution of the West Indian pulmonate Cenon (Gould, 1984b and c), a snail that encompasses its maximal diver¬ sity in overt form among populations within a constraining set of pervasive allometries in growth. I discuss some of this work in my text on the empirical validation of positive constraint (see Chapter 10, pages 1045-1051). My doubts on the third branch of extrapolationism and uniformity began even earlier, and in a more inchoate way, but then gained expression in my ef¬ forts in the history of science, and not so much in my direct empirical work—hence, in part, the reduced attention devoted to this theme (Chapters 6 and 12) compared with the first two branches of selection’s agency and efficacy. On a fieldtrip in my freshman geology course, my professor took us to a trav¬ ertine mound and argued that the deposit must be about 11,000 years old because he had measured the current rate of accumulation and then extrapo¬ lated back to a beginning. When I asked how he could assume such con¬ stancy of rate, he replied that the fundamental rule of geological inference, something called “the principle of uniformitarianism” permitted such infer¬ ences because we must regard the laws of nature as constant if we wish to reach any scientific conclusions about the past. This argument struck me as logically incorrect, and I pledged myself to making a rigorous analysis of the reasons. As a joint major in geology and philosophy, I studied this issue throughout my undergraduate years, producing a paper entitled “Hume and uniform¬ itarianism” that eventually transmogrified into my first publication (Gould, 1965), “Is uniformitarianism necessary?” (Norman Newell, my graduate ad¬ visor, urged me to send the paper to Science where, as I learned to my amuse¬ ment much later, my future “boss” at Harvard, the senior paleontology pro¬ fessor Bernie Kummel, rejected it roundly as a reviewer. Properly humbled— although I still regard his reasons as ill founded—I then sent the paper to a specialty journal in geology.) May I share one shameful memory of this otherwise iconoclastic first paper, from which I still draw some pride? In my undergraduate work on this theme, I made a personal discovery (as others did independently) that became impor¬ tant in late 20th-century studies of the history of geology. I had been schooled in the conventional view that the catastrophists (aka “bad guys”) had in¬ voked supernatural sources of paroxysmal dynamics in order to compress the earth’s history into the strictures of biblical chronology. I read and reread all
Defining and Revising the Structure of Evolutionary Theory the classical texts of late 18th and early 19th century catastrophism in their original languages—and I could find no claim for supernatural influences upon the history of the earth. In fact, the catastrophists seemed to be advanc¬ ing the opposite claim that we should base our causal conclusions upon a lit¬ eral reading of the empirical record, whereas the umformitarians (aka “good guys”) seemed to be arguing, in an opposing claim less congenial with the ste¬ reotypical empiricism of science, that we must make hypothetical inferences about the gradualistic mechanics that a woefully imperfect record does not permit us to observe directly. But, although I had developed and presented an iconoclastic exegesis of Lyell, I simply lacked the courage to state so general a claim for inverting the standard view about uniformitarians and catastrophists. I assumed that I must be wrong, and that I must have misunderstood catastrophism because I had not read enough, or could not comprehend the subtleties at this fledgling state of a career. So I scoured the catastrophist literature again until I found a quote from William Buckland (both a leading divine and the first reader in ge¬ ology at Oxford) that could be interpreted as a defense of supernaturalism. I cited the quotation (Gould, 1965, p. 223) and stuck to convention on this broader issue, while presenting an original analysis of multiple meanings— some valid (like the invariance of law) and some invalid (like my professor’s claim for constancy in range of rates)—subsumed by Lyell under the singular description of “uniformity” in nature. This work led me, partly from shame at my initial cowardice, and as others reassessed the scientific character of catastrophism, to a more general analysis of the potential validity of catastrophic claims, and particularly to an under¬ standing of how assumptions of gradualism had so stymied and constrained our comprehension of the earth’s much richer history. These ideas forced me to question the necessary basis for Darwin’s key assumption that observable, small-scale processes of microevolution could, by extension through the im¬ mensity of geological time, explain all patterns in the history of life—namely, the Lyellian belief in uniformity of rate (one of the invalid meanings of the hy¬ brid concept of uniformitarianism). This exegesis led to a technical book about concepts of time and direction in geology (Gould, 1987b), to an en¬ larged view that encouraged the development of punctuated equilibrium, and to a position of cautious favor towards such truly catastrophic proposals as the Alvarez theory of mass extinction by extraterrestrial impact—a concept ridiculed by nearly all other paleontologists when first proposed (Alvarez et al.f 1980), but now affirmed for the K-T event, and accepted as an empirical basis for expanding our range of scientifically legitimate hypotheses beyond the smooth extrapolationism demanded by this third branch of Darwinian central logic. In addition to these disparate accretions of revisionism on the three branches of Darwinian central logic, one further domain—my studies in the history of evolutionary thought—served as a sine qua non for wresting a co¬ herent critique from such an inchoate jumble of disparate items. Above all, if I had not studied Darwin’s persona and social context so intensely, I doubt
45
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THE STRUCTURE OF EVOLUTIONARY THEORY that I would ever have understood the motivations and consistencies—also the idiosyncrasies of time, place and manner—behind the abstract grandeur of his view of life. History, as I argued before (see p. 35), must not be dis¬ missed as a humanistic frill upon the adamantine solidity of “real” science, but must be embraced as the coordinating context for any broad view of the logic and reasoning behind a subject so close to the bone of human concern as the science of life’s nature and structure. (Of the two greatest revolutions in scientific thought, Darwin surely trumps Copernicus in raw emotional im¬ pact, if only because the older transition spoke mainly of real estate, and the later of essence.) Some of my historical writing appeared in the standard professional litera¬ ture, particularly my thesis about the “hardening” of the Modern Synthesis (Gould, 1980e, 1982a, 1983b), a trend (but also, in part, a drift) towards a stricter and less pluralistic Darwinism. Several full-time historians of science then affirmed this hypothesis (Provine, 1986; Beatty, 1988; Smocovitis, 1996). But much of the historical analysis behind the basic argument of this book had its roots (in my consciousness at least) in the 300 consecutive monthly essays that I wrote from 1974 to 2001 in the popular forum of Nat¬ ural History magazine, where I tried to develop a distinctive style of “mini in¬
tellectual biography” in essay form—attempts to epitomize the key ideas of a professional career in a biographic context, and within the strictures of a few thousand words. By thus forcing myself to emphasize essentials and to discard peripherals (while always searching out the truly lovely details that best exemplify any abstraction), I think that I came to understand the ma¬ jor ideological contrasts between the defining features of Darwinian theory and the centerpieces of alternative views. In this format, I first studied such structuralist alternatives as Goethe’s theory of the archetypal leaf, Geoffroy’s hypothesis on the vertebral underpinning of all animals, and on dorso-ventral inversion of arthropods and vertebrates, and Owen’s uncharacteristic English support for this continental view of life. I also developed immense sympathy for the beauty and raw intellectual power of various alternatives, even if I eventually found them wanting in empirical terms. And I came to understand the partial validity, and even the moral suasion, in certain proposals unfairly ridiculed by history’s later victors—as in reconsidering the great hippocam¬ pus debate between Huxley and Owen, and recognizing how Owen used his (ultimately false) view in the service of racial egalitarianism, while Huxley misused his (ultimately correct) interpretation in a fallacious defense of tradi¬ tional racial ranking. Finally, my general love of history in the broadest sense spilled over into my empirical work as I began to explore the role of history’s great theoretical theme in my empirical work as well—contingency, or the tendency of com¬ plex systems with substantial stochastic components, and intricate nonlinear interactions among components, to be unpredictable in principle from full knowledge of antecedent conditions, but fully explainable after time’s actual unfoldings. This work led to two books on the pageant of life’s history (Gould, 1989c; Gould, 1996a). Although this book, by contrast, treats gen-
Defining and Revising the Structure of Evolutionary Theory
eral theory and its broad results (pattern vs. pageant in the terms of this text), rather than contingency and the explanation of life’s particulars, the science of contingency must ultimately be integrated with the more conventional sci¬ ence of general theory as explored in this book—for we shall thus attain our best possible understanding of both pattern and pageant, and their different attributes and predictabilities. The closing section of the book (pp. 13321343 of Chapter 12) offers some suggestions for these future efforts. When I ask myself how all these disparate thoughts and items fell together into the one long argument of this book, I can only cite—and I don’t know how else to put this—my love of Darwin and the power of his genius. Only he could have presented such a fecund framework of a fully consistent theory, so radical in form, so complete in logic, and so expansive in implication. No other early evolutionary thinker ever developed such a rich and comprehen¬ sive starting point. From this inception, I only had to explicate the full origi¬ nal version, tease out the central elements and commitments, and discuss the subsequent history of debate and revision for these essential features, culmi¬ nating in a consistent reformulation of the full corpus in a helpful way that leaves Darwin’s foundation intact while constructing a larger edifice of in¬ terestingly different form thereupon. Clearly I do not honor Darwin by hagi¬ ography, if only because such obsequious efforts would make any honest character cringe (and would surely cause Darwin to spin in his grave, thus up¬ setting both the tourists in Westminster Abbey and the adjacent bones of Isaac Newton). I honor Darwin’s struggles as much as his successes, and I fo¬ cus on his few weaknesses as entry points for needed revision—his acknowl¬ edged failure to solve the “problem of diversity,” or his special pleading for progress in the absence of any explicit rationale from the operation of his cen¬ tral mechanism of natural selection. As a final comment, if this section has violated the norms of scientific dis¬ course (at least in our contemporary world, although not in Darwin’s age) by the liberty that I have taken in explicating personal motives, errors and cor¬ rections, at least I have shown how we all grope upward from initial stupid¬ ity, and how we would never be able to climb without the help and collabora¬ tion of innumerable colleagues, all engaged in the intensely social enterprise called modern science. I experienced no eureka moment in developing the long argument of this book. I forged the chain link by link, from initial pos¬ session of a few separate items that I didn’t even appreciate as pieces of a sin¬ gle chain, or of any chain at all. I made my linkages one by one, and then of¬ ten cut the segments apart, in order to refashion the totality in a different order. So many people helped me along the way—from long dead antecedents by their wise words to younger colleagues by their wisecracks—that I must view this outcome as a social project, even though I, the most arrogant of lite¬ rati, insisted on writing every word. Perhaps I can best express my profound thanks to the members of such an intellectual collectivity by stating, in the most literal sense, that this book would not exist without their aid and suffer¬ ance. My formal dedication to my two dearest and closest paleontological collaborators in this effort to formulate macroevolutionary theory records
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THE STRUCTURE OF EVOLUTIONARY THEORY the worthy apex of an extensive pyramid. Scientists fight and squabble as all folks do (and I have scarcely avoided a substantial documentation thereof in this book). But we are, in general, a reasonably honorable lot, and we do em¬ brace a tendency to help each other because we really do revel in the under¬ standing of nature’s facts and ways—and most of us will even trade some per¬ sonal acclaim for the goal of faster and firmer learning. For all the tensions and unhappinesses in any life, I can at least say, with all my heart, that I chose to work in the best of all enterprises at the best of all possible times. May our contingent future only improve this matrix for my successors.
Epitomes for a Long Development LEVELS OF POTENTIAL ORIGINALITY Most of this book can be described as extensive narration of work already done, and ideas already expounded elsewhere. But no one should write at such length merely to organize the conventional material of a field, and without an original structure, or a set of unconventional ideas, to propose. I wrote The Structure of Evolutionary Theory because I felt that I had fol¬ lowed a sufficiently idiosyncratic procedure to devise a sufficiently novel the¬ oretical structure that then yielded a sufficient number of original insights on specific matters to qualify as a justification for spending so many years of a career, and daring to ask readers for such a non-trivial chunk of their at¬ tention. As implied by the foregoing sentence, I think that whatever originality this work possesses might best be conceptualized at three levels of basic structure, primary justifications for the major components of theory, and specific in¬ sights or discoveries then developed under the aegis of this structure and the¬ ory. At the first level of basic structure, I believe that three features of organi¬ zation set the novelty of presentation: 1. Developing an exegesis of essential components in the logic of Darwin¬ ian theory, as expressed in the agency, efficacy, and scope of selection as an evolutionary mechanism (Chapter 2). 2. Explicating the history of evolutionary thought as a complex and ex¬ tended debate about these essential components, developed negatively at first by early evolutionists who sought alternative formulations to Darwinism (Chapters 3-6), and then positively in our times by scientists who recognized the need for extensive revisions and expansions that would build an enlarged structure upon a Darwinian foundation, rather than uproot the theoretical core of selectionism (Chapters 7-12). 3. Formulating an expanded theory that introduces substantial revisions on each branch of Darwinian central logic, but builds, in its ensemble, a coher¬ ently enlarged structure with a retained Darwinian base—moving from Dar¬ win’s single level of agency to a hierarchical theory of selection on the first branch; balancing positive sources of internal constraint (for both structural and historical reasons) with the conventional externalism of natural selection on the second branch; and recognizing the disparate inputs of various tiers
Defining and Revising the Structure of Evolutionary Theory of time, rather than trying to explain all phylogenetic mechanics by uniformitarian extrapolation from microevolutionary processes, on the third branch. At the second level of validation for proposed revisions in the structure of evolutionary theory, I have tried to develop broad arguments and empirical justifications for major changes and expansions on each of the three branches of Darwinian central logic. On the first branch of agency, the theory of punc¬ tuated equilibrium itself, initially formulated by Niles Eldedge and me, estab¬ lishes the species as a true and potent Darwinian individual, and grants a minimal guarantee of descriptive independence to macroevolution by requir¬ ing a treatment of trends as the differential success of stable species rather than the adaptive anagenesis of lineages by accumulated and extrapolated organismal selection alone. Beyond punctuated equilibrium, the general ra¬ tionale for a hierarchical theory of selection, as presented here through the interactor approach based on emergent fitnesses at higher levels, may estab¬ lish a complete (and tolerably novel) framework not only for grasping the consistent logic of hierarchical selection, but also for viewing each level as po¬ tent in its own distinctive way, and for recognizing the totality of evolution¬ ary outcomes as a realized balance among these potencies, and not as the achieved optimality of a single causal locus—a substantial difference from Darwinian traditions for conceiving the dynamics of evolutionary change. In working through the differences among levels—see Chapter 8, pp. 714744-—I was particularly struck by the surprising, but accurate and challeng¬ ing, analogies (Lamarckian inheritance at the organismal level with adaptive anagenesis at the species level, for example); and by the different modes of equally effective change implied by disparate structural reasons for the estab¬ lishment of individuality at various levels (particularly, the domination of se¬ lection over drift and drive at the organismal level vs. the potent balance among all three mechanisms at the species level). On the second branch of efficacy, I have tried to make the most compre¬ hensive case yet advanced for internal constraint as a positive director and channeler of evolutionary change, and not only as a negative brake upon pure Darwinian functionalism. I proceed by explicating two conceptually different forms of constraint—structural constraints as consequences of physical prin¬ ciples, and historical constraints as channels from particular pasts. I argue that each category challenges a different central tenet of Darwinism—struc¬ tural constraint by establishing a substantial space for non-selectionist origin of important evolutionary features, and historical constraint for explaining the markedly inhomogeneous filling of morphospace as flow down ancient internal channels of deep homology, and not primarily as a mapping of adap¬ tive design upon current ecological landscapes. Beyond any novelty in this general formulation, I have attempted to develop a conceptual space, and to establish practical criteria, for the identification of non-adaptive sequelae (spandrels), the evolutionary importance of their later cooptation for utility (exaptation), and the importance of such reservoirs of potential (exaptive pools) in explicating the important concept of “evolvability” in structural rather than purely adaptational terms.
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THE STRUCTURE OF EVOLUTIONARY THEORY On the third branch of scope, my contribution cannot claim much novelty, if only because I have not worked professionally in this area of paleonto¬ logical research. But I do explicate, perhaps more fully than before, both the historical and conceptual reasons for regarding catastrophic mass extinction, and catastrophic mechanics in general (within their limited scope of validity), not as anti-selectionist per se, but rather as fracturing the extrapolationist premise of Darwinian central logic, and requiring that substantial aspects of phyletic pattern be explained as interaction between temporal extensions of microevolution and different processes that only become visible and effective at higher tiers of time. I try to resolve “the paradox of the first tier” (the em¬ pirical failure of Darwin’s logically airtight argument for a vector of progress) by arguing that punctuated equilibrium at the second tier of phyletic trends, and mass extinction at the third tier of faunal overturn, impose enough of their own, distinctive and different, patterning to forestall the domination or pure imprint of extrapolated microevolutionary results upon the general pag¬ eant of life’s history. Finally, at the third level of those lovely details (where both God and the devil dwell, and where, ultimately, both the joy and power of science reside), I trust that any originality I have introduced at “higher” levels of theoretical structure gains primary expression and utility in the resolution of previously puzzling details, and in the identification of “little things” that had escaped previous notice or explicit examination. For example, most original analyses and discoveries in the historical first half of this book flow directly from my organizing theme of identifying essen¬ tial components in Darwinian logic, and then tracing both the early attempts to defeat, and our later efforts to modify and expand them through time. I was thus able to discover and identify Darwin’s major encounter with higher level selection not in his recognized discussion of group selection for human altruism, but in his previously unexplicated admission of species selection to resolve the problem of diversity (see Chapter 3, pp. 246-250). In this case, I “lucked out” through an odd reason for previous ignorance of such an im¬ portant textual revision—for Darwin omitted this material in his compressed and hasty discussion of diversity in Chapter 4 of the Origin (on this subject, the only Darwinian source generally known to professional biologists, who would immediately highlight the importance of any acknowledgment of spe¬ cies selection). But Darwin agonized over levels of selection at explicit length in the unpublished “long version” that only saw the light of printed day in 1975 (Stauffer, 1975), and that virtually no practicing biologist has ever read (whereas historians of science who do study this longer text usually lack suf¬ ficient knowledge of the technical debate about levels of selection to under¬ stand the meaning of Darwin’s passages or to appreciate their import). The same context led me to appreciate the previously unanalyzed develop¬ ment of a full hierarchical model by Weismann in his later works (Chapter 3, pp. 223-224), a formulation that Weismann himself identified as the most important theoretical achievement of his later career. Previous historians had written about his much longer and earlier explications of lower level selection
Defining and Revising the Structure of Evolutionary Theory (germinal selection in his terms), if only in the context of modern reductionistic breakdowns of Darwinism to selection among “selfish genes.” But they had missed his later reversal and expansion to a full hierarchical model, despite Weismann’s own emphasis. Similarly, de Vries’s clear understanding of Darwinian logic had also been ignored because de Vries, as an opponent of the efficacy of Darwinian organismal selection (a painful decision for him, given his psychological fealty to Darwin, also explored herein), applied the logic to higher levels, and even devised the term “species selection” (Chapter 5, pp. 446-451)—a concept and coining previously entirely unremarked by historians (much to the embarrassment of scientists, including yours truly, who coined and explicated the same term much later in full expectation of pristine originality!). Similarly, my sense of the logic in conflicts between constraint and adapta¬ tion (or internal vs. external, or formal vs. functional approaches) on the second branch helped me to pinpoint, or to make sense of, several important historical events and arguments that have not been properly treated or under¬ stood. Historians of science had not previously discussed orthogenetic theo¬ ries in this fairest light, and had not distinguished the very different formula¬ tions of Hyatt, Eimer, and Whitman in terms of their increasingly greater willingness to accommodate Darwinian themes as well (see Chapter 5). The same framework allowed me to identify the crucial importance, and brilliant epitomization, of this issue in the final paragraphs of Chapter 6 (“Difficulties on Theory”) in Darwin’s Origin, a significance that had not been highlighted before. I also traced the dichotomy of anglophonic preferences for functionalist accounts vs. continental leanings towards formalism back through the evo¬ lutionary reconstruction of the argument in the mid 19th century into the creationist formulations of Paley vs. Agassiz (Chapter 4), thus illustrating a pedigree for this fundamental issue in morphology that evolution may have recast in causal terms, but did not budge in basic commitments to the mean¬ ing of morphology. Among the little tidbits that emerge from such analy¬ ses, I even discovered that Darwin borrowed his clearest admission of co¬ opted utility from non-adaptive origins (unfused skull sutures in mammalian neonates, essential for passage through the birth canal, but also existing in birds and reptiles born from more capacious eggs) from the longer and more nuanced descriptions of Richard Owen, Britain’s anomalous defender of for¬ malism. I also included some historical analyses in the book’s second half on mod¬ ern advances because I thought they could make an original contribution to arguments usually developed only in contemporary terms and findings. I have already mentioned my analysis of how the initial pluralism of the Modern Synthesis (embracing any mode of change consistent with known genetic mechanisms) hardened through subsequent editions of the founding volumes into pronounced preferences for adaptationist accounts framed only in terms of natural selection (Chapter 7). In addition, I think that my reexhumation of the debate between Falconer and Darwin on fossil elephants provides a
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THE STRUCTURE OF EVOLUTIONARY THEORY good introduction to, punctuated equilibrium (Chapter 9, pp. 745-749). The largely unknown paradox of Lankester’s original definition of homoplasy as a category of homology, rather than in the opposite status held by the term to¬ day, provides the best entry I could devise for understanding the vital, but lit¬ tle appreciated and rarely acknowledged, theoretical differences between par¬ allelism and convergence. In the absence of this context and distinction, the key importance of evo-devo and the discovery of deep homology among dis¬ tant phyla cannot properly be grasped as a challenge and expansion of Dar¬ winian expectations (Chapter 10). I hope that my sympathetic portrayal of D’Arcy Thompson’s theory of form (Chapter 11), despite my general disagreement with his argument, will help colleagues to understand the thrust and potential power of this unusual formulation of structuralist constraint on external grounds of universal phys¬ ics. Although I am chagrined that I discovered Nietzsche’s account of the dis¬ tinction between current utility and historical origin so late in my work, I know no better introduction—from one of history’s greatest philosophers to boot, and in his analysis of morality, not of any scientific subject—to the the¬ oretical importance of spandrels and exaptation in the rebalancing of con¬ straint and adaptation within evolutionary theory (Chapter 11, pp. 12141218). In a final historical analysis of the second part, I think that Dar¬ win’s own rationale for progress (Chapter 12, pp. 1296-1303), rooted not in the mechanics of natural selection itself, but in an ecological argument for extrapolation of biotic competition through time in a perpetually crowded world—an aspect of Darwin’s thought that has very rarely been appreciated, formulated or discussed by historians—provided the best context I could de¬ vise for understanding why catastrophic mass extinction in particular, and non-extrapolation through tiers of time in general, play such havoc with Dar¬ win’s need for uniformity on the third branch of his essential logic. The original claims in the book’s second half on modern reformulations of evolutionary theory rest, necessarily and primarily, on theoretical insights and unusual conceptual parsings, rather than on novel data—if only be¬ cause custom dictates that my extensive empirical documentation be pre¬ sented in “review” format by collating published studies in support or refuta¬ tion of general themes under discussion. But I have sometimes presented existing data in novel contexts—as in my analysis of the proper category for understanding the exaptive value of genes lost by founder drift in establish¬ ing the social cohesion (albeit transient) that has made the Argentine ant Linepitbema humile such a successful invader of non-native Californian habi¬ tats (Chapter 11, pp. 1282-1284). I have also cited my own empirical studies, previously published but original in the more conventional sense, to support important pieces of more general arguments, including validation of punctu¬ ated equilibrium by dissection of a single bedding plane to reveal transition by absolute age dating of individual shells (Goodfriend and Gould, 1996), the “employment” of constraint by selection to yield several adaptive features by one heterochronic change in a case of neoteny in Gryphaea (Jones and Gould, 1999), and the explanation of most ordered geographic variation within
Defining and Revising the Structure of Evolutionary Theory a major subregion of Cerion as consequences of allometric correlations in growth (Gould, 1984b). I tried (and utterly failed) to compose a selective listing, as provided above for the book’s historical half, for original ideas about theoretical details devel¬ oped in revising the three branches of Darwinian central logic in the book’s second half on modern reformulations of evolutionary theory. I ripped up several attempts that read like the hodge-podge of a random laundry list rather than the ordered “sweet places” on a logical continuum. These high¬ lights, I finally recognized, have little meaning outside the broader context of a linearly developing argument for each branch, and I will therefore make a second attempt, within the more detailed epitome of the next and final sec¬ tion of this chapter, to designate the points that struck me with the force of “aha,” or that conveyed a hint of deeper, surprising, or more radical implica¬ tions for reasons that I couldn’t quite fathom directly, but that tickled my in¬ tuition at the edge of that wonderful, if elongate, German word: Fingerspitzengefuhl, or feeling at the tip of one’s finger. Most inchoate excitements of this sort lead to nowhere but foolishness and waste of time, but every once in a while, the following of one’s nose catches a whiff of novelty. At least we must trust ourselves enough to try—and not take ourselves so seriously that we forget to laugh at our more frequent and inevitable stumbles.
AN ABSTRACT OF ONE LONG ARGUMENT
I have insisted, borrowing Darwin’s famous line in my arrogance, that this “whole volume is one long argument,” flowing logically and sequentially from a clear beginning in Darwin’s Origin to our current reformulations of evolutionary theory. But this structural thread of Ariadne can easily become lost in the labyrinth of my tendencies to expatiate on little factual gems, or to follow the thoughts of leading scientists into small, if lovely, byways of their mental complexities. Hence, I need to present summaries and epitomes as guidelines. Long books, like large bureaucracies, can easily get bogged down in a ba¬ roque layering of summary within summary. The United States House of Representatives has a Committee on Committees (I kid you not), undoubt¬ edly embellished with subcommittees thereof. And we must not forget Jona¬ than Swift’s famous verse on the fractality of growing triviality in scholarly commentary: So, naturalists observe, a flea Hath smaller fleas that on him prey; And these have smaller still to bite ’em And so proceed ad infinitum. Thus every poet, in his kind, Is bit by him that comes behind. I wrote, on page 13, that this book includes three levels of embedding for this long argument—the summary in this chapter, the epitome of Darwin in
S3
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THE STRUCTURE OF EVOLUTIONARY THEORY Chapter 2, and the development of the totality. Now, and most sheepishly, I add two more, for a fractal total of five—the listed abstract, in pure “book order,” of this section, and (God help us) the epitome of this epitome, pre¬ sented now to introduce and guide the list. * I develop my argument throughout this book by asserting, first, that the central logic of Darwinism can be depicted as a branching tree with three ma¬ jor limbs devoted to selection’s agency, efficacy and scope. Second, that Dar¬ win, despite his heroic and explicit efforts, could not fully “cash out” his the¬ ory in terms of the stated commitments on each branch—and that he had to allow crucial exceptions, or at least express substantial fears, in each domain (admitting species selection to resolve the problem of diversity; permitting an uncomfortably large role for formalist correlations of growth as compromis¬ ers of strict adaptationism; expressing worry that mass extinction, if more than an artifact of an imperfect fossil record, would derail the extrapolationist premise of his system). Third, that the subsequent history of evolution¬ ary debate has focused so strongly upon the key claims of these three essential branches that we may use engagement with them as a primary criterion for distinguishing the central from the secondary when we need to gauge the im¬ portance of challenges to the Darwinian consensus. Fourth, that we should not be surprised by the prominence of these three themes, for they embody (in their biological specificity) the broadest underlying issues in scientific expla¬ nation, and in the nature of change and history: levels of structure and causal¬ ity, rates of alteration, directions of causal flow, the possibility of causal uni¬ fication by reduction to the lowest level vs. autonomy and interaction of irreducible levels, punctuational vs. gradual change, causal and temporal tier¬ ing vs. smooth extrapolation. Fifth, that the most interesting and impor¬ tant debates in our contemporary science continue to engage the same three themes, thus requiring the vista of history to appreciate the continuity and logical ordering that extends right back to Darwinian beginnings. Sixth, that our best modern understanding of the structure of evolutionary theory has re¬ versed the harmful dichotomization of earlier debates (Darwinian fealty vs. destructive attempts to trivialize or overturn the mechanism of selection) by confronting the same inadequacies of strict Darwinism, but this time intro¬ ducing important additions and revised formulations that preserve the Dar¬ winian foundation, but build a theory of substantial expansion and novelty upon a retained selectionist core. This logic and development may be defended as tolerably impersonal and universal, but any book of this length and complexity, and of so idiosyncratic a style and structure, must also own its authorial singularities. The Structure of Evolutionary Theory emerges, first of all, from my professional focus as a paleontologist and a student of macroevolution, defined, as explained on page 38, as descriptive phenomenology prior to any decision about the need for distinctive theory (my view) or the possibility of full subsumption un¬ der microevolutionary principles (the view of Darwin and the Modern Syn¬ thesis). The contingency of history guarantees that any body of theory will underdetermine important details, and even general flows, in the realized
Defining and Revising the Structure of Evolutionary Theory pageant of life’s phylogeny on Earth—and such a claim for nontheoretical in¬ dependence of macroevolution generates no dispute, even between rigid re¬ ductionists who grant no separate theoretical space to macroevolution, and biologists, like myself, who envisage an important role for distinctive macro¬ evolutionary theory within an expanded and reformulated Darwinian view of life. In his description of the reductionist view of classical Darwinism—his own opinion in positive support, not a simplistic caricature in opposition— Hoffman (1989, p. 39) writes: “The neodarwinian paradigm therefore asserts that this history of life at all levels—including and even beyond the level of speciation and species extinction events, embracing all macroevolutionary phenomena—is fully accounted for by the processes that operate within pop¬ ulations and species.” I dedicate my book to refuting this traditional claim, and to advocating a helpful role for an independent set of macroevolutionary principles that expand, reformulate, operate in harmony with, or (at most) work orthogonally as additions to, the extrapolated, and persistently relevant (but not exclusive, or even dominant) forces of Darwinian microevolution. This perspective of synergy confutes the contrary, and ultimately destruc¬ tive, attempts by late 19th and early 20th century macroevolutionists to de¬ velop substitute mechanisms that would disprove or trivialize Darwinism, and that spread such a pall of suspicion over the important search for nonreductionistic and expansive evolutionary theories—a most unfortunate (if historically understandable) trend that stifled, for several generations, the unification and fruitful expansion of evolutionary theory to all levels and temporal tiers of biology. Thus, for example, my attempt to develop a speciational theory of macroevolution (Chapters 8 and 9), with species treated as irreducible Darwinian individuals playing causal roles analogous to those oc¬ cupied by organisms in Darwinian microevolution, represents an extension of Darwinian styles of explanation to another hierarchical level of analysis (with interestingly different causal twists and resulting patterns), not a refuta¬ tion of natural selection from an alien realm. (Such a speciational theory, however, does counter Hoffman’s reductionistic claim of full theoretical suf¬ ficiency for “processes that operate within populations and species”—for, given the stasis of species under punctuated equilibrium, such macroevolu¬ tionary patterns originate by higher-order sorting among stable species, and not primarily by processes occurring anagenetically within the lifetime of these higher-level Darwinian individuals.) Similarly, the different rules of cat¬ astrophic mass extinctions require additions to the extrapolated Darwinian and microevolutionary causes of phyletic patterns, but do not refute or deny the relevance of conventional uniformitarian accretions through geological time. (In fact, a more comprehensive theory that seeks to integrate the rela¬ tive strengths, and interestingly disparate effects, of such different levels and forms of continuationist vs. catastrophic causality offers greater richness to Darwinian perspectives as both underpinnings and important contributors to a larger totality.) A second authorial input must arise from the distinctive ontogeny of past
56
THE STRUCTURE OF EVOLUTIONARY THEORY work. The Structure .of Evolutionary Theory occupies a much broader terri¬ tory than my first lengthy technical book of an earlier career, Ontogeny and Phytogeny (1977b). The motivating conceit of the first book rested upon my choice of a much smaller compass defined by a much clearer tradition of definition and research. I thought—thus my designation of this strategy as a conceit—that I could quote, in extenso and from original sources, every im¬ portant statement, from von Baer and before to de Beer and after, on the rela¬ tionship between development and evolution. This potential for comprehen¬ siveness brought me much pleasure and operational motivation. In fact, I soon realized that I could not succeed, even within this limited sphere—and I therefore punted shamelessly in the final result. I did manage to quote every important passage on the theoretical relationship between these central subjects of biology, but I passed, nearly completely, on the actual use of these putative relationships in specific proposals for phylogenetic recon¬ structions. And, as all historians of science and practitioners of evolutionary biology know, this genre of “phylogenizing” represented by far (at least by weight) the dominant expression of this theoretical rubric in the technical lit¬ erature. I would, by the way, defend my decision as entirely reasonable and proper, and not merely as practically necessary, because these specific phylo¬ genetic invocations made effectively no contribution to the development of evolutionary theory—my central concern in the book—and remained both speculative and transient to boot. But I do remember the humbling experi¬ ence of realizing that a truly full coverage could only represent a pipe dream, if applied to any important subject in a vigorous domain of research! My personal love of such thoroughness (with the necessary trade-off of limitation in domain) posed a substantial problem when I decided to expand my range from ontogeny and phylogeny to the structure of evolutionary the¬ ory. Of all genres in scholarship, I stand most strongly out of personal sympa¬ thy with broad-brush views that attempt to encompass entire fields (the his¬ tory of philosophy from Plato to Pogo, or of transportation from Noah to NASA) in a breathless summary paragraph for each of many thousand inci¬ dents. Even the most honorable efforts by great scholars—former Librarian of Congress Daniel Boorstin’s The Explorers, for instance—make me cringe for simplistic legends repeated and interesting complexities omitted. At some level, truly important and subtle themes can only be misrepresented by such a strategy. But how then to treat the structure of evolutionary theory in a reputable, even an enlightening, way? Surely we cannot abandon all hope for writing honorably about such broad subjects simply because the genre of comprehen¬ sive listing by executive summary must propagate more mythology and mis¬ information than intrigue or understanding. As a personal solution to this crucial scholarly dilemma, and in developing the distinctive strategy of this book, I employed a device that I learned by doing, through many years of composing essays—a genre that I pursued by writing comprehensive personal treatments of small details, fully documentable in the space available, but
Defining and Revising the Structure of Evolutionary Theory also conveying important and general principles in their cascading implica¬ tions. I vowed that I would try to encompass the structure of evolutionary theory in its proper intellectual richness, but that I would do so by exhaustive treatment of well-chosen exemplifying details, not by rapid summaries of in¬ adequate bits and pieces catalogued for all relevant participants. Under this premise, the central task then evolves (if I may use such a meta¬ phor) into an extended exercise in discrimination. The solution may be la¬ beled as elitist, but how else can selection in intellectual history be under¬ taken? One must choose the best and the brightest, the movers and shakers by the sieve of history’s harsh judgment (and not by the transiency of immedi¬ ate popularity)—and let their subtle and detailed formulations stand as a se¬ ries of episodes, each conveyed by an essay of adequate coverage. Luckily, the history of evolutionary thought—as one of the truly thrilling and expansive subjects of our mental lives—has attracted some of the most brilliant and fas¬ cinating doers and thinkers of intellectual history. Thus, we are blessed with more than adequate material to light the pathway of this particular odyssey in science. Luckily too, the founding figure of Darwin himself established such a clear basis of brave commitment that I could characterize, and then trace down to our own times, an essential logic that has defined and directed one of the most important and wide-ranging debates in the history of science into a coherent structure, ripe for treatment by my favored method of full coverage for the few truly central items (by knowing them through their fruits and logics, and by leaving less important, if gaudy, swatches gently aside in order to devote adequate attention to essential threads). A third, and final, authorial distinction—my treatment of history and my integration of the history of science with contemporary research on evolu¬ tionary theory—emerges directly from this strategy of coverage in depth for a small subset of essential items and episodes. My historical treatments tend to resolve themselves into a set of mini intellectual biographies (as exemplified and defended on page 46) for almost all the central players in the history of Darwinian traditions in evolutionary thought. I can only hope that this pecu¬ liar kind of intellectual comprehensiveness will strike some readers as enlight¬ ening for the “quick entree” thus provided into the essential work of the peo¬ ple who led, and the concepts that defined, the history of the greatest and most consequential revolution in the history of biological science. (In most cases—a Goethe, Cuvier, Weismann, de Vries, Fisher or Simpson, for exam¬ ple—I chose people for their intrinsic and transcendent excellence. In fewer instances—an Eimer or Hyatt as proponents of orthogenesis, for example—I selected eminently worthy scientists not as great general thinkers, but as best exponents of a distinctive approach to an important subject in the history of debate on essentials of evolutionary theory.) A few figures in history have been so prescient in their principal contribu¬ tion, and so acute and broad-ranging in their general perceptions, that they define (or at least intrude upon) almost any major piece of a comprehensive discussion (A. N. Whitehead famously remarked, for example, that all philos-
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THE STRUCTURE OF EVOLUTIONARY THEORY ophy might be regarded as a footnote to Plato). Evolutionary biology pos¬ sesses the great good fortune to embrace such a figure—Charles Darwin, of course—at the center of its origin and subsequent history. Thus, Darwin emerges again and again, often controlling the logic of discussion, through¬ out this book—in his own full foundational exegesis (Chapter 2); but then, in later chapters, as the principal subject, and best possible exemplification, of other important subbranches on all three boughs of his essential logic (his re¬ luctant acceptance of higher levels of selection in Chapter 3; his formalist contrast to his own functionalism in stressing “correlations of growth'1 in Chapter 4; his views on direction and progress in the history of life in Chapter 6, and, even in the book’s second half on modern developments, for his dis¬ cussion of discordance between historical origin and current utility as a point of departure for my treatment of exaptation in Chapter 11, and his attempt to underplay and undermine mass extinction as an introduction to my cri¬ tique of uniformitarianism and extrapolationism in the final Chapter 12). Who could ask for a more attractive and effective coordinating “device” to tie the disparate strands of such an otherwise disorderly enterprise together than the genial and brilliant persona of the man who first gave real substance to the grandeur in this view of life? Whatever my dubiety about the role and efficacy of abstracts (too often, as we would all admit in honest moments, our only contact with a work that we nonetheless then feel free to criticize in full assurance of our rectitude), I can¬ not deny that a work of this length, imbued moreover with a tendency to pen¬ etrate byways along a basic route that seems (at least to this author) ade¬ quately linear and logical, demands some attempt to list its principal claims in textual order. Hence, I now impose upon you the following abstract:
Chapter 2: An exegesis of the origin 1. All major pre-Darwinian evolutionary theories, Lamarck’s in particu¬ lar, contrasted a primary force of linear progress with a distinctly secondary and disturbing force of adaptation that drew lineages off a main line into par¬ ticular and specialized relationships with immediate environments. In his most radical intellectual move, expressing both the transforming depth and the conceptual originality of the theory of natural selection, Darwin denied the existence of a primary progressive force, while promoting the lateral force of adaptation to near exclusivity. In so privileging uniformitarian extrapola¬ tion as an explanatory device, Darwin imbued natural selection, the lateral force, with sufficient power to generate evolutionary change at all scales by accumulating tiny adaptive increments through the immensity of geological time. 2. The Origin of Species exceeds all other scientific “classics” of past centu¬ ries in immediate and continued relevance to the basic theoretical formula¬ tions and debates of current practitioners. Careful exegesis of Darwin’s logic and intentions, through textual analysis of the Origin, therefore assumes un¬ usual importance for the contemporary practice of science (not to mention its undeniable historical value in se).
Defining and Revising the Structure of Evolutionary Theory 3. Darwin famously characterized the Origin as “one long argument” without explicitly stating “for what?” Assumptions about the focus of this long argument have ranged from the restrictively narrow (for natural selec¬ tion, or even for evolution) to the overly broad (for application of the most general hypothetico-deductive model in scientific argument, as Ghiselm has claimed). I take a middle position and characterize the “long argument” as an attempt to establish a methodological approach and intellectual foundation for rigorous analysis in historical science—a foundation that could then be used to validate evolution. 4. The “long argument” for historical science operates at two poles—meth¬ odological and theoretical. The methodological pole includes a set of proce¬ dures for making strong inferences about phyletic history from data of an im¬ perfect record that cannot, in any case, “see” past causes directly, but can only draw conclusions from preserved results of these causes. Darwin devel¬ ops four general procedures, all based on one of the three essential premises of his theory’s central logic: the explanation of large-scale results by extrapo¬ lation from short-term processes. In order of decreasing information avail¬ able for making the required inference, these four procedures include: (1) ex¬ trapolation to longer times and effects of evolutionary changes actually observed in historic times (usually by analogy to domestication and horticul¬ ture); (2) exemplification and ordering of several phenomena as sequential stages of a single historical process (fringing reefs, barrier reefs and atolls as stages in the formation of coral reefs by subsidence of central islands, for ex¬ ample); (3) inference of history as the only conceivable coordinating explana¬ tion for a large set of otherwise disparate observations (consilience); and (4) inference of history from single objects based on quirks, oddities and imper¬ fections that must denote pathways of prior change. 5. The theoretical pole rests upon the three essential components of Dar¬ winian logic: (1) agency, or organismal struggle as the appropriate (and nearly exclusive) level of operation for natural selection; (2) efficacy, or natu¬ ral selection as the creative force of evolutionary change (with complexly co¬ ordinated sequelae of inferred principles about the nature of variation, and of commitments to gradualism and adaptationism as foci of evolutionary analy¬ sis); and (3) scope, or extrapolationism (as described in point 4 just above). The logical coordination of these commitments, and their establishment as a brilliantly coherent and intellectually radical theory of evolution, can best be understood by recognizing that Darwin transferred the paradoxical argument of Adam Smith’s economics into biology (best organization for the general polity arising as a side consequence of permitting individuals to struggle for themselves alone) in order to devise a mechanism—natural selection—that would acknowledge Paley’s phenomenology (the good design of organisms and harmony of ecosystems), while inverting its causal basis in the most radi¬ cal of all conceivable ways (explaining the central phenomenon of adaptation by historical evolution rather than by immediate creation, and recognizing nature’s sensible order as a side consequence of unfettered struggle among in¬ dividuals, rather than a sign of divine intent and benevolence).
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THE STRUCTURE OF EVOLUTIONARY THEORY 6. The first theme of agency: Darwin’s commitment to the organismal level as the effectively exclusive locus of natural selection occupies a more central, and truly defining, role than most historians and evolutionists have recog¬ nized. Invocation of this most reductionist^ locus then available (in igno¬ rance of the mechanism of inheritance) embodies the intellectual radicalism of Darwin’s theory—using Adam Smith to overturn Paley, and holding that all higher-order harmony, previously attributed to divine intention, arises only as a side-consequence of selfish “struggle” for personal advantage at the lowest organismal level. Darwin devoted far more of the Origin to defending this organismal locus than most exegetes have acknowledged, particularly in centering his only two chapters on specific difficulties in natural selection (7 on Instinct and 8 on Hybridism) to resolutions provided by insistence upon organismal agency—explaining the establishment of adaptive sterile castes in social insects by selection upon queens as individuals, and resolving sterility in interspecific crosses as an unselected sequel of differences accumulated by organismal selection in each of two isolated populations, rather than as a di¬ rect result of higher-level species selection, as Wallace affirmed and as Darwin strove mightily and consciously to avoid. We can also trace his struggle to af¬ firm organismal exclusivity in his reluctances, underplaymgs and walling off (as unique and unrepeated elsewhere in nature) of the one exception (for hu¬ man altruism) that the logic of his system forced upon his preferences. 7. For his defense of the second theme of efficacy—his assertion of natural selection as the only potent source of creative evolutionary change—Darwin recognized that his weak and negative force, although surely a vera causa (true cause), could only play this creative role if variation met three crucial re¬ quirements: copious in extent, small in range of departure from the mean, and isotropic (or undirected towards adaptive needs of the organism). I would argue that Darwin’s most brilliant intellectual move lay in his accurate identification, through the logical needs of his theory and not from any actual knowledge of heredity’s mechanism, of these three major attributes of varia¬ tion—because he recognized that natural selection could not otherwise oper¬ ate as a creative force in the evolution of novelties. 8. Gradualism enters Darwin’s svstem as another deductive intellectual j
consequence of asserting that natural selection acts as the creative mechanism of evolutionary change. Gradualism has three distinct meanings in Darwinian traditions, with only the second (or intermediate) statement relevant to the central assertion of selection’s creativity. First, gradualism as simple historical continuity of stuff or information underlies the basic factuality of evolution vs. creation, and does not validate any particular mechanism of evolutionary change. Second, gradualism as insensible intermediacy of transitional forms specifies the Goldilockean “middle position” required by the mechanism of natural selection to refute the possibility that saltational variation might en¬ gender creative change all at once, thus relegating selection to a negative role of removing the unfit. Third, gradualism as a geological claim for slowness and smoothness (but not constancy) of rate plays a crucial role in the third theme (see point 10 of this list) of selection’s scope, or the extrapolatability of microevolution to explain all patterns in geological time—and is therefore the
Defining and Revising the Structure of Evolutionary Theory aspect of gradualism that punctuated equilibrium refutes (for punctuated equilibrium questions Darwin’s uniformitarian and continuatiomst beliefs, but not his mechanism of natural selection). This parsing of three distinctly different forms of gradualism, all embraced by Darwin for different rea¬ sons, alleviates the misunderstanding behind some unfortunate terminologi¬ cal wrangles without substance that have generated much heat (but little light) in recent debates. 9. The adaptationist program as a primary strategy of research emerges as the third major implication of advocating natural selection as the primary creative force in evolutionary change—for this Darwinian style of evolution must proceed step by step, with each tiny increment of change rendering or¬ ganisms better adapted to alterations in local environments. To summarize all the key implications of this second theme of efficacy, the creativity of natural selection makes adaptation central, isotropy of variation necessary, and grad¬ ualism pervasive. 10. Restriction of agency to the organismal level, and assertions of selec¬ tion's creativity, set a biological basis for the third essential claim of Dar¬ winian logic—selection’s scope, or the argument that this incremental and gradualistic style of microevolution can, by smooth extrapolation through the immensity of geological time, build the full extent of life’s anatomical change and taxonomic diversity by simple accumulation. I focus my shorter discussion of this third essential theme not upon biological needs (already covered in the first two themes), but upon the requirement for similar grad¬ ualistic styles of change in the geological stage that must present the evolu¬ tionary play—particularly in Darwin’s embrace of Lyellian uniformity, and his denial of catastrophism (through arguments about the imperfection of the fossil record to allay the literal appearance of such rapidity in geological data), for even a fully consistent, intellectually sound, and operationally po¬ tent theory will not regulate actual events if surrounding conditions debar its operation. 11.1 use Kellogg’s brilliant approach to the evaluation of Darwinian theory (published in 1907 in anticipation of centennial celebrations for Darwin’s birth and the sesquicentenary of the Origin) to distinguish alternatives that deny the fundamental postulate of selection’s creativity from auxiliaries that enlarge, adumbrate, or reformulate the theory of natural selection in basically helpful and consistent ways. I show that Darwinism may be epitomized by its three essential claims of agency, efficacy, and scope—and that the history of debate has always centered upon these themes, with critiques focusing upon destructive alternatives or constructive auxiliaries. I argue, as the major thesis of this book, that modern debates have developed important and coherent auxiliary critiques on all three branches of essential Darwinian logic, and that these debates may lead to a fundamentally revised evolutionary theory with a retained Darwinian core.
Chapter 3: Seeds of hierarchy 1. Nearly all scientific revolutions originate as replacements and refuta¬ tions of previous explanatory schemes, not as pure additions to a former state
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THE STRUCTURE OF EVOEUTIONARY THEORY of acknowledged ignorance. Lamarck’s evolutionary theory, known to anglophonic readers as a first full account through the fair but critical descriptions of Lyell (in Volume 2, 1832, of the Principles of Geology), and from Cham¬ bers’s promotion in the Vestiges of 1844, provided a context for Darwin’s ref¬ utation. Darwin’s single-level theory, based on the full efficacy of locally adaptive changes at the smallest scale, countered the only available alter¬ native of Lamarckism by relocating the major phenomenon that generated change and required explanation (local adaptation for Darwin, general prog¬ ress for Lamarck), and (far more radically) by reversing the conventional Paleyan explanation for the good design of organisms and the harmony of ecosystems (direct divine construction at the highest level vs. sequelae of nat¬ ural selection working at the lowest level of organismal advantage). 2. Lamarck, a dedicated materialist with a two-factor theory of evolution as a contrast between linear progress up life’s ladder and tangential deflec¬ tions of diversity through local adaptation, has been widely misunderstood (and reviled), both in Darwin’s time and today, as a vitalist and pure expo¬ nent of “soft” or Lamarckian inheritance (which he accepted as the “folk wisdom” of his day, and invoked primarily to explain the secondary process of lateral adaptation). 3. Darwin’s theory of natural selection shared a functionalist basis with Lamarck in joint emphasis upon adaptation to external environment as the instigator of evolutionary change. But the two theories differ most radically in Darwin's citation of a single locus and mechanism of change—with the full range of evolutionary results proceeding by natural selection for local adapta¬ tion of populations to changing immediate environments, and all higher-level phenomenology emerging by sequential accumulation of such tiny incre¬ ments through the immensity of geological time. By contrast, Lamarck advo¬ cated a two-factor theory, with local adaptation as a merely secondary and diverging process (and, as we all know of course, arising by soft inheritance of acquired features generated by adaptive effort during an organism’s life, rather than by natural selection of fortuitous variation), set against a primary process of progressive complexihcation up the ladder of life. Thus, Darwin embraced Lamarck’s secondary force (instantiated by a different mechanism), denied the existence of Lamarck’s primary force, and argued that the second¬ ary force of local adaptation also produced the large-scale results attributed by Lamarck to the primary force. Thus, this first major debate between evolu¬ tionary alternatives contrasted Lamarck’s hierarchical theory with Darwin’s single-level account. Hierarchy has been an important issue from the start (al¬ though, obviously, modern versions of hierarchical selection theory, advo¬ cated as the centerpiece of this book, bear no relationship, either genealogical or ideological, to this false, but fascinating, Lamarckian original). 4. Darwin explicitly rejected Lamarck’s two-factor theory, correctly identi¬ fying the disabling paradox that rendered the theory nonoperational: “what is important cannot be observed or manipulated (the higher-level force of progress), and what can be observed and manipulated (the tangential force of local adaptation) cannot explain the most important phenomenon (progress
Defining and Revising the Structure of Evolutionary Theory in complexification).” Darwin developed the first testable and operational theory of evolution by locating all causality in the palpable mechanism of natural selection. 5. In the first generation of Darwinian debate, August Weismann, clearly the most brilliant theorist of his time, and the only biologist (besides Darwin) who fully grasped the logic and implications of selection, wrestled with levels of selection throughout his career, and along an interesting path, finally devel¬ oping a full hierarchical theory that he explicitly identified as the most impor¬ tant conclusion of his later work. He began by trying to refute Lamarckian inheritance (and Herbert Spencer’s vigorous defense thereof) by advocating the Allmacht (omnipotence, or literally “all might” or complete sufficiency) of natural selection. He first attributed the degeneration of previously useful structures (a bigger problem for Darwinism than the explanation of adaptive features) to what he called “panmixia” (not the modern meaning of the term, but the effect of recombination, in sexual reproduction, between adaptive ele¬ ments and inadaptive elements no longer subject to negative selection); then realized that this process could not explain complete elimination, thus lead¬ ing him to propose a lower level of subcellular selection, potentially acting in opposition to organismal selection, and called “germinal selection”; and finally recognized that if levels of selection existed below the organismal, then the same logic implies the existence and potency of supraorganismal levels as well. 6. Darwin himself provides the best 19th century example—previously un¬ recognized because Darwin omitted this material, originally written for the unpublished “long version,” from the Origin—of the need for a hierarchical theory of selection in any full account of the phenomenology of evolution. Entirely consistent single-level theories cannot be carried through to comple¬ tion. Darwin admitted important components of species selection in capping his (still unsatisfactory) explanation for an issue that he ranked second in im¬ portance only to explaining the anagenesis of populations by natural selec¬ tion: the resolution of organic variety and plenitude by a “principle of diver¬ gence” (his terminology). I document the largely unrecognized emphasis that he placed upon this principle of divergence (for example, the Origin's famous single figure does not illustrate natural selection, as generally misinterpreted, but rather the principle of divergence). Darwin struggled to explain this de¬ scriptively higher-level phenomenon of taxonomic diversification as a fully predictable consequence of ordinary organismal selection, but he could not proceed beyond an argument that he himself finally recognized as forced, and even a bit hokey: the claim that natural selection will always favor extreme variants at the tails of a distribution for a local population in a particular ecology (the Origin's diagram represents an exemplification of this claim). Eventually, Darwin realized that he needed to invoke species selection for a full explanation of the success of speciose clades—and this unknown argu¬ ment, rather than his well-documented defense of group selection for hu¬ man altruism, represents Darwin’s most generalized invocation of selection at supraorganismal levels.
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THE STRUCTURE OF EVOEUTIONARY THEORY 7. Hierarchical models of evolutionary processes (at least descriptively so, but causally as well) have been featured and defended by evolutionary theo¬ rists from the beginning of our science, although not always by good or valid arguments. This inadequately recognized theme explains the major contrast between Lamarck and Darwin, and coordinates the various disputes between Wallace and Darwin. Wallace simply didn’t grasp the concept of levels at all, and remained so committed to adaptationism that he ranged up and down the hierarchy, oblivious of the conceptual problems thus entailed, until he found a level to justify his adaptationist bent. Darwin, by contrast, com¬ pletely understood the problem of levels, and the reasons behind his strong preference for a reductionist and single-level theory of organismal agency— although he reluctantly admitted a need for species selection to resolve the problem of divergence. We can also understand why Wallace’s 1858 Ternate paper, sent to Darwin and precipitating the “delicate arrangement,” did not proceed as far to a resolution as later tradition holds, when we recognize Wallace’s conceptual confusion about levels of selection.
Chapter 4: Internalism and laws of form: pre-darwinian alternatives 1. In a brilliant closing section to his general chapter 6, entitled “difficulties on theory,” Darwin summarized the logical structure of the most important challenge to his system, and organized his most cogent defense for his func¬ tionalist theory of selection, by explicating the classical dichotomy between “unity of type” and “conditions of existence”—or the formalism of Geoffroy vs. the functionalism of Cuvier—entirely in selectionist terms, and to his ad¬ vantage. He attributed “conditions of existence” to immediate adaptation by natural selection, and then explicated “unity of type” as constraints of inheri¬ tance of homologous structures, originally evolved as adaptations in a distant ancestor. Thus, he identified natural selection as the underlying “higher law” for explaining all morphology as present adaptation or as constraint based on past adaptation. He also admitted, while cleverly restricting their range and frequency, a few other factors and forces in evolutionary explanation. 2. A fascinating, and previously unexplored, contrast may be drawn be¬ tween the strikingly similar dichotomy, although rooted in creationist ex¬ planations, of Paley’s functionalist and adaptationist theory of divine con¬ struction for individualized biomechanical optimality vs. Agassiz’s formalist theory of divine ordination of taxonomic structure as an incarnation of God’s thoughts according to “laws of form” reflecting modes and categories of eter¬ nal thought. Clearly, this ancient (and still continuing) contrast between structural and functional conceptions of morphology transcends and pre¬ dates any particular mechanism, even the supposedly primary contrast of cre¬ ation vs. evolution, proposed to explain the actual construction of organic di¬ versity. 3. In the late 18th century, the great poet (and naturalist) Goethe developed a fascinating (and, in the light of modern discoveries in evo-devo, more than partly correct) archetypal theory in the structuralist or formalist mode— and explicitly critical of functionalist, teleological and adaptationist alterna-
Defining and Revising the Structure of Evolutioitary Theory tives—for the diversity of organs growing off the stems and roots of plants. He viewed cotyledons, and all the standard parts of flowers (sepals, petals, stamens and carpels), as modifications of a leaf archetype. 4. The famous early 19th century argument, culminating in the public debate of 1830 between Georges Cuvier and Etienne Geoffroy St. Hilaire (and analyzed by Goethe in his final paper before his death), did not, as com¬ monly misinterpreted, pit evolutionary theories against creationist accounts (although Geoffroy favored a limited theory of evolution, while Cuvier re¬ mained strongly opposed), but rather represented the most striking and en¬ during incident in this older and persistent struggle between formalist (Geoffroy) and functionalist (Cuvier) explanations of morphology and taxo¬ nomic order. Geoffroy advocated the abstract vertebra as an archetype for all animals, beginning (largely successfully) with a common basis for anatomical differences between teleosts and tetrapods, moving to the putatively common design of insects and vertebrates (still with some success, partly confirmed by the Hoxology of modern evo-devo, but also including some “howlers” like the homology of arthropod limbs with vertebrate ribs), and crashing with the proposed homology of vertebrates and a cephalopod doubled back upon it¬ self (the comparison that sufficiently aroused Cuvier’s growing ire into a call for public debate). Geoffroy’s theory of dorso-ventral inversion between in¬ sects and vertebrates was not a silly evolutionary conjecture about “the worm that turned” (as later caricatures often portray), and did not represent an evo¬ lutionary explanation at all, but rather expressed a formalist comparison based upon a common underlying structure, ecologically oriented one way in vertebrates (central nervous system up), and the other way in arthropods. The common impression of Cuvier’s victory must be reassessed as a complex “draw,” with Geoffroy’s position abetted by the fortuity of his longer life and his courting of prominent literary friends as supporters (including Balzac and Georges Sand). 5. Adaptationist preferences have enjoyed a long anglophonic tradition, beginning with the treatises of Ray and Boyle, in Newton’s founding genera¬ tion, on final causes; then extending, in creationist terms, through Paley and the Bridgewater Treatises; and finally culminating in the radically reversed evolutionary explanations (but still retaining the same functionalist and adaptationist commitments) of Darwin, extending forward to Fisher and the Modern Synthesis. By contrast, continental traditions have favored formalist and structuralist explanations of morphology, from the creationist accounts of Agassiz, through the transitional systems of Goethe and Geoffroy, to the fully evolutionary accounts of Goldschmidt and Schindewolf in the mid 20th century. Interestingly, the complex views of Richard Owen, so widely misun¬ derstood as an opponent of evolution (when he only rejected the predomi¬ nant functionalism of traditional British approaches to morphology), may best be grasped when we understand him as a rare anglophonic exponent of a predominantly formalist theory. Owen, following Geoffroy, tried to explain the entire vertebrate skeleton, including the skull and limbs, as a set of modi¬ fications upon a vertebral archetype. 6. Darwin maintained a genuine interest in formalist constraints upon
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THE STRUCTURE OF EVOEUTIONARY THEORY adaptationist optimality for individualized features of anatomy—a theme that he epitomized as “correlations of growth.” But he developed an explicit framework and rationale, most thoroughly discussed not in the Origin but in his longest 1868 book on The Variation of Animals and Plants Under Do¬ mestication, that relegated such formalist effects to a clearly subservient and secondary status, compared with natural selection and adaptation, in evolu¬ tionary causality.
Chapter 5: Channels and saltations in post-Darwinian formalism 1. Galton’s Polyhedron, the metaphor and model devised by Darwin’s bril¬ liant and eccentric cousin Francis Galton, and then fruitfully used by many evolutionary critics of Darwinism, including St. George Mivart, W. K. Brooks, Hugo de Vries, and Richard Goldschmidt, clearly expresses the two great, and both logically and historically conjoined, themes of formalist (or structuralist, or internalist, in other terminologies) challenges to functionalist (or adaptationist, or externalist) theories in the Darwinian tradition. This model of evolution by facet-flipping to limited possibilities of adjacent planes in inherited structure stresses the two themes—channels set by internal con¬ straint, and evolutionary transition by discontinuous saltation—that struc¬ turalist alternatives tend to embrace and that pure Darwinism must combat as challenges to basic components of its essential logic (for channels direct the pathways of evolutionary change from the inside, albeit in potentially posi¬ tive and adaptive ways, even though some external force, like natural selec¬ tion, may be required as an initiating impulse; whereas saltational change vi¬ olates the Darwinian requirement for selection’s creativity by vesting the scope and direction of change in the nature and magnitude of internal jumps, and not in sequences of adaptive accumulations mediated by natural selection at each step). 2. Orthogenesis, as a general term for evolutionary directionality along channels of internal constraint, rather than external pathways of natural se¬ lection, existed in several versions, ranging from helpful auxiliaries to Dar¬ winism, to outright alternatives that denied any creative potency to selection. Theodor Eimer, who coined the term orthogenesis, presented a middle ver¬ sion that tried to integrate internal channels of orthogenesis with external pathways of functionalist determination. But Eimer defended Lamarckian mechanics for his functionalism, thus leading him to oppose natural selection (he spoke of the Ohnmacht, or “without power,” of selection, contrasted with Weismann’s Allmacht, or “all power”) despite his pluralistic linkage of formalist and functionalist explanations. 3. The orthogenetic theory of the late 19th century American paleontolo¬ gist Alpheus Hyatt embodied maximal opposition to natural selection, and must be viewed as alternative, rather than auxiliary, to Darwinism. Hyatt conceived the pathway of ontogeny, modified only by heterochronic changes permitted under the biogenetic law, as the internal directing channel that nat¬ ural selection could tweak, but not derail. Illustrating the influence of theory over perception, Hyatt found several parallel lineages of snails, running along
Defining and Revising the Structure of Evolutionary Theory different segments of a common pathway, but all supposedly living in an identical environment—where others had reconstructed typical Darwinian monophyletic trees of phylogeny from the same stratigraphic section of fresh¬ water planorbids. Hyatt, who engaged in a long and ultimately frustrating correspondence with Darwin on this subject, believed that lineages followed a preordained “ontogeny” of phyletic youth, maturity and old age, thus at¬ tributing the different internal responses of lineages living in the same envi¬ ronment to their residence in different stages of an ontogenetically fixed and shared phyletic pathway (a preset internal channel with a vengeance). 4. Charles Otis Whitman, a great early 20th century American naturalist, developed the most congenial auxiliary theory (to Darwinism) of ortho¬ genesis in his extensive work on the evolution of color patterns in Darwin’s own favorite organism, the domestic pigeon. Whitman argued that domestic pigeons in particular, and dove-like birds in general, followed a strong chan¬ nel of internal predisposition leading in one direction from checkers to bars, and eventually to the obliteration of all color. (Darwin, by interesting con¬ trast, argued for a reverse tendency from bars to checkers, but also held, as his basic theory obviously implies, that selection largely determines any par¬ ticular event and that no internal predisposition can trump the dictates of im¬ mediate function.) 5. In his 1894 book on Materials for the Study of Variation (where he coined the term homeosis), William Bateson presented an extensive catalog of cases in discontinuous variation among individuals in a population and be¬ tween populations of closely related organisms. He used these examples to develop a formalist theory of saltational evolution, strongly opposed to the adaptationist assumptions of Darwinian accounts. (Bateson’s acerbic criti¬ cisms of adaptationist scenario-building and story-telling in the speculative mode emphasize a common linkage between structuralist preferences for me¬ chanical explanation, and distaste for the adaptationist assumption that func¬ tional necessity leads and the evolution of form follows.) Although Bateson coined the term genetics, his personal commitment to a “vibratory” theory of heredity, based on physical laws of classical mechanics—an intuition that he could never “cash out” as a testable theory—prevented his allegiance to the growing influence of Mendelian principles. 6. Hugo de Vries, the brilliant Dutch botanist who understood the logic of selectionism so thoroughly and acutely (but largely in contrast with the only other biologists, Weismann and Darwin himself, who also grasped all the richness and range of implications, but with favor), developed a saltational theory of evolution, but explicitly denied any predisposition of lineages to follow internal channels of constraint. (He thus showed the potential inde¬ pendence of the frequently-linked formalist themes of channeling and salta¬ tion, a conjunction espoused by Bateson and Goldschmidt for example, but denied in the other direction by Whitman, who favored channeling but de¬ nied saltation by supporting a gradualist theory of orthogenetic change.) This fascinating scholar regarded Darwin as his intellectual hero and never forgot the kindness and encouragement conveyed by his mentor and guru during
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THE STRUCTURE OF EVOEUTIONARY THEORY their one personal meeting early in de Vries’s career. But de Vries, who devel¬ oped the theory of intracellular pangenesis (the ultimate source for the term “gene”) in the late 19th century, and then (quite fortuitously and long after he had reached saltational conclusions for other reasons) became one of Men¬ del’s rediscoverers, based his truly saltational theory of immediate macromutational origin of species on his work with the evening primrose, Oenothera lamarckiana, where he mistook an odd chromosomal organization that gen¬ erates occasional saltations for a biological generality. De Vries, who under¬ stood the logic of selectionism so well, who knew that his macromutational theory refuted several essential components of Darwinian logic, but who could not bear (for largely psychological reasons) to forsake his intellectual and personal hero, insisted upon his larger fealty to Darwin, even though he had banned Darwinian mechanisms from the master’s own realm of the ori¬ gin of species. So de Vries developed a hierarchical theory that, while denying selection for the origin of species, restored selectionist logic at the higher level of phyletic trends by explicitly proposing “species selection” (his term) as a mechanism for generating broader phylogenetic patterns. 7. By proposing a comprehensive formalist theory in the heyday of devel¬ oping Darwinian orthodoxy, Richard Goldschmidt became the whipping boy of the Modern Synthesis—and for entirely understandable reasons. Gold¬ schmidt showed his grasp, and his keen ability to utilize, microevolutionary theory by supporting this approach and philosophy in his work on variation and mtraspecific evolution within the gypsy moth, Eymantria dispar. But he then expressed his apostasy by advocating discontinuity of causality, and pro¬ posing a largely nonselectionist and formalist account for macroevolution from the origin of species to higher levels of phyletic pattern. Goldschmidt in¬ tegrated both themes of saltation (in his concept of “systemic mutation” based on his increasingly lonely, and ultimately indefensible, battle to deny the corpuscular gene) and channeling (in his more famous, if ridiculed, idea 0
of “hopeful monsters,” or macromutants channeled along viable lines set by internal pathways of ontogeny, sexual differences, etc.). The developmental theme of the “hopeful monster” (despite its inappropriate name, virtually guaranteed to inspire ridicule and opposition), based on the important con¬ cept of “rate genes,” came first in Goldschmidt’s thought, and always occu¬ pied more of his attention and research. Unfortunately, he bound this inter¬ esting challenge from development, a partially valid concept that could have been incorporated into a Darwinian framework as an auxiliary hypothesis (and now has been accepted, to a large extent, if under different names), to his truly oppositional and ultimately incorrect theory of systemic mutation, therefore winning anathema for his entire system. Goldschmidt may have acted as the architect of his own undoing, but much of his work should evoke sympathetic attention today.
Chapter 6: Pattern and progress on the geological stage 1. Darwin based his argument for a broad and general vector of progress in life’s history not on the “bare bones” operation of natural selection (where he
Defining and Revising the Structure of Evolutionary Theory had explicitly denied such an outcome as the most radical implication of his theory), but on subsidiary ecological claims for the predominance of biotic over abiotic competition, and for a geological history of plenitude in a persis¬ tently crowded ecological world, where one species must displace another to gam entry into ecosystems (the metaphor of the wedge). Darwin used these ecological sequelae, along with the gradualist and incrementalist logic of natural selection itself, as primary justifications for his third essential claim of selection’s scope, or the uniformitarian extension of small-scale microevolution, in a smoothly continuationist manner, to explain all patterns of macro¬ evolution by accumulation of increments through the immensity of geological time. 2. Such a claim requires that the geological stage operate in an appropri¬ ate, and “Goldilockean,” manner—not too much change to debar the opera¬ tion and domination of this slowly and smoothly accumulative biological mode, and not too little to provide insufficient impetus (within Darwin’s externalist and functionalist theory) for attributing the amount of change ac¬ tually observed to natural selection. 3. The primary claim of “too much” derived from the school of “catastrophism” in geology—a movement that has been unfairly stigmatized by later history, following Lyell’s successful and largely rhetorical mischaracterization (he was a lawyer by profession), as an unscientific defense of supernaturalism to cram the observed results of geology into the strictures of bibli¬ cal chronology, but that actually took the opposite position of strict empirical literalism (whereas uniformitarians argued that the numerous literal appear¬ ances of rapidity in the geological record must be “interpreted” as misleading consequences of how gradual change must be expressed in a woefully imper¬ fect set of strata). The great catastrophist Cuvier, in particular, was an En¬ lightenment rationalist, not a theological apologist—and he based his defense of catastrophism upon his literalist reading of the paleontological and geolog¬ ical record. 4. The primary claim of “too little” geology followed Lord Kelvin’s in¬ creasingly diminished estimates for the age of the earth (incorrectly made— although Kelvin accurately described the necessary, but (as it turned out) em¬ pirically false, logic required to validate his views—by assuming that heat now flowing from the earth represented a continuing loss from an originally molten state). Darwin worried intensely over Kelvin’s claims, even referring to him as an “odious spectre” in a letter to Wallace. Darwin feared that Kel¬ vin’s low estimates would not permit enough time to generate the history of life under his slowly acting theory of gradualistic and accumulative change. Although this story has been told often, and has become familiar to scientists, an important (and decisive) aspect of the tale has rarely been exposed: Dar¬ win fought this battle alone, and his strong distress illustrates the maximal, and unique, extent of his gradualistic and continuationist commitments. His closest colleagues, Wallace and Huxley, did not find Kelvin’s low estimates unacceptable, but argued that we had only been led to expect such slow change from our previous conception of the earth’s age, and that faster rates
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THE STRUCTURE OF EVOLUTIONARY THEORY of phyletic change, implied by Kelvin’s dates, were entirely acceptable under their reading of evolution.
Chapter 7: The modern synthesis as a limited consensus 1. From the anarchic situation that prevailed at the Darwinian centennial celebrations of 1909 (confidence in the factuality of evolution, linked with agnosticism about theories and mechanics, as the first fruits of Mendehsm seemed, initially, to refute the gradualism and incrementalism of natural se¬ lection), the Modern Synthesis eventually emerged in two stages (following the union of Darwinian and Mendelian perspectives in the work of Fisher and others): first, by a welcome restriction that eliminated Kellogg’s three al¬ ternatives in oppositional modes that would have destroyed Darwinism (Lamarckism as a substitute functionalism, and saltationism and orthogene¬ sis as formalist alternatives), and reasserted, now in a context of Mendelian particulate inheritance, the adequacy of natural selection as a creative force; and second, by an increasingly dubious hardening, culminating in centennial celebrations for the Origin in 1959, that substituted an increasingly rigid adaptationism for an earlier pluralism that embraced all mechanisms (includ¬ ing genetic drift) consistent with known genetic principles, while favoring se¬ lection as a primary force. 2. In his founding book of 1930, The Genetical Theory of Natural Selec¬ tion, R. A. Fisher showed how slow, gradualist evolution in large, panmictic populations (treated almost as an ahistorical system, analogous to effectively infinite populations of identical gas molecules free to move and diffuse by physical principles) could validate strict Darwinism under Mendelian partic¬ ulate inheritance (with Darwin’s own acceptance of blending inheritance ex¬ posed as a more serious impediment than Darwin himself had realized), and disprove saltational alternatives by the inverse correlation of frequency and magnitude in variation. To these mathematical and general chapters, Fisher appended a long closing section devoted to his eugenical theory that Western society had begun to degenerate seriously as a consequence of the social pro¬ motion of infertility (the rise in class level of “good” genetic stock, largely by their correlated tendency to have fewer children, thereby husbanding their economic resources to potentiate their social elevation). Fisher conceived this eugenical “blight” as entirely Darwinian in character—invisible in its gradual expression generation by generation, but ultimately more deadly than the ex¬ plicit saltational degenerations stressed by most eugenicists. 3. In contrast with the initial pluralism of Haldane and Huxley (in the book that coined the Modern Synthesis), and of the first editions of founding documents for the second phase of the Synthesis (Dobzhansky’s 1937 Genet¬ ics and the Origin of Species, Mayr’s 1942 Systematics and the Origin of Spe¬ cies, and Simpson’s 1944 Tempo and Mode in Evolution), later editions of these three documents encapsulated the hardening of this second phase, as initial pluralism yielded to an increasingly firm and exclusive commitment to adaptatiomst scenarios, and to natural selection as a virtually exclusive mech¬ anism of change. Even Sewall Wright’s views on genetic drift and shifting bal-
Defining and Revising the Structure of Evolutionary Theory ance altered from initial stress upon stochastic alternatives to selection to an auxiliary role for drift (as an impetus for the exploration of new, and poten¬ tially higher, adaptive peaks) as one aspect of a more inclusive and basically adaptatiomst process. The complex reasons for this hardening include some empirical documentations of selection, but also involve a set of basically so¬ cial and institutional factors not based on increasing factual adequacy. 4. If this hardening on the second Darwinian branch of selection’s efficacy reflects a general trend within evolutionary theory, then we should find a sim¬ ilar Darwinian strengthening (and narrowing) on the other two branches of selection’s agency (organismal vs. higher levels) and scope (adequacy to ex¬ plain the entire geological record by extrapolated microevolution). The tri¬ umph (for good reasons at the time) of Williams over Wynne-Edwards af¬ firms this trend for agency, although Williams’s important clarification then unfortunately hardened (among epigones) into a dogmatic and a priori re¬ jection of any hint of group selection. Similarly, the Synthesis’s increasing confidence in the exclusivity of gradualistic microevolution deprived paleon¬ tology of any independent theoretical space, and relegated the field to docu¬ mentation of an admittedly underdetermined pageant, built by the exclusive agency of microevolutionary principles. Several synthesists even denied the efficacy of differential speciation as an input to macroevolutionary pattern (branding the speciosity of some clades as a “luxury” rather than a crucial in¬ put to survival and flourishing), and attributed all higher-level change to ex¬ tensions of gradualistic and adaptive anagenesis within unbranched lineages. 5. The trends to development, initial pluralism and later hardening of the Modern Synthesis win clearest expression in two sources of data: comparison of statements by leading scientists at the two contrasting centennial cele¬ brations of 1909 and 1959 (for Darwin’s birth and for the publication of the Origin); and by documentation of hardening in the summary statements (and increasingly dogmatic dismissal of alternatives) in leading textbooks for sec¬ ondary and undergraduate courses in biology.
Chapter 8: Species as individuals in the hierarchical theory of selection 1. Selectionist mechanics, in the most abstract and general formulation, work by interaction of individuals and environments (broadly construed to include all biotic and abiotic elements), such that some individuals secure dif¬ ferential reproductive success as a consequence of higher fitness conferred by some of their distinctive features, leading to differential plurifaction of indi¬ viduals with these features (relative to other individuals with contrasting fea¬ tures), thus gradually transforming the population in adaptive ways. But the logic of this statement implies that organisms cannot be the only bio¬ logical entities that manifest the requisite properties of Darwinian individual¬ ity—properties that include both vernacular criteria (definite birth and death points, sufficient stability during a lifetime, to distinguish true entities from unboundable segments of continua), and more specifically Darwinian criteria (production of daughters, and inheritance of parental traits by daughters). In
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THE STRUCTURE OF EVOLUTIONARY THEORY particular, and by these criteria, species must be construed not only as classes (as traditionally conceived), but also as distinct historical entities acting as good Darwinian individuals—and therefore potentially subject to selection. In fact, a full genealogical hierarchy of inclusion—with rising levels of genes, cell lineages, organisms, demes, species and clades—features clearly definable Darwinian individuals, subject to processes of selection, at each level, thus validating (in logic and theory, but not necessarily in the potency of actual practice in nature) an extension and reformulation of Darwin’s exclusively organismal theory into a fully hierarchical theory of selection. 2. The validity of the “interactor approach” to defining the mechanics of selection, and the fallacy of the “replicator approach” expose, as logically in¬ valid, all modern attempts to preserve Darwinian exclusivity of level, but to offer an even more reductionistic account in terms of genes, rather than or¬ ganisms, as agents—with organisms construed as passive containers for the genes that operate as exclusive agents of natural selection. This false argu¬ ment, based upon the true but irrelevant identification of genes as faithful replicators, must be replaced by the conceptually opposite formulation of a hierarchical theory of selection, with genes identified as only one valid, and lowest, level in a hierarchy of equally potent, and interestingly different, lev¬ els of Darwinian individuality: genes, cell lineages, organisms, demes, species and clades. Replication identifies a valid and important criterion for the cru¬ cial task of bookkeeping or tracing evolutionary change; but replicators can¬ not specify the causality of selectionist processes, which must be based upon the recognition and definition of interactors with environments. Even Wil¬ liams and Dawkins, the two leading exponents of exclusive gene selectionism, have acknowledged and properly described the hierarchical causality of inter¬ action (while proferring increasingly elaborate and implausible verbal de¬ fenses of gene selection in arguments about parallel hierarchies and Necker cubing of legitimate alternatives rooted in criteria of replication vs. interac¬ tion). Thus, Williams and Dawkins seem to grasp the validity of hierarchical selection through a glass darkly, while still trying explicitly to defend their in¬ creasingly indefensible preferences for exclusive gene selectionism. 3. The logic of hierarchical selection cannot be gainsaid, and even Fisher admitted the consistency, even the theoretical necessity, while denying the em¬ pirical potency, of species selection. Fisher based his interesting and powerful argument on his assumption that low N for species in clades (relative to or¬ ganisms in populations) must debar any efficacy for species selection in a world of continuous and gradualistic anagenesis rooted in organismal selec¬ tion. However, Fisher’s argument, although logically tight, fails empirically because species tend to be stable and directionally unchanging (however fluctuating) during their geological lifetimes, and the theoretically “weaker” force of species selection may therefore operate as the “only game in town” for macroevolution. The arguments for potency of species selection are stron¬ ger than corresponding assertions for interdemic selection (largely because species actively maintain their boundaries as Darwinian individuals, whereas demes remain subject to breakup and invasion). But, despite these intrinsic
Defining and Revising the Structure of Evolutionary Theory weaknesses and problems, interdemic selection has now been empirically val¬ idated as an important force in evolution—thus strengthening a prima facie case for the even greater importance of species selection in macroevolution. 4. Two theoretical resolutions and clarifications have established both a sound theoretical basis, and a strong argument for the empirical potency, of species selection as an important component of macroevolution: first, the rec¬ ognition of differential proliferation rather than downward effect as the most operational criterion for defining and recognizing species selection; second, the acknowledgment that emergent fitnesses under the interactor approach, rather than emergent features treated as active adaptations of the species, constitute the proper criterion for identifying species selection. The former in¬ sistence upon emergent features (by me and other researchers, and in error), while logically sound and properly identifying a small subset of best and most interesting cases, relegated the subject to infrequent operational utility, and thus to relative impotence. The proper criterion (under the interactor ap¬ proach) of emergent fitness universalizes the subject by permitting general identification in the immediacy of the current mechanics of selection, and not requiring knowledge—often unavailable given the limits of historical ar¬ chives—of adaptive construction and utility in ancestral states. 5. The six levels recognized for convenience, and not accompanied by any claim of completion or exclusivity—gene, cell lineage, organism, deme, spe¬ cies and clade—feature two important principles that make the theory of hi¬ erarchical selection so different from, while still in the lineage and tradition of, exclusivistic Darwinian organismal selection. First, adjacent levels may in¬ teract in the full range of conceivable ways—in synergy, orthogonally, or in opposition. Opposition has been stressed in the existing literature, but only because this mode is easier to recognize, and not for any argument of greater importance in principle. Second, the levels operate non-fractally, with fas¬ cinating and distinguishing differences in mode of functioning, and relative importance of components, for each level. For example, the different mecha¬ nisms by which organisms and species maintain their equally strong individu¬ ality dictate that selection should dominate at the organismal level, while selection, drift, and drive should all play important and balanced roles at the species level. 6. To cite just one difference (from conventions of the organismal level) for each nonstandard level, and to make the key point about distinctiveness of levels in an almost anecdotal manner: random change may be most promi¬ nent in relative frequency at the level of the gene-individual; true gene selec¬ tion also plays an important, if limited, role (largely in the mode that has been given the unfortunate name—for its implication of opposition, almost in ethical terms, to the supposed standard of proper organismal selection—of “selfish DNA”); however, the Dawkinsian argument for exclusivity of genic selection only records the confusion of a preferred level of bookkeeping with an erroneous claim for a privileged locus of selection. Selection among celllineages, although ancestrally important in the evolution of multicellular or¬ ganisms, has largely been suppressed by the organismal level in the interests
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THE STRUCTURE OF EVOLUTIONARY THEORY of its own integrity; failure of this suppression leads to the pyrrhic victory of cell-lineages that we call cancer. Interdemic selection, although once so widely rejected, probably plays an essential role in the evolution of social coopera¬ tion in general, and not only for such specific phenomena as human altruism. Species-level selection, combined with other species-level properties of drive and drift, establishes the independent basis for a distinctive speciational the¬ ory and reformulation of macroevolution. The highest level of clade selec¬ tion, although sometimes operative, may be relatively weak by an extension of Fisher’s argument about low N. 7. I explore the distinctive differences between levels of selection by trying to exemplify and “play out” the detailed disparities in a “grand analogy” be¬ tween the conventional operation of organismic selection and the relative conceptual novelty of species selection. As an idiosyncratic sample of poten¬ tial reforms and surprises, consider the following claims: First, the formula¬ tion of a general taxonomy for sources of change in hierarchically ordered systems, based on a primary distinction of “drive” for directed changes aris¬ ing within an individual, based on change among lower-level individuals as constituent parts; and “sorting,” with two causally distinct subcategories of “selection” and “drift” for change based on alterations of relative frequencies among individuals at the focal level itself. Second, the recognition, by follow¬ ing the logic of the analogy, of some strikingly counterintuitive comparisons that become both interesting and revealing upon subsequent reflection—in¬ cluding the likeness of Lamarckian change, construed as ontogenetic drive at the organismal level, with standard anagenetic transformation as organismal drive at the species level (transformation by directional change of constituent parts of a higher-level individual, in this case the organisms of a species); this similarity may also highlight the rather different reasons for general unimpor¬ tance of both levels of drive—Lamarckism for the well-known reason of the¬ oretical non-occurrence in a Mendelian world, and anagenesis based on the controversial claim for its evident plausibility in theory (as a basic Darwinian process), but rarity in fact, given the dominant relative frequency of punctu¬ ated equilibrium. Third, the establishment of a framework for distinguishing directional speciation as a form of reproductive drive (inherently biased dif¬ ferences in autapomorphies of descendant species vs. ancestral states) from true species selection as a higher order sorting among daughter species that arise with phenotypic differences randomly distributed about parental means. I believe that we have missed this crucial distinction because the ana¬ log of directional speciation at the organismal level—drives induced by muta¬ tion pressure—occur so rarely (for conventional reasons of organismal selec¬ tion’s power to suppress them) that we haven’t considered the greater potency of analogous processes at other levels. Fourth, the importance of testing “Wright’s Rule”—the claim that speciation is random with respect to the di¬ rection of evolutionary trends within clades—because the major alternative of directional speciation as the cause of trends holds such potential power at the species level, whereas its analog (drives of mutation pressure) assumes so little importance at the organismal level. Fifth, the potentially far greater im-
Defining and Revising the Structure of Evolutionary Theory portance of drift (both species drift and founder drift) vs. selection as a mech¬ anism of sorting at the species level, but not at the orgamsmal level, where se¬ lection predominates in standard formulations. Sixth, the identification of an intrinsically, and probably unbreakable (in most cases), negative correlation between speciation and extinction propensities as the primary constraint op¬ erating to prevent the takeover of life by a few megaclades (which might dom¬ inate by enhancing speciation while retarding extinction among constituent species—or perhaps the Coleoptera have prevailed by this means). Seventh, the recognition that the organismal level operates uniquely in securing the in¬ tegrity of its individuals by devices (physiological homeostasis among organs, and spatial bounding by an external surface) that “clear out” both drive from below and drift at its own level as mechanisms operating at high relative fre¬ quency—thus leaving selection in its most dominant position at this level. Perhaps our Darwinian prejudice for regarding selection as by far the most effective, or virtually the only important, process of evolutionary change arises more from the parochialism of our organismal focus (given our own personal residence in this category) than from any universal characterization of all levels in evolution.
Chapter 9: Punctuated equilibrium and the validation of macro evolutionary theory 1. The clear predominance of an empirical pattern of stasis and abrupt geo¬ logical appearance as the history of most fossil species has always been ac¬ knowledged by paleontologists, and remains the standard testimony (as doc¬ umented herein) of the best specialists in nearly every taxonomic group. In Darwinian traditions, this pattern has been attributed to imperfections of the geological record that impose this false signal upon the norm of a truly gradualistic history. Darwin’s argument may work in principle for punctuational origin, but stasis is data and cannot be so encompassed. 2. This traditional argument from imperfection has stymied the study of evolution by paleontologists because the record’s primary (and operational) signal has been dismissed as misleading, or as “no data.” Punctuated equilib¬ rium, while not denying imperfection, regards this signal as a basically accu¬ rate record of evolution’s standard mode at the level of the origin of species. In particular, before the formulation of punctuated equilibrium, stasis had been read as an embarrassing indication of absence of evidence for the de¬ sired subject of study—that is, of data for evolution itself, falsely defined as gradual change—and this eminently testable, fully operational, and intellec¬ tually fascinating (and positive) subject of stasis had never been subjected to quantitative empirical study, a situation that has changed dramatically during the last 25 years. 3. The key empirical ingredients of punctuated equilibrium—punctuation, stasis, and their relative frequencies—can be made testable and defined oper¬ ationally. The theory only refers to the origin and development of species in geological time, and must not be misconstrued (as so often done) as a claim for true saltation at a lower organismal level, or for catastrophic mass extmc-
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THE STRUCTURE OF EVOLUTIONARY THEORY tion at a higher faunal level. Punctuation must be scaled relative to the later duration of species in stasis, and we suggest 1-2 percent (analogous to human gestation vs. the length of human life) as an upper bound. Punctuated equi¬ librium can be distinguished from other causes of rapid change (including anagenetic passage through bottlenecks and the traditional claim of imperfect preservation for a truly gradualistic event) by the criterion of ancestral sur¬ vival following the branching of a descendant. Punctuations can be revealed by positive evidence (rather than inferred from compression on a single bed¬ ding plane) in admittedly rare situations, but not so infrequent in absolute number, of unusual fineness of stratigraphic resolution or ability to date the individual specimens of a single bedding plane. Stasis is not defined as abso¬ lute phenotypic immobility, but as fluctuation of means through time at a magnitude not statistically broader than the range of geographic variation among modern populations of similar species, and not directional in any pre¬ ferred way, especially not towards the phenotype of descendants. Punctuated equilibrium will be validated, as all such theories in natural history must be (including natural selection itself), by predominant relative frequency, not by exclusivity. Gradualism certainly can and does occur, but at very low relative frequencies when all species of a fauna are tabulated, and when we overcome our conventional bias for studying only the small percentage of species quali¬ tatively recognized beforehand as having changed through time. 4. Punctuated equilibrium emerges as the expected scaling of ordinary allopatric speciation into geological time, and does not suggest or imply radi¬ cally different evolutionary mechanisms at the level of the origin of species. (Other proposed mechanisms of speciation, including most sympatric modes, envision rates of speciation even faster than conventional allopatry, and are therefore even more consistent with punctuated equilibrium.) The theoreti¬ cally radical features of punctuated equilibrium flow from its proposals for macroevolution, with species treated as higher-level Darwinian individuals analogous to organisms in microevolution. 5. The difficulty of defining species in the fossil record does not threaten the validity of punctuated equilibrium for several reasons. First, in the few studies with adequate data for genetic and experimental resolution, paleospecies (even for such difficult and morphologically labile species as colonial cheilostome bryozoans) have been documented as excellent surrogates, com¬ parable as units to conventional biospecies. Second, the potential underesti¬ mation of biospecies by paleospecies only imposes a bias that makes punctu¬ ated equilibrium harder to recognize. The fossil record’s strongly positive signal for punctuated equilibrium, in the light of this bias, only increases the probability of the pattern’s importance and high relative frequency. Third, the potential overestimation of biospecies by paleospecies is probably false in any case, and also of little practical concern because no paleontologist would as¬ sert punctuated equilibrium from the evidence of oversplit taxa in faunal lists, but only from direct biometric study of stasis and punctuation in actual data. 6. We originally, and probably wrongly, tried to validate punctuated equi¬ librium by asserting that, in principle, most evolutionary change should be
Defining and Revising the Structure of Evolutionary Theory concentrated at events of speciation themselves. Subsequent work in evolu¬ tionary biology has not confirmed any a priori preference for concentration in such episodes. Futuyama’s incisive macroevolutionary argument—that re¬ alized change will not become geologically stabilized and conserved unless such change can be “tied up" in the unalienable individuality of a new spe¬ cies—offers a far richer, far more interesting, and theoretically justified ra¬ tionale for correlating episodes of evolutionary change with speciation. 7. Section III presents a wide-ranging discussion of why proposed empirical refutations of punctuated equilibrium either do not hold in fact, or do not bear the logical weight claimed in their presentation. Refutations for single cases are often valid, but do not challenge the general hypothesis because we anticipate a low relative frequency for gradualism, and these cases may reside in this minor category. Claims for predominant gradualism in the entire clade of planktonic forams may hold as exceptional (although, even here, the ma¬ jority of lineages remain unstudied, in large part because they seem, at least subjectively, to remain in stasis, and have therefore not attracted the attention of traditional researchers, who wish to study evolution, but then equate evo¬ lution with gradualism). However, in these asexual forms with vast popula¬ tions, gradualism at this level may just represent the expected higher-level expression of punctuational clone selection, as Lenski has affirmed in the most thorough study of evolution in a modern bacterial species—and just as gradual cladal trends in multicellular lineages emerge as the expected conse¬ quences of sequential punctuated equilibrium at the species level (trends as stairsteps rather than inclined planes, so to speak). Claims for genetic gradu¬ alism do not challenge punctuated equilibrium, and may well be anticipated as the proper expression at the genic level (especially given the high relative frequency of random nucleotide substitutions) of morphological stasis in the phenotypic history of species. Punctuated equilibrium has done well in tests of conformity with general models, particularly in the conclusion that exten¬ sive polytomy in cladistic models may arise not only (as usually interpreted) from insufficient data to resolve a sequence of close dichotomies, but also as the expectation of punctuated equilibrium for successive branching of daugh¬ ter species from an unchanged parental form in stasis. In fact, the frequency of polytomy vs. dichotomy may be used as a test for the relative frequency of punctuated equilibrium in well resolved cladograms—a test well passed in data presented by Wagner and Erwin. 8. Section IV then summarizes the data on empirical affirmations of punc¬ tuated equilibrium, first on documented patterns of stasis in unbranched lin¬ eages; second on punctuational cladogenesis affirmed by the criterion of an¬ cestral survival; third on predominant relative frequencies for punctuated equilibrium in entire biotas (with particularly impressive affirmations by Hallam, Kelley, and Stanley and Yang for mollusks; and by Prothero and Heaton for Oligocene Big Badlands mammals, where a study of all taxa yielded 177 species that followed the expectations of punctuated equilibrium and three cases of potential gradualism, only one significant); fourth on predominant relative frequencies for punctuated equilibrium in entire clades, with empha-
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THE STRUCTURE OF EVOLUTIONARY THEORY sis on Vrba’s antelopes and, especially, Cheetham’s rigorously quantitative and multivariate data of evolution in the bryozoan genus Metrarabdotos, perhaps the best documented and most impressive case of exclusive punctu¬ ated equilibrium ever developed. Finally, we*can learn much from variation in relative frequencies among taxa, times, and environments—and interesting inferences have been drawn from recorded differences, particularly in Shel¬ don’s counterintuitive linkage of stasis to rapidly changing, and gradualism to stable, environments. 9. Among many reasons proposed to explain the predominance of stasis, a phenomenon not even acknowledged as a “real” and positive aspect of evo¬ lution before punctuated equilibrium gave it some appropriate theoretical space, habitat tracking (favored by Eldredge), constraints imposed by the nature of subdivided populations (favored by Lieberman), and normalizing clade selection (proposed by Williams) represent the most novel and interest¬ ing proposals. 10. Among the implications of a predominantly punctuational origin of stable species-individuals for macroevolutionary theory, we must rethink trends (the primary phenomenon of macroevolution, at least in terms of dedi¬ cated discussion in existing literature) as products of the differential success of certain kinds of species, rather than as the adaptive anagenesis of lin¬ eages—a radical reformulation with consequences extending to a new set of explanations no longer rooted (as in all traditional resolutions) in the adap¬ tive advantages conferred upon organisms, but potentially vested in such structural principles as sequelae (by hitchhiking or as spandrels) of fortuitous phenotypic linkage to higher speciation rates of certain taxa. In further exten¬ sions, macroevolution itself must be reconfigured in speciational terms, with attendant implications for a wide range of phenomena, including Cope’s rule (structurally ordained biases of speciation away from a lower size limit occu¬ pied by founding members of the clade, rather than adaptive anagenesis to¬ wards organismal benefits of large size), living fossils (members of clades with persistently minimal rates of speciation, and therefore no capacity for ever generating much change in a speciational scheme, rather than forms that are either depauperate of variation, or have occupied morphological optima for untold ages), and reinterpretation of cladal trends long misinterpreted as tri¬ umphs of progressive evolution (and now reevaluated in terms of variational range in species numbers, rather than vectors of mean morphology across all species at any time—leading, for example, to a recognition that modern horses represent the single surviving twig of a once luxurious, and now de¬ pleted, clade, and not the apex of a continually progressing trend). By the same argument, generalized to all of life, we understand the stability and con¬ tinued domination of bacteria as the outstanding feature of life’s history, with the much vaunted progress of complexity towards mammalian elegance rein¬ terpreted as a limited drift of a minor component of diversity into the only open space of complexity’s theoretical distribution. But, to encompass this re¬ formulation, we need to focus upon the diversity and variation among life’s species, not upon the supposed vectors of its central tendencies, or even its pe-
Defining and Revising the Structure of Evolutionary Theory npheral superiorities. Hominid evolution must also be rethought as reduction of diversity to a single species of admittedly spectacular (but perhaps quite transient) current success. In addition, the last 50,000 years or more of hu¬ man phenotypic stability becomes a theoretical expectation under punctuated equilibrium, and not the anomaly so often envisaged (and attributed to the suppression of natural selection by cultural evolution) both by the lay public and by many professionals as well. 11. Further extensions of punctuated equilibrium include the controversial phenomenon of "‘coordinated stasis,” or the proposition that entire faunas, and not merely their component species, tend to remain surprisingly stable in composition over durations far longer than any model based on independent behavior of species (even under punctuated equilibrium) would allow, al¬ though other researchers attribute the same results to extended consequences of sudden external pulses and resulting faunal turnovers, while still others deny the empirics of coordination and continue to view species as more inde¬ pendent, one from the other, even in the classical faunas (like the Devonian Hamilton Group) that serve as “types” for coordinated stasis. 12. Punctuated equilibrium has inspired several attempts, of varying suc¬ cess in my limited judgment, to construct mathematical models (or to simu¬ late its central phenomena in simple computer systems of evolving “artificial life”) that may help us to identify the degree of generality in modes of change that this particular biological system, at this particular level of speciation, exemplifies and records. Punctuated equilibrium has also proved its utility in extension by meaningful analogy (based on common underlying principles of change) to the generation of punctuational hypotheses at other levels, and for other kinds of phenomena, where similar gradualistic biases had prevailed and had stymied new approaches to research. These extensions range from phyletic and ecological examples below the species level to interesting ana¬ logs of both stasis and punctuation above the species level. Non-trending, the analog of stasis in large clades, for example, had been previously disre¬ garded—following the same fate as stasis in species—as a boring manifesta¬ tion of non-evolution, but has now been recognized and documented as a real and fascinating phenomenon in itself. Punctuational analogs have proven their utility for understanding the differential pace of morphological innova¬ tion within large clades, and for resolving a variety of punctuational phenom¬ ena in ecological systems, including such issues of the immediate moment as rates of change in benthic faunas (previously the province of hypotheses about glacially slow and steady change in constantly depauperate environ¬ ments), and such questions of broadest geological scale as the newly recog¬ nized stepped and punctuational “morphology” (correcting the hypothetical growth through substantial time of all previous gradualistic accounts) of mutual biomechanical improvement in competing clades involved in "arms race,” and generating a pattern known as “escalation.” 13. Punctuational models have also been useful, even innovative in break¬ ing conceptual logjams, in nonbiological fields ranging from closely cognate studies of the history of human tools (including extended stasis in the Homo
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THE STRUCTURE OF EVOEUTIONARY THEORY erectus toolkit), and nontrending, despite classical (and false) claims to the contrary by both experts, the Abbe Breuil and Andre Leroi-Gourhan, for the 25,000 year history of elegance in parietal cave art of France and Spain—and extending into more distant fields like learning theory (plateaus and innova¬ tive punctuations), studies of the dynamics of human organizations, patterns of human history, and the evolution of technologies, including a fascinating account of the history of books, through punctuations of the clay tablet, the scroll, the codex, and our current electronic reformation (wherever it may lead), and long periods of morphological stasis (graced with such vital inno¬ vations as printing, imposed upon the unaltered phenotype of the codex, or standard “book”). 14. In a long and final section, I indulge myself, and perhaps provide some useful primary source material for future historians of scientific conflicts, by recording the plethora of non-scientific citations, ranging from the absurd to the insightful, for punctuated equilibrium (including creationist misuses and their politically effective exposure by scientists in courtroom trials that de¬ feated creationist legislative initiatives; and the treatment of punctuated equi¬ librium, often very good but sometimes very bad, by journalists and by au¬ thors of textbooks—the primary arenas of vernacular passage). I also trace and repudiate the “dark side” of non-scientific reactions by professional col¬ leagues who emoted at challenges to their comfort, rather than reacting criti¬ cally and sharply (as most others did, and as discussed extensively in the main body of the chapter) to the interesting novelty, accompanied by some promi¬ nent errors of inevitable and initial groping on our part, spawned by the basic hypothesis and cascading implications of punctuated equilibrium.
Chapter 10: The integration of constraint and adaptation: historical constraint and the evolution of development 1. Although the directing of evolutionary change by forces other than natu¬ ral selection has loosely been described as “constraint,” the term, even while acknowledged as a domain for exceptions to standard Darwinian mecha¬ nisms, has almost always been conceived as a “negative” force or phenome¬ non, a mode of preventing (through lack of variation, for example) a popula¬ tion’s attainment of greater adaptation. But constraint, both in our science (and in vernacular English as well), also has strongly positive meanings in two quite different senses: first, or empirically, as channeled directionality for reasons of past history (conserved as homology) or physical principles; and second, or conceptually, as an nonstandard force (therefore interesting ipso facto) acting differently from what orthodoxy would predict. 2. The classical and most familiar category of internal channeling (the first, or empirical, citation of constraint as a positive theme) resides in preferred di¬ rections for evolutionary change supplied by inherited allometries and their phylogenetic potentiation by heterochrony. As “place holders” for an exten¬ sive literature, I present two examples from my own work: first, the illustra¬ tion of synergy with natural selection (to exemplify the positive, rather than oppositional, meaning), where an inherited internal channel builds two im-
Defining and Revising the Structure of Evolutionary Theory portant adaptations by means of one heterochronic alteration, as neoteny in descendant Gryphaea species of the English Jurassic produces shells of both markedly increased size (by retention of juvenile growth rates over an un¬ changed lifetime) and stabilized shape to prevent foundering in muddy envi¬ ronments (achieved by “bringing forward” the proportions of attached juve¬ niles into the unattached stage of adult ontogeny); second, an illustration of pervasiveness and equal (or greater) power than selective forces (to exemplify the strength and high relative frequency of such positive influences), as geo¬ graphic variation of the type species, Cerion uva, on Aruba, Bonaire, and Curasao, a subject of intense quantitative study and disagreement in the past, becomes resolved in multivariate terms, with clear distinction between local adaptive differences and the pervasive general pattern of an extensive suite of automatic sequelae, generated by nonadaptive variation in the geometry of coiling a continuous tube, under definite allometric regularities for the genus, around an axis. 3. For the second, or conceptually positive, meaning of constraint as a term for nonstandard causes of evolutionary change, I present a model that com¬ pares the conventional outcomes of direct natural selection, leading to local adaptation, with two sources that can also yield adaptive results, but for rea¬ sons of channeling by internal constraints rather than by direct construc¬ tion under external forces of natural selection. In this triangular model for aptive structures, the functional vertex represents features conventionally built by natural selection for current utilities. At the historical vertex, cur¬ rently aptive features probably originated for conventionally adaptive rea¬ sons in distant ancestors; but these features are now developmentally chan¬ neled as homologies that constrain and positively direct both patterns of immediate change and the inhomogeneous occupation of morphospace (espe¬ cially as indicated by “deep homologies” of retained developmental patterns among phyla that diverged from common ancestry more than 500 million years ago). At the structural vertex, two very different reasons underlie the origin of potentially aptive features for initially nonadaptive reasons: physical principles that build “good” form by the direct action of physical laws upon plastic material (as in D’Arcy Thompson’s theory of form), and architectural sequelae (spandrels) that arise as nonadaptive consequences of other features, and then become available for later cooptation (as exaptations) to aptive ends in descendant taxa. These two structural reasons differ strongly in the ahistoricist implications of direct physical production independent of phyletic context vs. the explicit historical analysis needed to identify the particular foundation for the origin of spandrels in any individual lineage. 4. As a conceptual basis for understanding the importance of recent ad¬ vances in evo-devo (the study of the evolution of development), the largely unknown history of debate about categories of homology, particularly the distinction between convergence and parallelism, provides our best ordering device—for we then learn to recognize the key contrast between parallelism as a positive deep constraint of homology in underlying generators (and therefore as a structuralist theme in evolution) and convergence as the oppo-
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THE STRUCTURE OF EVOEUTIONARY THEORY site sign of domination for external natural selection upon a yielding internal substrate that imposes no constraint (and therefore as a functionalist theme in evolution). As a beginning paradox, we must grasp why E. Ray Lankester coined the term homoplasy as a category of homology, whereas today’s termi¬ nology ranks the concepts as polar opposites. Lankester wanted to contrast homology of overt structure (homogeny in his terms, or homology sensu stricto) with homology of underlying generators (later called parallelism) building the same structure in two separate lineages (homoplasy, or homol¬ ogy sensu lato, in Lankester’s terms). Because parallelism could not be cashed out in operational terms (as science had no way, until our current revolution in evo-devo, to characterize, or even to recognize, these underlying genera¬ tors), proper conceptual distinctions between parallelism and convergence have generally not been made, and the two terms have even (and often) been united as subtypes of homoplasy (now defined in the current, and utterly nonLankesterian sense, as opposite to homology). I trace the complex and con¬ fused history of this discussion, and show that structuralist thinkers, with doubts about panadaptationism, have always been most sensitive to this is¬ sue, and most insistent upon separating and distinguishing parallelism as the chief category of positive developmental constraint—a category that has now, for the first time, become scientifically operational. 5. I summarize the revolutionary empirics and conceptualizations of evodevo in four themes, united by a common goal: to rebalance constraint and adaptation as causes and forces of evolution, and to acknowledge the perva¬ siveness and importance—also the synergy with natural selection, rather than opposition to Darwinian themes—of developmental constraint as a positive, structuralist, and internal force. The first theme explores the implications— for internally directed evolutionary pathways and consequent clumping of taxa in morphospace—of the remarkable and utterly unanticipated discovery of extensive “deep homology” among phyla separated at least since the Cam¬ brian explosion, as expressed by shared and highly conserved genes regulat¬ ing fundamental processes of development. I first discuss the role and ac¬ tion of some of these developmental systems—the ABC genes of Arabidopsis in regulating circlets of structures in floral morphology, the Hox genes of Drosophila in regulating differentiation of organs along the AP axis, and the role of the Pax-6 system in the development of eyes—in validating (only par¬ tially, of course) the archetypal theories of 19th century transcendental mor¬ phology, long regarded as contrary to strictly selectionist views of life’s his¬ tory—particularly Goethe’s theory of the leaf archetype, and Geoffroy’s idea of the vertebral groundplan of AP differentiation. I then discuss the even more exciting subject of homologically conserved systems across distant phyla, as expressed in high sequence similarity of important regulators, com¬ mon rules of development (particularly the “Hoxology” followed in both ar¬ thropod and vertebrate ontogeny), and similar action of homeotic mutations that impact Hoxological rules by loss or gain of function. Geoffroy was partially right in asserting segmental homology between arthropods and ver¬ tebrates, particularly for the comparison of insect metameres with rhom-
Defining and Revising the Structure of Evolutionary Theory bomenc segments in the developing vertebrate brain (a small part, perhaps, of the AP axis of most modern vertebrates, but the major component of the earliest fossil vertebrates), where the segments themselves may form differ¬ ently, but where rules of Hoxology then work in the same manner during later differentiation. I also defend the substantial validity of Geoffroy’s other “crazy” comparison—the dorso-ventral inversion of the same basic body plan between arthropods and vertebrates. 6. The second theme stresses the even more positive role of parallelism, based on common action of regulators shared by deep homology, in directing the evolutionary pathways of distantly related phyla into similar channels of adaptations thus more easily generated (thereby defining this phenomenon as synergistic and consistent with an expanded Darwinian theory, and not con¬ frontational or dismissive of selection). I discuss such broadscale examples as the stunning discovery of substantial parallelism in the supposedly classical, “poster boy” expression of the opposite phenomenon of convergence—the development of eyes in arthropods, vertebrates, and cephalopods. The overt adult phenotype, of course, remains largely convergent, but homology of the underlying regulators demonstrates the strong internal channeling of parallel¬ ism. The vertebrate and squid version of Pax-6 can, in fact, both rescue the development of eyes in Drosophila and produce ectopic expression of eyes in such odd places as limbs. I also discuss smaller-scale examples of “conver¬ gence,” reinterpreted as parallelism, for even more precise similarities among separate lineages within coherent clades—particularly the independent con¬ version of thoracic limbs to maxillipeds, by identical homeotic changes in the same Hox genes, in several groups of crustaceans. Finally, I caution against overextension and overenthusiasm by pointing out that genuine developmen¬ tal homologies may be far too broad in design, and far too unspecific in mor¬ phology, to merit a designation as parallelism, as in the role of distal-less in regulating “outpouchmgs” so generalized in basic structure, yet so different in form, as annelid parapodia, tunicate ampullae and echinoderm tube feet. I designate these overly broad similarities (that should not be designated as parallelism, or used as evidence for constraint by internal channeling) as “Pharaonic bricks”—that is, building blocks of such generality and multi¬ purpose utility that they cannot be labeled as constraints (with the obvious reductio ad absurdum of DNA as the homological basis of all life). By con¬ trast, the “Corinthian columns” of more specific conservations define the proper category of important positive constraint by internal channelings of parallelism based on homology of underlying regulators (just as the specific form of a Corinthian column, with its acanthus-leafed capital, represents a tightly constrained historical lineage that strongly influences the particular shape and utility of the entire resulting building). 7. My third and shorter theme—for this subject, though “classical” throughout the history of evolutionary thought, holds, I believe, less validity and scope than the others—treats the role of homologous regulators in pro¬ ducing rapid, even truly saltational, changes channeled into limited possibili¬ ties of developmental pathways (as in Goldschmidt’s defense of discontinuous
S3
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THE STRUCTURE OF EVOEUTIONARY THEORY evolution based upoti mutations in rate genes that control ontogenetic trajec¬ tories). I discuss the false arguments often invoked to infer such saltational changes, but then document some limited, but occasionally important, cases of such discontinuous, but strongly channeled, change in macroevolution. 8. The fourth theme of top-down channeling from full ancestral comple¬ ments, rather than bottom-up accretion along effectively unconstrained path¬ ways of local adaptation, explores the role of positive constraint in establish¬ ing the markedly non-random and inhomogeneous population of potential morphospace by actual organisms throughout the history of life. Ed Lewis, in brilliantly elucidating the action of Hox genes in the development of Drosophila, quite understandably assumed (albeit falsely, as we later discov¬ ered to our surprise) that evolution from initial homonomy to increasing complexity of AP differentiation had been achieved by addition of Hox genes, particularly to suppress abdominal legs and convert the second pair of wings to halteres. In fact, the opposite process of tinkering with established rules, primarily by increased localization of action and differentiation in tim¬ ing (and also by duplication of sets, at least for vertebrate Hox genes), has largely established the increasing diversity and complexity of differentiation in bilaterian phyla. The (presumably quite homonomous) common ancestor of arthropods and vertebrates already possessed a full complement of Hox genes, and even the bilaterian common ancestor already possessed at least seven elements of the set. Moreover, the genomes of the most homonomous modern groups of onycophorans and myriapods also include a full set of Hox genes—so differentiation of phenotypic complexity must originate as a de¬ rived feature of Hox action, exapted from a different initial role. The Cam¬ brian explosion remains a crucial and genuine phenomenon of phenotypic diversification, a conclusion unthreatened by a putatively earlier common an¬ cestry of animal phyla in a strictly genealogical (not phenotypic) sense. The further evolution of admittedly luxuriant, even awesome, variety in major phyla of complex animals has followed definite pathways of internal channel¬ ing, positively abetted (as much as negatively constrained) by homologous developmental rules acting as potentiators for more rapid and effective selec¬ tion (as in the loss of snake limbs and iteration of pre-pelvic segments), and not as brakes or limitations upon Darwinian efficacy.
Chapter 11: The integration of constraint and adaptation: structural constraints, spandrels, and exaptation 1. D’Arcy Thompson’s idiosyncratic, but brilliantly crafted and expressed, theory of form (1917, 1942) presents a 20th century prototype for the gener¬ alist, or ahistorical, form of structural constraint: adaptation produced not by a functionalist mechanism like natural selection (or Lamarckism), but di¬ rectly and automatically impressed by physical forces operating under invari¬ ant laws of nature. This theory enjoyed some success in explaining the corre¬ lation of form and function in very simple and labile forms (particularly as influenced by scale-bound changes in surface/volume ratios). But similarly nongenetic (and nonphyletic) explanations do not apply to complex crea-
Defining and Revising the Structure of Evolutionary Theory tures, and even D’Arcy Thompson admitted that his mechanism could not en¬ compass, say, “hipponess,” but, at most, only the smooth transformations of these basic designs among closely related forms of similar Bauplan (the true theoretical significance of his much misunderstood theory of transformed co¬ ordinates). In summary, D’Arcy Thompson, the great student of Aristotle, erred in mixing the master’s modes of causality—by assuming that the adap¬ tive value (or final cause) of well designed morphology could specify the physical forces (or efficient causes) that actually built the structures. 2. Stuart Kauffman and Brian Goodwin have presented the most cogent modern arguments in this tradition of direct physical causation. These argu¬ ments hold substantial power for explaining some features of relatively sim¬ ple biological systems, say from life’s beginnings to the origin of prokaryotic cells, where basic organic chemistry and the physics of self-organizing sys¬ tems can play out their timeless and general rules. Such models also have sub¬ stantial utility in describing very broad features of the ecology and energy dynamics of living systems in general terms that transcend any particular tax¬ onomic composition. But this approach founders, as did D’Arcy Thompson’s as well, when the contingent and phyletically bound histories of particular complex lineages fall under scrutiny—and such systems do constitute the “bread and butter” of macroevolution. Nonetheless, Kauffman’s powerful notion of “order for free,” or adaptive configurations that emerge from the ahistoric (even abiologicai) nature of systems, and need not be explained by particular invocations of some functional force like natural selection, should give us pause before we speculate about Darwinian causes only from evidence of functionality. This “order for free” aids, and does not confute, such func¬ tional forces as selection by providing easier (even automatic) pathways to¬ wards a common desideratum of adaptive biological systems. 3. I then turn to the second, and (in my judgment) far more important, theme of structural constraint in the fully historicist and phyletic context of aptive evolution by cooptation of structures already present for other reasons (often nonadaptive in their origin), rather than by direct adaptation for cur¬ rent function via natural selection. The central principle of a fundamental logical difference between reasons for historical origin and current functional utility—a vital component in all historical analysis, as clearly recognized but insufficiently emphasized by Darwin, and then unfortunately underplayed or forgotten by later acolytes—was brilliantly identified and dissected by Friedrich Nietzsche in his Genealogy of Morals, where he contrasted the ori¬ gin of punishment in a primal will to power, with the (often very different) utility of punishment in our current social and political systems. 4. Darwin himself invoked this principle of disconnection between histori¬ cal origin and current utility both in the Origin's first edition, and particularly in later responses to St. George Mivart’s critique (the basis for the only chap¬ ter that Darwin added to later editions of the Origin) on the supposed inabil¬ ity of natural selection to explain the incipient (and apparently useless) stages of adaptive structures. Darwin asserted the principle of functional shift to ar¬ gue that, although incipient stages could not have functioned in the manner
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THE STRUCTURE OF EVOLUTIONARY THEORY of their final form, they might still have arisen by natural selection for a dif¬ ferent initial utility (feathers first evolved for thermoregulation and later co¬ opted for flight, for example). Darwin used this principle of cooptation, or functional shift, in two important ways tha* enriched and expanded his the¬ ory away from a caricatured panselectionist version—as the primary ground >
'
of historical contingency in phyletic sequences (for one cannot predict the di¬ rection of subsequent cooptation from different primary utilities), and as a source of structural constraint upon evolutionary pathways. But these Dar¬ winian invocations stopped short of a radical claim for frequent and impor¬ tant nonadaptive origins of structures coopted to later utility. That is, Darwin rarely proceeded beyond the principle of originally adaptive origin for differ¬ ent function, with later cooptation to altered utility. 5. This important principle of cooptation of preexisting structures origi¬ nally built for different reasons has been so underemphasized in Darwinian traditions that the language of evolutionary theory does not even include a term for this central process—which Elisabeth Vrba and I called “exaptation” (Gould and Vrba, 1982). (The available, but generally disfavored, term “preadaptation” only speaks of potential before the fact, and has been widely re¬ jected in any case for its unfortunate, but inevitable, linguistic implication of foreordination in evolution, the very opposite of the intended meaning!) 6. I present a list of criteria for recognizing exaptations and separating them from true adaptations. 1 also discuss some outstanding examples of exaptation from the recent literature, with particular emphasis on the multi¬ ple exaptation of lens crystallins (in part for their fortuitous transparency, but for many other cooptable characteristics as well) in so many vertebrates and from so many independent and different original functions. 7. The exaptation of structures that arose for different adaptive reasons re¬ mains within selectionist orthodoxy (while granting structural constraint a large influence over historical pathways, in contrast with crude panadaptationism) by confirming a Darwinian basis for the adaptive origin of struc¬ tures, whatever their later history of exaptive shift. On the other hand, the theoretically radical version of this second, or historicist, style of structural constraint in evolution posits an important role for an additional phenome¬ non in macroevolution: the truly nonadaptive origin of structures that may later be exapted for subsequent utility. Many sources of such nonadaptive origin may be specified (see point 10 below), but inevitable architectural con¬ sequences of other features—the spandrels of Gould and Lewontin’s termi¬ nology (1979)—probably rank as most frequent and most important in the history of lineages. 8. Spandrels (although unnamed and ungeneralized) have been acknowl¬ edged in Darwinian traditions, but relegated to insignificant relative frequen¬ cies by invalid arguments for their rarity, their structural inconsequentiality (the mold marks on an old bottle, for example), or their temporally subse¬ quent status as sequelae—with the first two claims empirically false, and the last claim logically false as a further confusion between historical origin and current utility.
Defining and Revising the Structure of Evolutionary Theory 9. I affirm the importance and high relative frequency of spandrels, and therefore of nonadaptive origin, in evolutionary theory by two major argu¬ ments for ubiquity. First, for intrinsic structural reasons, the number of po¬ tential spandrels greatly increases as organisms and their traits become more complex. (The spandrels of the human brain must greatly outnumber the im¬ mediately adaptive reasons for increase in size; the spandrels of the cylindri¬ cal umbilical space of a gastropod shell, by contrast, may be far more limited, although exaptive use as a brooding chamber has been important in several lineages.) Second, under hierarchical models of selection, features evolved for any reason at one level generate automatic consequences at other levels—and these consequences can only be classified as cross-level spandrels (since they are “injected into” the new level, rather than actively evolved there). 10. The full classification of spandrels and modes of exaptation offers a re¬ solving taxonomy and solution—primarily through the key concept of the “exaptive pool”—for the compelling and heretofore confusing (yet much discussed) problem of “evolvability.” Former confusion has centered upon the apparent paradox that ordinary organismal selection, the supposed ca¬ nonical mechanism of evolutionary change, would seem (at least as its pri¬ mary overt effect) to restrict and limit future possibilities by specializing forms to complexities of immediate environments, and therefore to act against an “evolvability” that largely defines the future macroevolutionary prospects of any lineage. The solution lies in recognizing that spandrels, al¬ though architecturally consequential, are not doomed to a secondary or un¬ important status thereby. Spandrels, and all other forms of exaptive potential, define the ground of evolvability, and play as important a role in macroevolutionary potential as conventional adaptation does for the immediacy of microevolutionary success. I emphasize the centrality of the exaptive pool for solving the problem of evolvability by presenting a full taxonomy of catego¬ ries for the pool’s richness, focusing on a primary distinction between “frank¬ lins” (or inherent potentials of structures evolved for other adaptive roles— that is, the classical Darwinian functional shifts that do not depart from adaptationism), and “miltons” (or true nonadaptations, arising from several sources, with spandrels as a primary category, and then available for later cooptation from the exaptive pool—that is, the class of nonadaptive origins that does challenge the dominant role of panadaptationism in evolutionary theory). 11. I argue that the concept of cross-level spandrels vastly increases the range, power and importance of nonadaptation in evolution, and also unites the two central themes of this book by showing how the hierarchically ex¬ panded theory of selection also implies a greatly increased scope for non¬ adaptive structural constraint as an important factor in the potentiation of macroevolution.
Chapter 12: Tiers of time and trials of extrap olationism 1. Darwin clearly recognized the threat of catastrophic mass extinction to the extrapolationist and uniformitarian premises underlying his claim for full
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THE STRUCTURE OF EVOLUTIONARY THEORY explanation of macroevolutionary results by microevolutionary causes (and not as a challenge to the efficacy of natural selection itself). Darwin therefore employed his usual argument about the imperfection of geological records to “spread out” apparent mass extinction oveu sufficient time for resolution by ordinary processes working at maximal rates (and therefore only increasing the intensity of selection). 2. The transition of the impact scenario (as a catastrophic trigger for the KT extinction) from apostasy at its proposal in 1980 to effective factuality (based on the consilience of disparate evidence from iridium layers, shocked quartz and, especially, the discovery of a crater of appropriate size and age at Chicxulub) has reinstated the global paroxysms of classical catastrophism (in its genuinely scientific form, not its dismissive Lyellian caricature) as a legiti¬ mate scientific mechanism outside the Darwinian paradigm, but operating in conjunction with Darwinian forces to generate the full pattern of life’s his¬ tory, and not, as previously (and unhelpfully) formulated, as an exclusive al¬ ternative to disprove or to trivialize Darwinian mechanisms. 3. If catastrophic causes and triggers for mass extinction prove to be gen¬ eral, or at least predominant in relative frequency (and not just peculiar to the K-T event), then this macroevolutionary phenomenon will challenge the cru¬ cial extrapolationist premise of Darwinism by being more frequent, more rapid, more intense and more different in effect than Darwinian biology (and Lyellian geology) can allow. Under truly catastrophic models, two sets of reasons, inconsistent with Darwinian extrapolationism by microevolu¬ tionary accumulation, become potentially important agents of macroevolu¬ tionary patterning: effectively random extinction (for clades of low N), and, more importantly, extinction under “different rules” from reasons regulating the adaptive origin and success of autapomorphic cladal features in normal times. 4. Catastrophic mass extinction, while breaking the extrapolationist credo, may suggest an overly simplified and dichotomous macroevolutionary model based on alternating regimes of “background” vs. “mass” extinction. Rather, we should expand this insight about distinctive mechanisms at different scales into a more general model of several rising tiers of time—with conven¬ tional Darwinian microevolution dominating at the ecological tier of short times and intraspecific dynamics; punctuated equilibrium dominating at the geological tier of phyletic trends based on interspecific dynamics (with species arising in geological moments, and then treated as stable “atoms,” or basic units of macroevolution, analogous to organisms in microevolution); and mass extinction (perhaps often catastrophic) acting as a major force of over¬ all macroevolutionary pattern in the global history of relative waxing and waning of clades. (I also contrast this preferred model of time’s tiering with the other possible style of explanation, which I reject but find interesting nonetheless, for denying full generality to smooth Darwinian upward extrap¬ olation from the lowest level—namely, an equally smooth and monistic downward extrapolation from catastrophic mortality in mass extinction to
Defining and Revising the Structure of Evolutionary Theory diminishing, but equally random and sudden, effects at all scales, as proposed in Raup’s “field of bullets” model.) 5. In a paradoxical epilogue, I argue (despite my role as a longtime cham¬ pion of the importance and scientific respectability of unpredictable con¬ tingency in the explanation of historical patterns) that the enlargement and reformulation of Darwinism, as proposed in this book, will recapture for gen¬ eral theory (by adding a distinctive and irreducible set of macroevolutionary causes to our armamentarium of evolutionary principles) a large part of macroevolutionary pattern that Darwin himself, as an equally firm supporter of contingency, willingly granted to the realm of historical unpredictability because he could not encompass these results within his own limited causal structure of strict reliance upon smooth extrapolation from microevolu¬ tionary processes by accumulation through the immensity of geological time. A FINAL THOUGHT. May I simply end by quoting the line that I wrote at the
completion of a similar abstract (but vastly shorter, in a much less weighty book) for my first technical tome, Ontogeny and Phytogeny (1977b, p. 9): “This epitome is a pitiful abbreviation of a much longer and, I hope, more subtle development. Please read the book!”
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PART
HE HISTORY OF DARWINIAN LOGIC
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ment must be expected to occur up to the end of their existence. 2. Fitness, although measured by a uniform method, is qualitatively different for every different organism, whereas entropy, like tempera¬ ture, is taken to have the same meaning for all physical systems. 3. Fitness may be increased or decreased by changes in the environ¬ ment, without reacting quantitatively upon that environment. But what do these exceptions express in ordinary biological parlance? Con¬ tingency, individuality, and interaction, for the three points respectively. Could anyone have presented a better list of the peculiarly biological proper¬ ties that make organisms and their history so intrinsically unlike simpler physical systems that operate by timeless and general laws? Do these differ¬ ences between physical thermodynamics and Darwinian biology then rank as exceptions or invalidations? The
eugenical
chapters.
Fisher’s Genetical Theory has generally
been acknowledged, and properly so, as the keystone of 20th century evolu¬ tionary theory. Yet few contemporary biologists have actually read the book in extenso, and one feature of this common neglect seems especially puzzling. The last five chapters, nearly 40 percent of the entire volume, present a single coherent (if fatally flawed) argument in eugenics—a claim that modern indus¬ trial society (particularly the British version) has entered a potentially fatal decline as a result of “social promotion of the relatively infertile.” In essence, Fisher argues that people who rise socially, by dint of moral or intellectual superiority, also tend to express ineluctable genetic propensities (not revers¬ ible, environmentally induced preferences) for infertility. This superior upper stratum will therefore be swamped by greater reproduction of less worthy so¬ cial classes. Throughout human history, most great civilizations have declined for this reason. Society should fight this decay by rewarding gifted and highly fecund members of the lower classes, thereby helping them to rise and rejuve¬ nate the reproduction of higher social strata. A tradition of discreet silence has enveloped these chapters. Provine (1971), for example, relegates this material to a single sentence in his impor¬ tant book (pp. 153-154): “In the concluding five chapters he extended his genetical ideas to human populations.” This discretion, I suppose, reflects our embarrassment that such a paragon of our profession should have ended his canonical book with such a long argument for a politically discredited move¬ ment (see Gould, 1991c, for an analysis and critique of Fisher’s eugenical ar¬ guments). This professional silence surely cannot reflect a belief that these chapters bear no connection to the rest of the book, and that Fisher merely appended this material to grind his political axe—for Fisher states that he could have dispersed this 40 percent among the other chapters, and then adds
The Modern Synthesis as a Limited Consensus
(p. x): “The deductions respecting Man are strictly inseparable from the more general chapters.’’ I regard the conspiracy of silence about these chapters as both unscholarly and overly fastidious. First of all, how can we justify silence about integral parts of an important thinker’s work because we now recoil at his beliefs? (Wagner’s anti-semitism retains intimate linkage with his musical produc¬ tions, but we cannot ban such glorious operas.) Second, even if we wish to de¬ fend such posthumous cleansing, Fisher’s eugenics can only be judged as “garden variety” material for his time, and not as especially benighted or vengeful. His visions of proper social stratification may surely be judged elitist (scarcely a rare attitude for an Oxbridge don in class-conscious Brit¬ ain), but anachronistic exponents of modern political correctness will appre¬ ciate other facets of his argument. (Fisher, for example, cautiously advocates racial mixing for its role in increasing genetic variance, thereby supplying more material at the right tail of the human distribution, even though admix¬ ture with a “lower race”—Fisher did not espouse egalitarian beliefs!—might depress the mean.) But the central relevance of these final chapters lies in the consonance of Fisher’s eugenical argument with his commitment to a general and statisti¬ cal Darwinism. Fisher’s eugenics provides our most interesting and incisive affirmation of his evolutionary philosophy. Darwinian triumph must be mea¬ sured as differential reproductive success, statistically defined in large pop¬ ulations—not as particularistic victory for nifty bits of morphology (or men¬ tality) in Tennyson’s world of “nature red in tooth and claw.” Moreover, Fisher maintains that our current pattern of degeneration arises from differ¬ entials in birth rate, not from selective superiority in resisting death—so Dar¬ winian “success” can only be viewed as statistical leverage in components of reproductive advantage, not as improvement in any social or vernacular sense: “Even the highest death-rate in this period, that in the first year of life, must be quite unimportant compared with slight differences in reproduction; for the infantile-death rate has been reduced in our country to about seven percent of the births, and even a doubling of this rate would make only about a third as much difference to survival as an increase in the family from three children to four” (1930, p. 194). Finally, this eugenical example illuminates the central Darwinian claim for the power of slight statistical advantage. A truly effective, and truly Darwin¬ ian, eugenics, Fisher argues, will focus on apparently tiny reproductive differ¬ entials, and not on the elimination of rare and overt “saltations”—steriliza¬ tion of the genetically diseased or mentally defective, as in the programs favored by most eugenicists who did not grasp the Darwinian imperative. We might regard small differences in birth rates as trifling, and unlikely to exert much effect upon the rapid time scales of human history. But anything that can be measured at all over the minimal span of a generation or two trans¬ lates to an enormous effect in evolutionary time. Thus, the social promotion of relative infertility, however “invisible” in comparison to the devastation of war or the progress of technology, will yield an evolutionary degeneration far
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THE STRUCTURE OF EVOEUTIONARY THEORY in excess of almost any other Darwinian change in nature. In evolutionary time, Fisher laments, our social structures disintegrate rapidly; we had better pay heed: “Civilized man, in fact, judging by the fertility statistics of our own time, is apparently subjected to a selective process of an intensity approach¬ ing a hundredfold the intensities we can expect to find among wild animals, with the possible exception of groups which have suffered a recent and pro¬ found change in their environment” (1930, p. 199).
J. B. S. HALDANE AND THE INITIAL PLURALISM OF THE SYNTHESIS
Flaldane purposely included a plural in the title of his book—The Causes of Evolution (1932)—for he believed that nothing so encompassing could be so unifactorial. But Haldane wrote his book in the tradition of restriction, pri¬ marily to debunk Kellogg’s triad of alternatives by showing the power of nat¬ ural selection. He states (p. v) that his book began as a series of lectures enti¬ tled “A Re-examination of Darwinism,” and he then announces his primary aim in the preface (p. vi): “To prove that mutation, Lamarckian transforma¬ tion, and so on, cannot prevail against natural selection of even moderate in¬ tensity.” (Haldane treats the same subject more formally in the book’s lengthy mathematical appendix, thus uniting both the front and back matter for a single purpose.) Haldane presents a conventional account of the revivification of Darwin¬ ism and the rejection of alternatives. Darwinism had fallen on bad times be¬ fore the synthesis: “Criticism of Darwinism has been so thoroughgoing that a few biologists and many laymen regard it as more or less exploded” (p. 32). The Darwinian resurrection followed from the recognition that continuous, small-scale variation could also claim a Mendelian basis (p. 71) and, espe¬ cially, that tiny selection pressures, working in a cumulative manner on such minor variations, could effectively explain all evolution: “But however small may be the selective advantage the new character will spread, provided it is present in enough individuals of the population to prevent its disappearance by mere random extinction. . . . An average advantage of one in a million will be quite effective in most species” (1932, p. 100). The development of mathematical population genetics establishes the cen¬ terpiece of Darwinian revival. Haldane even begins the tradition of a found¬ ing trinity in stating, however immodestly (p. 33): “I can write of natural se¬ lection with authority because I am one of the three people who know most about its mathematical theory.” However, in contrast to Fisher’s quest for pervasive and abstract generality, Haldane felt compelled to bring the smaller and more particular puzzles of natural history under his theoretical umbrella. Here he allows a substantial range of exceptions to Darwinism, albeit at subsidiary frequency—thus illus¬ trating the predominant pluralism of the early synthesis. Haldane rejects Lamarckism outright, as contrary in principle to the known workings of in¬ heritance. But, in a remarkable passage, he finds some space, in chinks and
The Modern Synthesis as a Limited Consensus comers of the new world of fusion between Darwin and Mendel, for the two internalist theories in Kellogg’s triad of alternatives—saltation and ortho¬ genesis. (In fact, Haldane even repeats the “standard” anti-Darwinian claim for selection’s merely subsidiary and negative role in enhancing and stabiliz¬ ing a saltational change arising by other means—even though Haldane re¬ gards this alternative mechanism as rare in nature.) Gabon’s polyhedron cannot be fully rounded by the emerging Darwinian consensus: But if we come to the conclusion that natural selection is probably the main cause of change in a population, we certainly need not go back completely to Darwin’s point of view. In the first place, we have every reason to believe that new species may arise quite suddenly, sometimes by hybridization, sometimes perhaps by other means. Such species do not arise, as Darwin thought, by natural selection. When they have arisen they must justify their existence before the tribunal of natural selection, but that is a different matter. . . . Secondly, natural selection can only act on the variations available, and these are not, as Darwin thought, in every direction. In the first place, most mutations lead to a loss of complexity (e.g. substitution of leaves for tendrils in the pea and sweet pea) or reduction in size of some organ (e.g. wings in Drosophila). . . . Mutations only seem to occur along certain lines (1932, pp. 138139). Two modes of non-Darwinian change especially intrigued Haldane. First, though he tried to reinterpret as many cases as possible in a Darwinian manner, Haldane accepted some paleontological claims for supposedly orthogenetic trends, and he admitted that the developing Darwinian synthesis could find no place for such phenomena: “Many such cases—for example the development of large size or large horns—can, I think, be put down to the ill effects of competition between members of the same species. Others, such as the exaggerated coiling of Gryphaea cannot at present be explained with any strong degree of likelihood” (1932, p. 141). (This example seems especially ironic in retrospect, because Gryphaea's supposed overcoiling to necessary extinction never occurred, and the claim rested upon misreported and misin¬ terpreted data—see Chapter 10, pp. 1040-1045 and Gould, 1972.) As his favorite general argument for awarding a small space to ortho¬ genesis, Haldane cited the putatively higher frequency of degenerational over progressive evolution, arguing that such a tendency probably required an internalist explanation rooted in a bias for deletional mutations: “Degenera¬ tion is a far commoner phenomenon than progress. It is less striking because a progressive type, such as the first bird, has left many different species as progeny, while degeneration often leads to extinction, and rarely to a wide¬ spread production of new forms . . . But if we consider any given evolutionary level we generally find one or two lines leading up to it, and dozens leading down” (1932, pp. 152-153). Second, Haldane accepted the common wisdom of taxonomists in his gen¬ eration that most differentia of species expressed no adaptive significance. He
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THE STRUCTURE OF EVOEUTIONARY THEORY also acknowledged this factual substrate as a primary source of legitimate doubts, then common among taxonomists, about Darwinism: “But when we have pushed our analysis as far as possible, there is no doubt that innumera¬ ble characters show no sign of possessing selective value, and, moreover, these are exactly the characters which enable a taxonomist to distinguish one species from another. This had led many able zoologists and botanists to give up Darwinism” (1932, pp. 113-114). Haldane even presents the interesting argument that we have been fooled into accepting a dominant frequency for adaptation by a pronounced bias in the fossil record—the differential preservation of species with persistently large populations subject to control by small Fisherian differentials in natural selection. Perhaps most species exist as much smaller populations, and there¬ fore become subject to Wrightian dynamics of genetic drift—even if such spe¬ cies rarely enter the fossil record and therefore fail to leave evidence for their dominant relative frequency. Haldane even cites the highest of all authorities to buttress this idea: But Wright’s theory certainly supports the view taken in this book that the evolution in large random-mating populations, which is recorded by paleontology, is not representative of evolution in general, and perhaps gives a false impression of the events occurring in less numerous species. It is a striking fact that none of the extinct species, which, from the abun¬ dance of their fossil remains, are well known to us, appear to have been in our own ancestral line. Our ancestors were mostly rather rare crea¬ tures. “Blessed are the meek: for they shall inherit the earth” (1932, pp. 213-214).
J. S. HUXLEY: PLURALISM OF THE TYPE
As with Haldane, Huxley also credited a well received lecture that he had pre¬ sented on Darwinism as the stimulus for writing his much longer book—a 1936 presidential address to the British Association on “Natural selection and evolutionary progress.” Huxley maintained the focus of this lecture in presenting a thoughtful, but partisan, defense of Darwinism throughout Evo¬ lution, The Modern Synthesis, beginning with a wry comment on the exten¬ sive pessimism so common before the movement he christened: “The death of Darwinism has been proclaimed not only from the pulpit, but from the bio¬ logical laboratory; but, as in the case of Mark Twain, the reports seem to have been greatly exaggerated, since today Darwinism is very much alive” (1942, p. 22). Huxley encapsulates the central logic of Darwinism in much the same way, and with the same intent, that I advocate in this book. He recognizes the three main characteristics of variation as central (pp. 22-24)—copiousness (though not pervasive enough for mutation pressure to overwhelm selection), small¬ ness of phenotypic effect, and nondirectionality—and he credits Mendelism
The Modern Synthesis as a Limited Consensus with providing the physical explanation for what Darwin could only deduce from first principles of natural selection, while hoping for later confirmation from discoveries about the basis of heredity. In an interesting discussion on the nature of theories and their central logic, Huxley disputes Lancelot Hogben’s claim that the Mendelian fusion had so altered Darwin’s own notion of mechanics, that the reformulation of Fisher, Haldane, and Wright should neither bear Darwin’s name nor even retain the term '‘natural selection” for its central mechanism. Huxley replies that all theories must change by growth, but that the proper standard for mainte¬ nance of a name must be defined by continuity in key precepts in a central logic: Hogben is perfectly right in stressing the fact of the important differences in content and implication between the Darwinism of Darwin or Weismann and that of Fisher or Haldane. We may, however, reflect that the term atom is still in current use and the atomic theory not yet rejected by physicists, in spite of the supposedly indivisible units having been di¬ vided. This is because modern physicists still find that the particles called atoms by their predecessors do play an important role, even if they are compound and do occasionally lose or gain particles and even change their nature. If this is so, biologists may with a good heart continue to be Darwinians and to employ the term Natural Selection, even if Darwin knew nothing of mendelizing mutation (1942, p. 28). Huxley also follows the English tradition (see pp. 116-119) for central em¬ phasis upon adaptation in the definition of evolutionary mechanisms. He speaks of “a functionally-guided course of evolution” (p. 39), and almost claims an a priori status for panadaptationism: “Our enumeration will also serve as a reminder of the omnipresence of adaptation. Adaptation cannot but be universal among organisms, and every organism cannot be other than a bundle of adaptations, more or less detailed and efficient, coordinated in greater or lesser degree” (1942, p. 420). But as further evidence for pluralism in the early synthesis, and despite this emphasis upon the ubiquity of adaptation, Huxley then speaks favorably of the same challenges and exceptions that intrigued Haldane—orthogenesis and nonadaptation. Whereas he does claim (correctly) that most cases of sup¬ posed orthogenesis only represent instances of phyletic constraint, he also provides an interesting taxonomy of genuine examples. Mirroring our mod¬ ern distinction between positive and negative meanings of constraint (see Chapter 10, pp. 1025-1061, and Gould, 1989a), Huxley speaks of dominant and subsidiary orthogenetic restriction: True orthogenetic restriction depends on a restriction of the type and quantity of genetic variation. When dominant it prescribes the direction of evolution: when subsidiary it merely limits its possibilities. . . . Domi¬ nant orthogenetic restriction [is] very rare, if indeed it exists at all. . . .
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THE STRUCTURE OF EVOLUTIONARY THEORY Subsidiary orthogenetic restriction is probably frequent, but we are not yet able to be sure in most cases whether a limitation of variation as ac¬ tually found in a group is due to a limitation in the supply of mutations or to selection, or to other causes. It is, however, certain that some mutational effects recur regularly in some allied species, and probable that this phenomenon is widespread (1942, p. 524). Huxley also cites overcoiling in Grypbaea (Trueman, 1922)—the classic case of his time, though since invalidated (Gould, 1972)—as a primary puz¬ zlement and most promising example for “dominant orthogenesis”: We must provisionally face an explanation in terms of orthogenesis—i.e. of evolution predetermined to proceed within certain narrow limits, irre¬ spective of selective disadvantage except where this leads to total extinc¬ tion. It should be noted that, even if the existence of orthogenesis in this cause [sic, for case] be confirmed, it appears to be a rare and exceptional phenomenon, and that we have no inkling of any mechanism by which it may be brought about. It is a description, not an explanation. Indeed its existence runs counter to fundamental selectionist principles (1942, p. 509). Despite his general commitment to adaptation, Huxley also granted some importance (beyond mere existence) to Wright’s genetic drift in the formation of species with small population sizes (p. 58). He even extended the power of this non-adaptational force to the origin of generic differences, though not beyond: “It may be presumed, on somewhat indirect evidence, that 'useless’ non-adaptive differences due to isolation of small groups may be enlarged by the addition of further differences of the same sort to give generic distinction, though it seems probable that differences of family or higher rank are always or almost always essentially adaptive in nature” (1942, p. 44). Thus, the early synthesis, in the view of both its founders and its namegiver, reinstated Darwinism as the centerpiece of evolutionary theory by rejecting any substantial role for the full spate of previously popular alternatives. (I should say “instated,” for Darwinism had never before attained majority ap¬ peal as a mechanism, even during Darwin’s lifetime.) But the early synthesists, with Fisher’s exception, also left a few facets intact on Gabon’s polyhedron. Their interest lay in showing that our increasing knowledge of the Mendelian world could establish natural selection as the primary cause of evolutionary change, not in staking a claim for Darwinian exclusivity.
Synthesis as Hardening THE LATER GOAL OF EXALTING SELECTION’S POWER
Evolutionists have generally depicted the second phase of the Synthesis as a gathering of traditional subdisciplines under an umbrella constructed during
The Modern Synthesis as a Limited Consensus the first phase by fusing Mendel with Darwin. I learned something fundamen¬ tal about this second phase as a participant at the conference, entitled Work¬ shop on the Evolutionary Synthesis, that Ernst Mayr convened in Boston in 1974. This conference—an amazing experience for a young evolutionist at the beginning of a career—included every major living participant in the Syn¬ thesis except Bernhard Rensch, who was ill; G. G. Simpson, who was angry; and Sewall Wright, whom Mayr simply would not invite, despite pleas from yours truly and several others. I don’t think I ever experienced a greater mo¬ ment of pure “academic awe” than my first impression, when I looked across from “our” side of the table (where Mayr had placed the “young” historians and evolutionists) and saw Dobzhansky, Mayr, Stebbins, Ford, and Darling¬ ton all together on the other side. This marvelous conference was marred (in terms of its stated purpose) only by a severe difficulty in keeping these men to the intended subject of their reminiscences about past accomplishments. They all remained so passion¬ ately involved in modern research that, whenever the planned reminiscences began, someone would make a reference to the latest paper revising some view or another—and they would immediately begin a learned discussion about current events, fueled by delight at new findings that forced revisions of their old certainties! (A difficulty for the conference’s stated aim perhaps, but personally one of the most memorable events that I have ever witnessed. If the best practitioners can maintain such openness and involvement to the end of their lives, then scholarship need not fear ossification. Such traits do not, however and alas, represent the norm in science—so I did come to under¬ stand the special excellence of these extraordinary men, and I did achieve some visceral grasp of why they, and not others, made the Synthesis.) I had always viewed the books of the second phase as coequal. But the con¬ ference discussions emphasized a major point previously unclear to me: the preeminence of Dobzhansky’s 1937 book, Genetics and the Origin of Species. This volume did not merely happen to enjoy the luck of first publication in a series—a temporal primus inter pares, so to speak. Dobzhansky’s volume provided a direct and primary inspiration for the books that followed. Speaker after speaker rose to state that his own contribution had been prod¬ ded by reading Dobzhansky’s account first. And now the irony—and the key point about disjunction between the two phases of the Synthesis. If we wish to argue that the first phase of synthe¬ sis featured the construction of population genetics by Fisher, Haldane and Wright, while the second phase brought traditional subdisciplines into this framework, we should expect the primary translator to be fluent in the lan¬ guage of transfer. In one sense, Dobzhansky did possess the requisite flu¬ ency—uniquely (at least for English-speaking scientists), and for an interest¬ ing reason of national traditions. As I also learned at the 1974 conference, only in Russia had Mendelian experimental work been merged, extensively and successfully, with traditional taxonomy and natural history. Dobzhansky, after all, had developed expertise as both a skilled Drosophila experimen-
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THE STRUCTURE OF EVOLUTIONARY THEORY talist and a specialist on the taxonomy of coccinellid beetles (ladybirds). In America and Western Europe, experimentalism and field biology occupied two different and largely hostile worlds. Could the second phase of the syn¬ thesis have emerged from a Western Drosophila lab like T. H. Morgan’s (where field biology held low status and enjoyed no practice—see p. 532), or a museum program in comprehensive systematics (with virtually no experi¬ mental facilities)? Dobzhansky exported a fusion that Western science, in ig¬ norance of the Russian language and in hostility to communist politics, had failed to recognize—even though H. J. Muller had brought the first Dro¬ sophila stocks to Russia, thus fueling Dobzhansky’s optimal training with a Western trigger. But if Dobzhansky could integrate the Mendelian experimental world with natural history, what about the supposed centerpiece of mathematical popu¬ lation genetics? Here, by his own repeated, almost gleeful, admission, Dob¬ zhansky remained a near dunce. He did not study, nor could he even under¬ stand, the details of this literature. Of his long and fruitful collaboration with Sewall Wright, Dobzhansky simply said that he had followed the principle of “father knows best”—that is, he bypassed Wright’s mathematical manipula¬ tions and accepted his English explanations on faith. In fact, of all the great second-phase synthesists only G. G. Simpson possessed sufficient mathemati¬ cal background to read and understand these papers. Dobzhansky’s willingness to accept an incomprehensible literature, and the later acquiescence of so many leaders from other subdisciplines (largely via Dobzhansky’s “translation”), testify to a powerful shared culture among evolutionists—a set of assumptions accepted without fundamental question¬ ing or perceived need to grasp the underlying mechanics. Such a sense of community can lead to exhilarating, active science (but largely in the accu¬ mulative mode, as examples cascade to illustrate accepted principles). As a downside, however, remaining difficulties, puzzles, anomalies, unresolved corners, and bits of illogic may retreat to the sidelines—rarely disputed and largely forgotten (or, by the next generation, never learned). This situation may sow seeds of an orthodoxy that can then become sufficiently set and un¬ challenged to verge on dogma—as happened in many circles, at least among large numbers of epigones, at the acme of the Synthesis in the late 1950’s and 1960’s. In this section, I shall try to illustrate one example in extenso—the central and defining case, I believe—of the narrowing suffered by a synthesis that be¬ came augmented in power but downgraded in the art and tactic of question¬ ing. I call this increasing confidence, bordering on smugness, the “hardening” of the Synthesis. Thus I contrast the positive restriction of the first phase—the elaboration of a generous and comprehensive theory, and the invalidation of false and fruitless alternatives—with the negative tightening that occurred during the ontogeny of the second phase. This hardening—still our legacy to¬ day—must serve as a starting point for any current attempt to introduce more amplitude into evolutionary theory. The hard version of the Synthesis pro¬ vides a standard for judging (by contrast) the interest and importance of
The Modern Synthesis as a Limited Consensus modern revisions—from neutralism, * to punctuated equilibrium, to a com¬ mon feeling that the theme of developmental constraints not only gives sub¬ stance to an old truth, but also confutes the hardened version’s commitment to Darwin’s (I should really say Fisher’s) billiard ball against Gabon’s poly¬ hedron. My example shall trace the transformation of adaptation from an option to be ascertained (albeit favored and granted a dominant relative frequency) to an a priori assumption of near ubiquity (save in trivial or derivative situa¬ tions without evolutionary importance)—in other words the burnishing of Gabon’s polyhedron to the billiard ball of pure functionalism (allowing no significant pushing back from internal structure upon the direction of evolu¬ tionary change). This hardening buttressed (or rather, in my view, overly rigidified and sclerotised) one leg on the essential Darwinian tripod of sup¬ port—the second theme of functionalism against internalist and structuralist forces (see Chapters 2, 4, and 5). But hardening pervaded all major themes of Darwinian central logic, and the other two legs of the tripod also experienced their own form of petrifac¬ tion (treated in less detail in Section 4 of this chapter). Pluralistic (and, admit¬ tedly, often loose) thinking about levels of selection yielded to an explicit pro¬ mulgation of organismic selection as the only acceptable mode—as a virtual campaign to root out group selection accompanied the battle of Williams (1966) against Wynne-Edwards (1962). Thirdly, a willingness to grant some independence, or at least some puzzlement, to patterns in macroevolution (see Haldane and Huxley’s respectful view of orthogenesis, as discussed in the last section), ceded to the hard view that all phenomena measured in millions of years must be explained by smooth extrapolation from palpable causes on generational scales in modern populations—and that the paleontological re¬ cord can therefore only present a pageant of products generated by known causes, and not provide an independent theory or even a set of additional causal principles. I have used a particular method to demonstrate the hardening of the Syn¬ thesis—textual comparison of early and later works by key authors. Ontog¬ eny can be an unconscious trickster. In trying to forge sense and continuity in
*If the Synthesis had retained the pluralism of its early years, Kimura’s neutral theory would have been welcomed from the first, under the criterion that any result legitimated by the mechanics and mathematics of known genetic processes thereby secured a rightful place (Wrightful in this particular case)—though Kimura’s claim would have been viewed as sur¬ prising in the light of adaptationist preferences. But when the Synthesis hardened, and adaptationism itself became the primary criterion for acceptability, Kimura’s theory seemed beyond the pale to many evolutionists. I shall never forget a decisive moment in my own early career, when I began to understand the difference between theoretical power and po¬ tentially dangerous overconfidence: Ernst Mayr rising (at the annual meeting of the Evolu¬ tion Society in New York) to confute the claim for neutralism in synonymous third position substitutions. Such changes could not, a prion and in principle he stated, be neutral. Alter¬ ations in the third position must impart some difference, perhaps energetic, that selection can “see” even if the coded amino acid does not alter. This must he so, he stated, because we now know that all substantial change is adaptive.
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THE STRUCTURE OF EVOEUTIONARY THEORY our own lives, we, often forget or “reconstruct” the actualities of our early years—thus subtly recasting our former selves as miniatures of our current beliefs. Therefore, direct interview can be a notoriously unreliable technique (while representing, ostensibly, the most direct and empirical of all scholarly sources)—for an older person may become a very unreliable chronicler of his own past. But written records stand as frozen testimonies, unaltered fossils of a time that may not be personally recoverable with high accuracy. I received my first insight into the hardening of the Synthesis by a proper (if gentle) pedagogical correction. During my graduate student years, I presented a report on paleontology in the Modern Synthesis to a seminar at the Ameri¬ can Museum of Natural History. In the characteristically naive manner of a young and awestruck protoscholar, I explicated the views of Simpson and others as jewels of reforming consistency, lux in tenebris and complete from the first. Bobb Schaeffer, a wonderful teacher, stopped me as I was explaining Simpson’s complex idea of “quantum evolution” (see p. 530). I had done well, he said, for the concept as presented in Simpson’s 1953 book, but had I ever studied the original version in Tempo and Mode in Evolution (1944)? I replied that I had not read this initial formulation, for I had assumed that the first account could only represent a less developed, and therefore pale and trifling, version of later subtlety. Schaeffer said that the two discussions dif¬ fered fundamentally, but that Simpson had minimized the appearance of change by retaining the same terms while profoundly altering their meaning. (Schaeffer also told me that he had argued the issue with Simpson for years, and that the essence of Simpson’s change, for which Schaeffer took some credit—a shift from nonadaptionist to selectionist interpretation of interme¬ diate forms in major phyletic transitions—had only strengthened the general argument, even though Simpson had covered up his changes.) I did not be¬ lieve that most of the profession could have missed such a major shift, but I checked. Schaeffer was entirely right. My personal failure piqued my interest and I began to wonder whether Simpson’s change had been idiosyncratic or part of an unrecognized pat¬ tern. I began to check early and late works of other key figures, particularly Dobzhansky and Mayr. All had moved from pluralism to strict adaptationism—and along a remarkably similar path. I began to view this transition as the major ontogenetic event of the Synthesis during its second phase. I chris¬ tened this change as the “hardening” of the Synthesis and wrote four papers on the subject (Gould, 1980e, 1982d, 1983b and c). The rest of this section documents my three favorite cases—Dobzhansky through the three editions (1937, 1941, and 1951) of his seminal book, Mayr (1942 vs. 1963), and Simpson (1944 vs. 1953)—and reproduces a good deal of material from my earlier articles. Several historians have tested my hypothesis by application to other key figures, and have affirmed the adaptational hardening as general (e.g., Beatty, 1988, and Smocovitis, 1996, 2000). Sewall Wright, subject of Provine’s mas¬ sive biography (1986; see also Provine, 1971), provides the most interesting and revealing case. Wright’s name, of course, immediately evokes the phe¬ nomenon of genetic drift, generally called the “Sewall Wright effect” in arti-
The Modern Synthesis as a Limited Consensus
cles of the early Synthesis. One would therefore regard Wright as the man most likely to speak for the importance of nonadaptation, and against any functionalist hardening. In fact, when interviewed late in life, as both Provine and I can attest, Wright complained bitterly that his views on the evolutionary role of genetic drift had been consistently misinterpreted (Wright died in 1988 at age 98, sharp as ever to the very end). Since genetic drift describes stochastic change in gene frequencies by sampling error, one might assume that Wright had ad¬ vocated a radically non-Darwinian approach to evolutionary change by de¬ moting selection and adaptation, and boosting the importance of accident. But Wright strongly denied such an interpretation of his views. He argued, with evident justice apparent to anyone who reads the works of his last thirty years, that his theory of “shifting balance,” while providing an important role for genetic drift, remains strongly adaptationist—though adaptation generally arises at a level higher than the traditional Darwinian focus on organisms. In brief (see p. 555 for a fuller account), Wright asserted that he had invoked genetic drift primarily as a generator of raw material to fuel an adaptationist process of interdemic selection. If the founding deme of a new species occupies one adaptive peak on a complex landscape (to use standard Wrightian imagery), movement to additional peaks requires genetic drift— for this stochastic process permits small demes to descend slopes and enter valleys, where selection can then draw a deme up to another peak. When demes within a single species populate several peaks, interdemic selection can operate as a powerful mechanism of adaptation. Wright therefore (and accurately) depicted his later shifting balance theory as adaptationist, and as invoking drift only for a source of variation among demes. But Wright, though estranged in many ways from the developing synthesis (see Section 4), followed its trend toward increasingly exclusive emphasis upon adaptation in evolutionary change. The version of shifting balance that Wright advocated during the last 30 years of his life did not orig¬ inate by sudden creation, complete in this final form. Shifting balance em¬ phasized different themes and arguments in Wright’s earlier work, and these articles, written during the pluralistic phase of the synthesis, granted a much greater role to randomness and nonadaptation in evolutionary change. In fact, Wright often, and explicitly, invoked drift as a non-Darwinian agent of change in articles written during the early pluralistic phase of the syn¬ thesis. Wright presents a striking example of the principle that later recollections may be inferior, as historical sources, to written testimony from the time in question. Provine (1986) has catalogued Wright’s ambiguities and multiple intents during the crucial period of 1929-1932. The later selectionist view al¬ ready stands in the wings, but most passages of these early articles advocate the nonadaptationist role for drift that Wright would later reject (and deny he ever held). Wright wrote in 1931 (p. 158), for example, that shifting balance “originates new species differing for the most part in nonadaptive respects.” In the following year, he stated (1932): “That evolution involves nonadaptive
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THE STRUCTURE OF EVOLUTIONARY THEORY differentiation to a large extent at the subspecies and even the species level is indicated by the kinds of differences by which such groups are actually distin¬ guished by systematists. It is only at the subfamily and family levels that clearcut adaptive differences become the rule*The principal evolutionary mecha¬ nism in the origin of species must then be an essentially nonadaptive one" (pp. 363-364). Provine (1986) concludes: “The careful reader in 1932 would almost certainly conclude that Wright believed nonadaptive random drift was a primary mechanism in the origin of races, subspecies, species, and perhaps genera. Wright’s more recent view that the shifting balance theory should lead to adaptive responses at least by the subspecies level is found nowhere in the 1931 and 1932 paper.”
INCREASING EMPHASIS ON SELECTION AND ADAPTATION BETWEEN THE FIRST (1937) AND LAST (1951) EDITION OF DOBZHANSKY’S GENETICS AND THE ORIGIN OF SPECIES Dobzhansky’s original probe (1937) toward synthesis operated more as a methodological claim for the sufficiency of genetics than a strong substantive advocacy of any particular causal argument—although he clearly states his general Darwinian preferences in this first edition. Dobzhansky held, con¬ trary to his own Russian mentor Filipchenko, that the methods of experi¬ mental genetics can provide enough principles to encompass evolution at all levels. But Dobzhansky did not play favorites among the admitted set of legit¬ imate principles. He did not, in particular, proclaim the pervasive power of natural selection leading to adaptation as a predominant style and outcome of evolutionary change. Some inkling of the chaotic and depressed state of evolutionary theory be¬ fore the Synthesis can be glimpsed in a simple list of previously popular argu¬ ments that Dobzhansky regarded as sufficiently important to refute—claims that denied his hope for synthesis by treating Mendelian processes observed in the laboratory as different from the genetic modes for regulating “impor¬ tant” evolutionary change in nature. Dobzhansky rebuts the following argu¬ ments explicitly: Continuous variation in nature is non-Mendelian and differ¬ ent in kind from discrete mutational variation in laboratory stocks (p. 57); Mendelian variation can only generate differences between taxa of low rank (races to genera), while higher taxa owe their distinctions to another (and un¬ known) genetic process (p. 68); chromosomal changes are always destructive and can only lead to degeneration of stocks (p. 83); differences between taxa of low rank are directly induced by the environment and have no genetic or evolutionary basis (p. 146); Johannsen’s experiments on pure lines prove the ineffectiveness of natural selection as a mechanism of evolutionary change (p. 150); selection is too slow in large populations to render evolution, even in geological time (p. 178); genetic principles cannot account for the origin of reproductive isolation (p. 255). Dobzhansky’s fifth chapter, on “variation in natural populations,” stresses the pluralism of the early synthesis. Observable genetic phenomena provide
The Modern Synthesis as a Limited Consensus
a source for all evolution; we can trace full continuity from studies in the lab¬ oratory, to variation within natural populations, to formation of races and species: It is now clear that gene mutations and structural and numerical chro¬ mosome changes are the principal sources of variation. Studies of these phenomena have been of necessity confined mainly to the laboratory and to organisms that are satisfactory as laboratory objects. Nevertheless, there can be no reasonable doubt that the same agencies have supplied the materials for the actual historical process of evolution. This is at¬ tested by the fact that the organic diversity existing in nature, the differ¬ ences between individuals, races, and species, are experimentally resolv¬ able into genic and chromosomal elements which resemble in all respects the mutations and the chromosomal changes that arise in the laboratory (1937, p. 118). But what forces mold and preserve this variation in nature? Dobzhansky stresses natural selection (p. 120), but he does not grant this process the dom¬ inant role that later “hard” versions of the synthesis would confer. He em¬ phasizes genetic drift (which he calls “scattering of the variability”) as a fun¬ damental mode of evolutionary change in nature, not as an odd phenomenon occurring in populations too small to leave any historical legacy. He argues that local races can form without influence from natural selection, and he supports Crampton’s (1916, 1932) interpretation of the nonadaptive and in¬ determinate character of substantial racial differentiation in the Pacific land snail Partula. He emphasizes that evolutionary dynamics depend, in large measure, upon the size of populations because selection does not always con¬ trol the outcome (and we therefore need information about numbers of indi¬ viduals and their mobility in order to assess the effects of drift, migration, and isolation). He coins the term “microgeographic race” and argues that most group distinctions at this level may be both nonadaptive and genetically based, contrary to the opinions of many naturalists who then regarded such races as adaptive and nongenetic. The sixth chapter then treats natural selection explicitly. Dobzhansky be¬ gins by clearing away some early Mendelian misconceptions about the impo¬ tence of natural selection (logical errors in interpreting Johannsen’s experi¬ ments on pure lines, for example). He then poses a central question: Darwin devised the theory of natural selection to explain adaptation; admitting Dar¬ win’s success in this area, may we then extrapolate and argue that selection controls the direction of all evolutionary change (p. 150)? Dobzhansky an¬ swers that we cannot defend such an extension of selection’s power. He then criticizes the strict selectionism of Fisher (p. 151), and praises a book that would later be castigated by all leading synthesists as a remnant of older and unproductive ways of thought—Robson and Richards (1936), with their de¬ fense of a nonadaptive origin for most subspecific and even interspecific dif¬ ferences in closely related forms. A long concluding section (pp. 185-191) supports Wright’s “island model”
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THE STRUCTURE OF EVOLUTIONARY THEORY of selection among semi-isolated demes occupying different peaks of an adap¬ tive landscape. Dobzhansky pleads for more study of “the physiology of pop¬ ulations” since Wright’s model proclaims three factors as important in differ¬ ent ways, while not granting inherent predominance to any: genetic drift, migration, and natural selection: “Since evolution as a biogenic process obvi¬ ously involves an interaction of all of the above agents, the problem of the rel¬ ative importance of the different agents unavoidably presents itself. For years this problem has been the subject of discussion. The results of this discussion so far are notoriously inconclusive; the ‘theories of evolution’ arrived at by different investigators seem to depend upon the personal predilections of the theorist” (p. 186). Dobzhansky does, however, suggest that Wright’s model may validate the common conviction of naturalists that the morphological differentia of races and species must often be nonadaptive.
Genetics and the Origin of Species went through three editions (1937, 1941, and 1951). As in the successive versions of Darwin’s Origin, the differ¬ ences among these editions cannot be dismissed as trivial or cosmetic, for they convey a major change in emphasis—an alteration that set the research program for most evolutionary biologists until the past few years. As the Syn¬ thesis developed, the adaptationist program grew in influence and prestige, and other modes of evolutionary change fell into disrepute, or became rede¬ fined as locally operative but unimportant in the overall picture. Dobzhansky’s third edition (1951) clearly reflects this hardening. He still insists, of course, that not all change can be called adaptive. He attributes the frequency of some traits to equilibrium between opposed mutation rates (p. 156) and doubts the adaptive nature of racial variation in blood types. He asserts the importance of genetic drift (pp. 165, 176) and does not accept as proof of panselectionism one of the centerpieces of the adaptationist pro¬ gram—A. J. Cain’s work on frequencies of banding morphs in the British land snail Cepaea (p. 170). But inserted passages and shifting coverage convey, as their common focus, Dobzhansky’s increasing faith in the scope and power of natural selection, and in the adaptive nature of most evolutionary change. He deletes the two chapters that contained most material on nonadaptive or nonselected phe¬ nomena (polyploidy and chromosomal changes, though he includes their ma¬ terial, in much reduced form, within other chapters). He adds a new chapter on “adaptive polymorphism” (pp. 108-134). Moreover, he now argues that anagenesis, or “progressive” evolution, works only through the optimizing, winnowing agency of selection based on competitive deaths; species adapting by increased fecundity in unpredictably fluctuating environments do not con¬ tribute to anagenesis (p. 283). But the most remarkable addition occurs right at the beginning. I label these passages remarkable because I doubt that Dobzhansky really believed what he literally said; I feel confident that he would have modified his words had anyone pointed out how his increasing fascination for adaptationism had led him to downgrade the deepest and oldest of evolutionary themes to effec-
The Modern Synthesis as a Limited Consensus
tive invisibility (see Chapter 10, pp. 1175-1178 for the modern relevance and refutation of this striking image). Dobzhansky poses the key question of organic form and taxonomy: why do organisms form discrete and clearly nonrandom “clumps” in populating morphological space? Why does the domain of mammalian carnivores con¬ tain a large cluster of cats, another of dogs, a third of bears, leaving so much unoccupied morphological space between? Dobzhansky begins by “promot¬ ing” Wright’s model of the “adaptive landscape” to an inappropriate level. In so doing, Dobzhansky subtly shifts the model’s meaning from an explanation for nonoptimality (with important aspects of nonadaptation) to an adaptationist argument about best solutions. Wright devised his model to explain differentiation among demes within a species. He proposed the metaphorical landscape to justify a fundamentally nonadaptationist claim: If a “best solu¬ tion” exists for the phenotype of a species (the highest peak in the landscape), why don’t all demes reside there? But if we “upgrade” the model to encom¬ pass differences between species within a clade, then metaphorical landscapes mutate into a framework for strict adaptationism. Each peak now becomes the optimal form for a single species (not the nonoptimal form for some demes within a species). And related peaks represent a set of best solutions as the various adaptations of separate evolutionary entities within a clade. Dobzhansky then attempts to solve the problem of clumping with an adaptationist argument based upon the organization of ecological space into pre¬ existing optimal “places” where good design may find a successful home. Evolution has produced a cluster of cats because an “adaptive range,” stud¬ ded with adjacent peaks, exists in the economy of nature, waiting, if you will, for creatures to move in. In other words, discontinuity in taxonomic space maps discontinuity in optimal form for available environments, with adapta¬ tion as the agent for mapping. The enormous diversity of organisms may be envisaged as correlated with the immense variety of environments and of ecological niches which exist on earth. But the variety of ecological niches is not only im¬ mense, it is also discontinuous . . . The adaptive peaks and valleys are not interspersed at random. “Adjacent” adaptive peaks are arranged in groups, which may be likened to mountain ranges in which the separate pinnacles are divided by relatively shallow notches [sic, Dobzhansky does indeed mean “notches” in this passage, not “niches” (as later in the quotation)]. Thus, the ecological niche occupied by the species “lion” is relatively much closer to those occupied by tiger, puma, and leopard than to those occupied by wolf, coyote, and jackal. The feline adaptive peaks form a group different from the group of the canine “peaks.” But the feline, canine, ursine, musteline, and certain other groups of peaks form together the adaptive “range” of carnivores, which is separated by deep adaptive valleys from the “ranges” of rodents, bats, ungulates, pri¬ mates, and others. In turn, these “ranges” are again members of the
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THE STRUCTURE OF EVOLUTIONARY THEORY adaptive system of mammals, which are ecologically and biologically segregated, as a group, from the adaptive systems of birds, reptiles, etc. The hierarchic nature of the biological classification reflects the objec¬ tively ascertainable discontinuity of adaptive niches, in other words the discontinuity of ways and means by which organisms that inhabit the world derive their livelihood from the environment (pp. 9-10). Thus, Dobzhansky renders the hierarchical structure of taxonomy as a fitting of clades into preexisting ecological spaces. Discontinuity emerges not so much as a function of history, but as a reflection of adaptive topography. But this interpretation cannot hold; surely, the cluster of cats exists primarily as a consequence of homology and historical constraint. All felines share a basic morphology because they arose from the common ancestor of this clade alone. We doubt neither the excellent adaptation of this common ancestor nor the claim that all descendants may fit equally well into their current envi¬ ronments. But the feline group and the gaps that separate this cluster from other families of carnivores reflect history above all, not the current organiza¬ tion of ecological topography. All feline species have inherited the unique
Bauplan of cats, and cannot deviate far from this commonality as they adapt, each in its own particular way. Genealogy, not current adaptation, provides the primary source for clumped distribution in morphological space.
THE SHIFT IN G. G. SIMPSON’S EXPLANATION OF “QUANTUM EVOLUTION” FROM DRIFT AND NON ADAPTATION (1944) TO THE EMBODIMENT OF STRICT ADAPTATION (1953)
Although Simpson, probably more than Dobzhansky, personally favored se¬ lectionist arguments in the initial version of his seminal work (1944), he also adopted a pluralistic stance at first. In fact, at the crux of his book, Simpson proposed an explicitly nonadaptationist theory to resolve the greatest anom¬ aly in the fossil record; he also considered this theory of “quantum evolu¬ tion” as the crowning achievement of his book. Like Dobzhansky in his first edition (1937), Simpson (1944) espoused con¬ sistency of all evolutionary change with principles of modern genetics as his primary assertion for a general and synthetic theory. The major challenge to unity and consistency arose from the infamous “gaps” or discontinuities of the fossil record—particularly at the largest scale of appearances for new
Bauplane without fossil intermediates. Simpson wrote: The most important difference of opinion, at present, is between those who believe that discontinuity arises by intensification or combination of the differentiating processes already effective within a potentially or really continuous population and those who maintain that some essen¬ tially different factors are involved. This is related to the old but still vital problem of micro-evolution as opposed to macro-evolution ... If the two proved to be basically different, the innumerable studies of micro-
The Modern Synthesis as a Limited Consensus
evolution would become relatively unimportant and would have minor value to the study of evolution as a whole (1944, p. 97). To explain these discontinuities, Simpson relied, in part, upon the classical argument of an imperfect fossil record. But he also conceded that such a prominent pattern could not be interpreted as entirely artificial—and he rec¬ ognized that his favored process of gradualistic Darwinian selection in the phyletic mode would not provide a full explanation. He therefore proposed his book’s only major departure from explanations based upon selection lead¬ ing directly to adaptation—and thus, in his most striking and original contri¬ bution, framed the hypothesis of quantum evolution. Simpson clearly took great pride in this novel theory, for he ended his book with a twelve-page defense of quantum evolution, identified as “perhaps the most important outcome of this investigation, but also the most controversial and hypothetical” (p. 206). Faced with the prospect of abandoning strict se¬ lection in the gradual, phyletic mode, he framed a hypothesis that adhered rigidly to his more important goal—the proviso that macroevolution must be rendered by genetical models and mechanisms operating within species, and amenable to study in living populations. Thus, he focused upon the only ma¬ jor phenomenon in the literature of population genetics that permitted a mechanism other than selection to serve as a basis for directional change— Sewall Wright’s genetic drift. He envisaged major transitions as occurring within small populations (where drift might be effective and preservation in the fossil record virtually inconceivable). He chose the phrase “quantum evolution” because he con¬ ceived the process as an “all-or-none reaction” (p. 199) propelling a small population across an “inadaptive phase”—explicitly so named—from one stable adaptive peak to another. Since selection could not initiate this depar¬ ture from an ancestral peak, he called upon drift to carry the population into an unstable intermediary position, where it must either die, retreat, or be drawn rapidly by selection to a new stable position. Simpson felt that, with quantum evolution, he had carried his consistency argument to completion by showing that genetical models could encompass the most resistant and mysterious of all evolutionary events—the rapid origin of novel phenotypes at high taxonomic levels. Quantum evolution, he wrote, is “believed to be the dominant and most essential process in the origin of taxonomic units of rela¬ tively high rank, such as families, orders, and classes. It is believed to include circumstances that explain the mystery that hovers over the origins of such major groups” (p. 206). Simpson could, therefore, conclude: “The materials for evolution and the factors inducing and directing it are also believed to be the same at all levels and to differ in mega-evolution only in combination and in intensity” (p. 124). Simpson’s emphasis on quantum evolution underscores a central feature of his explanatory preferences in 1944—his pluralistic view of evolutionary mechanisms. He wished to render all of macroevolution as the potential con¬ sequence of microevolutionary processes, not to rely dogmatically upon any
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THE STRUCTURE OF EVOLUTIONARY THEORY single process. Although he favored selection leading to adaptation as a pri¬ mary theme, he explicitly denied that all evolution could be adaptive and un¬ der selective control. He concluded: “The aspects of tempo and mode that have now been discussed give little support to the extreme dictum that all evolution is primarily adaptive. Selection is a truly creative force and not >
*
solely negative in action. It is one of the crucial determinants of evolution, al¬ though under special circumstances it may be ineffective, and the rise of char¬ acters indifferent or even opposed to selection is explicable and does not con¬ tradict this usually decisive influence” (1944, p. 180). When pressured for a new edition of Tempo and Mode, Simpson realized that evolutionary theory had developed too much in the intervening ten years to permit a reissue or even a simple revision. The field that he pioneered had stabilized and flourished: “It was [in the late 1930s] to me a new and exciting idea to try to apply population genetics to interpretation of the fossil record and conversely to check the broader validity of genetical theory and to extend its field by means of the fossil record. That idea is now a commonplace” (1953, p. ix). Thus, Simpson followed the outline of Tempo and Mode, but wrote a new book more than double the length of its ancestor-—T/?£ Major
Features of Evolution, published in 1953. The two books differ in many ways (see p. 522 for my personal and profes¬ sional introduction to the distinctions), most notably in Simpson’s increasing confidence that selection within phyletic lineages must represent the only im¬ portant cause of substantial change. Consider the following addition to the 1953 book, a speculative comment on trends in titanothere horns, with its prompt dismissal—tinged with impatience, if not incipient dogmatism—of the venerable argument that no evident function can be ascribed to the incipi¬ ent stages of useful structures: “This long seemed an extremely forceful argu¬ ment, but now it can be dismissed with little serious discussion. If a trend is advantageous at any point, even its earliest stages have some advantage. Thus if an animal butts others with its head, as titanotheres surely did, the slightest thickening as presage of later horns already reduced danger of fractures by however small an amount” (p. 270). But the most dramatic difference between the two books lies in Simpson’s demotion to insignificance of the concept that had formerly been, by his own reckoning and explicit announcement, his delight and greatest pride—quan¬ tum evolution. This hypothesis embodied the pluralism of his original ap¬ proach—a reliance on a range of genetical models. For he had advocated ge¬ netic drift to propel small populations off adaptive peaks into an ultimately untenable inadaptive phase. And he had explicitly christened quantum evolu¬ tion as a mode different in kind, not only in rate, from phyletic transforma¬ tion within lineages. But now, as the adaptationist program of the Synthesis hardened, Simpson decided that genetic drift could not trigger any major evo¬ lutionary event: “Genetic drift is certainly not involved in all or in most ori¬ gins of higher categories, even of very high categories such as classes or phyla” (p. 355). In an “intermediate stage” of his personal ontogeny—his presentation to
The Modern Synthesis as a Limited Consensus
the Princeton conference on genetics, paleontology and evolution—Simpson (1949, p. 224) had emphasized the dominance of selection in quantum evolu¬ tion, while not denying other factors. But by 1953, he had completed his per¬ sonal transition. Quantum evolution now merits only four pages in an en¬ larged final chapter on modes of evolution. More importantly, this concept has now mutated to a meaning that Simpson had explicitly denied before: merely a name for phyletic evolution when the process operates at a maximal rate—an evolutionary tempo differing only in degree from the leisurely, grad¬ ual transformation of populations in ordinary geological time. Quantum evo¬ lution, he now writes, “is not a different sort of evolution from phyletic evo¬ lution, or even a distinctly different element of the total phylogenetic pattern. It is a special, more or less extreme and limiting case of phyletic evolution” (p. 389). He lists quantum evolution as one category among the four styles of phyletic evolution (p. 385)—with all four characterized by “the continuous maintenance of adaptation.” The bold hypothesis (1944) of an absolutely inadaptive phase has been replaced by the semantic notion of a relatively inadaptive phase (an intermediary stage inferior in design to either the ances¬ tral or the descendant Bauplan). But relative inadaptation poses no threat to the adaptationist paradigm. Even the strictest Darwinian will feel no Angst if the fit of phenotype to environment decreases for an intermediate form in a new habitat, relative to the ancestor in a different original place; (the two forms, after all, cannot directly compete). Even less Angst will then accom¬ pany an acknowledgment that this intermediate form may be less well de¬ signed than its own future descendant (for selection should engender increas¬ ing adaptation through time, especially as a population adjusts to a strikingly new environment). In short, such relatively inadaptive populations can only be regarded as adequately adapted to their own environments at their own time (unsubjected, as they must be, to competition with better adapted ances¬ tors in a different habitat, or with improved future descendants in this new world). Quantum evolution, by linguistic redefinition, therefore moves com¬ fortably under the umbrella of the adaptationist program. Simpson now even suggests that quantum evolution may be more rigidly controlled by selection than any other mode of evolution (though he still invokes inadaptation for the initial trigger): “Indeed the relatively rapid change in such a shift is more rigidly adaptive than are slower phases of phyletic change, for the direction and the rate of change result from strong selection pressure once the thresh¬ old is crossed” (p. 391).
MAYR AT THE INCEPTION (1942) AND CODIFICATION (1963): SHIFTING FROM THE “GENETIC CONSISTENCY” TO THE “ADAPTATIONIST” PARADIGM
If we consider the synthesis as a fusion of three equally robust disciplines— experimental genetics, population genetics, and studies of natural history ex¬ pressed primarily by systematics (and not as an imposition of the first two, as modernisms, upon a hidebound, or even moribund, third mode of study)—
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THE STRUCTURE OF EVOEUTIONARY THEORY then the role played by Mayr and other field naturalists in building the syn¬ thesis becomes fully constitutive and not only derivative. Mayr (1980), wear¬ ing his historian’s hat, has strongly defended such an account of the Synthesis against the reductionist tradition that regards genetics as paramount, and the second phase of the Synthesis largely as a whipping of older disciplines into line. I do not deny Mayr’s partisan motives in advancing this interpretation, but I also concur with his judgment. Dobzhansky, as argued above, became the beacon of the second phase be¬ cause he represented the only tradition, Russian genetics, that tried to fuse ex¬ perimental Mendelism with systematics and natural history, rather than im¬ posing the first upon the second (or ignoring the second entirely). At Mayr’s 1974 conference, Dobzhansky vividly recalled the impediments to synthe¬ sis within American traditions. He had originally left Russia to work with Thomas Hunt Morgan, America’s premier experimental geneticist. Dobzhan¬ sky recalled Morgan’s attitude to natural history: “Naturalist” was a word almost of contempt with him, the antonym of “scientist.” Yet Morgan himself was an excellent naturalist, not only knowing animals and plants but aesthetically enjoying them . . . Morgan was profoundly skeptical about species as biological and evolutionary realities. The species problem simply did not interest him. . . . Biology had to be strictly reductionistic. Biological phenomena had to be ex¬ plained in terms of chemistry and physics. Morgan himself knew little chemistry, but the less he knew the more he was fascinated by the powers he believed chemistry to possess. There was no surer way to impress him than to talk about biological phenomena in ostensibly chemical terms (1980, p. 446). Morgan, Dobzhansky also remembered, “liked to say that genetics can be studied without any reference to evolution.” Could the Synthesis have taken root in such soil? Dobzhansky brilliantly set a different task for evolutionary theory—an enterprise embodied in Darwin’s title (but not treated as a major theme in his book), and emerging from traditions of systematics and natural history (while scarcely conceivable for someone with Morgan’s, and to a large ex¬ tent Darwin’s, views on the unreality of species): how can a theory originally constructed to describe continuous change in natural populations also ex¬ plain the discontinuous structure of nature’s taxonomic diversity? The central problem of evolution, Dobzhansky asserted, is the origin of discontinuity among species. This statement sounds commonplace today, but only because Dobzhansky and the Synthesis moved the question to center stage. Morgan and virtually all experimentalists had argued that the origin and nature of variation, and its manner of spread through populations, defined the key issues in evolutionary theory. Morgan disavowed the species problem as, at best, a hang-up of dull taxonomists and, at worst, a bogus issue because species have no reality in
The Modern Synthesis as a Limited Consensus
the flow of nature. (We name species, under this view, only because our poor minds can’t handle continuity.) Dobzhansky didn’t deny the importance of Morgan’s questions. But he argued that evolution operates on a series of levels, and that the primary gaze of natural history must not be focused upon these lower levels, but upon the broader phenomenon of the origin of species itself (Darwin’s title, after all). Diversity represents the primary fact of nature (and the first topic of chapter 1 in Dobzhansky’s book). Diversity arises by the splitting of lineages—that is by speciation. Speciation produces discontinuity in nature. How can a continu¬ ous process of genetic change yield such bounded separations? The origin of discontinuities between species must therefore be recast as the key problem in evolutionary theory. Only a naturalist (better yet, a trained systematist) could have reset the stage for synthesis in such a fruitful way. The origin of hereditary variations is, however, only a part of the mecha¬ nism of evolution. . . . These variations may be compared with building materials, but the presence of an unlimited supply of materials does not in itself give assurance that a building is going to be constructed . . . Mu¬ tations and chromosomal changes are constantly arising at a finite rate, presumably in all organisms. But in nature we do not find a single greatly variable population of living beings which becomes more and more vari¬ able as time goes on; instead, the organic world is segregated into more than a million separate species, each of which possesses its own limited supply of variability which it does not share with the others. . . . The ori¬ gin of species . . . constitutes a problem which is logically distinct from that of the origin of hereditary variation (Dobzhansky, 1937, p. 119). Mayr (1942) then furthered Dobzhansky’s program by dedicating an entire book to modes of speciation, and to realigning taxonomic practice with in¬ sights of the developing Synthesis. He even formulated his title in conscious parallel to Dobzhansky’s (while both, of course, also claim and honor Dar¬ win)—and as a manifesto for the centrality of his field: Systematics and the
Origin of Species. Mayr’s first paragraph (1942, p. 3) sets his theme and tone: The rise of genetics during the first thirty years of this century had a rather unfortunate effect on the prestige of systematics. The spectacular success of experimental work in unraveling the principles of inheritance and the obvious applicability of these results in explaining evolution have tended to push systematics into the background. There was a ten¬ dency among laboratory workers to think rather contemptuously of the museum man, who spent his time counting hairs or drawing bristles, and whose final aim seemed to be merely the correct naming of his speci¬ mens. A welcome improvement in the mutual understanding between ge¬ neticists and systematists has occurred in recent years. Mayr (1942) follows the characteristic pluralism of the early synthesis in listing all valid evolutionary principles that can explain the data of systemat¬ ics. His major aim therefore follows the program of “healthy restriction”—
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THE STRUCTURE OF EVOLUTIONARY THEORY the focus of the first phase of the Synthesis (see pp. 503-508). Thus, Mayr ex¬ plicitly rejects such fallacies as Larmackian inheritance, and the idea that higher taxa arise by different and mysterious routes—thereby invoking an ar¬ gument by elimination to make evolutionary change at all levels fully consis¬ tent with principles of genetics at work in modern populations and subject to experiment in the laboratory or observation in the field. Whereas Mayr’s ma¬ jor themes remain Darwinian, he still invokes the full panoply of legitimate genetic forces. Note in particular that selection (leading to adaptation), while listed first, represents only one force in an array collectively responsible for the formation of species. Adaptation holds no exclusivity, or even any partic¬ ular pride of place: “First, there is available in nature an almost unlimited supply of various kinds of mutations. Second, the variability within the small¬ est taxonomic units has the same genetic basis as the differences between the subspecies, species, and higher categories. And third, selection, random gene loss, and similar factors, together with isolation, make it possible to ex¬ plain species formation on the basis of mutability, without any recourse to Lamarckian forces” (1942, p. 70). Mayr reemphasizes this pluralistic theme at the end of his book in asserting the essential integrative claim that all phenomena of macroevolution can also be subsumed by the Synthesis. Inclusion within the Synthesis implies explana¬ tion by principles of modern genetics, not a commitment to any particular mode of genetic change: “It is feasible to interpret the findings and generali¬ zations of the macroevolutionists on the basis of the known genetic facts (random mutation) without recourse to any other intrinsic factors” (1942, p. 292). Mayr then lists the eight key principles of modern genetics that he re¬ gards as necessary for accomplishing the integration. Only one, number seven on the list, mentions selection and adaptation (p. 293). As a more positive argument against adaptationist exclusivity, Mayrs own taxonomy of “factors involved in speciation” (p. 216) grants explicit and equal weight to adaptation and nonadaptation as the two primary categories of divergence. He writes (p. 216): “We may classify these factors as (1) those that either produce or eliminate discontinuities and (2) those that promote or impede divergence. The latter may be subdivided further into adaptive (selec¬ tion) and non-adaptive factors.” Within this important category of nonadaptation, Mayr includes many prominent phenomena that he would later ascribe to selection.
1. Nearly all polymorphism within species: There is, however, considerable indirect evidence that most of the char¬ acters that are involved in polymorphism are completely neutral, as far as survival value is concerned. There is, for example, no reason to believe that the presence or absence of a band on a snail shell would be a notice¬ able selective advantage or disadvantage. Among the many species of birds which occur in several clear-cut color phases, there is, with one or two exceptions, no evidence for selective mating or any other advantage of any of the phases (p. 75).
The Modern Synthesis as a Limited Consensus
2. Most geographic variation in dines: It is difficult to see why the gradual decrease from the north to the south in the number of the bridled individuals (ringvia) in populations of the Atlantic murre (Uria aalge) should have an adaptational significance . . . The convergent development in several species of Draco also seems to belong to the category of non-adaptive dines (p. 96). 3. Much geographic variation in general: It should not be assumed that all the differences between populations and species are purely adaptational and that they owe their existence to their superior selective qualities. . . . Many combinations of color pat¬ terns, spots, and bands, as well as extra bristles and wing veins, are prob¬ ably largely accidental. This is particularly true in regions with many sta¬ tionary, small, and well-isolated populations, such as we find commonly in tropical and insular species. . . . We must stress the point that not all geographic variation is adaptive (p. 86). Mayr’s later book (1963) expanded to more than twice the number of pages, and became even more weighty in its assurances. This work shaped my own evolutionary thinking more than any other book—and I am confident that most naturalists of my generation would offer the same testimony. As I reread Animal Species and Evolution in preparing to write this chapter—and examined my old marginalia, pencilled in preparation for the deciding oral exam of my Ph.D. program—I came to appreciate even more (now that I know the genre’s difficulty through personal experience) the enormous labor and creative thought involved in bringing so much material together. And I finally understood the defining word that once puzzled me in Julian Huxley’s review of Mayr’s book—“magisterial.” (The etymological source does not re¬ side in “magnificent” or “majestic,” though Mayr’s book surely merits either of these accolades, but in magister, the Latin word for teacher. A great magister is not a schoolroom pedant, but a wise preceptor who holds mastery within his teaching authority, or magisterium. Magisterial, above all else, means authoritative. And to what greater virtue, after all, may an author aspire?) Although Mayr’s 1963 book covers the same general material, and in simi¬ lar order, as the 1942 version, the works differ profoundly, and Mayr chose a new title (just as Simpson had done in noting the changes between his 1944 and 1953 volumes). I would specify two thematic changes as most important. 1. The primary role of geographic isolation as a sine qua non, and the con¬ sequent near universality of allopatric speciation, has consistently formed the centerpiece of Mayr’s worldview. But, in 1942, pure continuationism reigned. Populations split into roughly equal divisions and each subgroup then func¬ tioned as a microcosm of the ancestral mass—as in the model now called “dumb-bell allopatry” and considered (by Mayr at least) both rare and rela¬ tively ineffective in producing new species. In other words, Mayr (1942) orig¬ inally identified no distinctive properties promoting speciation in certain kinds of isolated populations vs. others. Isolation itself, and the severing of
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THE STRUCTURE OF EVOLUTIONARY THEORY gene flow, rendered any population ripe for speciation: “The big gaps which we find between species are preceded by the little gaps which we find between subspecies and by the still lesser gaps which we find between populations. Of course, if these populations are distributed as a complete continuum, there are no gaps. But with the least isolation, the first minor gaps will appear” (1942, p. 159). But by 1963, Mayr had developed the full apparatus of the distinctive the¬ ory that he later called “peripatric speciation” to emphasize a sharp separa¬ tion from his original, continuationist version of allopatry. For the peripatric model promotes the role of small populations, isolated at the periphery of pa¬ rental ranges, and subject to a special maelstrom of influences including greatly enhanced selection and random effects of the founder principle—all leading to potential achievement of specific status with relative speed by a “genetic revolution.” Mayr says (personal communication) that he intro¬ duced this new apparatus in a paper (1954) that achieved no impact, but nonetheless represents his most important idea and best work. (Nihil sub sole
novum. He published this paper in a symposium volume—the greatest reposi¬ tory of unread literature, both then and now.) 2. Mayr’s 1942 book included little explicit material about adaptation, since this volume emphasized the origin and development of discontinuity be¬ tween species, and said little about anagenetic change within populations. This context of minimal consideration reflects Mayr’s pluralism and lack of commitment to strict adaptationism at this time. (This claim may sound para¬ doxical, but should not be so read. Views expressed in passing—by their sim¬ ple acknowledgment of an unchallenged belief—tend to record a professional consensus more clearly than material explicitly touted as central and distinc¬ tive.) But, in 1963, Mayr added a full consideration of variation and change within populations—the main reason for a much longer book. Here the hard¬ ened, panadaptationist position of the later Synthesis reigns supreme, per¬ haps more strongly than in any other book of comparable influence. In the mid 1990’s, Mayr himself (in litt. and personal communications— see end of this section), while continuing to explicate and defend his favored themes of 1963, denies any substantial change between the volumes of 1942 and 1963 on questions of adaptation. This difference between current mem¬ ory and textual record, previously discussed as a general principle (see p. 521), provides a fascinating illustration of how scholars can slowly and un¬ consciously imbibe a shifting professional consensus, thus imposing a subjec¬ tive and personal impression of stability upon a virtual transmogrification. I find this unconscious alteration all the more ironic in Mayr’s case because his first category of major change in ideas about speciation—his intellectual move from the dumbbell to the peripatric model—so strongly encourages a widened space for nonadaptationist themes (for many evolutionists have interpreted his notions of genetic revolutions and founder effects in small peripheral isolates as a powerful antidote to the classical panadaptationist model of Fisherian panmixia in large populations). Yet Mayr never translated the implications of these changes in his own ideas about speciation into
The Modern Synthesis as a Limited Consensus
doubts about adaptation in his chapters on variation and change within pop¬ ulations. No good naturalist, living in our complex universe of relative frequencies, could ever become an uncompromising dogmatist on the subject of adapta¬ tion. Mayr therefore mentions occasional inadaptive features (1963, p. 156), or acknowledges the importance of developmental constraint (p. 608). But these statements function more as footnotes or placeholders in the logic of an argument; for Mayr does not treat alternatives to adaptation as operational imperatives in the ordinary analysis of cases. Moreover, Mayr laces his plu¬ ralistic admissions with hedges and caveats. Note, for example, how Mayr frames his main admission of potential nonadaptation only as an argument against optimality, not as a denial of selection—and how his closing hedge anticipates a movement of even these least promising cases into the adaptationist camp: Each local population is the product of a continuing selection process. By definition, then, the genotype of each local population has been se¬ lected for the production of a well-adapted phenotype. It does not follow from this conclusion, however, that every detail of the phenotype is max¬ imally adaptive. If a given subspecies of ladybird beetles has more spots on the elytra than another subspecies, it does not necessarily mean that the extra spots are essential for survival in the range of that subspecies. It merely means that the genotype that has evolved in this area as the result of selection develops additional spots on the elytra . . . Yet close analysis often reveals unsuspected adaptive qualities even in minute details of the phenotype (1963, p. 311). Selection holds primacy of place as the ruling force of evolution: “Every species is the product of a long history of selection and is thus well adapted to the environment in which it lives. There is no doubt that the phenotype as a whole, including its physiological properties, is adaptive and is produced by a genotype that is the result of natural selection. This is not contradicted by the fact that an occasional component of the phenotype is adaptively irrelevant'’ (1963, p. 60). Above all else, Mayr regards one conclusion as especially well confirmed by observation: adaptation rules in “every local population” as selection to “exacting requirements” of local environments produces an “optimal pheno¬ type.” One could hardly state the adaptationist position more boldly: “One conclusion emerges from these observations more strongly than any other: every local population is very precisely adjusted in its phenotype to the exact¬ ing requirements of the local environment. This adjustment is the result of a selection of genes producing an optimal phenotype” (1963, p. 318). Mayr’s treatment of potential alternatives illustrates his adherence to the rule of adaptation, both as a methodological preference and an empirical claim. Geographic trends that he formerly attributed to incidental allometries have now become active adaptations: “A particularly impressive result of studies of ecogeographical rules is the discovery of the extreme sensitivity of
S3 7
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THE STRUCTURE OE EVOLUTIONARY THEORY body proportions, to natural selection. The former belief that proportions are determined by ‘built-in’ allometry factors and change automatically with changes in body size is not supported by these findings” (1963, p. 324). Neutral genes become improbable, almost nonsensical in principle, once we recognize the pervasive monitoring of nature by selection: Entirely neutral genes are improbable for physiological reasons. Every gene elaborates a “gene product,” a chemical that enters the develop¬ mental stream. It seems unrealistic to me to assume that the nature of the particular chemical (enzyme or other product) should be without any ef¬ fect whatsoever on the fitness of the ultimate phenotype. A gene may be selectively neutral when placed on a particular genetic background in a particular temporary physical and biotic environment. However, genetic background as well as environment change continually in natural popu¬ lations and I consider it therefore exceedingly unlikely that any gene will remain selectively neutral for any length of time (1963, p. 207). Consequently, even the most apparently trivial features probably origi¬ nated by direct selection. “One can never assert with confidence that a given structure does not have selective significance. The peculiar tarsal combs of the males in certain species of Drosophila turned out to have an important func¬ tion during copulation; the color patterns of Cepaea snails have cryptic sig¬ nificance, mitigating predator pressure” (1963, p. 190). In 1963, Mayr repudiated all three major classes of nonadaptation that he had defended in 1942: polymorphisms, dines, and much geographic varia¬ tion in general. Explicitly refuting his own former view, Mayr now argues (1963, p. 162) that the ubiquity of selection must imply an adaptive basis for polymorphisms (see also pp. 158 and 167): “Such neutral polymorphism, it was claimed, was maintained by ‘accident.’ Now that the cryptic physiologi¬ cal effects of ‘neutral’ genes have been discovered, it is evident that such genes are anything but selectively neutral. It is altogether unlikely that two genes would have identical selective values under all the conditions in which they may coexist in a population.” In a remarkable statement, he then urges that polymorphisms and dines be viewed as evidence for adaptation a priori: “Selective neutrality can be ex¬ cluded almost automatically wherever polymorphism or character dines are found in natural populations . . . Virtually every case quoted in the past as caused by genetic drift due to errors of sampling has more recently been rein¬ terpreted in terms of selection pressures” (1963, pp. 207-208). As for geographic variation, what else could such a phenomenon represent but adaptation to an altered environment, with selection as an efficient and omnipresent watchdog: “The geographic variation of species is the inevitable consequence of the geographic variation of the environment. A species must adapt itself in different parts of its range to the demands of the local environ¬ ment. Every local population is under continuous selection pressure for maxi¬ mal fitness in the particular area where it occurs. . . . Each local environment
The Modern Synthesis as a Limited Consensus
exerts a continuous selection pressure on the localized demes of every species and models them thereby into adaptedness” (1963, pp. 311-312). Throughout Mayr’s 1963 book—with a cadence that sounds, at times, almost like a morality play—phenomenon after phenomenon falls to the explanatory unity of adaptation, as the light of nature’s truth expands into previous darkness: non-genetic variation (p. 139), homeostasis (pp. 57, 61), prevention of hybridization (p. 109). Former standard bearers of the opposi¬ tion fall into disarray, finally succumbing to defeat almost by definition: “It is now evident that the term ‘drift’ was ill-chosen and that all or virtually all of the cases listed in the literature as ‘evolutionary change due to genetic drift’ are to be interpreted in terms of selection” (p. 214). All particular Goliaths have been slain (although later genetic studies would revivify this particular old warrior): “The human blood-group genes have in the past been held up as an exemplary case of ‘neutral genes,’ that is, genes of no selective significance. This assumption has now been thoroughly disproved” (p. 161). However, Mayr’s most interesting expression of movement towards a hard¬ ened adaptationism occurs not so much in these explicit claims for near ubiq¬ uity, but even more forcefully in the subtle redefinition of all evolutionary problems as issues in adaptation. The very meaning of terms, questions, groupings and weights of phenomena, now enter evolutionary discourse un¬ der adaptationist presumptions. Not only have alternatives to adaptation been routed on an objective playing field, Mayr claims in 1963, but the con¬ ceptual space of evolutionary inquiry has also become so reconfigured that hardly any room (or even language) remains for considering, or even formu¬ lating, a potential way to consider answers outside an adaptationist frame¬ work. Major subjects, the origin of evolutionary novelty for example, now reside exclusively within an adaptationist framework by purely functional defini¬ tion: “We may begin by defining evolutionary novelty as any newly acquired structure or property that permits the performance of a new function, which, in turn, will open a new adaptive zone” (p. 602). In a world of rapid and pre¬ cise adaptation, morphological similarity between distantly related groups can only arise through convergence imposed by similar adaptive regimes upon fundamentally different genetic material. The older, internalist view (constraint-based and potentially nonadaptationist)—the claim that we might attribute such similarities to parallelism produced by homologous genes—is dismissed as both old-fashioned and wrong-headed. (In modern hindsight, this claim provides a particularly compelling example of how hardened adaptationism can suppress interesting questions—for such homologues have now been found in abundance. Their discovery ranks as one of the most important events in modern evolutionary science—see Chapter 10, p. 1092, where we will revisit this particular Mayrian claim): “In the early days of Mendelism there was much search for homologous genes that would account for such similarities. Much that has been learned about gene physiol¬ ogy makes it evident that the search for homologous genes is quite futile ex¬ cept in very close relatives” (1963, p. 609).
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THE STRUCTURE OF EVOLUTIONARY THEORY Subjects that might have seemed challenging or exceptional now achieve a place within the adaptationist framework by expanded definition. Nonfunc¬ tional pleiotropic consequences, for example, become an aspect of orthodoxy because they now enter a hardened discaurse in the redefined guise of features subsidiary to a main effect of adaptive significance. (I do not challenge the particular assertion in this case, but I do feel that such an important subject deserves consideration from a structuralist perspective as well): “Pleiotropic gene action is the key to the solution of many other puzzling phenomena . . . Color, pattern, or some structural detail may be merely an incidental by-prod¬ uct of a gene maintained in the gene pool for other physiological properties. The curious evolutionary success of seemingly insignificant characters now appears in a new light” (1963, p. 162). All potential anomalies yield to a more complex selectionist scenario, often presented as a “just-so-story.” Why did the crown height of molars increase so slowly, if hypsodonty became so advantageous once horses shifted to vegetational regimes of newly-evolved grasses with high silica content? Mayr devises a story—sensible, though empirically wrong in this case—and regards such a hypothetical claim for plausibility as an adequate reason to affirm a selectionist cause. (The average increase may have been as small as the figure cited by Mayr, but horses did not change in anagenetic continuity at constant rates. Horses probably evolved predominantly by punctuated equilibrium— see Prothero and Shubin, 1989, and Chapter 9. The average of a millimeter per million years represents a meaningless amalgam of geological moments of rapid change during speciation mixed with long periods of stasis): “An in¬ crease in tooth length (hypsodonty) was of selective advantage to primitive horses shifting from browsing to grazing in an increasingly arid environment. However, such a change in feeding habits required a larger jaw and stronger jaw muscles, hence a bigger and heavier skull supported by heavier neck mus¬ cles, as well as shifts in the intestinal tract. Too rapid an increase in tooth length was consequently opposed by selection, and indeed the increase aver¬ aged only about 1 millimeter per million years” (1963, p. 238). In 1991, I asked Ernst Mayr about changes between his 1942 and 1963 books. He acknowledged the structural alterations, of course—particularly his addition of several chapters emphasizing adaptational themes. But he strongly denied any personal augmentation of adaptationist preferences through the intervening years, citing the interesting argument that, as a Lamarckian in his evolutionary youth (well before both books), he had al¬ ways favored adaptationism. He even wrote me a fascinating letter the day af¬ ter our lunchtime conversation: Dear Steve, I gave considerable thought to your question how my 1963 book dif¬ fered from the 1942 one, and why adaptation was so much more fea¬ tured in the later volume. I think I now have the answer. Remember that I consider evolution by and large to consist of two
The Modern Synthesis as a Limited Consensus
processes: 1) the maintenance and improvement of adaptedness, and 2) the origin and development of diversity. Since (2) was so almost totally ignored by the pre-Synthesis geneti¬ cists, I focussed in 1942 on (2). By the 1950s the study of diversity had been fully admitted to evolutionary biology, owing to the efforts of Dobzhansky, myself, Rensch and Stebbins, and in my 1963 book I could devote a good deal of attention to (1). This was rather easy because, as you know, I used to be a Lamarckian. And Lamarckians are adaptationists. Hence, it is not that from 1942 to 1963 I had become an adaptationist, rather I reconciled in 1963 my adaptationist inclination with the Darwinian mechanism (Letter of December 20, 1991). (Mayr then added a handwritten footnote, demoting to insignificance the one subject for which he did acknowledge a reversal of opinion between the two books: “Neutral polymorphism is an infinitesimal percentage of all evolu¬ tionary phenomena. Don’t make a mountain out of this little mole-hill.”) I do not deny Mayr’s stable adaptationist preferences (through his onto¬ genetic change in explanatory preferences from Lamarck to Darwin). This personal stability provides an even better reason for regarding as important, and therefore generally indicative, the textual evidence of transition from plu¬ ralism in 1942 to adaptationist hardening in 1963 (for Mayr’s 1942 text may therefore, by implications of his own testimony, be reporting the conven¬ tional pluralistic wisdom of the time despite Mayr’s own personal preference for adaptationism). On the subject of adaptation—not the major concern of either book (for both treat speciation and the production of diversity as their primary topics)—Mayr recorded a professional consensus both times, largely passively I suspect (hence his personal inattention to the alteration). Scientists must struggle to identify and understand these influences of “shared culture,” for such a “background” consensus fuels the sources of unconscious bias for each of us at every moment of our careers.
WHY HARDENING?
I have documented the adaptationist hardening of the Modern Synthesis in some detail, but I have not addressed an obvious and pressing question: why did this conceptual trend occur? Several aspects of an answer seem clear, but I can offer no full or satisfying resolution. The culture of science trains us to believe that such major shifts of empha¬ sis record improvements in knowledge won by empirical research and discov¬ ery. I do not deny that observation did play a significant role, at least in il¬ lustrating, with some elegant examples, the power of adaptation. Consider, for example, the “ecological genetics” of E. B. Ford and his panselectionist school in England. Their commitment to adaptationist explanations of effec¬ tively all variation among populations, and their documentation of strong selection coefficients in nature, buoyed the strict Darwinian faith. Dobzhan-
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THE STRUCTURE OF EVOLUTIONARY THEORY sky’s own empirical work increased his belief in the power of selection. In 1937, he tended to attribute chromosomal inversion frequencies in natural populations of Drosophila to genetic drift, but he then discovered that these frequencies fluctuate in a regular and rq^eatable way from season to season, and he therefore decided (with evident justice) that such systematic and iter¬ ated change must be adaptive. But empirical discovery cannot supply the entire (or even, I think, the major) reason for adaptationist hardening, for each favorable case can be matched by a failure (often hedged or unacknowledged), and no adequate as¬ sessment of an overall relative frequency has ever been achieved—to this day. Thus, any judgment, in either direction, must represent the fashionable impo¬ sition of a few well-documented cases upon an unstudied plethora. For exam¬ ple, A. J. Cain and colleagues did win a major victory for adaptation by showing that banding-morph frequencies in the land snail Cepaea, a former mainstay for claims about genetic drift, reflected selection based upon visual predation by birds, and upon climatic factors (Cain and Sheppard, 1950, 1952, 1954). But Cam and his colleagues then recognized and named the outstand¬ ing pattern of “area effects” (Cain and Currey, 1963a and b)—abrupt geo¬ graphic changes in banding-morph frequencies occurring with no perceptible alteration in any environmental factor that might impose a selection pressure. In what can only be labeled an article of faith, Cain attributed area effects to selection based upon “cryptic [meaning truly unmeasured and unperceived by any investigator, not merely subtle] environmental differences”—a re¬ markable affirmation of an a priori preference based upon not finding the necessary empirical confirmation. Good evidence has since been presented for a non-adaptive explanation of area effects as historical remnants of previous patterns in land use, and not as an outcome of current regimes in selection (Cameron et al., 1980; see review of the entire case in Gould and Woodruff, 1990). (Area effects rank as anomalies under selectionist presuppositions— hence Cam’s need to supply an orthodox adaptationist explanation, even in the absence of required evidence. Under a “legacy of history” explanation, such discordance of morphology with present geography presents no anom¬ aly and need not even receive a special name.) If adaptationist hardening cannot be explained as simply and empiri¬ cally driven, we might turn to historical and sociological themes. Smocovitis (1996), as previously mentioned (see p. 503), presents the intriguing thesis that renewed optimism following the wreckage of World War II (including the hope inspired by the newly constituted United Nations) launched a strong push for scientific defenses of potential human improvement and evolution¬ ary progress—an impetus that became a semi-official movement spurred by positivistic theories of knowledge proffered as antidotes for older irratio¬ nalisms. Smocovitis writes: If selection had enough agency (and at the same time were a mechanical principle) then all the more rapid and possible the “improvement” of hu-
The Modem Synthesis as a Limited Consensus mans. . . . More strongly selectionist models would be favored by biolo¬ gists who modelled themselves after physicists at the same time they pointed the way to the “improvement” of humanity and painted a pro¬ gressive and optimistic picture of the world . . . Evolutionary models fa¬ voring random genetic drift, which enforced a stochastic view of evolu¬ tion—and culture—would not be favored in a post-war frame of mind seeking to “improve” the world. So powerful would be the need for a progressive and selectionist framework in the 1940’s that even Dobzhansky and Wright were to adopt more strongly selectionist models."' Some complex mixture of empirical and sociological themes may explain the adaptationist hardening of the synthesis, but we must not neglect the ad¬ ditional impetus of a cultural analog to drift and founder effects in small pop¬ ulations. The community of evolutionary biologists is sufficiently small, and sufficiently stratified—a few lead and many follow, as in most human activi¬ ties—that we need not necessarily invoke some deep and general scientific or societal trend to explain a change in opinion by a substantial community of evolutionists in different nations. A reassessment by a few key people, bound in close contact and mutual influence, might trigger a general response. The three leading exponents of hardening in America—Dobzhansky, Simpson, and Mayr—worked together as colleagues in a “New York Mafia” centered at Columbia University and the American Museum of Natural History. Add another seemingly eternal principle of human affairs—that founders tend to be brilliant and subtle, and to keep all major difficulties constantly in mind, while epigones generally promulgate the faith and disregard, or never learn, the problems, exceptions, and nuances—and we may then wish to view the adaptationist hardening as ultimately inadaptive for the broadest goal of understanding evolution aright. Bandwagons might well be construed as cul¬ tural analogs of internalist drives in nonfunctional orthogenesis. Theories can grow tired. Theories can also harden and lose their bearings when compla¬ cency occupies the driver’s seat.
Hardening on the Other Two Legs of the Darwinian Tripod To illustrate the hardening of the Modern Synthesis, I have documented its most significant ontogenetic trend in extenso—increasingly exclusive empha¬ sis on adaptation as the sign of natural selection’s pervasive power. But if we epitomize the Synthesis as Darwinism reclothed in Mendelian understanding, *1 am not generally drawn to sociological proposals in this mode, and I reacted nega¬ tively at first to Smocovitis’s suggestion. But I have since read widely in the just post-World War II literature, and I only now understand the fervor and hope of “never again,” follow¬ ing all the devastation, and the heartrending impact (and inspired shame) as knowledge of the Holocaust surfaced. I was too young, when the war ended, to experience viscerally both this horror and hope, but I do grasp the character of this unusual time with a pervasive theme and agenda—and I accept the idea that humanistically inclined scientists must have hoped fervently that their own field might contribute to the reconstruction.
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THE STRUCTURE OF EVOLUTIONARY THEORY then, following this book’s focal argument that the minimal commitments of Darwinian logic encompass three central themes, the other two legs of this es¬ sential tripod should experience corresponding changes as the Synthesis hard¬ ened. I will not provide so extensive a discussion of these other legs—levels of selection and extrapolation into geological time—but I do wish to record that the literature of these subjects also experienced the same ontogeny of solidi¬ fication (and unjustified neglect of reasonable alternatives).
LEVELS OF SELECTION
Darwin, as we have seen (pp. 125-137), viewed organisms as nearly exclusive agents of selection—for deep reasons situated at the core of both the logic (the invisible hand of Adam Smith transferred to nature) and the psychology (the inversion of Paley’s world) of his theory. But few Darwinians grasped the rationale or centrality of this principle, and a tradition of vagueness and loose thinking about levels of selection developed. Some, like R. A. Fisher, rode Darwin’s wave and wrote explicitly and cogently about reasons for choosing individual organisms as the proper locus, and for disregarding, as effectively impotent, other levels that must be deemed conceivable in theory (1958, on species selection—see my critique of Fisher on pp. 644-652). But others, dat¬ ing back to A. R. Wallace himself (see pp. 131-132), never understood the full logic and implications of this issue, and ranged indiscriminately up and down potential levels, without grasping the theoretical problems entailed by such excursions. Thus, a fluid situation prevailed on this issue at the time of the Darwinian centennial celebrations of 1959—my point of reference for the triumphal height of the Modern Synthesis in its strongly adaptationist version. Adapta¬ tion had become all the rage, but vagueness shrouded the key issue of selec¬ tion’s focus and level—and for two reasons. First, and less important because the position attracted few supporters, a few evolutionists explicitly advocated a multi-level view of both selection and adaptation. A group of Chicago ecologists, authors of an important text¬ book known by its acronym of AEPPS (Allee, Emerson, Park, Park and Schmidt, Principles of Animal Ecology, 1949), generated and led this small movement. Emerson spoke at the Chicago centennial symposium, and pre¬ sented his multi-level view in both content and title: “The evolution of adap¬ tation in population systems.” Emerson begins by acknowledging the conventional Darwinian preference for individual organisms (and reminding us that he will not neglect this usual argument). But he then stakes his higher claim: “It is my intention in this es¬ say to emphasize the evolution of adaptation in population systems without, however, negating the data or the major interpretations of the roles of indi¬ viduals in evolutionary history or processes” (1960, p. 307). I find Emerson’s article frustrating, for his arguments are so reasonable in some places, and so very wrong, to the point of illogic, in others. On the one hand, he presents a defendable and properly philosophical criterion for
The Modern Synthesis as a Limited Consensus higher-level selection based on features of populations that cannot be expli¬ cated as additive results of organismal properties—in other words, “emer¬ gent” characters. He correctly defines a population ripe for selection at its own level as “an inclusive entity with emergent characteristics that tran¬ scend the summation of the attributes of the component individuals” (1960, p. 307). But, having legitimately defined the problem, he then launches into an almost rhapsodic, and simply illogical, claim that almost anything with defin¬ able boundaries can be recognized as a unit of natural selection: “Natural selection operates at each level of integration from the gene and complex polygenic characters within the individual, to the whole individual, and to various levels of intraspecific population systems and interspecific interadapted community systems and ecosystems” (1960, p. 340). This argument can be defended in theory so long as the higher unit oper¬ ates as an interactor with surrounding environments and remains in a genea¬ logical nexus engaged in differential reproduction (see Chapter 8). But how can Darwinian selection possibly operate directly on an ecosystem? However we may choose to define such an entity, ecosystems do not mate and do not produce children (see Chapter 8, pp. 597-613 on criteria of Darwinian indi¬ viduality). No argument can be made about their differential reproductive success, and no Darwinian calculus can therefore be applied to their history through time. Emerson doesn’t seem to grasp that selection works by differential repro¬ ductive success, not by design for immediate, self-serving utility: “It would be extremely difficult,” he writes (1960, p. 319), “to explain the evolution of the uterus and mammary glands in mammals or the nest-building instincts of birds as the result of natural selection of the fittest individual.” But if milkrich mammary glands promote the survival of offspring, then the mother acts in her own Darwinian interest. In short, Emerson’s paper gives us an unin¬ tended insight into the confusing lack of definition that natural selection has always suffered, even at the moment of its greatest explicit influence. Second—and more important in its virtual ubiquity—leading evolutionists, though well aware that orthodoxy identified individual organisms as the fo¬ cus of selection, did not grasp the logical necessity or centrality of such a claim in Darwinian theory, and therefore often indulged in vague, perhaps unconscious, and often fuzzy, statements about the efficacy of higher levels. (I say “fuzzy” because most of these claims about populations and groups only invoked the non-emergent effects of organismic characters—and therefore do not necessarily qualify as valid statements about higher-level selection. I don’t think that many evolutionists had properly formulated this crucial issue at the time.) Dobzhansky, for example (1957, p. 392), states that selection operates on organisms, but then proposes that such phenomena as heterozygote advan¬ tage might record some populational “extra” in exposing the reduced fitness of homozygotes as a kind of organismic sacrifice “for” the group: “Natural selection operates through differential survival and differential fertility of in-
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THE STRUCTURE OF EVOLUTIONARY THEORY dividuals, and yet at some times brings about such forms of integration of the gene pool of the population which lead to the sacrifice of some of the indi¬ vidual members of the population. The phenomenon of balanced polymor¬ phism, with highly fit heterozygotes contrasting with less fit homozygotes, is one of such forms of genetic integration of Mendelian populations.” Mayr’s most authoritative book (1963) provides an excellent illustration of orgamsmic orthodoxy amidst statements, lacking clear definition, about se¬ lection at higher levels. Mayr surely recognizes the usual form of proper Dar¬ winian argument—that apparent benefits to populations should be ex¬ plained, whenever possible, as effects of selection upon organisms: “The solution usually proposed for the difficulty raised by the conflict between a benefit for the individual and one for the population is to make the popula¬ tion rather than the individual the unit of selection. ... It would seem prefera¬ ble to search for solutions based on the selective advantage of individual ge¬ notypes, such as Fisher’s explanation of an even sex ratio” (1963, pp. 198— 199). In rereading Mayr’s 1963 book with the hindsight of thirty years, however, I was struck by the number of passages and arguments that either speak loosely about explicit advantages to groups and populations (rather than for¬ tuitous beneficial effects arising as side consequences of selection on organ¬ isms), or seem to state an explicit claim for selection at the population level. Most of these statements focus on the virtues of genetic variability. Mayr asks (1963, p. 158): “Why are not all individuals of a population identical in ap¬ pearance? Is it because diversity is of selective advantage to the population?” He then argues (1963, p. 308) that the primary function of chromosomal variation lies in the flexibility thus accorded to populations: “They appear to have, as primary function, either the increase of adaptability and adaptedness of these populations through balanced polymorphism of entire chromosome sections or the regulation of the amount of recombination.” In fact, Mayr’s mam justification for regarding polymorphism as adaptive—a major shift in his own adaptationist hardening from the examples used to support non¬ selectionist claims in his 1942 book—focuses on advantages to populations (1963, p. 251). Polymorphism is based on and produced by definite genetic mechanisms, such as genes for differential niche selection and the heterosis of hetero¬ zygotes. A population that has not responded to selection for such mech¬ anisms and therefore lacks polymorphic diversity is more narrowly adapted, more specialized, and therefore more vulnerable to extermina¬ tion. The widespread occurrence of genetic mechanisms that produce and maintain polymorphism is directly due to selection and is in itself a component of adaptiveness. It seems appropriate, therefore, to speak of “adaptive polymorphism.” A rally around the flag of orgamsmic selection, and an explicit (and vocif¬ erous) denial of higher levels, became a major movement in evolutionary the¬ ory during the 1960’s. The hardening of adaptationism had occurred largely
The Modern Synthesis as a Limited Consensus during the 1940’s and 1950’s (in time for the Darwinian centennial of 1959); but the refinement of adaptationist arguments to nearly exclusive operation at the organismic level followed later. This reform * emerged largely within the held of animal behavior, where the ethological tradition, particularly in the work of Konrad Lorenz, had long promulgated a loose and largely uncon¬ sidered approach to multilevel selection. The primary impetus to explicit debate appeared with the publication, in 1962, of V. C. Wynne-Edwards’s Animal Dispersion in Relation to Social Behavior. Since most evolutionists now regard Wynne-Edwards’s primary ar¬ gument as wrong (and I do not dispute this consensus), we greatly undervalue his work and misconstrue its importance. Most evolutionists today, many of whom have never read Wynne-Edwards, know his book only by reputation as a dumb argument for group selection that George Williams and others thoroughly demolished. I regard this assessment as entirely unfair. WynneEdwards’s claim for group selection may be wrong, but I can cite few other theories, presented within evolutionary biology during my career, that could be deemed so challenging in implication, so comprehensive in claims, so fasci¬ nating in extension, and so thought-provoking. First of all, and essential for grasping the book’s sweep, “Animal Disper¬ sion” presents a theory about organic self-regulation of population numbers, not primarily an argument for group selection in general (although group se¬ lection serves as a fundamental feature of the proposed mechanism). WynneEdwards begins, as so many others have done (including Darwin), with an analogy to human institutions. When predators show no restraint in the midst of plenty, ecosystems may crumble as both predators and prey suc¬ cumb. He speaks with feeling about the collapse of Arctic whaling (1962, p. 5): “The stocks of the two right whales have never recovered, and the pop¬ ulation of Greenland-whaling men and of those who ministered to them has become effectively extinct.” Wynne-Edwards then generalizes from carnivory to food-based limitation in any kind of eating, and to the transcendent need for regulating population sizes of consumers in any ecosystem. I am fascinated that Wynne-Edwards, in this statement, invokes the same, largely metaphysical, argument that Darwin proposed in specifying a summum bonum for the construction of nature (a situation established by very different mechanisms in Wynne-Edwards’s and Darwin’s systems): the old principle of plenitude, or maximization of the kinds and numbers of organisms in any given segment of earthly real estate (see p. 229 for Darwin’s version):
"'Although I strongly advocate a hierarchical model of multilevel selection (see Chapters 8 and 9), I regard this restriction to organismic selection as an important and positive re¬ form. Earlier claims for group and higher level selection had been formulated so vaguely and falsely that they impeded our understanding of both this important concept and of the theory of selection in general. This salutary reform tore down erroneous standards and in¬ sisted that no further claims be made until the logical edifice could be properly rebuilt—no examples without a proper substructure; no paintings without a strong frame.
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THE STRUCTURE OF EVOLUTIONARY THEORY The need for restraint in the midst of plenty, as it turns out, must apply to all animals whose numbers are ultimately limited by food whether they are predators in the ordinary sense of the word or not. . . . Where we can still find nature undisturbed by human interference, whether un*
der some stable climax of vegetation or proceeding through a natural succession, there is generally no indication whatever that the habitat is run down or destructively overtaxed. On the contrary the whole trend of ecological evolution seems to be in the very opposite direction, leading towards the highest state of productivity that can possibly be built up within the limitations set by the inorganic environment. Judging by ap¬ pearances, chronic over-exploitation and mass poverty intrude them¬ selves on a mutually-balanced and thriving natural world only as a kind of adventitious disease, almost certain to be swiftly suppressed by natu¬ ral selection. It is easy to appreciate that if each species maintains an optimum population-density on its own account, not only will it be pro¬ viding the most favorable conditions for its own survival, but it will au¬ tomatically offer the best possible living to species higher up the chain that depend on it in turn for food. Such pnma facie argument leads to the conclusion that it must be highly advantageous to survival, and thus strongly favored by selection, for animal species (1) to control their own population-densities, and (2) to keep them as near as possible to the opti¬ mum level for each habitat they occupy (1962, pp. 8-9). In Darwinism, this regulation proceeds by a fundamentally Malthusian method—imposed from outside by a hecatomb on populations that outstrip resources. Wynne-Edwards holds that such an indirect and inefficient mode of external imposition wreaks havoc upon ecosystems, and that existing sta¬ bilities therefore imply the operation of an entirely different system for regu¬ lating populations—internally, by complex sets of behaviors that limit re¬ production and match population sizes to appropriate resources. Since the Darwinian imperative leads organisms to maximize their own reproductive success, such internal limitation can only be achieved by mechanisms of group selection powerful enough to counteract the personal gains of individ¬ ual organisms from conventional Darwinian selection. The ingenuity of Wynne-Edwards’s theory lies largely in the range of be¬ havioral phenomena that he interprets as devices evolved by group selection for limitation of population size. In fact, Wynne-Edwards ascribes the origin of social organization itself to this need for limitation upon the size of popu¬ lations. Note that by “conventional competition” he does not mean the ver¬ nacular “orthodox” or “ordinary” (which would then become Darwinian, or the opposite of his personal intention), but rather apparent competition by bluff, ritual and display—convention in this sense—rather than actual (and potentially destructive) fighting: Undisguised contest for food inevitably leads in the end to overexploi¬ tation, so that a conventional goal for competition has to be evolved in its stead; and it is precisely in this—surprising though it may appear at first sight—that social organization and the primitive seeds of all so-
The Modern Synthesis as a Limited Consensus
cial behavior have their origin . . . Putting the situation the other way around, a society can be defined for our purposes as an organization ca¬ pable of providing conventional competition: this, at least, appears to be its original, most primitive function, which indeed survives more or less thinly veiled even in the civilized societies of man (1962, p. 14). Almost all the rich repertoire of putative Darwinian behaviors become, for Wynne-Edwards, devices evolved by groups of organisms to limit their popu¬ lation size. Dominance hierarchies and pecking orders become group-selected controls, exercised by denying reproductive rights to large numbers of poten¬ tial breeders. The chorusing of frogs, insects and birds become censusing de¬ vices, whereby populations may judge their numbers and trigger appropriate behaviors of regulation. Such homeostatic adaptations exist in astonishing profusion and diver¬ sity, above all in the two great phyla of arthropods and vertebrates. There we shall find machinery for regulating the reproductive output or recruitment rate of the population in a dozen different ways—by varying the quota of breeders, the number of eggs, the resorption of embryos, survival of the newborn and so on; for accelerating or retarding growthrate and maturity; for limiting the density of colonization or settlement of the habitat; for ejecting surplus members of the population, and even for encompassing their deaths in some cases in order to retrieve the cor¬ rect balance between population-density and resources (1962, p. 9). Such massive suppression of the Darwinian game could only be achieved by group selection—that is, by the differential success of groups with emer¬ gent social behaviors that debar reproduction for many members, thus limit¬ ing population size from within, and winning temporal persistence by avoid¬ ing collapse through overexploitation: “We have met already with a number of situations—and shall later meet many more—in which the interests of the individual are actually submerged or subordinated to those of the community as a whole” (1962, p. 18). Wynne-Edwards surely understood the stringent requirements for such a mechanism. He recognized, for example, that demes or social groups must be persistent and genealogically exclusive in order to act as higher-level “individ¬ uals” in a selective process—as in this epitome of his views on group selection (1962, p. 144). To understand group-selection we ought first to recognize that normally local populations are largely of common descent, self-perpetuating and potentially immortal. They are the smallest subdivisions of the species of which this is true, and can be adapted to safeguard their own future. What is actually passed from parent to offspring is the mechanism for re¬ sponding correctly in the interests of the group in a wide range of cir¬ cumstances. What is at stake is whether the group itself can survive or will become extinct. If its social adaptations prove inadequate, the stock will decline or disappear and its ground be colonized by neighboring
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THE STRUCTURE OF EVOLUTIONARY THEORY stocks with more successful systems: it must be by this process that group-characters slowly evolve. The abstract logic of this argument cannot be faulted, but we must ask if the required conditions are encountered frequently enough in nature. Do so¬ cial groups remain sufficiently exclusive; is group selection strong enough to overcome Darwinian organismic selection; do social behaviors originate by organismal or group selection? Nearly all evolutionists would now agree that groups rarely maintain the required cohesion, and that group selection (in Wynne-Edwards’s mode) will usually be far too weak a force to prevail over the conventional Darwinian mechanism of organismic selection. George Williams’s brilliant book, Adaptation and Natural Selection (1966), provided the historical focus for general rejection of WynneEdwards’s theory. Williams wrote with other sources and targets in mind as well. (He told me, for example, that he had originally been most strongly mo¬ tivated by the false arguments of Allee, Emerson and the Chicago school.) But Wynne-Edwards stood out as the main group-selectionist game in town when Williams wrote his book. Williams’s book won deserved influence by its incisiveness of logic and ar¬ gument, and for its persuasive style of composition. (I know no better exam¬ ple of a work that prevailed primarily by the entirely honorable sense of the unfairly maligned word “rhetoric,” properly defined as “the effective use of language.”) Williams begins by characterizing adaptation as an “onerous” concept that should be invoked only when all simpler explanation fails. We should then become all the more impressed when we find that we need to in¬ voke adaptation so often! Having established one “tough” criterion by per¬ mitting the invocation of adaptation only when all else fails, Williams then proposes another—this time governing the advocacy of levels higher than the conventional Darwinian focus upon organisms. In short, Williams states, don’t make a claim about higher levels unless both logic and empirics permit no other alternative. Adaptation is onerous enough in any case; if we must call upon such a mechanism, we should do so at the lowest possible level of the genealogical hierarchy. This appeal to some form of parsimony or reduc¬ tion leads Williams to reject all claims for group selection, so long as Darwin¬ ian organismic selection can render the same phenomenon in principle. Wil¬ liams presents his argument largely as a theoretical proposition, and only rarely as an empirical claim. If the phenomenology of a situation can be ren¬ dered by an organismic interpretation, he asserts, one should then advocate this lower level of causality—even if a group selectionist scenario violates no tenet of logic or plausibility. In his introductory pages, Williams tells us that claims for group selection have inspired his attempt at cleansing and simplification: Even among those who have expressed the opinion that selection is the sole creative force in evolution, there are some inconsistent uses of the concept. With some minor qualifications to be discussed later, it can be said that there is no escape from the conclusion that natural selection . . .
The Modern Synthesis as a Limited Consensus can only produce adaptations for the genetic survival of individuals. Many biologists have recognized adaptations of a higher than individual level of organization. A few workers . . . postulate that selection at the level of alternative populations must also be an important source of ad¬ aptation, and that such selection must be recognized to account for ad¬ aptations that work for the benefit of groups instead of individuals. I will argue . . . that the recognition of mechanisms for group benefit is based on misinterpretation, and that the higher levels of selection are impotent and not an appreciable factor in the production and maintenance of ad¬ aptation (1966, pp. 7-8). This statement includes several interesting features, suggesting useful definitions and frameworks that I will follow throughout this book (while of¬ ten disagreeing with Williams’s own conclusions): (1) The term natural selec¬ tion shall refer only to Darwin’s process at the organismic level; selection at higher levels requires a different name. (2) The logic of group (and higher level) selection cannot be denied; we may only reject the process as impotent relative to natural selection, not as inconceivable. (3) The criterion advanced by group selectionists—the existence of properties that work for the benefit of groups, but at the expense of individual organisms—may be sound in the¬ ory but inapplicable in fact, for virtually all proposed cases either have been misinterpreted or remain subject to recasting in terms of advantages for or¬ ganisms alone. Williams then states his “doctrine,” frankly so designated: “The ground rule—or perhaps doctrine would be a better term—is that adaptation is a spe¬ cial and onerous concept that should be used only where it is really necessary. When it must be recognized, it should be attributed to no higher a level of or¬ ganization than is demanded by the evidence. In explaining adaptation, one should assume the adequacy of the simplest form of natural selection, that of alternative alleles in Mendelian populations, unless the evidence clearly shows that this theory does not suffice” (1966, pp. 4-5). Williams’s doctrine then serves as a hammer against group selection. This higher level process poses no problem in theory, for “there can be no sane doubt about the reality of the process. Rational criticism must center on the importance of the process and on its adequacy in explaining the phenomena attributed to it” (1966, p. 109). But group adaptation is both methodologi¬ cally onerous (more so than Darwinian adaptation, which is onerous enough already), and theoretically impotent (though potentially operative). If there are many adaptations of obvious group benefit which cannot be explained on the basis of genic selection, it must be conceded that group selection has been operative and important. If there are no such adap¬ tations, we must conclude that group selection has not been impor¬ tant, and that only genic selection—natural selection in its most austere form—need be recognized as the creative force in evolution. We must al¬ ways bear in mind that group selection and biotic adaptation are more onerous principles than genic selection and organic adaptation. They
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THE STRUCTURE OF EVOEUTIONARY THEORY should only be invoked when the simpler explanation is clearly inade¬ quate (1966, pp. 123-124). (Note that Williams here introduces the ultimate causal reduction to genes as units of selection. He speaks of adaptation at the organismic level—but only as the consequence of genic selection. Thus Williams’s book also becomes the manifesto for the ultimate—and, I think, erroneous—Darwinian reductionism still popular today as “selfish gene” thinking in such fields as socio¬ biology and evolutionary psychology. See Chapter 8 for a critique.) In closing, Williams waxes messianic in his pointed comparison of natural selection with the teachings of Jesus (see John 8:12 and 14:6): “Perhaps to¬ day’s theory of natural selection, which is essentially that provided more than 30 years ago by Fisher, Haldane, and Wright, is somewhat like Dalton’s atomic theory. It may not, in any absolute or permanent sense, represent the truth, but I am convinced that it is the light and the way” (1966, p. 273). I love Williams’s book; his austere and incisive argument shaped my think¬ ing and that of all evolutionists in my generation. But Williams’s central thesis includes a disabling problem in logic, one that produced unfortunate effects in evolutionary practice. Parsimony, or Occam’s razor, embodies an impor¬ tant logical principle when properly applied. William of Occam, a 14th cen¬ tury English philosopher and divine (a Franciscan), strongly espoused nomi¬ nalism against the Platonic concept of ideal types as entities in a realm higher than material existence. (For nominalists, our designations of general catego¬ ries only have standing as names [nomina] based on abstraction from objects in the material world, not as ideal and “excess” archetypes in a non-material realm.) Occam devised his famous motto, “non sunt multiplicanda entia praeter necessitatem” (entities are not to be multiplied beyond necessity), as a weapon in this philosophical battle—an argument against the existence of an ideal Platonic realm (for nominalists regard names of categories only as men¬ tal abstractions from material objects, and not as descriptions of higher reali¬ ties, requiring an additional set of unobserved ideal entities, or essences). Occam’s razor, in its legitimate application, therefore operates as a logical principle about the complexity of argument, not as an empirical claim that nature must be maximally simple. Williams’s key invocation of parsimony— to reject group selection when an explanation based on organismic selection can be devised for the same results—fails as a general argument, and does not use Occam’s razor in a valid manner, on two grounds: 1. Whereas Occam’s razor holds that we should not impose complexities upon nature from non-empirical sources of human argument, the factual phe¬ nomena of nature need not be maximally simple—and the Razor does not ad¬ dress this completely different issue at all. The Famarckian one-step route to adaptation, for example, operates more simply and directly than the Darwin¬ ian two-step process of variation and selection. But nature happens to follow Darwin’s path. Similarly, the simultaneous operation of several hierarchical levels in selection may represent a more complex system than the idea that se-
The Modern Synthesis as a Limited Consensus
lection works only on organisms. But nature may (and does) work in this hierachical manner. 2. We should recognize Williams’s claim as a statement about reductionism, not (as he thought) as an invocation of parsimony. Organismic selec¬ tion is not intrinsically “simpler” than group or species selection. (One could only call organismic selection “simpler” in the obviously invalid psychologi¬ cal sense of affirming our habits and legacies as Darwin’s intellectual chil¬ dren.) Consider Williams’s argument: “Various levels of adaptive organiza¬ tion, from the subcellular to the biospheric, might conceivably be recognized, but the principle of parsimony demands that we recognize adaptation at the level necessitated by the facts and no higher. It is my position that adaptation need almost never be recognized at any level above that of a pair of parents and associated offspring” (1966, p. 19). Lower levels in a hierarchy cannot be deemed inherently simpler, either to conceive or to operationalize, than higher levels. If we had been brought up in an intellectual world that emphasized populations, rather than organisms, as primary entities, we would probably regard interdemic selection as maxi¬ mally simple, and organismic selection as an unwelcome complication. A pri¬ ori preference for lower levels represents a claim for reductionism, not parsi¬ mony. I do not say that such a preference therefore becomes invalid; I simply ask evolutionists to recognize the proper status of Williams’s claim as an ar¬ gument about reductionism—and also to acknowledge that reductionism, as a cultural prejudice, may be far harder to defend than true parsimony, when properly invoked as a logical principle (though aspects of our preferences for parsimony may rank as cultural prejudice as well)/1' In Western science, which developed with such strong traditions for expla¬ nation by analytic division into constituent parts, claims for reduction have often been mistakenly advanced in the name of parsimony—most notably in biophilosopher C. Lloyd Morgan’s early 20th century dictum that no human activity should be explained by a higher psychological faculty when a lower faculty suffices. This inappropriate invocation of parsimony did not disable Williams’s ar¬ gument because he usually proceeded beyond this theoretical point. That is, Williams extended his argument further by presenting direct evidence favor¬ ing the organismic mode in particular cases. He wrote: “This conclusion sel¬ dom has to rest on appeals to parsimony alone, but is usually supported by specific evidence” (1966, p. 19). But subsequent developments force us to
"‘Footnote added in proof stage: Just as I submitted this completed book to the publisher, the press conference on Darwin’s birthday (Feb. 12, 2001), announcing the very low num¬ ber of genes in the human genome, struck the deepest blow of our lifetimes against the conventions of reductionism, and for the irreducibility of proteomic (and full phenotypic) explanation to simple properties of codes at lower levels. Combinations, replete with emer¬ gent properties, and the specifics of contingent phyletic histories, must become a key part¬ ner, if not a primary locus, for biological explanation (see Gould, 2001).
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THE STRUCTURE OF EVOLUTIONARY THEORY consider one of the most troubling phenomena in the sociology of science— the principle of epigones and bandwagons. Williams himself did not abuse, rigidify, misconstrue, or unduly simplify his criteria—but his followers did, both early and often (to cite the classical principle for voting in Boston local elections), as Williams’s “doctrine” be¬ came a dogma among his epigones. Few aspects of academic life can be more distressing and ironic than the common observation that a fine scholar often becomes a victim of his own success in this manner—but subtle po¬ sitions can be trivialized to sound bites in science as well as in political culture. “Genic or organismic selection only” became the bandwagon slogan of the late 60’s and 70’s. Combined with a strong preference, already established as the Synthesis developed, for hardline adaptationism in general (see previous section), this restriction set a predisposition strong and exclusive enough to be labeled as a dogma: interpret all substantial phenotypic characters as ad¬ aptations built by natural selection in the organismic mode (or lower). This dictum did not always function as a cleansing wind in a former stable, but all too often as a narrow and misdirected tunnel that carried a necessary reform too far. Moreover, many epigones used the dogma as a kind of linguistic game rather than a guide to research: “Can I tell a clever story to render this or that puzzling phenomenon as an organismic, rather than a group, adaptation?” For some evolutionists, the ability to spin such a tale, and to answer such a challenge as a theoretical affirmation, became the goal of a supposedly scien¬ tific effort. I have never witnessed a more distressing bandwagon in science, or seen any idea of such salutary origin pushed so far in the direction of thoughtless orthodoxy. (Pardon a personal incident, but I remember raising a question, early in my career, at a session of the first ICSEB meeting in 1973. I asked a speaker, fol¬ lowing his formal presentation, if the dwarfed size of Pleistocene mammals on Mediterranean islands might have been favored by resistance to extinction afforded by the correlated effect of larger population sizes (than full-bodied hippos and elephants could have maintained in such small places). I hadn’t thought the issue through, and I may well have been making a dumb sugges¬ tion, but the speaker’s response floored me (and stunned me into silence at this ontogenetic stage of early diffidence). He said this and only this—and his words, with their intended dripping irony, still cut through me—“are you re¬ ally satisfied with a group selectionist argument like that?” He made no at¬ tempt to rebut my suggestion with any content whatever; the stigma of group selection sufficed for refutation.) As a final illustration of how reform, once established, can turn into the opposite phenomenon of rigidification, I interviewed Sewall Wright several times during the last decade of his life. He felt hurt by what he interpreted as his exclusion from the Modern Synthesis (beyond the ritualistic invocation of his name within the founding trinity of population genetics). “I was out of it,” he told me. He explained this passage into obscurity as the failure of a new generation of evolutionists to understand either his intended role for ge-
The Modern Synthesis as a Limited Consensus netic drift, or his proposed mode for the operation of drift within his “shift¬ ing balance” theory of evolution. Most evolutionists of the 1960’s viewed genetic drift only as a random force of evolutionary change—a prime anomaly under adaptationist harden¬ ing (or at least a factor relegated to the marginal role of efficacy only in tiny populations at the brink of extinction). Since genetic drift bore such a promi¬ nent association with Wright’s name (extending to its original designation as the “Sewall Wright effect” in early days of the Synthesis), such demotion to marginality relegated the author to a similar fate under hardline adaptationism. This situation is surely unfair enough in itself, but—and now the irony— Wright had also participated in the adaptationist hardening of the Synthesis (see pp. 522-524), and his later interpretation of genetic drift invoked this concept primarily as an aid to an enlarged style of adaptationism, and not as a contrary force in evolutionary change (as he had originally argued). So if Wright had tried to be helpful in the service of orthodoxy, why did he become so misunderstood and relegated to the sidelines? Many reasons might explain Wright’s fall into limbo, including the opaque character of his highly mathematical writing and the fact that he had invoked genetic drift as a nonadaptive force for change in his earlier work (see p. 523). But the major factor almost surely resides in a failure of evolutionists to un¬ derstand the multi-leveled character of his theory—the aspect that allows drift to serve as an input to an adaptive process. Wright told me (in an inter¬ view in 1981) that he originally intended to call his shifting-balance process the “two-level theory,” for the full process relies on essential components of both organismic and mterdemic sorting. (Wright also told me that he re¬ garded “exclusive focus on individual selection” as the major error of the Synthesis.) In Wright’s later formulation of “shifting balance,” drift enters into a cre¬ ative and selective process in the following manner: The founding population of a new species moves by selection to an adjacent adaptive peak in a larger landscape. The chief problem for adaptationism then intrudes: this initial peak will probably not represent the most favored spot in the landscape (for other unoccupied peaks probably stand higher), but how can daughter demes ever move to these better places? Valleys of lesser fitness surround this local peak. If evolution always proceeds towards adaptation, then the initial popu¬ lation must remain stuck on the first peak forever. But drift allows small groups to enter valleys, and to cross troughs into areas where selection may then draw populations up to a higher peak. When expanding populations, by this process, occupy several peaks in the landscape, a process of interdemic (mterpeak) sorting can occur, eventually leading to a mean increase in adap¬ tation for the species as a whole. Thus, genetic drift does not operate as a ran¬ dom force against adaptation in Wright’s mature theory, but as a source of variability for fueling a higher level process of interdemic sorting. In other words, drift operates as part of a process that enhances adaptation through higher-level sorting.
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THE STRUCTURE OF EVOEUTIONARY THEORY This argument seems clear enough in logic (its validity, or relative fre¬ quency, in the world of real populations raises a different issue that can only be resolved empirically). Why was Wright’s theory misunderstood or, even worse, simply ignored? I suggest the following (and unfortunate) primary reason: How could Wright’s argument be grasped by an intellectual commu¬ nity now committed to the exclusivity of organismic selection? The concep¬ tual tools no longer exist under such a stricture: adaptation arises by “strug¬ gle” for reproductive success among genes or organisms; drift causes a population to depart from a place attained by such struggle. How then could drift possibly act as a helpmate to adaptation? To grasp Wright’s view, one must allow for a higher level of interdemic sorting, and one must understand the logic of hierarchical models, with sorting operating at several nested levels. The intellectual space for viewing drift as an aid to interdemic selection doesn’t exist in a context of exclusive commitment to selection at genic or organismic levels. Wright’s idea then gets demoted to a status even lower than “merely wrong” in such a world; “shifting balance,” in Wright’s own sense, becomes inconceivable, and therefore intellectually inaccessible. Erro¬ neous ideas can at least be expressed and made available to others with po¬ tentially different opinions. But the definitions of orthodoxy simply erased Wright’s multiple-level theory—in much the same way that evolutionary sta¬ sis could not be recognized as interesting, or even grasped as a phenomenon at all, within a community committed to gradualism. When we think an idea through, and then reject the notion, we have at least made an intellectual decision (perhaps wrong, perhaps overly rigid). But when we maintain an unarticulated and unexamined commitment, and then use such a premise, al¬ beit unconsciously, to render interesting ideas inconceivable, then we have fallen under the spell of dogma. Sewall Wright—unlike Schubert, Wegener and a host of historical figures deemed tragic—lived long enough to witness his vindication (and to partici¬ pate mightily in his renewed respect by writing a four-volume mathemati¬ cal treatise, largely during his eighties—see Wright, 1978). But his period of unjustified eclipse should warn us all about the dangers of bandwagons and unexamined commitments.
EXTRAPOLATION INTO GEOLOGICAL TIME
A good flavor of the confidence, even the dogmatism, of the hardened synthe¬ sis, as presented at the Darwinian centennial celebrations of 1959 (see pp. 569-576), shines forth in Mayr’s introductory proclamation from his 1963 book. Mayr pronounces the “complete unanimity” of competent professional opinion, the “colossal ignorance” of the “few dissenters,” and the conse¬ quent “waste of time” involved in any refutation of the intellectual stragglers: When we reread the volumes published in 1909, on the occasion of the 50th anniversary of the Origin of Species, we realize how little agree-
The Modern Synthesis as a Limited Consensus ment there was at that time among the evolutionists. The change since then has been startling. Symposia and conferences were held all over the world in 1959 in honor of the Darwin centennial, and were attended by all the leading students of evolution. If we read the volumes result¬ ing from these meetings ... we are almost startled at the complete una¬ nimity in the interpretation of evolution presented by the participants. Nothing could show more clearly how internally consistent and firmly established the synthetic theory is. The few dissenters, the few who still operate with Lamarckian and finalistic concepts display such colossal ig¬ norance of the principles of genetics and of the entire modern literature that it would be a waste of time to refute them. The essentials of the modern theory are to such an extent consistent with the facts of genetics, systematics, and paleontology that one can hardly question their correct¬ ness (1963, p. 8). Later on, Mayr proposes a succinct definition of the Synthesis, emphasiz¬ ing all three legs (or branches) of the essential tripod (or tree) of Darwinian logic “The proponents of the synthetic theory maintain that all evolution is due to the accumulation of small genetic changes, guided by natural selec¬ tion, and that transpecific evolution is nothing but an extrapolation and magnification of the events that take place within populations and species” (1963, p. 586). I regard Mayr’s balance of emphases as particularly revealing. His defini¬ tion includes two phrases. The first statement buttresses the two legs of Dar¬ win’s tripod that receive most attention in this work—control of direction by external selection rather than internal constraint (with attendant gradualism of change), and operation of the process through differential reproductive success of organisms (implicit in the term “natural,” as opposed to some other form or level of selection). But the second, and longer, statement affirms Mayr’s appreciation for the importance of the third leg—the complete suf¬ ficiency of microevolutionary theory to explain the entire history of life—so long as the earth’s geological behavior sets a proper stage, and does not derail a full extrapolation of microevolutionary mechanics into all geological time and to the entire extent of phylogenetic change. For how can we celebrate the power and generality of a beautiful, sufficient and completely validated mechanism of change, established for the immediacy of an ecological here and now, if the same processes cannot render the broad pattern of life’s his¬ tory as well? This theme of extrapolation becomes, in many ways, the most comprehen¬ sive issue of all. In Chapter 2, I distinguished two separate aspects of Dar¬ win’s radicalism in proposing the theory of natural selection—a methodologi¬ cal pole to grant operational status to the study of evolution by asserting that observable events, however apparently trivial and inconsequential, can ex¬ plain change at all scales by extension; and an ideological pole to present a radical mechanism of evolutionary change for rendering all “higher" attri¬ butes of good design and organic harmony as side-consequences of a process
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THE STRUCTURE OF EVOLUTIONARY THEORY working only by a> struggle among organisms for personal reproductive suc¬ cess. Strict Darwinians must defend extrapolation as crucial at both poles— for observable events at small scale cannot generate the full panoply of phylogeny without such a principle; while d&ily happenings cannot accumulate into totalities if the background setting does not “behave” properly—if, for example, the ecological stage explodes every once in a geological while, fortu¬ itously dumping most of the improved and accumulated inhabitants into a vat of extinction. For these two reasons—to validate the entire methodological pole and to uphold the third leg of the Darwinian tripod supporting the ideological pole—the principle of extrapolation represents the key to the validity of the Synthesis as a fully general theory of evolution. And extrapolation—-the es¬ sential Lyellian postulate that Darwin imbibed from his most important men¬ tor (see p. 94)—embodies two central aspects of what Lyell and his school called “uniformitarianism”: (i) the complete theoretical sufficiency of cur¬ rently acting small-scale changes to produce, by successive and imperceptible increments, the entire panoply of large-scale phenomena; and (ii) the proper “behavior” of the earth, with geological change sufficiently slow and steady that trends produced by gradualistic, accumulative natural selection will not be derailed often enough to yield a history of life patterned more by these geological upsets than by biological accumulations. For this last leg of the tripod, the Synthesis did not so much harden as be¬ come emboldened during its ontogeny. We have seen (pp. 514-518) how Haldane and Huxley, in the early days of the Synthesis, still respected (even feared) apparent paleontological exceptions to natural selection as the cause of trends. But the balance of power had shifted by the 1960’s. Simpson (1944, 1953) and others had forged their “consistency argument”—the principle that all known phenomena of the fossil record can, in principle, be explained by modern mechanisms of genetics and selection (even though no direct proof of sufficiency can be derived from paleontological evidence). Paleontology had been tamed, taken in by the synthesis, and told to behave. (And I do in¬ tend “taken in” in the metaphorical sense as well, as I shall argue more ex¬ plicitly in Chapter 9.) Paleontology could retain the archives of actual phe¬ nomenology as its particular bailiwick in exchange for giving up the conceit of believing that the fossil record could say anything distinctive about the causes of evolutionary change. The distinguished panel on “the evolution of life” at the Chicago cen¬ tennial celebration of 1959 included Julian Huxley as chairman, Th. Dobzhansky, E. B. Ford, Ernst Mayr, Ledyard Stebbins, and Sewall Wright. After an orthodox discussion of mechanisms, Huxley shifted the topic to “the course, the process, of evolution as shown in the fossils” (Tax and Callender, 1960, volume 3, p. 127)—and paleobotanist D. I. Axelrod rose to present a summary in advance, characterizing the history of life as a stately process of unfolding to more and better: “I think most of us are in full agreement about the gradual change in time: increasing diversification; then, gradual transfor-
The Modern Synthesis as a Limited Consensus mation, so new categories gradually arise, first at smaller and then at higher levels” (in Tax and Callender, 1960, volume 3, p. 127). In the panel’s stated agenda of 16 points, only one even hints at nonadaptive phenomena, and only as an adjunct to selectionist orthodoxy. Huxley addressed this topic midway through the discussion: “Natural selec¬ tion may lead to side effects, which at the same time are of no adaptive value but may later provide the basis for adaptive changes” (Tax and Callender, 1960, volume 3, p. 125). Vertebrate paleontologist E. C. Olson, the sympo¬ sium’s lone (and very gentle) doubter (see pp. 574-576), ventured a percep¬ tive comment on this point. But his words, as Moses said of Pharaoh’s chari¬ ots, promptly “sank as lead in the mighty waters.” The entire discussion of this topic occupies less than a page. Olson said: “This is the general area in which we can include events that are random with respect to the adaptive value of the genotype of populations. I refer to the simple matter of acci¬ dent—for example, the effects of a forest fire on a population . . . This sort of side effect, the impact of accidents and other factors producing non-adaptive shifts, may cause very rapid changes and give completely new shape to the course of evolution. I think this is an extremely important evolutionary fac¬ tor” (in Tax and Callender, 1960, volume 3, p. 125). If the Synthesis viewed the entire history of life, the full tree itself, as grow¬ ing by an adaptive and stately unfolding, then the history of single branches —trends in lineages, the primary topic of macroevolutionary study—also re¬ ceived a thoroughly adaptationist reading in the extrapolationist mode. At the same Chicago conference, Simpson defended the adaptationist postulate for all geometries, parallel as well as diverging lineages, and even (in principle and without direct evidence) for “erratic” features where selective control “is not apparent.” The selectionist theory is that a trend is adaptive for the lineage involved, that it continues only as long as it is adaptive, that it stops when adapta¬ tion is as complete as selection can make it in given circumstances, and that it changes or the group becomes extinct if a different direction of evolution becomes adaptive. Often the adaptive nature of a trend seems apparent. Often it is not apparent, but the postulate seems required to account for otherwise erratic features of trends. In instances of parallel evolution the selectionist theory is that changes actually occurring in parallel are adaptive over the whole ecological range occupied by the group, while those divergent (radiating) within the group are adapta¬ tions to special niches within that range (Simpson, 1960, pp. 170-171). Under these precepts, a procedure of building scenarios in the strictly adap¬ tationist mode, based on assumption and conjecture, often passed for ade¬ quate explanation. The second half of the Chicago panel on the “evolution of life,” supposedly dedicated to the actual record of evolutionary change, in¬ cluded almost no discussion about paleontology, and relied on theoretical inferences about the past based on knowledge of modern organisms. Ernst
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THE STRUCTURE OF EVOLUTIONARY THEORY Mayr, however, did offer the following conjecture—wrong in many details (as we now know), yet firm in its confident adaptationist scenario—for the evo¬ lution of lungs. (Devonian fishes already possessed lungs, for the trait is symplesiomorphic in tetrapods and their aquatic ancestors, with the swim bladder of later fishes as its derived homolog. But note Mayr’s confidence in his erroneous conjecture for the easy construction of such a novelty—from scratch, gradually, and in pure adaptive continuity with unchanging func¬ tion): I think the development of lungs is now pretty well understood. Certain fishes during the Devonian period lived in stagnant, fresh water swamps, where oxygen was so scant that respiration through the skin and the gills no longer provided the necessary oxygen. Apparently they came to the surface and gulped air, from which the membranes of the digestive tract took up oxygen. When that stage was reached, there was a tremendous selection pressure for developing diverticles and enlarging this respira¬ tory surface of the digestive tract. As soon as the necessary gene combi¬ nation providing such diverticles appeared, selection pressure could push this tendency further and further, and this led quite naturally to the de¬ velopment of lungs (in Tax, 1960, volume 3, p. 136). As documented in Chapter 6, the putative domination of biotic over abiotic competition provided Darwin with a rationale for defending general progress in the history of life. The synthesists upheld this orthodoxy as well, thereby imparting broad predictability to the stately unfolding of life. Huxley offered a clear assessment of relative frequencies in the founding document (1942, p. 495): “Sometimes the inorganic environment changes markedly, as when there is a climatic revolution, such as occurred at the end of the Creta¬ ceous; but in general it is the organic environment which shows the more rapid and important alterations.” In his concluding address to the entire Chicago symposium, Huxley then remarked, with the expanded scope and surer resolve of nearly two decades in hardening: “Improved organization gives biological advantage. Accord¬ ingly, the new type becomes a successful or dominant group. It spreads and multiplies and differentiates into a multiplicity of branches. This new biologi¬ cal success is usually achieved at the biological expense of the older dominant group from which it sprang or whose place it had usurped. Thus, the rise of the placental mammals was correlated with the decline of the terrestrial rep¬ tiles, and the birds replaced the pterosaurs as dominant in the air” (1960, p. 250). In citing the canonical example of dinosaurs and mammals, Huxley ex¬ poses the heart of the extrapolationist error—the assumption that large-scale pattern can be inferred by extending, through immense time, the small effects of observable processes (in this case the supposed general and overall “superi¬ ority'1 of mammals over reptiles in most cases of immediate competition). In explaining trends, the greatest threat to this orthodoxy lies in occasional but profound environmental catastrophe that disrupts and resets the pattern ac-
The Modern Synthesis as a Limited Consensus cumulating during “normal” times. The theory and factuality of catastrophic mass extinction has now broken this orthodoxy (see Chapter 12), but sim¬ ple knowledge of mass extinction had always posed a threat, especially in Cuvier’s original paroxysmal interpretation (see pp. 484-492). The synthesists therefore treated this apparent phenomenon in the conventional and congenial way, either by dismissing a catastrophic cause, or by “spreading out” the time of extinction so that all deaths could be encompassed by tradi¬ tional competitive mechanisms, perhaps enhanced in intensity by rapid envi¬ ronmental changes, and therefore propelling adaptive evolution even more rigorously. Huxley (1942, p. 446), for example, held that tough physical con¬ ditions only accelerated the competitive takeover by superior groups: “The worsening of the climate at the end of the Mesozoic reduced the general adaptiveness of the dinosaurs, pterosaurs, and other reptilian groups, while increasing that of the early mammals and birds.” Ernst Mayr, in his characteristically forthright way, then linked the denial of catastrophic extinction, via uniformitarianism, to the crucial second state¬ ment in his definition of the Synthesis (see p. 557), the requirement for ex¬ trapolation into geological vastness: “Yet it has become clear that there is nothing in the past history of the earth that cannot be interpreted in terms of the processes that are known to occur in the Recent fauna. There is no need to invoke unknown vital forces, mutational avalanches, or cosmic catastro¬ phes. Geographic speciation, adaptation to the available niches (guided by se¬ lection), and competition are largely responsible for the observed phenomena”(1963, p. 617). In short, by viewing trends as adaptive and anagenetic phenomena, pro¬ pelled by competition and building, by a lengthy process of stepwise summa¬ tion, the principal pattern of life’s history, the Synthesis encompassed the most salient phenomenon of paleontology within its favored framework of extrapolation. All causality could reside in the accessible here and now. How then, we must ask, did the Synthesis treat the two phenomena—speciation and extinction—now viewed as crucial in breaking the extrapolationist or¬ thodoxy (for if trends must be expressed as the differential success of some kinds of species vs. others, with most species formed in geological moments, then the adaptive struggles of populations don’t extrapolate smoothly to changes of mean and modal phenotypes within clades)? The developing orthodoxy generally acknowledged speciation and then de¬ moted its importance and distinctiveness. According to Huxley, for example, all radiations should be treated as adaptive and each event of speciation therefore represents an independent, gradualistic expression of an anagenetic trend (1942, p. 487): “The adaptive radiation is seen to be the result of a number of gradual evolutionary trends, each tending to greater specializa¬ tion—in other words to greater adaptive efficiency in various mechanisms subservient to some particular mode of life. . . . Each single adaptive trend also shows the phenomenon of successional speciation.” In a statement that I find charming, however wrongheaded, Nicholson (1960, p. 518, at the Chicago centennial symposium) extolled speciation
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THE STRUCTURE OF EVOLUTIONARY THEORY as a device to provide more opportunities for adaptation to work its “un¬ trammeled” magic: “The splitting of organisms into the genetically isolated groups we call ‘species’ has played a very important part in evolution, for it has permitted selection to proceed untrammeled within each group, so per¬ mitting adaptations of innumerable kinds in the different groups. Had organ¬ isms not divided into genetically isolated groups, the numerous and beautiful adaptations so characteristic of living things could not have evolved, nor could organisms have used the resources of the world in the efficient way they do” (Nicholson, 1960, p. 518). More commonly, however, speciation received short shrift rather than glory. Evolution required such a process of multiplication, of course, lest fa¬ vorable trends disappear through the extinction of single species bearing their fruits. Speciation therefore became a hedge against death by parcelling out, into several iterated lines, a set of adaptations built anagenetically—so that the extinction of one species could not abort the general benefit. The trend itself remains anagenetic (see Fig. 7-2), for speciation does not contribute to the directionality of evolutionary change. (Under later views, including punctuated equilibrium, differential speciation constructs the trend, and anagenetic main trunks do not even exist.) Simpson held strongly to this view, and even ventured a quantitative de¬ fense, in his repeated assertions that speciation represents only a minor mode in evolution because 90 percent of important changes arise anagenetically in the phyletic mode (1944, 1953; Simpson recognized three major modes of change: speciation, phyletic evolution, and quantum evolution). Huxley, in a grand prose flourish, then branded speciation as a pretty little epiphenomenon, a luxurious patina upon the grand pattern of evolution—never realiz-
7-2. Standard view of the role of speciation in evolutionary trends under the Modern Synthesis. Speciation certainly plays an important part in iterating fa¬ vorable variations produced by anagenesis within species. (If this iteration did not occur, lineages would quickly become extinct because individual species must eventually die.) But the trend in morphology arises almost entirely hy anagenetic directionalism within the geological duration of individual species.
The Modern Synthesis as a Limited Consensus ing that the pattern itself might be built by higher-level sorting, operating through the differential success of certain kinds of species! The formation of many geographically isolated and most genetically iso¬ lated species is thus without any bearing upon the main processes of evo¬ lution. . . . Much of the minor systematic diversity to be observed in na¬ ture is irrelevant to the main course of evolution, a mere thrill of variety superimposed upon its broad pattern. We may thus say that, while it is inevitable that life should be divided up into species, and that the broad processes of evolution should operate with species as units of organiza¬ tion, the number thus necessitated is far less than the number which ac¬ tually exist. Species-formation constitutes one aspect of evolution; but a large fraction of it is in a sense an accident, a biological luxury, without bearing upon the major and continuing trends of the evolutionary pro¬ cess (Huxley, 1942, p. 389). Amidst this attempt to relegate the origin of the primary unit of macro¬ evolution to irrelevancy at larger scales, one prominent voice within the Syn¬ thesis spoke up for the centrality of speciation in constructing large-scale pat¬ tern. In a cautious, but prophetic statement, Ernst Mayr (1963, p. 587) wrote: “To state the problems of macroevolution in terms of species and pop¬ ulations as ‘units of evolution’ reveals previously neglected problems and sometimes leads to an emphasis on different aspects.” (Much of macroevo¬ lutionary theory, as developed later, begins with this proposition, and Mayr therefore becomes an inspiration—ironically in a sense, for several key con¬ cepts in this developing body of thought have challenged other aspects of the Synthesis that Mayr so strongly championed. For example, the theory of punctuated equilibrium rests upon a proper translation into geological time of Mayr’s peripatric theory of speciation—see Eldredge and Gould, 1972, and Chapter 9. Directly refuting Huxley’s charge that speciation only ranks as a frill and luxury in the overall pattern of evolutionary change, Mayr wrote: I feel that it is the very process of creating so many species which leads to evolutionary progress. Species, in the sense of evolution, are quite com¬ parable to mutations. They also are a necessity for evolutionary prog¬ ress, even though only one out of many mutations leads to a significant improvement of the genotype. Since each coadapted gene complex has different properties and since these properties are, so to speak, not pre¬ dictable, it requires the creation of a large number of such gene com¬ plexes before one is achieved that will lead to real evolutionary advance. Seen in this light, it appears then that a prodigious multiplication of spe¬ cies is a prerequisite for evolutionary progress. . . . Without speciation, there would be no diversification of the organic world, no adaptive radi¬ ation, and very little evolutionary progress. The species, then, is the key¬ stone of evolution (1963, p. 621).
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THE STRUCTURE OF EVOLUTIONARY THEORY A world of difference separates the negative view held by most synthesists—that speciation merely iterates (and therefore buffers) adaptations pro¬ duced by a different, anagenetic process—from Mayr’s recognition that adap¬ tations may be pieced together through accumulated events of speciation, each chancy in itself and not directed towards the eventual novel phenotype. In this sense, Mayr’s view becomes the root for those branches of modern macroevolutionary theory that treat speciation as a higher-order analog of organismic birth—leading to a concept of trends as the product of differential sorting within the multitude of units thereby produced, and not as the extrap¬ olated result of organismic selection within anagenetic lineages. If most of the synthesists viewed speciation as trivial, they didn’t grant even this modicum of concern to the counterpart process of extinction. Although they acknowledged the death of species (for a process affecting 99 percent of all species that ever lived can’t be entirely ignored), they viewed extinction en¬ tirely in a negative light—as a loss of adaptation, and therefore as a failure in evolution, something to be recognized but not extensively discussed in polite company. Even Ernst Mayr, who understood so clearly how speciation could enter a higher-level process of sorting, didn’t grasp the logical corollary—that any selective process must pair survivals with eliminations, and that “de¬ feats” can therefore teach us as much as “victories.” Instead, Mayr professed puzzlement as to why such a profoundly negative phenomenon should be so common: We find so many cases of extreme sensitivity of natural selection, doing the most incredible and impossible things; and yet the whole pathway of evolution is strewn left and right with the bodies of extinct types. The frequency of extinction is a great puzzle to me (in Tax, 1960, volume 3, P- 141). Natural selection comes up with the right answer so often that one is sometimes tempted to forget its failures. Yet the history of the earth is a history of extinction, and every extinction is in part a defeat for natural selection. . . . Natural selection does not always produce the needed im¬ provements (1960, pp. 375-376). The Synthetic approach to macroevolution can be encapsulated in a few dicta: view life as stately unfolding under adaptive control; depict trends as accumulative and anagenetic within lineages according to the extrapolationist model; downplay or ignore the macroevolutionary calculus of birth and death of species. These propositions leave little role for the actual archives of life’s history—the fossil record—beyond the documentation of change. The causes of change must be ascertained elsewhere, and entirely by neontologists (my profession’s term for the folks who study modern organisms). Thus the Synthesis held paleontology at arm’s length. (I suppose we deserved this deni¬ gration in retaliation for the plethora of poorly-conceived, anti-Darwinian assertions and speculations that so many earlier paleontologists had falsely based upon the fossil record—see Chapter 4. In this sense, our later demo¬ tion, however unfairly extended, became part of the salutary cleansing ac-
The Modern Synthesis as a Limited Consensus complished by the early Synthesis in its first phase of restriction—see pp. 503508.) Huxley (1942, p. 41) spoke of “the illegitimacy of using data on the course of evolution to make assertions as to its mechanism.”* He continued: As admitted by various paleontologists ... a study of the course of evo¬ lution cannot be decisive in regard to the method of evolution. All that paleontology can do in this latter field is to assert that, as regards the type of organisms which it studies, the evolutionary methods suggested by the geneticists and evolutionists shall not contradict its data. For in¬ stance, in face of the gradualness of transformation revealed by paleon¬ tology in sea-urchins or horses it is no good suggesting that large muta¬ tions of the sort envisaged by de Vries shall have played a major part in providing the material for evolutionary change (1942, p. 38). Even so iconoclastic a morphologist as D. Dwight Davis, who would later tweak strict adaptationism so effectively in discussing formal and historical constraints in his classic monograph on the giant panda (Davis, 1964), wrote for the Princeton meeting on genetics, paleontology, and evolution (1949, p. 77): “Paleontology supplies factual data on the actual rates of change in the skeleton and the patterns of phyletic change in the skeleton. Because of the inherent limitations of paleontological data, however, it cannot perceive the factors producing such changes. Attempts to do so merely represent a su¬ perimposition of neobiological concepts on paleontological data.” I admit, of course, that paleontologists have no access to mechanisms re¬ quiring direct observation of ontogeny and ecological interaction. But to say, as Davis does, that we cannot ever derive concepts of evolutionary mecha¬ nisms from paleontological data—and must therefore gain all our causal un¬ derstanding from “neobiology”—seems excessively pessimistic, and consigns paleontology to impotence. If paleontologists cannot gain insights about mechanisms, then historical science of any kind becomes impossible, for all scientific study of the past must make causal inferences from results of pro¬ cesses that cannot be directly observed. Moreover, if historical data hold such limited promise, then the conse¬ quences become even more serious for science in general. For if we acknowl¬ edge that extrapolationism can’t suffice in principle because much of macro¬ evolution proceeds by patterns of differential birth and death among species, and if we cannot generate any theory about such higher-level sorting because we cannot observe the constituent events directly, then much of evolution be¬ comes unknowable in principle. Fortunately, such pessimism may be firmly *But Huxley’s own later contentions belie this strong claim. For example, he argues against uniform internal drives in parallelism, and for control by external selection, by not¬ ing that characters do not always correlate in the same manner within parallel trends ob¬ served in different lineages of fossils: “In all cases where fossils are abundantly preserved over a considerable period, we find the same phenomena. The change of form is very grad¬ ual. It is often along similar lines in related types. And in general it appears that different characters vary independently” (1942, p. 32). But doesn’t this statement qualify as an ex¬ ample of “using data on the course of evolution to make assertions as to its mechanism”?
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THE STRUCTURE OF EVOLUTIONARY THEORY rejected (see Gould, 1986, on Darwin’s use of historical science, and 1989c, on applications to the history of life at greatest scale). Much of the best sci¬ ence—inevitably and properly—relies upon inference and insight, not always upon direct sight. Young scientists can easily succumb to the thrall of such proclamations by >
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leaders. The stupidest passage I ever wrote occurs in the heart of a contribu¬ tion to independent macroevolutionary theory—our original piece on punc¬ tuated equilibrium, where I stated (this excerpt comes from my part of a joint text with Niles Eldredge): First, we must emphasize that mechanisms of speciation can be studied directly only with experimental and field techniques applied to living or¬ ganisms. No theory of evolutionary mechanisms can be generated di¬ rectly from paleontological data. Instead, theories developed by students of the modern biota generate predictions about the course of evolution in time. . . . We can apply and test, but we cannot generate new mecha¬ nisms. If discrepancies are found between paleontological data and the expected patterns, we may be able to identify those aspects of a general theory that need improvement. But we cannot formulate these improve¬ ments ourselves (Eldredge and Gould, 1972, pp. 93-94). Stanley (1975) then properly rebuked us for such unwarranted subservi¬ ence. Our current partnership with “neobiology,” based on the “bonded in¬ dependence” of macroevolutionary theory—the recognition that we can gen¬ erate and test novel concepts but cannot come close to a fully adequate account of macroevolution without the vital input of microevolutionary the¬ ory—produces a better balance of subdisciplines. This mutually sustaining in¬ teraction must benefit paleontology, but such an enlarged view will also aid anyone, in any evolutionary subdiscipline, who wishes to comprehend the “grandeur in this view of life.”
From Overstressed Doubt to Overextended Certainty A TALE OF TWO CENTENNIALS
Darwin did all Americans a mnemonic favor by entering the world on the same day as Abraham Lincoln—February 12, 1809. He also made life sim¬ pler for conference organizers by publishing the Origin of Species in 1859, at age 50—thus intensifying the force of commemorations and cutting their re¬ quired number in half. We have indeed celebrated mightily at the requisite times, with the usual array of resulting Festscbriften. As others have noted, and as I have stated throughout this chapter, the two celebrations of the 20th century occurred at maximally disparate moments in the history of evolution¬ ary theory: in 1909 at the heyday of doubt about natural selection as a potent mechanism, and in 1959 at the apotheosis of certainty about the nearly exclu¬ sive power of selection as an agent of evolutionary change. A comparison of
The Modern Synthesis as a Limited Consensus the two centennials therefore provides a striking example and epitome of the success (and rigidification) of the Modern Synthesis. Consider two of the leading symposia in 1909: the “official” celebration held in Cambridge (Seward, 1909), and the major American vernacular Fest¬ schrift, published as a special issue of Popular Science Monthly in 1909. The cardinal message reeks with ambiguity (for a celebration of Darwin’s accom¬ plishments): complete confidence in the fact of evolution; lavish praise for Darwin as midwife of the factual confirmation; admission that no consensus has been reached on mechanisms of evolutionary change; and a general feel¬ ing that natural selection plays, at most, a minor role. A few strong selectionists restated their claims, most notably the two sur¬ viving members of Darwin’s inner circle: Joseph Hooker and Alfred Russel Wallace. But even Wallace, the most ardent of selectionists, could no longer muster the confidence and enthusiasm of former years. The qualifiers in his “triumphalist” statement could not be more revealing—for he can now only assert that selection has been adopted as a “satisfying” solution by “a large number” of qualified experts: “And this brings me to the very interesting question: Why did so many of the greatest intellects fail, while Darwin and myself hit upon the solution of this problem—a solution which this celebra¬ tion proves to have been (and still to be) a satisfying one to a large number of those best able to form a judgment on its merits” (Wallace, 1909, p. 398). The range of opinions expressed at the Cambridge symposium illustrates the turmoil in evolutionary theory at Darwin’s 100th birthday. Participants spanned the full spectrum from August Weismann’s defense of selection’s Allmacht (all-sufficiency) to Bateson’s claims for selection’s impotence (ac¬ companied by lavish praise for Darwin’s other achievement in establishing the fact of evolution—see p. 396). More commonly, authors tried to assimi¬ late Darwin to their own disparate views, thereby turning the profession’s hero into a chameleon. For de Vries, Darwin became a closet saltationist (see p. 416 on editor Seward’s annoyance at de Vries’ false and self-serving rein¬ terpretation of Darwin). For Haeckel, Darwin ranked as a pluralist, a true kin to the speaker who had dedicated volume 2 of his Generelle Morphologie col¬ lectively to Darwin, Lamarck, and Goethe! (Haeckel, 1866). Haeckel wrote (1909, pp. 140-141), trying to distance Darwin from Weismann’s position (called “neoDarwinism” at the time), and to reinvent the symposium’s hero as a man in the middle between selectionism and Lamarckism: “It seems to me quite improper to describe this [Weismann’s] hypothetical structure as ‘NeoDarwinism.’ Darwin was just as convinced as Lamarck of the transmis¬ sion of acquired characters and its great importance in the scheme of evolu¬ tion . . . Natural selection does not of itself give the solution of all our evolu¬ tionary problems. It has to be taken in conjunction with the transformism of Lamarck, with which it is in complete harmony.” The strategy of Henry Fairfield Osborn (in heaping praise on Darwin while denying any substantial power to natural selection) well illustrates the most consistent theme of both 1909 symposia. “There is no denying,” Osborn writes (1909, p. 332), “that there is today a wide reaction against the central
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THE STRUCTURE OF EVOLUTIONARY THEORY feature of Darwin’s thought and this leads us to consider the merit of this re¬ action.” Osborn then invoked a time-honored diplomatic tactic by defining Darwin’s achievement as threefold: establishment of the “law of evolution” itself, documentation of the fact of evolution, and development of the theory of natural selection. Since the first two propositions cannot be gainsaid, why fuss over the third, even if Darwin overemphasized the role of natural selec¬ tion? Osborn notes (1909, p. 332): “There is some lack of perspective, some egotism, much onesidedness in modern criticism. The very announcement, ‘Darwin deposed,’ attracts such attention as would the notice ‘Mt. Blanc re¬ moved.’” Osborn correctly identifies two claims at the core of Darwinian theory: (1) selection operates on undirected variability to cause evolutionary change (legs one and two of my tripod); and (2) gradualism rules in geological time (leg three and the methodological pole): “In the operations of this intimate circle of minute variations within organisms, he was inclined to believe two things: first that the fit or adaptive always arises out of the accidental, or that out of large and minute variations without direction selection brings direc¬ tion and fitness; second, as a consistent pupil of Lyell, he was inclined to be¬ lieve that the chief changes in evolution are slow and continuous” (Osborn, 1909). But Osborn then gently chides Darwin for putting too much faith in the power of selection: “There can be no question, however, that Darwin did love his selection theory, and sometimes overestimated its importance” (1909, p. 336). Then, as a consummate politician and administrator, Osborn put a positive spin on his criticism—granting ultimate praise with only a faint damn. He emphasized the most common of all anti-Darwinian arguments— that selection can only operate as a negative force (“judicial” rather than “creative”). But he then converted Darwin’s weakness to centennial strength with a remarkable diplomatic move: Darwin’s problems arose from his igno¬ rance of heredity, but he set a great task for us thereby, and we must per¬ severe: Selection is not a creative principle, it is a judicial principle. It is one of Darwin’s many triumphs that he positively demonstrated that this judi¬ cial principle is one of the great factors of evolution. Then he clearly set our task before us in pointing out that the unknown lies in the laws of variation and a stupendous task it is. At the same time he left us a legacy in his inductive and experimental methods by which we may blaze our trail. Therefore, in this anniversary year, we do not see any decline in the force of Darwinism but rather a renewed stimulus to progressive search. This diplomatic theme—that Darwin did not discover an adequate mecha¬ nism of evolution, but we celebrate his centennial because he opened up a new world of research—became a virtual litany for symposiasts. For exam¬ ple, William Morton Wheeler virtually threw selection in the ashcan as he praised Darwin: “And even if we go so far as to say that natural selection may eventually prove to be an unimportant factor in evolution, to be consigned to
The Modern Synthesis as a Limited Consensus the limbo of defunct hypotheses, together with Darwin’s views on Pangenesis, sexual selection and the origin of species from fluctuating variations, we must, I believe, still admit that the great English naturalist opened up before us a vast new world of thought and endeavor” (Wheeler, 1909, p. 385). T. H. Morgan, who would later become a strong supporter of natural selec¬ tion, began his centennial contribution by expressing the standard argument that Darwin’s importance transcends the limitations of natural selection: “The loyalty that every man of science feels towards Darwin is something greater than any special theory. I shall call it the spirit of Darwinism, the point of view, the method, the procedure of Darwin” (Morgan, 1909, p. 367). Morgan then ended his article, entitled “For Darwin,” by heaping tangential scorn on natural selection while praising the liberating generality of evolution itself: “We stand today on the foundations laid 50 years ago. Darwin’s method is our method, the way he pointed out we follow, not as the advocates of a dogma, not as the disciples of any particular creed, but the avowed adherents of a method of investigation whose inauguration we owe chiefly to Charles Darwin. For it is this spirit of Darwinism, not its formulae, that we proclaim as our best heritage” (1909, p. 380). William Bateson, the least Darwinian of the symposiasts, began his article on the same theme, and then stated his own view right up front: “Darwin’s work has the property of greatness in that it may be admired from more as¬ pects than one. For some the perception of the principle of Natural Selection stands out as his most wonderful achievement to which all the rest is subordi¬ nate. Others, among whom I would range myself, look up to him rather as the first who plainly distinguished, collected, and comprehensively studied that new class of evidence from which hereafter a true understanding of the process of Evolution may be developed” (Bateson, 1909, p. 85). Bateson then added (see p. 596 for more on this quotation and Bateson’s general views), in a statement that strikes me as the most appropriately generous, genuine and cogent expression of an argument that could be advanced with equal validity today: “We shall honor most in him not the rounded merit of finite accom¬ plishment, but the creative power by which he inaugurated a line of discovery endless in variety and extension” (1909, p. 85). Finally, consider the dilemma of A. C. Seward, general editor of the “of¬ ficial” Cambridge celebration, who followed a British tradition of fairness in inviting all sides, but then struggled to find some coherence amidst the Babel of papers he received: “The divergence of views among biologists in regard to the origin of species and as to the most promising directions in which to seek for truth is illustrated by the different opinions of contributors. Whether Dar¬ win’s views on the modus operandi of evolutionary forces receive further con¬ firmation in the future, or whether they are materially modified, in no way af¬ fects the truth of the statement that, by employing his life ‘in adding a little to Natural Science,’ he revolutionized the world of thought” (Seward, 1909, p. vii). I can imagine no contrast more stark, no reversal so complete, as the com¬ parison of these doubts in 1909 with the confidence and near unanimity ex-
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THE STRUCTURE OF EVOLUTIONARY THEORY pressed fifty years later at the Origin's centennial in 1959, The success of the Modern Synthesis established the difference. Beginning as a pluralistic mar¬ riage of Darwin and Mendel in the 1930’s, the Synthesis had hardened by 1959 into a set of core commitments that, at least among epigones and aco¬ lytes, had become formulaic and almost catechistic, if not outright dogmatic. Again, I will consider two leading Festschnften of this later centennial, the two major American celebrations in this case: the American Philosophical So¬ ciety’s annual general meeting in Philadelphia, and the elaborate festival held in Chicago in 1959 (published as a three volume compendium, edited by Sol Tax, in 1960). Major speakers at both meetings attributed the remarkable uniformity of opinion on all major issues to the success of the Synthesis, par¬ ticularly to a consensus on the paramount, virtually exclusive, role of natural selection as the cause of evolutionary change. Ledyard Stebbins, appropri¬ ately for the City of Brotherly Love, spoke in Philadelphia about the unifying power of natural selection: “The last quarter of the century which has elapsed since the publication of The Origin of Species has seen the gradual spread and an almost universal acceptance by biologists actively working with problems of evolution of some form of the neodarwinian concept of evolutionary dy¬ namics. This concept may be broadly defined as one which, like Darwin’s original concept, maintains that the direction and rate of evolution have been largely determined by natural selection” (Stebbins, 1959, p. 231). Mean¬ while, in Chicago, Julian Huxley gave a capsule history of Darwinism, ascrib¬ ing the same binding role to natural selection: “The emergence of Darwinism, I would say, covered the fourteen-year period from 1859 to 1872; and it was in full flower until the 1890’s, when Bateson initiated the anti-Darwinian re¬ action. This in turn lasted for about a quarter of a century, to be succeeded by the present phase of Neo-Darwinism, in which the central Darwinian concept of natural selection has been successfully related to the facts and principles of modern genetics, ecology, and paleontology” (Huxley, 1960, p. 10). Michael Lerner’s development of the argument (in the Philadelphia sympo¬ sium) may be viewed as typical for this confident time. He begins with the venerable (if cryptic) motto of the Greek poet Archilochus: “The fox knows many things, but the hedgehog knows one big thing.” As a profession, Lerner states, we marched along the path from Darwin to the Modern Synthesis as urchins, following the “one big thing” of natural selection, and ultimately re¬ jecting the major alternatives as “sins against Occam’s razor.” Lerner wrote: Their one big thing, natural selection, set at rest the doctrine of special creation. In combination with our knowledge of Mendelian inheritance acquired since Darwin’s day, it rendered obsolete such alternative theo¬ ries of evolution as were based on extra-mechanical agencies, or on di¬ rect adaptation of organisms to their immediate environment (that is, on inheritance of acquired characters), and exposed them as sins against Occam’s razor. Natural selection furnished the binding principle for a general or unified theory of historical change in the living world (Lerner, 1959, p. 173).
The Modern Synthesis as a Limited Consensus Lerner felt so confident that he proclaimed natural selection necessarily dominant a priori, not merely validated by evidence: “There is no longer any doubt that natural selection is more than a theoretical possibility—it is un¬ questionably a logically imperative necessity in any accounting for evolution” (1959, p. 174). He acknowledges, of course, that selection cannot manufac¬ ture, but can only shape, the physical material of organisms, but he compares selection’s role to Michelangelo’s claim that a great sculptor works to liberate beautiful forms from the blocks of stone that begin as their raw material—a lovely, poetic rendition of the standard argument for selection’s creativity (see Chapter 2). In so doing, Lerner trivializes the role of potential constraints, even suggesting that a sow’s ear might not represent an impossible starting point for a silk purse: In the same way, natural selection does not originate its own building blocks in the form of mutations of genes. But from them it does create complexes; it solves in a diversity of ways the great variety of problems that successful individuals and populations face; it builds step by step, even if by trial and error, entities of infinite complexity, ingenuity, and be one inclined to say so, beauty. Granted that it needs appropriate raw ma¬ terials, that it may not necessarily be able to make a silk purse out of a sow’s ear; yet, interacting with other evolutionary mechanisms, it has created the human species out of stuff which in its primordial stage may have looked no more promising (1959, p. 179). Lerner’s conclusion bears more than a whiff of similarity to the apostolic creed, suitable for multiple repetition by the faithful: “Evolution is the most fundamental biological law yet discovered. Natural selection is the basic mechanism implementing it. The principle of descent with modification, cre¬ atively, albeit opportunistically, husbanded by natural selection, is as firmly established as any concept in biology” (1959, pp. 181-182). (I don’t disagree with the content; I just don’t feel fully at ease with the triumphalist presen¬ tation.) If Lerner verges on the overconfident, some centennial expressions treated any conceivable alternative with disdain. I have already cited Mayr’s as¬ sertion of “complete unanimity” in competent professional opinion and of the “colossal ignorance” of remaining doubters. In Chicago, Mayr even re¬ sorted to theological language in citing “the opposing evils of Lamarckism and saltationism” (Mayr, 1960, p. 350). Others noted, but with some sense of unfairness, the vilification of Lamarck. C. H. Waddington regretted that “Lamarck is the only major figure in the history of biology whose name has become, to all intents and purposes, a term of abuse” (1960, p. 383); while Marston Bates noted that “Lamarck remains some kind of horrible example of wrong thinking in the introductory textbooks” (1960, p. 548). But the prevailing tenor of these symposia does not display pugnaciousness towards opponents (which would imply an existing and meaningful conflict of uncertain resolution), but smugness in the confidence that a total victory has, at last, been achieved after a long battle (cigars and a drink around the
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THE STRUCTURE OF EVOLUTIONARY THEORY fireplace at night,,to cite an androphilic metaphor of past and privileged gen¬ erations). Evolutionary theory is now essentially complete; we know how the process works and now only need to supply some details. G. G. Simpson had written in a 1950 essay (reprinted in Sifnpson, 1964, p. 14): “This general theory is now supported by an imposing array of paleontologists, geneticists, and other biological specialists. Differences of opinion on relatively minor points naturally persist and many details remain to be filled in, but the es¬ sentials of the explanation of the history of life have probably now been achieved.” The Chicago symposiasts continually asserted their agreement with this confident consensus. Tinbergen spoke of “the all-pervading power of selec¬ tion” (1960, p. 609). Huxley (in Tax and Callender, 1960, volume 3, p. 45) defined the future task of evolutionary biology as filling in the blanks: “We are no longer having to bother about establishing the fact of evolution, and we know that natural selection is the major factor causing evolutionary change. Our problems now concern working out in detail how natural selec¬ tion operates, defining what we mean by ‘increase of organization,’ tracing the general trends that appear in the course of evolution, and so on.” He then described the range of phenomena that selection can fashion—in short, every¬ thing that might happen in evolution! “It produces branching; it produces in¬ creasing adaptation, improvement, progress, or whatever you like to call it; and it produces horizontal persistence of branches, or stabilization” (in Tax and Callender, 1960, volume 3, p. 139). I argued in the last section that “hardening” of the Synthesis gains clearest expression in an increasing faith that adaptation must be both the impetus and result of nearly any evolutionary change. The 1959 symposia continually stress this theme. Panadaptationism became a premise for Chicago’s major panel discussion on “the evolution of life,” not an issue to be adjudicated by participants. Panelists received a list of assumptions, including the statement that “transformation always leads to adaptive or, better, teleonomic results” (in Tax and Callender, 1960, volume 3, p. 109). Confidence in adaptation grew so great that many symposiasts presented their arguments in a “can’t fail” manner, by delimiting a set of supposedly in¬ clusive outcomes, each validating adaptation for any conceivable result.51' Mayr, for example, argued that the general ecological rules of Bergmann and others enjoy good adaptive explanations, but that the numerous exceptions also affirm adaptation because local (and opposite) factors can override the
*But what scientific good can derive from a theory that includes no possibility of refuta¬ tion from within? (A Mormon friend once told me that archaeologists of his church would either one day find direct evidence that the people of Mormon and Moroni had migrated from the Near East and lived in the New World until the 4th century ad, which would sup¬ port the testimony of the Book of Mormon, or they would not find such evidence, which would also support the doctrines of his church by illustrating God’s challenge to his people to keep faith in the absence of empirical support.) In such cases, one can only suggest alter¬ native theories from without, and try to persuade people of good will that these alternatives provide better explanations for the purely empirical evidence.
The Modern Synthesis as a Limited Consensus
general trend: “In recent years the analysis of these rules has shown that, as we stressed earlier, all phenotypes are compromises among a variety of con¬ flicting selection pressures. As a result, there are many so-called exceptions to such rules, where a new selection pressure takes over and adjusts an organism or a local population in a different way” (in Tax and Callender, 1960, volume 3, p. 138). In another example of victory by virtual definition, Tinbergen acknowl¬ edged that randomness might provide a theoretical alternative to adaptation in the evolution of a behavior. But since he construed randomness only as an absence of evidence for selection, and since he regarded the variety of con¬ ceivable adaptationist explanations as effectively inexhaustible, how would one ever validate randomness in any particular case? “This task [of explain¬ ing the results of evolutionj really amounts to an assessment of the relative importance of the contribution made by random variation, on the one hand, and by adaptation directed by selection, on the other. Since randomness is, per definition, detectable only by elimination of every conceivable directedness, it is natural that this approach should lead to a quest for directedness” (Tinbergen, 1960, p. 602). Adaptation pervades Tinbergen’s discourse and world of thought. He even proposes a turnabout from Darwin’s own, eminently sensible, view that nonadaptive features of conservative inheritance (deep homologies) provide opti¬ mal characters for taxonomic definition, since more recent adaptations tend to be homoplastic (as easily convergeable with similar features in indepen¬ dent lineages) and nondistinctive. Tinbergen, on the other hand, states that his paper will focus upon “the extent to which taxonomic characters must be assumed to be due to natural selection” (1960, p. 595). He then carries his adaptationist paean even further by arguing that evolutionists (as opposed to other scientists who might need to classify for different reasons) must divide organisms and designate their characters in terms of adaptive complexes, thus assuring his preferred interpretation by predefining the structure of ob¬ servation itself: The conclusion that adapted features are systems of functionally related components forces us to reconsider once more the question What is a taxonomic character? The answer is, of course, that it depends on the aims which the scientist has in mind. The classifier is fully entitled to use, e.g., the tameness of the kittiwakes, their nest-building behavior, the black neck band of their young, and their nidicolous habits as four sepa¬ rate characters. But the evolutionist is not entitled to treat them as four independent characters. To him, the correct description of the character¬ istics of the species would be in terms of adapted systems, such as (1) cliff breeding; (2) pelagic feeding; (3) orange inside of the mouth and related characteristics of posturing (Tinbergen, 1960, p. 609). These symposia not only featured adaptation as the centerpiece of the bio¬ logical world, but also extended the concept to all other fields included within their program of lectures. Robert McC. Adams, then a young anthropology
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THE STRUCTURE OF EVOLUTIONARY THEORY
professor at Chicago, but later the Secretary of the Smithsonian Institution, confessed an initial skepticism about the symposium, based on “uncertainty about finding anything in common to talk about with representatives of other disciplines” (in Tax and Callender, 1%0, volume 3, p. 268). But he dis¬ covered relevance in learning to view human societies as “adaptive mecha¬ nisms,” and in using this idea to grant an “evolutionary role” to culture, thus equating adaptation with the entire realm of potential evolutionary insight: As man evolves, he superadds culture to his genetic equipment, and by this new addition he is enabled to adapt in a whole series of much more effective and complex ways—to spread himself over the entire globe, to construct very complex societies, and, in fact, frequently to direct the evolution of species all around him. Human societies are adaptive mech¬ anisms; they have to be understood as having an evolutionary role rather than as uniquely human creations that are not to be compared with the evolutionary development of other organisms (Adams in Tax and Callender, 1960, volume 3, p. 268). Only one “interloper,” historian Ilza Veith, dared to suggest that nonadaptive phenomena might be important in evolution, but Julian Huxley firmly dismissed these worries:
Veith: In my field, perhaps the most rewarding line would be to find those moments or those evolutionary processes that will present weak¬ nesses, where maladaptation will occur, and where the mind will not continue to function in its normal manner.
Huxley: I am sorry you wish to concentrate on maladaptation. I should think it would be much better to concentrate on adaptation from the positive angle (Tax and Callender, 1960, volume. 3, p. 269). Sweetness surely triumphed in Chicago, but perhaps at the expense of light. The panel discussions ended in a virtual orgy of agreement, with Darwin as hero and adaptation as king. Even Sewall Wright, who had approached selec¬ tion with ambiguity for years but had finally made his peace with the hard¬ ened consensus (though in his own idiosyncratic way—see pp. 522-524), ended his paper by writing: “From a more general standpoint, all of this is merely an elaboration in terms of modern genetics of the conception of evolu¬ tion by natural selection advanced by Darwin in the Origin of Species a hun¬ dred years ago” (Wright, 1960, p. 471). Wright became even more accom¬ modating in his role on the “evolution of life” panel. As the discussion wound down, Wright presented a simple comment as a last word before Julian Huxley’s summary: “I agree with everybody.” Yet a bit of rain, as our mottoes proclaim, must fall on any long parade. One skeptic and whistle blower did speak out in Chicago, unsurprisingly a paleontologist who doubted the sufficiency of synthetic adaptationism as a complete explanation for the fullness of events in geological time. The Ameri¬ can vertebrate paleontologist E. C. Olson had become disturbed by the in¬ creasingly dogmatic, peremptory, and exclusivist tone that many synthesists
The Modern Synthesis as a Limited Consensus
had adopted in this period of hardening. He spoke, with some irony, of the consensus that “has come to be known as the ‘synthetic theory of evolution5 but has also been variously termed ‘selection theory,5 ‘neo-Mendelian theory,5 and ‘neo-Darwinian theory.5 It is unfortunate that occasionally it is called The theory of evolution,5 as if no other could exist55 (Olson, 1960, p. 524). Olson then identified three aspects of the logic and sociology of the syn¬ thetic theory that, in veering towards dogmatism, made him uncomfortable. First, the theory had become flexible enough to encompass all possible results almost a priori, thus setting itself no challenges for potential refutation: The feeling of a slight sense of frustration in the elasticity involved in de¬ veloping a universal explanation is hard to avoid . . . There is little or nothing that cannot be explained under the selection theory, and, at pres¬ ent, this theory appears to be unique in this respect (1960, p. 530) . . . This possible danger is amply revealed in some studies of the last decade which seem more concerned with fitting results into the current theory than with evaluation of results in terms of a broader outlook. Further, of course, much research is conceived and carried out within the frame¬ work of the theory, and, no matter what its excellence, is not likely to break out of this framework (1960, p. 536). Second, the synthesists themselves often haughtily dismiss those who dis¬ agree as misguided, if not obtuse: “The statement is made, in effect, that those who do not agree with the synthetic theory do not understand evolution and are incapable of so doing, in most cases because they think typologically . . . Some avid proponents of the synthetic theory would appear to . . . eliminate as competent students of evolution, because of their inability to understand
the theory, those who may disagree55 (1960, pp. 526-527; Olson’s italics). Third, the success of consensus and consequent derision has silenced most doubters, but their numbers may be large and their questions cogent: “There exists, as well, a generally silent group of students engaged in biological pur¬ suits who tend to disagree with much of the current thought but say and write little ... It is, of course, difficult to judge the size and composition of this si¬ lent segment, but there is no doubt that the numbers are not inconsiderable. Wrong or right as such opinion may be, its existence is important and cannot be ignored or eliminated as a force in the study of evolution55 (1960, pp. 523524). As a paleontologist, Olson expressed most unhappiness with the “con¬ sistency argument55 that awarded the synthetic theory hegemony over all scales of macroevolution—a misplaced confidence achieved by extrapolating, by fiat more than by evidence, a process that undoubtedly works in the ecological here and now to a sufficient explanation for all major changes oc¬ curring over hundreds of millions of years (1960, pp. 531 and 533). Yet, however cogent Olson’s doubts, his attempt to inject more pluralism and skepticism into evolutionary theory ultimately failed—and for a valid reason from the orthodox point of view. A successful whistle blower must proceed beyond the exposure of faults in his boss’s domain; he must also sug¬ gest a path towards greater accuracy and fuller explanation. And, on this
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THE STRUCTURE OF EVOLUTIONARY THEORY constructive side, Olson could offer nothing. He ventured a few comments about cytoplasmic inheritance as a possible mechanism that might not follow all synthetic rules, but such a limited and inadequate speculation could not fuel such a comprehensive set of doubtsl Revisionists would gain no hearing until they could propose an extensive and positive set of extensions or alter¬ natives—and I write this book because I believe that such an affirmative pro¬ gram has now been formulated. Olson’s critique achieved no currency, and the hardened version descended from the empyrean academy into the vernac¬ ular world of textbooks, the ultimate test of establishment by social imposi¬ tion as well as by professional consensus.
ALL QUIET ON THE TEXTBOOK FRONT Professional writing tends to be nuanced and judicious. Even the strongest partisan finesses his commitment and adds at least a footnote or tangential comment, so that any charge of oversimplification or dogmatism may be countered by stating: “but look on page 381 (in the small print); you see, I raised that caveat myself.” To learn the unvarnished commitments of an age, one must turn to the textbooks that provide “straight stuff” for introductory students. Yes, text¬ books truly oversimplify their subjects, but textbooks also present the central tenets of a field without subtlety or apology—and we can grasp thereby what each generation of neophytes first imbibes as the essence of a field. Moreover, many textbooks boast authorship by the same professionals who fill their technical writings with exceptions, caveats, and complexities. I have long felt that surveys of textbooks offer our best guide to the central convictions of any era. What single line could be more revealing, more at¬ tuned to the core commitment of a profession that bathed in the blessings of Victorian progressivism, and aspired to scientific status in Darwin’s century, than the epigram that Alfred Marshall placed on the title page to innumerable editions of his canonical textbook, Principles of Economics: “natura non
facit saltum. ” The changing foci of 20th century textbooks provide direct insight into the history of evolutionary thought and the eventual triumph of Darwinism. In particular, if the Synthesis truly hardened, as I have argued, then texts fol¬ lowing the 1959 centennial celebrations—the apogee of strict selectionism— should describe evolution in unambiguously panadaptationist language, and should extol the sufficiency of natural selection to craft the entire range of evolutionary phenomena at all scales, ecological to geological. This section does not present a systematic survey of texts, though I have consulted everything I could find, including nearly all major American books for introductory college biology (and several high school textbooks as well). A more complete search, extended back in time to cover the early days of the Synthesis, and the pre-synthetic period as well, would provide a fascinating topic for a dissertation in the history of science or education. This field of ver¬ nacular expression has been neglected by scholars, though the subject would
The Modern Synthesis as a Limited Consensus
yield great insight (for such material obviously represents the only formal contact that most students ever receive with any given discipline). I apologize for my almost anecdotal approach, but I think that I have iden¬ tified a robust pattern supporting the hypothesis of hardening. I will focus on the two topics that authors of texts found most congenial in their efforts to explain synthetic evolutionism to introductory audiences: the centrality of adaptation, and the sufficiency of synthetic microevolution to explain events at all scales. (I consider here only the evolution chapters of comprehensive bi¬ ology texts for introductory courses, not entire textbooks on evolution. These short, unvarnished and straight-line accounts of adaptation and extrapola¬ tion appear in the context of such epitomes. Full texts on evolution, which cannot be called “introductory” or “elementary” (for such courses have al¬ ways been taught at intermediate or advanced levels in American universi¬ ties), do treat the subject more comprehensively, with a proper listing, often called “textbooky” in our jargon, of divergent views.)
Adaptation and natural selection In this age of sound bites, even short chapters include final summaries to tell students the pith of what they must remember. Consider the following from Nelson, Robinson, and Boolootian (1967, p. 249), written to summarize a chapter entitled “Evolution, Evidences and Theories.” I cite the entire state¬ ment, not an excerpt: Principles 1. Charles Darwin proposed a theory of evolution based on variation, competition, and consequent natural selection. 2. The basic mechanism of evolution is now known to be changes in gene frequencies of populations through time, guided by natural selec¬ tion. On the subject of exclusivity, Darling and Darling (1961, p. 199) tell us that “any organism is a bundle of interacting adaptations. Most all the fea¬ tures of all living things are adaptations.” Howells (1959, p. 24), a great evo¬ lutionary anthropologist publishing his popular text in Darwin’s centennial year, discussed natural selection with his usual panache and good humor, but also in the all-encompassing celebratory mode: “So much for natural selec¬ tion, the external force, that finger beckoning to the otherwise unguided he¬ redity of an animal type. All other principles and facts of evolution may be satisfactorily related to it or explained by it, and the century following 1859 has seen Darwin triumphant.” Simpson, Pittendrigh and Tiffany (1957, p. 405), an excellent text that dominated the market for years (and featured a leading architect of the Syn¬ thesis as first author), also stated that any nonrandom evolution must be adaptive: “The evolutionary changes that result from nonrandom reproduc¬ tion are clearly adaptive: the changes are always, necessarily, of such a kind as to improve the average ability of the population to survive and reproduce in the environments that they inhabit.”
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Many paeans to adaptation proceeded beyond mere claims about omni¬ presence to assert optimalized excellence, or near organic perfection, as well. Convictions about the exclusive power (as well as the range) of natural selec¬ tion emerge most clearly from such statements, as by Telford and Kennedy (1965, p. 3) (Kennedy later became the president of Stanford University and editor of Science magazine): It is of profound importance for the nature of the organism that, due to natural selection, the evolutionary changes in organisms have either moved relentlessly in the direction of efficiency or have kept them at¬ tuned to a changing environment. . . . Evolutionary adaptation thus sug¬ gests an extremely fine attunement between organism and environment. The organism doesn’t merely get along; its whole life mode has been tem¬ pered and refined by the successful competition of generations of its an¬ cestors with a multitude of differing genotypes. Thus even in the finest details of their organization, organisms are constructed and operate in a manner which makes sense in terms of the way they make their living. From this assertion of omnipresence for adaptation in morphologies, phys¬ iologies and behaviors of the moment, these texts then proceed to ascribe the second great phenomenon of evolution—the production of diversity—to nat¬ ural selection as well. Simpson et al. (1957, p. 405) extend selection’s scope to all phenomena at all scales by writing: “The evolutionary process, viewed in broad perspective, is characterized by two major features: it produces diver¬ sity among living things, and it gives rise to their adaptation, their fitness to survive and reproduce efficiently in the environments they inhabit. These two features are interdependent: life’s diversity is largely a diversity in adapta¬ tion.” Speciation, although replete with nonadaptive elements in Mayr’s canoni¬ cal formulation, usually receives a textbook description as an even stronger affirmation of natural selection (because the process now operates in two sep¬ arated lines, working its differential effects to produce just the right adapta¬ tions in both distinct and varying environments). Nelson et al. entitling their section “Speciation: The Results of Adaptation,” write in summary (1967, p. 235): “Natural selection operating on the variability present in the geno¬ types of populations can cause better adaptation of organisms to their envi¬ ronment. Coupled with reproductive isolation, these adaptations bring about speciation.” Jones and Gaudin (1977, p. 548) introduce their discussion of speciation with a scenario of pure adaptation and extrapolation. (Their full text dis¬ cusses other mechanisms, including polyploidy—but note the pride of place awarded to adaptation, and the argument that so separate and important a phenomenon as geographic isolation only provides an impetus by setting new selection pressures in a different environment): The accumulation of adaptations can lead to the production of new spe¬ cies, a process called speciation. . . . Suppose a population of gophers liv-
The Modern Synthesis as a Limited Consensus
ing in a valley is divided in two by a river that cuts a channel through their valley. The two segments of the population are now effectively sep¬ arated from one another, and any environmental differences that exist between the two regions of the valley will result in adaptations restricted to one side or the other of the river. . . . Different selective pressures now will be operating on opposite sides of the river. Given sufficient time, the two gopher populations may diverge quite extensively. With speciation thus explained as an extended consequence of adaptation under certain environmental circumstances, the same argument can then be smoothly extended to life’s full pattern in geological time. Alexander (1962, p. 826) tells students that all phylogeny flows from “the fact of adaptation.” I can hardly imagine a more gradualistic and meliorist account of evolution, with all death for improved existence, and all life in continual motion to¬ wards more and better: We need only accept the fact of adaptation—the idea that organisms are fitted for the particular environments in which they live—to see the ne¬ cessity for a process of organic evolution. The environment in which organisms live has not been constant . . . Organisms, of course, do not exist under conditions for which they are not adapted. They have, there¬ fore, met these various conditions at different times and places; in order to persist under a changing environment they themselves have had to change. We may think of organic evolution, therefore, as the progressive change of plants and animals in harmony with the changes in their envi¬ ronments. The unadapted die out and disappear. Those organisms whose descendants can fit into the new conditions survive, expand in numbers and kinds, and take over the changing habitat.
Reduction and trivialization of macroevolution The hypothesis of selection’s Allmacht, and adaptation’s ubiquity, rests upon the validity of extrapolation to the full range of geological time, for what power (or generality) can a well-formulated theory of local adaptation assert if the same process, by uniformitarian extension, cannot explain the origin of multicellularity, the rise of mammals, and the eventual emergence of human intelligence? Paradoxically perhaps, this extrapolationist assertion becomes, at the same time, the most vulnerable and the most essential of all synthetic propositions—vulnerable in necessary reliance upon a “consistency argu¬ ment” in the absence of empirical proof, and essential because the theory be¬ comes such a paltry and limited device if its explanatory range cannot extend beyond the compass of its directly observable effects. No evolutionary assertion has been more commonly advanced in text¬ books, or more superficially (and almost nonchalantly) proclaimed by fiat, than the claim that adaptation by natural selection must be fully sufficient to render life’s entire history. In the last section, I documented the “promotion” of arguments about pervasiveness of adaptation in local circumstances, to speciation, to the entire tree of life. Capping this sequence, Howells (1959,
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THE STRUCTURE OF EVOEUTIONARY THEORY p. 28) writes that “all this exploring, stopping, and rushing, in the pursuit of profitable adaptation, has resulted in the great family tree of the animals.” Nelson et al. (1967, p. 239) briefly extol the full sequence—from the rule of selection in local populations, through speciation, to the origin and di¬ versification of phyla: “Evolution in its simplest and broadest sense means changes in gene frequency over a period of time. Natural selection guides these changes . . . Over long periods the accumulation of changes may be suf¬ ficient to separate once similar populations into distinct groups. In the course of evolutionary history this divergence has apparently led to different classes (mammals, birds, fish, etc.), different phyla (insects and corals, for example), and even different kingdoms (plants and animals).” The dominant high school text of the 1960’s and 70’s depicts the standard equine example of macroevolution as anagenetic gradualism guided by natu¬ ral selection, thus making any definition of chronospecies arbitrary: “The fos¬ sil record shows that all these differences are the result of a series of many gradual changes. Each change that became established through natural selec¬ tion must have been very slight; only when many such changes accumulated did they result in detectable differences. How can this long sequence of horses be divided into species?” (Biological Sciences Curriculum Study, Green Ver¬ sion, 1973, p. 621). The accompanying figure of the phylogeny of horses de¬ picts the actual (and copiously branching) bush as a smooth ladder of prog¬ ress (see Fig. 7-3). Bonner (1962, pp. 52-53), another leading evolutionist who also wrote a popular text, argued that paleontologists can’t study the mechanics of evolu¬ tion directly, but professed complete confidence in the efficacy of microevo¬ lutionary selection: Paleontologists as well as ecologists have been for some years studying the evolutionary factors we have discussed, and have continuously at¬ tempted to see how the fossil record, on the one hand, or the present-day distribution of animals and plants, on the other, fit in with this scheme. There seem to be no major discrepancies, and a general feeling that the mechanism of evolution is understood prevails, particularly in regard to the importance of selection and the method of formation of new species. . . . Some groups such as the mollusks have been exceedingly slow in their progress while others, such as the mammals, have been very rapid. Again this can be totally understood in terms of selection in particular environments. No other hypothetical mechanisms seem to be necessary to account for the facts as we know them. Such confidence in microevolutionary sufficiency can only lead to a down¬ grading of paleontology—either to theoretical irrelevance, or to a status as a mere repository for results of processes that can only be elucidated by study¬ ing modern organisms (and may then be smoothly extrapolated across a mil¬ lion millennia). I do not think that this derogatory judgment originated by the conscious intent of most textbook authors. Rather, the marginalization of paleontology flows directly from the logic of pure extrapolation. The basic
MUZZLE
FORELEG
MOLAR TOOTH
7-3. Standard textbook misdepiction of a copiously branching evolutionary lin¬ eage as a ladder of progress. This canonical view of the evolution of horses ap¬ peared in the 1973 Green Version of the most popular and most widely re¬ spected high school textbook produced by the Biological Sciences Curriculum
Study.
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THE STRUCTURE OF EVOEUTIONARY THEORY argument takes two forms. Some authors explicitly exclude paleontology from the theoretical game: Evolution can be studied on the population level only with living organ¬ isms. The fossil record provides too few data to allow such treatment; it merely allows paleontologists to reconstruct the history of animal and plant groups. The population approach makes it possible to ask such questions as: what is the rate of evolution in a given species? What fac¬ tors influence the course or rate of evolution? What conditions are neces¬ sary for evolution to begin or cease? (Baker and Allen, 1968, p. 524). fl do not see why paleontologists cannot address all three of these ques¬ tions with data from the morphology of fossils and their temporal distri¬ bution.] But I must confess that a stronger and more focused form of this argument has long evoked my deeper distress, and has served, in substantial measure, as the impetus for personal career choices in research, and for my eventual deci¬ sion to write this book. I refer to the claim, repeated almost as a catechism, and obviously copied from textbook to textbook, that macroevolution poses no problem not resolvable by a further understanding of allelic substitutions directed by natural selection in contemporary populations. We may move smoothly from one gene to an entire Bauplan, and extrapolate upwards from a few generations to a geological era. No additional problems arise in tempo¬ ral vastness. Macroevolution becomes little more than industrial melanism writ large. But can we even imagine, in a world dominated by effects of scale, that such a maximal extension of form and time will engage not a single force or principle beyond the factors fully in evidence at the lowest level? Can the smallest scales really provide an entirely sufficient model for the largest? Can a uniformitarianism this rigid truly be sustained? If so, then paleontology only represents a playground for the full display of microevolutionary mus¬ cle—and textbooks need not consider the fossil record as more than an ar¬ chive of the pathways carved by this power. Most standard textbooks make this confident assertion based on little be¬ yond hope and tradition—thus making macroevolution a nonsubject. Bonner (1962, p. 48), for example, writes: “There is no reason to believe that these large changes are not the result of the very same mechanics of the small changes of industrial melanism. One involves a small step over a few years; the other involves many many thousands of steps over millions of years.” Curtis (1962, p. 712), in a best-selling text of the 60’s, begins her short sec¬ tion on macroevolution by stating: “Can the same processes that slowly shape the seed of mustard weed or change the color of the peppered moth cre¬ ate the differences between elephants and daisies or between butterflies and redwood trees? Darwin believed so—all he felt that was needed was time, millions of years of slow change. Today, almost all evolutionists are, in princi¬ ple, in general agreement with Darwin’s conclusions.” Several texts even present this canonical argument as their only statement about macroevolution. I end this chapter by quoting two striking examples of
The Modern Synthesis as a Limited Consensus this trivialization and marginalization of macroevolution, each from the most important source in its respective genre. As mentioned above, BSCS text¬ books (written by a semiofficial consortium of private and governmental sources, The Biological Sciences Curriculum Study) virtually cornered highschool markets during post-Sputnik years of the 1960’s and 1970’s. The 1968 version of Biological Science: An Inquiry Into Life includes a heading on “The Origin of Genera and Larger Groups.” But the text contains only two paragraphs, fully reproduced below: The final question which we must ask about the forces of evolution is this: can mutation, recombination, selection, and barriers to cross-breed¬ ing explain the major trends of evolution, such as the divergence of cat¬ like from doglike animals and the evolution of the horse from its small primitive ancestors? The mechanisms that govern these major trends of evolution cannot be studied directly: they took place many thousands or millions of years ago. Nevertheless, a study of populations today, and of fossils, provides strong evidence that the same evolutionary forces in operation today have guided evolution in the past. One species evolves into two (or more). All the new species continue to evolve, becoming more different from one another until eventually we would classify them as different genera (1968, p. 203). Life on Earth (1973) surely ranks as the most distinguished textbook of in¬ troductory college biology published during the 1970’s. Written by a team of eight authors, and headed by two of the world’s leading evolutionists (E. O. Wilson and T. Eisner), this book staked an explicit claim for groundbreaking novelty by linking appropriate expertise at the highest level with accessibility in style, and excellence in design and illustration. Chapter 28 on “The Process of Evolution” ends with the heading “Macroevolution.” The quotation be¬ low may seem limited in content, particularly for a college text, but I do not cite an excerpt. I have reproduced the book’s entire section on macro¬ evolution! In this passage, the history of life becomes a simple extension of the story of the raspberry eye-color gene. (For the second edition, the authors switched to the standard case of industrial melanism, but did not alter the general ar¬ gument at all.) Paleontologists may be burdened with an incomplete record, the authors assert, but as they look more carefully, the gap between the rasp¬ berry gene and the Cambrian explosion closes continually. I can only express my astonishment at such a limited, but definitive, assertion by applying Ethel Barrymore’s famous closing line to this dismissal of macroevolution as a sub¬ ject: “That’s all there is, there isn’t any more.” Each of the examples of microevolution examined, involving shifts in the frequencies of small numbers of genes, could be multiplied a hundred¬ fold from reports in the scientific literature. Biologists have been privi¬ leged to witness the beginnings of evolutionary change in many kinds
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THE STRUCTURE OF EVOEUTIONARY THEORY of plants and animals and under a variety of situations, and they have used this opportunity to test the assumptions of population genetics that form the foundations of modern evolutionary theory. The question that should be asked before we proceed to fiew ideas is whether more exten¬ sive evolutionary change, macroevolution, can be explained as an out¬ come of these microevolutionary shifts. Did birds really arise from rep¬ tiles by an accumulation of gene substitutions of the kind illustrated by the raspberry eye-color gene? The answer is that it is entirely plausible, and no one has come up with a better explanation consistent with the known biological facts. One must keep in mind the enormous difference in time scale between the ob¬ served cases of microevolution and macroevolution. Under natural con¬ ditions the nearly complete substitution of the melanic gene of the pep¬ pered moth took 50 years. Evolution of the magnitude of the origin of the birds usually, perhaps invariably, takes many millions of years. As paleontologists explore the fossil record with increasing care, transitions are being documented between increasing numbers of species, genera, and higher taxonomic groups. The reading from these fossil archives suggests that macroevolution is indeed gradual, paced at a rate that leads to the conclusion that it is based upon hundreds or thousands of gene substitutions no different in kind from the ones examined in our case his¬ tories (1973, p. 792). But, pace Ms. Barrymore, there is so much more—as research in the vi¬ brant field of macroevolution, filling the pages of numerous journals (all founded after these dismissive comments), attests; as the development of a tight and powerful theory of hierarchical selection embodies (see Chapters 8 and 9); as the union of developmental with evolutionary biology displays (see Chapters 10 and 11); as our advancing understanding of genomic complexity asserts. Can we not feel the frustration of E. C. Olson as he queried the titans of the Modern Synthesis in Chicago? Can we not understand why a few icon¬ oclasts never made their peace with such a comfortable and limiting ortho¬ doxy? Can we not gain a visceral (and not only an intellectual) sense of C. H. Waddington’s isolation and irritation when he made his famous comment on the limitations of population genetics (Waddington, 1967), and won admira¬ tion for his panache but no consideration for his content: “The whole real guts of evolution—which is, how do you come to have horses and tigers, and things—is outside the mathematical theory.”
Segue to Part Two
Contemporary challenges to all three central commitments of Darwinism (the legs of the tripod in my chosen metaphor, or the “essence” of the theory in the legitimate use of a word generally shunned by evolutionary biologists) prompted me to write this book. Such forms of debate set the mainsail of scholarly life, and cynics may be excused for suspecting the academic equiva¬ lent of glitz and grandstanding when their colleagues proclaim major unhap¬ piness with received wisdom. This cynicism merits special attention when Charles Darwin serves as a target—for the demise of Darwinism has been trumpeted more often than the guard changes at Buckingham Palace, not¬ withstanding the evident fact that both seem to stand firm as venerable Brit¬ ish institutions. (I state nothing new here: Kellogg (1907) began his won¬ derful book, the basis for my style of exposition, by refuting a German proclamation, then current, about the Sterbelager, or death-bed, of Darwin¬ ism—see Dennert, 1904, for an English translation of the book that inspired Kellogg’s long and celebrated rejoinder.) If continuity breeds respect—and what other criterion could an evolution¬ ist propose in this volatile vale of tears?—then the most persuasive rejoinder to a charge of superficial and ephemeral grandstanding must lie in the docu¬ mentation of long persistence and serious attention for a given critique. Per¬ sistence, of course, need not imply cogency; Lord only knows the lengthy pedigree of stupidity. But an analog to natural selection also operates in the world of ideas, and truly silly notions do get weeded out at certain levels of intellectual competence. Moreover, only a small subset of our forebears rank as brilliant thinkers. When we can designate a critique as both longstanding in general and seriously supported by scholars in this subset, then such argu¬ ments should command our respect and attention. (Brilliance, of course, only implies cogency, not correctness. Cultural biases and simple lack of informa¬ tion can lead even the most gifted minds to firm convictions that seem risible today. But I do assert that brilliant scholars, while often as wrong as anyone else, devise their positions for interesting and instructive reasons. We may now reject Lyell’s strict views about substantive uniformity, and Paley will find few modern devotees for his natural theology. But we must not write these men off—and we will learn much by studying the reasons for their dis¬ tinctive attitudes.)
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THE STRUCTURE OF EVOEUTIONARY THEORY In this context, bask readers to consider two points about the tripod of nec¬ essary support for Darwinism: First, even during the period of current orthodoxy, beginning with the co¬ alescence and spread of the Modern Synthesis; the three supports have never been particularly firm, or adequately defended. The first support—restriction of selection to the organismal level—received little explicit defense, but rather prevailed in a fuzzy sort of way by convention in practice. Sloppy statements implying group selection abounded in the literature (as documented in Chap¬ ter 7, pp. 544-556), and some disciplines, notably the classical ethology of Lorenz and his disciples, frequently cited arguments about supraorganismal selection without understanding their consequences for Darwinian the¬ ory. This situation changed dramatically when Wynne-Edwards (1962) ad¬ vanced his explicit argument for group selection at a predominant relative frequency—and Williams (1966) wrote his spirited defense of the Darwinian straight and narrow by setting out the centrality of organismal selection so forcefully. The second support—the validation of selection as a nearly exclusive mechanism of evolutionary change, as embodied in the adaptationist pro¬ gram—received strong verbal approbation, and elegant illustration in a few cases, but won orthodox status largely as a bandwagon effect prompted by the urgings of a few central figures, notably Mayr and Dobzhansky, and the subsequent acquiescence of most professionals to the assertion of such lead¬ ing figures, and not to the data of convincing demonstrations (see Chapter 7 for a detailed defense of this claim, as embodied in my hypothesis on the “hardening” of the Modern Synthesis). In particular, taxonomic orthodoxy just before the Synthesis (Kinsey, 1936; Robson and Richards, 1936) re¬ garded most geographic variation within species as nonadaptive. The oppo¬ site opinion triumphed as the Synthesis reached a height of prestige and or¬ thodoxy, but few actual cases had been overturned by data. Rather, a shifting theoretical preference led to assertions of dominant relative frequency based on documentation inadequate to affirm either view. (I do not regard earlier arguments for nonadaptation as inherently more cogent. On the contrary, I am convinced that we still have no good idea about the relative frequencies of adaptive and nonadaptive effects in geographic variation. I only claim that the shift to adaptationist preferences resulted more from a bandwagon effect than from direct evidence—and that this second leg of the tripod therefore never enjoyed adequate buttressing.) The third support—extrapolation, explicitly discussed here in terms of the surrogate proposition of geological uniformity, and so necessary to provide a stage that would nurture, or at least not disrupt, Darwin’s hope for explain¬ ing the entire history of life by “pure” extension of principles derived from the small and palpable—prevailed more by assumption than by active valida¬ tion, with Kelvin’s defeat and Rutherford’s proof of the earth’s great age read as adequate defense (a logically insufficient argument, by the way, because time may be long, but change still concentrated in rare catastrophic episodes). By the largely arbitrary and contingent sociology of disciplines, paleontology
The Modern Synthesis as a Limited Consensus belonged to the geologists, and students of fossils therefore received virtually no training in evolution. With few exceptions, notably the work of G. G. Simpson, paleontology (at least during the first half of the 20th century) played little role in the development of a theory to account for its own subject matter. Second, and more important in summarizing the first half of this book: not only can we identify basic logical weaknesses in standard defenses for the three Darwinian supports, but cogent critiques have also been persistent, in¬ deed omnipresent though changing in form, throughout the history of evolu¬ tionary thought. These histories may not be widely known to current practi¬ tioners, but the best minds of our profession have struggled continuously with themes of the essential tripod—and their arguments deserve our atten¬ tion and respect. (Without this knowledge, we tend to imbue orthodoxies with false permanence or, even worse, to lose sight of basic principles in the surrounding silence, thereby converting dubious but central postulates into hidden assumptions. History can and should be liberating.) For the first leg of the tripod of essential support, hierarchy theory and multiple levels of selection do not represent only a modern gloss upon Dar¬ winism. Rather, contemporary versions of these concepts have reinvigorated the oldest issue of our profession. In Chapter 3, I showed that the first two evolutionary systems well known to English-speaking naturalists—Lamarck’s and Chambers’s—relied on a causal hierarchy that contrasted progress with diversification. Darwin explicitly combatted these ideas with a single-level theory based on extrapolating the small and observable results of natural se¬ lection, operating on organisms in local populations, to render all evolution¬ ary phenomena at all scales of time and effect. Weismann, and Darwin him¬ self as he struggled to explain diversity, then considered hierarchical models of selection formulated very much in the spirit of modern versions. Weis¬ mann, after much internal debate, leading to eventual rejection of his previ¬ ous commitment to the strict Darwinism of single-level selection on organ¬ isms alone, eventually advocated a full hierarchy of levels, explicitly citing this concept as the most distinctive innovation and centerpiece of his mature evolutionary views. The internalist critique of adaptationism (the main subject of modern criti¬ cism on the second, or “creativity,” leg of the tripod) boasts an even more venerable pedigree. I showed, in Chapters 4 and 5, how this critique defined the major difference between British (Paleyan) and continental versions of natural theology in preevolutionary days. I then demonstrated that the same division, transformed as the structuralist-functionalist dichotomy, served as a focus for evolutionary debate—pitting the functionalism (adaptationism) of such disparate theories as Darwinian selectionism and Lamarckian soft inher¬ itance against the great continental schools of structuralism, as advocated by such scientists as Goethe (and most of the German Naturphilosophen), Geoffroy Saint-Hilaire (and the French transcendental morphologists), and, in a rare move across the Channel, in major themes of the complex and much misunderstood evolutionary views of Richard Owen. Finally, I traced the two
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THE STRUCTURE OF EVOLUTIONARY THEORY major lines of pos.t-Darwinian internalist thought (orthogenesis and saltationism), and documented the continuity of this pedigree, after the Mendelian rediscovery, in the macromutationism of Hugo de Vries and (combining both strands of constraint and saltation) in the*apostasy of Richard Goldschmidt. Modern critiques of adaptationism rest upon an ancient legacy. For the third leg of extrapolationism, as illustrated here by the surrogate theme of Darwin’s geological needs (for the other major aspects of extrapola¬ tionism fall into the theoretical domains of the first two essential postulates), I showed, in Chapter 6, that classical catastrophism operated as good science, and also represented the literal empiricism of its time. I then argued that Lyell’s uniformitarian victory arose largely as a triumph of skilled but dubi¬ ous rhetoric. The aspects of catastrophism that posed the strongest challenges to Darwin’s ideas on the origin of macroevolutionary pattern never received a convincing critique, and have now experienced a legitimate rebirth in modern views on mass extinction. Nonetheless, although each leg of the Darwinian tripod faces a venerable indictment from the fullness of history, the path of modern reform surely does not he with these classical critiques, for each embodies fatal flaws in each of two debilitating ways: Anchors in cultural biases. The attempt to validate human supe¬ riority by the doctrine of progress identifies the heaviest burden imposed by Western culture upon evolutionary views of all stripes. In their nineteenth century versions, all three critiques of the essential postulates of Darwinism sunk their major root (and fallacy) in the concept of progress. For the first leg, the original hierarchical models of Tamarck and Chambers construed their higher level of large-scale change as a force of progress orthogonal to a palpa¬ ble cause of local adaptation. For both Lamarck and Chambers, the two forces of general progress and local adaptation are not only geometrically or¬ thogonal (upwards vs. sidewards), but also conceptually opposed, as the lat¬ eral force “pulls” lineages from their upward course into dead ends of local specialization. For the second leg, most structuralist visions postulated an in¬ herent increase of complexity and progress mediated by laws of form and in¬ ternal principles of living matter. These internalist theories proved attractive because, in contrast with the Darwinian contingency of shifting local adapta¬ tion, they offered more promise as validations for the great psychic balm of progress. On the third leg, catastrophism might seem inherently opposed to ideas of regulated and predictable increase in life’s complexity, but the classi¬ cal versions advocated an intimate connection between progressionism and paroxysmal change—for pre-Darwinian catastrophists generally postulated renewed faunas of increasing excellence after each episode of extinction. This theme became such an intrinsic component of classical catastrophism that many scholars now designate this movement as the “directionalist synthesis” or as “progressionism,” and not by the paroxysmal dynamics of catastrophic change. Failures of logic. The three critiques, in their nineteenth century versions, are explicitly anti-Darwinian. That is, they propose alternative
The Modern Synthesis as a Limited Consensus causes of evolution that either deny natural selection entirely, or demote Dar¬ win’s preferred cause to insignificance. These alternative forces are, in any case, opposed to—clearly not synergistic with—Darwin’s principle of natural selection at the organismic level. The explicit, often vociferous, invocation of these critiques against Dar¬ winism set the primary agenda for scientific debate from the very beginning of modern evolutionary theory. On the first leg, the Lamarck-Chambers tra¬ dition of a primary force of progress (effectively inaccessible to empirical study), opposed to a palpable cause of immediate adaptation (eminently op¬ erational for research, but decidedly secondary in significance), acted as a ma¬ jor spur to Darwin’s development of a fully operational theory with causes working at a single and accessible level. On the second leg, internalist notions of orthogenesis and saltation denied creativity to natural selection and de¬ fined the major versions of late 19th century anti-Darwinism. On the third leg, classical catastrophism became Darwin’s personal bete noire, an obstacle that he surmounted by allegiance to Lyellian uniformity. Later in Darwin’s ca¬ reer, the appearance of a similar threat in a different guise—the claim for an earth too young to render the results of evolution by natural selection in a gradualistic mode—led Darwin to characterize Lord Kelvin as an “odious specter.” I believe that the historical tradition for using these critiques as supposed confutations of Darwinism has engendered a great deal of unnecessary and unproductive wrangling in our own time, as markedly different versions of the same critiques needlessly evoke old fears. I also believe that we can find the way to a better (and healing) taxonomy by following the lead of Kellogg’s fine presentation (1907), already much praised in this book. Kellogg, as pre¬ viously discussed (pp. 163-169), divided critical commentary about Darwin¬ ism into arguments “auxiliary to” and “alternative to” natural selection—en¬ largements and confutations, if you will. In the past, critiques of the Darwinian tripod have usually been advocated in Kellogg’s alternative, or de¬ structive, mode—and a tradition for quick (often ill-considered) and defen¬ sive reaction by Darwinians has developed whenever the critical buzz-words rise again: rapid change, group selection, mass extinction, directed mutation, for example. But all these critiques can also generate powerful versions in Kellogg’s auxiliary, or helpful and expansive, mode—as Kellogg himself rec¬ ognized when he classified Weismann’s theory of hierarchy as one of the two most significant auxiliary propositions of his time. The older, alternative mode of these critiques did lead to a series of dead ends, rightly rejected by the resurgent Darwinism of the Modern Synthesis. Older versions of hierarchy (the first leg) foundered in the mysticism of super¬ organisms and harmonious ecologies; constraint and laws of form (the sec¬ ond leg) became mired in invalid macromutationism or lingering orthogene¬ sis; catastrophic geology (representing the third leg) languished in the failure of all proposed mechanisms for global paroxysm. The old versions, freighted by the cultural bias of progress, and rooted in false arguments for the demise of Darwinism, richly deserved the rejection they received.
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THE STRUCTURE OE EVOLUTIONARY THEORY But modern forms of these critiques are now being advanced in different and helpful versions within Kellogg’s auxiliary mode—that is, as ideas to ex¬ pand, while substantially changing, the Darwinian core. For the first leg, and most importantly, the hierarchical theory of multi-level selection retains Dar¬ win’s emphasis on the centrality of selection as a mechanism, but rejects the notion that the organismal level must hold nearly exclusive sway as a causal locus of change (while wondering if this conventional Darwinian level can even claim dominant status—Chapters 8 and 9). On the second leg, modern ideas of constraint and channeling deny the crucial isotropy of variation, so necessary to the logic of selection as the primary directional force in evolu¬ tion, and therefore envision important roles for structural and internal causes as patterning agents of evolutionary change (Chapters 10 and 11). These in¬ ternal channels work with selection as conduits for its impetus—that is, as auxiliary (not alternative) forces to natural selection. For the third leg, cur¬ rent notions of mass extinction do not challenge the Darwinian mechanism of selection per se, but suggest that any full explanation of macroevolutionary pattern must integrate the accumulated Darwinian effects of normal times with the profound restructurings of diversity that occur in environmental epi¬ sodes too rapid or too intense for adaptive response by many species and clades (Chapter 12). Therefore, in modern versions of the three critiques, classical Darwinism either becomes expanded (in the theory of hierarchical selection), or dynami¬ cally counterposed with other causal forces working in concert with selection to produce the patterns of life’s history, either at a conventional microevo¬ lutionary scale (internal channels as conduits for selection) or as interact¬ ing regimes through geological time (mass extinctions and selective replace¬ ments). But we cannot fairly portray these expanded views as pure sweetness and light for orthodox Darwinism. Much that has been enormously com¬ fortable must be sacrificed to accept this enlarged theory with a retained Dar¬ winian core—particularly the neat and clean, the simple and unifocal, no¬ tion that natural selection on organisms represents the cause of evolutionary change, and (by extrapolation) the only important agent of macroevolu¬ tionary pattern. On the first leg, the theory of hierarchical selection differs substantially from classical Darwinism in basic logic and concept—for explanations of both stability and change must now be framed as compound results of a bal¬ anced interaction of levels, working in all possible ways (in concert, in con¬ flict, or orthogonally), and not as shifting optimalities built at a single level. On the second leg, an emphasis on constraints and channels implies a new set of operational concerns, and a revamping of the evolutionary research pro¬ gram. Internally imposed biases upon directions of change become a major subject for study—and the role of developmental patterns must again become prominent in evolutionary theory. (I must confess great personal pleasure in observing the rapid progress of this integration, as the wall between these two subjects seemed so frustratingly impenetrable when I published my first book, Ontogeny and Phylogeny, not so long ago in 1977.) On the third leg, a re-
The Modern Synthesis as a Limited Consensus newed appreciation for the shaping power of mass extinction must reinstate paleontology as a source of theory, and not merely a repository for the histor¬ ical unfolding of processes fully illuminated by microevolutionary studies. Thus, the new theory produced by the confluence of these critiques and their integration with classical Darwinism will not emerge from a simple act of generosity or noblesse oblige by previous orthodoxy. The new theory may re¬ main Darwinian in spirit (a “higher Darwinism,” see Gould, 1982b), but its development requires a wrenching from several key assumptions of classical Darwinism—not simply a smooth evolution from conventional precepts—as embodied in both the tripod of essential theoretical support, and the method¬ ology of uniformitarian extrapolation (the theoretical and methodological poles of Darwinism, as discussed in Chapter 2). Substantial change in any domain usually follows such a scenario, and cannot unfold in smooth and untroubled gradualistic continuity. The venera¬ ble Hegelian triad of thesis^antithesis->synthesis may not adequately de¬ scribe all examples of important change (see p. 23 for more on this general concept), but this classic philosophical model of tension and (often episodic) resolution seems more in tune with nature—or, to use the terminology of this book, higher in relative frequency among patterns of change. Human thought, unlike the evolution of life, does include the prospect of meaningful progress as a predictable outcome, especially in science where increasingly better understanding of an external reality can impose a fundamental orga¬ nizing vector upon a historical process otherwise awash in quirks of indi¬ vidual personalities, and changing fashions of cultural preferences. Surely our views on the nature of taxonomic order have progressed (in the sense of better consonance with the true causes of diversity ) from the eclecticism of Aldrovandi, to the coherent creationism of Tinnaeus, to Cuvier’s addition of a temporal dimension, to Darwin’s evolutionary synthesis of space, form, and time. The Hegelian triad proceeds by confrontation between old and new sys¬ tems (thesis and antithesis), and by their melding into a novel theory pre¬ serving worthy aspects of both—synthesis. But the continuing interplay of confrontation and reconstitution does not spin in an endless circle, proceed¬ ing nowhere. Useful synthesis builds a transformed structure, and does not merely shuffle an unaltered deck (or raise an unstable house of cards pre¬ cariously built from unaltered parts). Darwin constructed a powerful anti¬ thesis to older evolutionary views rooted in predictable progress and internal drives. Modern versions of the three critiques now present a worthy antithe¬ sis to the limitations of strict Darwinism. The second part of this book pre¬ sents this Hegelian antithesis, written in the hope and expectation of synthe¬ sis and improvement. The synthesis that must eventually emerge will build a distinct theoretical architecture, offering renewed pride in Darwin’s vision and in the power of persistent critiques—a reconstitution and an improve¬ ment, waiting for the next antithesis that must lead us onward to the next of many future syntheses in the wondrous, eternal play of mind and nature.
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PART II
TOWARDS A REVISED AND EXPANDED EVOLUTIONARY THEORY
k
V
CHAPTER EIGHT
Species as Individuals in the Hierarchical Theory of Selection
The Evolutionary Definition of Individuality AN INDIVIDUALISTIC PROLEGOMENON The perceived excesses of the French Revolution may have sapped English en¬ thusiasm for the tenets of Enlightenment Rationalism—the faith of Darwin’s grandfather Erasmus. The subsequent romantic movement stressed opposite themes of emotion vs. logic, and national variety vs. universal reason. Charles Darwin, who revered his grandfather but also loved Wordsworth’s poetry, re¬ ceived a firm grounding in both great philosophical and aesthetic traditions. He also—and perhaps as a direct result—maintained strong fascination for a central theme common to both movements, but for different reasons: the role of individuals as agents of change in larger systems. (The Enlightenment fo¬ cussed on individuals as effective intellectual agents and inherent bearers of rights—“unalienable” in Jefferson’s memorable phrase—and therefore as pri¬ mary causal and moral agents in themselves, not as expendable items of a larger collectivity. The Romantics exalted individual effort as the motive force of social change through the actions of occasional heroes of higher sen¬ sibility.) In any case, and whatever the deeper source, we do know that, as Darwin stitched together his theory of natural selection in 1838, he centered his ma¬ jor intellectual struggle in the few weeks before his “Malthusian” insight (Schweber, 1977) upon the role of individuals as primary causal agents of evolutionary pattern, even at largest scales (see full discussion in Chapter 2). He first studied the economic theory of Adam Smith through the major sec¬ ondary source then available—Dugald Stewart’s On the Life and Writing of Adam Smith. He expressed special fascination for Smith’s distinctive notion that the overall optimality of an economy might best (and paradoxically) be fostered by allowing individuals to maximize personal profit without re¬ straint (the doctrine known ever since as laissez faire, or “let do"—more roughly, “leave ’em alone”). He then read an extensive analysis of the work of the great Belgian statistician Adolphe Quetelet—particularly his central notion of Vbomme moyen (average man), based on the aggregation of indi¬ vidual attributes into collectivities. 595
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THE STRUCTURE OF EVOEUTIONARY THEORY In the context of this conscious and directed search, we should not be sur¬ prised that Darwin’s theory of natural selection rests upon the same central paradox that fueled Adam Smith’s system: postulate a cause based on individ¬ uals ruthlessly pursuing their own benefits; an ordered polity will then arise as an incidental side consequence. No dismissal of Paley’s omniscient God as the direct creator of general order could possibly have been more incisive, or more radical. We can therefore understand why Darwin insisted so strongly upon a single-level theory of natural selection—with struggle among individ¬ ual bodies as a virtually exclusive locus of causality (see Chapters 2 and 3 for extended analysis). The downward shift of agency, from a purposively benev¬ olent deity to the amoral self-interest of organisms, embodies the most dis¬ tinctive and radical aspect of Darwinism. Given Darwin’s intense and conscious desire to restrict causality to compet¬ ing organisms, I have been particularly struck, in researching and writing this book, by the inability of all the most diligent, and most thoughtful, early selectionists to make such a system work fully and consistently—despite in¬ tense and clearly focussed efforts to “cash out” Darwin’s vision. As discussed in Chapters 3 and 5, only two early evolutionists fully grasped the meaning of natural selection, and the logic behind Darwin’s restriction to the level of or¬ ganisms. The first, August Weismann, defended Darwin’s system with utmost zeal, as he spoke with pride about the Allmacht (all-might, or omnipotence) of natural selection. He began by advocating rigid adherence to Darwin’s level of organisms. But, in fighting the resurgent Lamarckism of late 19th cen¬ tury thought, Weismann had to descend a notch to postulate important “ger¬ minal selection” at the level of hereditary units. Late in his career, he recog¬ nized the logical generality enjoined by his admission of a second locus—that selection can work on objects with requisite properties at any level of the genealogical hierarchy. Weismann therefore articulated a fully hierarchical model of selection operating at several levels both below and above individ¬ ual organisms. Moreover, he developed this full theory not in retreat or as a hedge, but as a compelling extension of selection’s central logic—and as fur¬ ther testimony to an Allmacht even more inclusive. The second, Hugo de Vries, postulated a macromutational mechanism that logically precluded the production of new species by gradual selection of intrapopulational variation. In so doing, he committed intellectual parricide against his personal hero, Charles Darwin—and this mental act caused him great psychological distress. He assuaged his feelings of guilt, and showed his understanding of the abstract logic of selection, by insisting that he remained loyal to Darwinism—but at a higher level of selection among species, rather than among organisms (see pp. 446-451). We must also not neglect the man who had invested the most effort in holding the line at organismal selection, and who had the most to lose if such a restriction could not work—Charles Darwin himself. Darwin struggled mightily to bring all evolutionary phenomena, including a host of apparently exceptional items from hymenopteran colonies to prevention of interspecific hybridization, under the umbrella of organismic selection, often with truly in-
Species as Individuals in the Hierarchical Theory of Selection genious formulations—and he ultimately failed, as all others had. The logic of his argument led Darwin to postulate higher level selection at two crucial points (see pp. 127-137)—to explain the evolution of altruism in human so¬ cieties by interdemic selection, and to encompass multiplication of species un¬ der his essential “principle of divergence” by a partial appeal to species selec¬ tion. If none of the most rigorous and savvy early Darwinians could render evolution without some appeal to selection at levels higher than individual organisms, shall we not tentatively conclude that both the logic of the theory and nature’s empirical record compel such an expansion, and the attendant notion of hierarchy?
THE MEANING OF INDIVIDUALITY AND THE EXPANSION OF THE DARWINIAN RESEARCH PROGRAM
We may agree with the strictest formulation of agency in Darwinian theory: natural selection works by a struggle (actual or metaphorical) among individ¬ uals for personal reproductive success. In other words, selection occurs when properties of a relevant individual interact with the environment in a causal way to influence the relative representation of whatever the individual con¬ tributes to the hereditary make-up of future generations. If we place the theory’s causal focus so squarely upon individuals as agents, then we might suppose that Darwin’s unitary perspective must apply: all results at all evolu¬ tionary scales must cascade from the causal process of selection among indi¬ viduals, defined in the conventional vernacular manner as the bodies of or¬ ganisms. But, as Hamlet said of the fears that prevent suicide (an adaptation, some would no doubt argue, for keeping humans viable as Darwinian agents), “ay, there’s the rub.” What is an individual? Are vernacular bodies the only objects in nature that merit such a designation—especially when discrete “bodiness” doesn’t always define an unambiguous individual at the focal level of Darwin’s intent (not to mention the difficulties encountered in trying to characterize entities at levels above and below bodies in the genealogical hierarchy of nature)? For example, biologists spent more than a fruitless century trying to decide whether the parts of siphonophores are “persons” in a colony or organs of an organism—only to recognize that the question cannot be answered because both solutions can justly claim crucial and partial merit (see Gould, 1984d). Are grass blades or bamboo stalks bodies in their own right (as some aspects of functional organization suggest), or parts (called ramets) of a larger evolu¬ tionary individual (called a genet)? Do our feelings about definition shift when ramets become spatially discrete and therefore look just like conven¬ tional bodies—as in the parthenogenetic offspring of an aphid stem-mother (designated, in their totality, as a single El, or evolutionary individual, by Janzen, 1977)? And what shall we do with discrete bodies that maintain some genetic variation among themselves (and cannot, therefore, form a set of identical ramets), but operate together as differentiated items (analogs of or-
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THE STRUCTURE OF EVOLUTIONARY THEORY gans) in a larger “totality” like a beehive or ant colony with a single queen? Wilson and Sober (1989) have urged a revival for the old concept of “super¬ organism” in such circumstances. As so much uncertainty surrounds the Lssue of how we define an “individ¬ ual” at the supposedly unambiguous level of Darwin’s own intent, we should not be surprised that attempts to restrict the concept to organic bodies have yielded more confusion than resolution. Perhaps we should try a different and more general approach. Perhaps we should attempt to specify a set of minimal properties required to designate an organic entity as an “individ¬ ual”—and then ask whether any objects at levels above or below traditional bodies possess these properties, and therefore qualify for inclusion under an expanded concept of individuality. If so, we might obtain a useful definition divorced from the happenstances of scale, and therefore sufficiently general to provide a deeper (and clearer) understanding for this central concept in Darwinism. This subject has generated an enormous and often confusing literature throughout the history of Darwinian thought—and more so than ever before during the past twenty years. Some colleagues may wish to throw up their hands and brand the entire enterprise with labels usually invoked pejoratively by scientists—merely “semantic” or “philosophical.” Indeed, several of the finest contemporary philosophers of science have devoted considerable at¬ tention to this issue—see, for example and in alphabetical order, Brandon (1982), Hull (1980), Lloyd (1988), Sober (1984), and Wimsatt (1981). But I believe that both the volume and the confusion arise for two reasons that compel primary attention to the subject: the issue is both exceedingly difficult and enormously important/1' The best scholars tend to gravitate to the most fascinating and portentous questions—and the confluence of extensive con¬ sideration by the most prominent philosophers of science (as mentioned above) and the most thoughtful evolutionary biologists from early days (Dar¬ win, Weismann, de Vries, as discussed above) to current times cannot be accidental or wrong-headed. As a testimony to this current concern, and again in alphabetical order, I cite as a small sample: Arnold and Fristrup *1 have struggled with this issue all my professional life, and have often wondered why the questions raised seem so much more recalcitrant, and so much more cascading in impli¬ cations, than for any other major problem in Darwinian theory. I don’t think that mere per¬ sonal stupidity underlies my puzzlement—or rather, if so, the mental limitations must be largely collective, because other participants share the same struggle and express the same frustrations. I don’t mean to sound either grandiloquent or exculpatory, but I seriously wonder if some of the difficulties might not arise largely from limitations in the common mental machinery of Homo sapiens. Levi-Strauss and the French structuralists may well be correct in holding that human brains work best as dichotomizing machines at single levels. We make our fundamental divisions by two (nature and culture or “the raw and the cooked” in Levi-Strauss’s terms, night and day, male and female), and we therefore experi¬ ence great mental difficulty with continua, and with any system other than a two-valued logic (hence Aristotle’s law of the excluded middle, and other similar guides). We are espe¬ cially ill-equipped to think hierarchically, and to juggle simultaneous influences from sev¬ eral nested levels upon the foci of our interest. The hierarchical theory of natural selection rests upon all these intrinsically difficult modes of reasoning.
Species as Individuals in the Hierarchical Theory of Selection (1982), Dawkins (1976, 1982), Eldredge (1985a), Fisher (1958), Ghiselin (1974a and b), Leigh (1977), Lewontin (1970), Maynard Smith (1976), Stan¬ ley (1975, 1979), Vrba (1980; Gould and Vrba, 1982; Vrba and Gould, 1986) , Williams (1966, 1994), D. S. Wilson (1983), and Wright (1980). Col¬ laborations between philosophers and biologists have also added to the inter¬ est (for example, Wilson and Sober, 1994; Sober and Wilson, 1998; Lloyd and Gould, 1993; Gould and Lloyd, 1999). Discussion of this most difficult and most important subject may be orga¬ nized in a hundred different ways. I have chosen a point of entry that may seem peculiar or indulgent as an abstract philosophical question tenuously re¬ lated to the “real” biology of organic objects: are species individuals or classes? As a twofold justification for this strategy, I found, first and person¬ ally, that I could best organize this material and place all subjects into logical sequence, if I started here and worked systematically outward through a par¬ ticular net of implications. (Others, no doubt, would choose different begin¬ nings and construct just as sensible and comprehensive a sequence.) Second and collectively, this particular philosophical question has been widely and passionately discussed in the biological literature, and has struck several sci¬ entists (e.g. Eldredge, 1995) as a potential centerpiece unwisely relegated to a peculiar periphery by many scholars. In 1974, Michael T. Ghiselin published an article in Systematic Zoology under a title that I found insufferably self-indulgent at the time (especially since his manuscript directly followed my own densely empirical article on lo¬ cal geographic variation in the land snail Cerion bendalli on the Bahamian is¬ land of Abaco), but have since come to view as adequately justified: “A radi¬ cal solution to the species problem.” In short, Ghiselin argued that many classical problems about species (not primarily or especially related to this chapter’s topic of levels in selection) could be instantly resolved if we—in the Pauline manner of “scales falling from the eyes”—reversed our customary definition of species as classes (or universal categories that can “house” ob¬ jects) and reconceptualized them instead as individuals (or particular things). A species then becomes a singular item—an evolutionary entity defined by both a unique historical genesis and a current particular cohesion. I will not trace the large and complex trail that Ghiselin’s proposal gener¬ ated in the scientific and philosophical literature (see, for example, Ghiselin, 1987) . In my reading and understanding, I do not think that any clean resolu¬ tion can be stated, or even any consensus described. Perhaps we might best acknowledge, with Mayr (1982a and b), that the term “species,” as conven¬ tionally used and understood, includes statements about both classes and in¬ dividuals. In this sense, the extensive discussion of Ghiselin’s proposal sharp¬ ened our thinking, but provided no closure. In another sense, however, and following a common (largely sociological) pathway in science, the explicit airing of such an interesting theme launched, or at least impacted in major ways, a substantial set of theoretical issues, in¬ cluding two of central importance to this book: the nature of evolution as a historical discipline, and the definition of individuality as crucial to the “units
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THE STRUCTURE OF EVOLUTIONARY THEORY of selection” problem. In his initial article, Ghiselin (1974b, p. 543) dimly perceived the key implication for hierarchical selection if species be con¬ strued as individuals rather than classes, and selection (by Darwin’s defini¬ tion) works on individuals—namely, that ^election must also operate among species-individuals (and, by extension, potentially at several levels in a hierar¬ chy of units, each properly construed as an “individual”). But Ghiselin did not complete his argument and grant full evolutionary individuality to spe¬ cies. “Species are units, and they have evolutionary importance, but the same may be said of organisms. Doubtless both organisms and species specialize. And probably organisms become adapted but species do not, except in so far as they consist of adapted organisms” (Ghiselin, 1974b, p. 543). David Hull (1976), in the first major philosophical extension of Ghiselin’s proposal, firmly linked the concept of species as individuals to the older is¬ sue of units (or levels) of selection, thus properly tying the rationale for a causal theory of hierarchical selection to the generalization of Darwin’s key insight that selection can only operate by the differential reproductive success of “individuals”: “Entities at various levels of organization can function as units of selection if they possess the sort of organization most clearly exhib¬ ited by organisms: and such units of selection are individuals, not classes” (Hull, 1976, p. 182). In his important later article—the locus classicus of the pivotal distinction between “replicators” and “interactors” (see next section of this chapter)—Hull then added (1980, p. 315): “Individuality wanders from level to level, and as it does, so too does the level at which selection can occur. ” If the rationale for a hierarchical theory of selection resides in the expan¬ sion of “individuality” to several levels of biological organization (see Gould and Lloyd, 1999), then we must specify a set of criteria that any material con¬ figuration must meet to merit designation as an “individual.” We may, I think (see Gould, 1994), most usefully divide these criteria into two categories: (1) requirements in ordinary language for granting individuality to any configu¬ ration (vernacular criteria); and (2) requirements in Darwinian theory for re¬ garding any entity as an evolutionary individual, or potential agent of selec¬ tion (evolutionary criteria). (I trust that, despite a traditional ethos contrary to such an admission, all thoughtful and self-introspective scientists no longer feel threatened or disloyal in acknowledging that all definitions must be the¬ ory laden—see Kuhn, 1962, for the classic statement.) We must also resolve one other terminological confusion before listing the criteria of individuality. What word shall we use as a general term for the dis¬ crete “thing” that can serve as a unit of selection at various hierarchical lev¬ els? In an important article, a manifesto for reviving interest in the power of group selection and the validity of the hierarchical model in general, D. S. Wilson and E. Sober (1994) suggest that we use the term “organism” for the generality (and therefore speak of “species organisms,” “gene organisms,” and so forth), while restricting the word “individual” to organic bodies (you, me, the oak tree, and the barnacle) at the conventional level of Darwinian concern. They choose this definition because they emphasize—I would say
Species as Individuals in the Hierarchical Theory of Selection overemphasize (see Gould and Lloyd, 1999)—functional cohesiveness among their general criteria of “thingness,” a property better captured by “organ¬ ism” than by “individual” in vernacular English. I strongly urge the opposite and more conventional solution. This issue, I fully recognize, only concerns words, not the empirical world. But we get so muddled, and waste so much time, when we fail to be clear about words and definitions—especially when various scholars use the same word in different, or even opposite, ways—as in the classic confusion generated when molecu¬ lar biologists began to use “homology” for the percent of similarity in genetic sequence between two organisms, rather than for the well-established and en¬ tirely different concept of joint possession due to common ancestry. Fortu¬ nately, in this case, we evolutionists have apparently managed to persuade our molecular colleagues to respect conventional usage, and to call their im¬ portant concept “sequence similarity,” or some such. No one would create such a muddle on purpose, but this particular confu¬ sion already exists—and some common ground must therefore be established if we wish to address this growing and important literature without a peren¬ nial need to stop, translate, and bear linguistic idiosyncrasies continually in mind whenever we read a paper. At the moment, most authors use “organ¬ ism” for the Darwinian body (me and thee), and “individual” for the general¬ ized unit of selection at any hierarchical level—while others (like Wilson and Sober) employ reversed definitions. I strongly urge the former course—organisms as conventional vernacular bodies, and individual for the generalized term—for two reasons. First, this decision represents more common usage, both in vernacular English and among biologists. (Several academic departments include the phrase “organismic biology” in their title to defend a continuing focus on entire bodies against molecular claims for hegemony. But if genes are organisms as well, this ploy will not work!) Second, the technical definition of an “individual” in academic philosophy, and the spread of this terminology in the large litera¬ ture inspired by Ghiselin’s arguments for species as individuals (summarized above), grants priority to “individual” as the general term, and “organism” as the restricted body. Ghiselin (1974b, p. 536) clearly defended this usage in his original definition: “In logic, ‘individual’ is not a synonym for ‘organism.’ Rather, it means a particular thing.” And Hull (1976, p. 175) explicitly la¬ beled the application of “organism” to higher-level objects as misleading because vernacular language so strongly equates “organism” with discrete bodies. He then urged “individual” as the general term, as advocated here: “From the point of view of human perception, organisms are paradigm indi¬ viduals. In fact, biologists tend to use the terms ‘organism’ and ‘individual’ interchangeably. Thus, biologists who wish to indicate the individualistic character of species are reduced to terming them ‘superorganisms.’ The same claim can be expressed less misleadingly by stating that both organisms and species are individuals.” In discussing criteria of individuality, I will focus on species as paradigms for higher-level evolutionary entities for two reasons: (1) because I believe
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THE STRUCTURE OF EVOLUTIONARY THEORY that a proper theory of macroevolution, the central concern of this book, rests upon such a proposition; and (2) because species seem so maximally un¬ like discrete “things” to many biologists, thus making the correction of this false impression a prerequisite for accepting the full hierarchical model of se¬ lection. But species can claim no favored status in the hierarchical model, and I use them here only as an example—so that the argument may then proceed to a full set of levels, each characterized by a valid kind of individual acting in a distinctive way.
Criteria for vernacular individuality When we apply the term “individual” in ordinary English, we envisage a set of properties centered upon uniqueness, discreteness, functionality, and cohe¬ sion considered both spatially and temporally. To be a unique “thing,” and not just a part of a continuum, a named object must clearly begin and end— and must remain its definable self throughout a continuous existence. We may, I think, best summarize this intuition in three criteria. To be called an in¬ dividual, a material entity must have: ♦ a discrete and definable beginning, or birth; ♦ an equally discrete and definable ending, or death; and ♦ sufficient stability (defined as coherence of substance and constancy of form) during its lifetime to merit continuous recognition as the same “thing.” I realize, of course, that the third criterion amalgamates several crucial no¬ tions into a single statement. We might specify at least four properties in¬ volved in our ordinary concept of “sufficient stability” for individuality: Change. An individual may undergo some, even substantial, change during its lifetime, but not so much either to become unrecognizable or to en¬ courage redefinition as a different thing—and, particularly, for temporal se¬ quences of individuals, not so much alteration that late stages come to resem¬ ble the next-named individual in a sequence more than the early stages of the same individual. Discreteness and cohesion. An individual must maintain clear and coherent boundaries during its lifetime. Parts should not “ooze out” into other individuals, while components of other individuals should not enter and become incorporated. Continuity. An individual cannot fade in and out of existence during its lifetime, but must maintain material continuity throughout. Members of classes, on the other hand, are not so constrained, for classes are defined by common properties, not by historical continuity. As Hull (1980) argues, the class of gold atoms does not require continuity or filiation. If all gold disap¬ peared, its position on the periodic table would remain—and an element later reconstituted with the right atomic particles and requisite properties would still, and legitimately, be called gold. But if all peacocks die, the species-indi¬ vidual Pavo cristatus disappears forever. Even if some human engineer re-
Species as Individuals in the Hierarchical Theory of Selection tained an electronic record of the entire Pavo cristatus genome, and future technology permitted chemical reconstitution from nucleotides, we couldn’t call the resurrected creature a member of Pavo cristatus, even if the reconsti¬ tuted object looked and acted like an extinct peacock of old. Functionality, or organization.
We expect that, at least in some
crucial ways, the parts of an individual will work together so that the individ¬ ual functions in a distinctive and cohesive way. This criterion, though crucial as we will see to the second set of evolutionary criteria, may be the least im¬ portant (perhaps even dispensable) for vernacular definitions. If a bounded object maintained all the other listed properties, but failed to do anything as an entity (and acted, instead, largely as a repository of separate parts), we would still call the object an individual, however inert and uninteresting. Conventional organisms certainly possess all these properties—as well they must, for the bodies of complex animals established our vernacular Western paradigm for the general concept of individuality. Yet note that, even here, at the point of maximal clarity, some fuzziness and indefiniteness plague every criterion. Consider human bodies, the inevitable exemplar of the paradigm. Our lives have reasonably discrete beginnings—but if a true moment could be defined without ambiguity, then our social and political debates about abor¬ tion would require new terms and engage different issues. Death might seem even more definable and momentary—but, again, fuzziness and ambiguity plague our definitions, leading to complex, and often heart wrenching, medi¬ cal and legal wrangles. Perhaps our bodies pass the criterion of “sufficient stability” with more clarity. We don’t fuse with others, or rise again from the dead (at least in a material world that science can adjudicate). We are cer¬ tainly designed to operate discretely, even if our actions be dysfunctional. All our chemicals, and most of our cells, undergo periodic replacement—but I re¬ main myself and continue to look sufficiently like my baby pictures (though not much like my early embryonic form with tail and gill slits!). So organisms surely pass muster as individuals. But we encounter prob¬ lems, including several classic issues subject to endless discussion in the litera¬ ture, when we try to assign individuality at other (particularly higher) levels of the organic hierarchy. For example, the standard objection to interdemic selection (see pp. 648-652) holds that too many demes fail the criterion of “sufficient stability”—for they may not persist long enough to matter in evo¬ lution, and their borders may be too “leaky” as organisms move in and out in the absence of reproductive isolation between the parts (organisms) of differ¬ ent demes. All too frequently, demes may operate, in Dawkin’s apt metaphor (1976, p. 36), “like clouds in the sky or dust storms in the desert . . . tempo¬ rary aggregations or federations.” Defenders of classical “group” (i.e. interdemic) selection recognize these problems of course, and all workable models have been purposely con¬ structed to overcome such objections by specifying conditions that will per¬ mit demes to fulfill the defining criteria listed above. In fact, the classical em¬ pirical issue of our literature on group selection asks whether demes can
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THE STRUCTURE OF EVOEUTIONARY THEORY “hold together” long and discretely enough so that the differential prolifera¬ tion and survival of some demes vs. others can propel the general increase of an “altruistic” allele (promoting demic success), even while the allele’s fre¬ quency declines within groups as “selfish” alternatives prevail in conven¬ tional natural selection among bodies. If demes can “hold together” by this operational criterion of evolutionary outcomes, then they possess “sufficient stability” to be regarded as individuals on functional grounds within selec¬ tionist theory. Traditionally, biologists have not been willing to imbue species with these requisite criteria for individuality. Species, in an argument dating to both Lamarck and Darwin, have often been construed as mere names of conve¬ nience attached to segments of evolving continua without clear borders. Un¬ der this gradualistic and anagenetic view (see Fig. 8-1), a species near the end of its arbitrary existence must be phenotypically more similar to a forthcom¬ ing descendant than to the initial ancestor. (Indeed, under strict gradualism, we even face the definitional absurdity that the last generation of an ancestor should be reproductively isolated from its own offspring—that is, the first generation of a new descendant. Some creatures may eschew such incest on moral or adaptive grounds, but no one would gainsay the biological possibil¬ ity.) Thus, on this traditional view, species cannot maintain sufficient tempo¬ ral stability to be called individuals. In addition, species do not have discrete birth points if they branch from their ancestors at rates no different from characteristic tempos of transformation during their subsequent anagenetic lifetimes (see Fig. 8-1). At most, some species display clear termination in ex¬ tinction (but others evolve gradually to descendants.) Thus, species do not function as good vernacular individuals if gradualism and anagenesis pervade the history of life. Even so—or as long as most species arise by splitting of lineages rather
/ Species 3
/
/ Transitional forms ^ species uncertain /
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Transitional forms species uncertain
cS Species 1
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c> Morphology
8-1. The traditional view (as depicted, but not defended, in Lipps, 1993) of why species cannot be construed as proper biolog¬ ical individuals but only as arbitrary segments of a smooth and unbreakable continuum.
Species as Individuals in the Hierarchical Theory of Selection
than by wholesale transformation, no matter how gradual the tempo of branching—the individuality of species may be maintained in some technical sense, though only by violation of our vernacular intuitions. After all (see Fig. 8-2), so long as branching points (or fuzzy intervals) can be temporally lo¬ cated at all, then species do have definable intervals of existence, and can be individuated on this basis, even if their life courses violate our usual notions about sufficient morphological stability. Many evolutionary biologists have failed to recognize that the so-called cladistic revolution in systematics rests largely upon this insistence that spe¬ cies (and all taxa) be defined as discrete historical individuals by branching (leading to the rule of strict monophyly)—and not as classes with “essential” properties by appearance (leading to the acceptance of paraphyletic groups). Many biologists reject (and regard as nonsense) the cladistic principle that no species name can survive the branching off of a descendant—and that both branches must receive new names after such an event, even if the ancestral line remains phenotypically unchanged. But this counterintuitive rule makes sense within cladistic logic—for cladists define new entities only as products of branching (the word clade derives from a Greek term for branch). A trans¬ forming species that does not branch cannot receive a new name even if the final form bears no phenotypic resemblance or functional similarity to the original ancestor. Thus, if such extensive transformation occurs in un¬ branched lineages, a cladist, by failing to designate a truly different anatomy with a distinctive name, retains the technical individuality of species at the price of a severe assault against legitimate intuition. Can we find any solution to this dilemma? Must we either deny that species can be viewed as individuals, or else accept a logically “pure” definition based on branching, but strongly in violation of vernacular usage? I suggest that this issue can be resolved empirically, and need not persist as a defini¬ tional or philosophical conundrum. If gradualism and anagenesis prevail in
8-2. A repeat of Figure 7-2 to show that, even under the most gradualistic and anagenetic models, species can still be individuated under a conception of evolu¬ tion as a branching process at the species level.
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THE STRUCTURE OF EVOLUTIONARY THEORY nature, then all the aforementioned problems cannot be avoided and apply in *
force. But suppose, as Eldredge and I have long argued in our theory of punc¬ tuated equilibrium (see Chapter 9), that gradualistic anagenesis occurs only rarely in nature, and that the great majority of species remain essentially sta¬ ble throughout their geological lifetimes. (Our concept of stasis recognizes that species fluctuate mildly throughout time, to an extent no different from ordinary geographic variation among demes of a species at any one moment, but we hold that mean values of phenotypes generally do not change in a cu¬ mulative or directional manner.) Suppose also that species, on geological scales, branch in unresolvable “moments.” (In nearly all geological circum¬ stances, single bedding planes amalgamate the events and accumulated results of several thousand years.) If species tend to originate in thousands to tens of thousands of years—that is, with glacial slowness by the inappropriate crite¬ rion of a human lifetime in potential observation—and then to persist in sta¬ sis for millions of years, their origin becomes instantaneous in geological time, and species arise as discrete individuals at this proper macroevolu¬ tionary scale. Of course, some fuzziness must attend the origin of a species, for we acknowledge that macromutational beginnings in leaps of a single generation rarely, if ever, occur. But when “fuzziness” occupies only a thou¬ sand years in a million—that is one tenth of one percent of later existence in stasis—then geological indefiniteness surely does not exceed even the relative duration of the fuzziness (9 months in some 80 years) attending the embryological beginning of human personhood! Under punctuated equilibrium, the remaining criterion of discrete death achieves even clearer definition—for nearly all species disappear by extinc¬ tion (“living on” only through their progeny of daughter species with new names and individualities), and not by gradual bodily transformation into something else. Species deaths, at geological scales, are surely more discrete and “momentary” than human deaths scaled against the lengths of our life¬ times. In summary, then, species that originate by branching can be individuated even under the assumption that gradualistic anagenesis prevails during the history of most species lifetimes (but only by violating our vernacular concep¬ tion of “personhood” or individuality). However, if punctuated equilibrium prevails as an empirical proposition (see Chapter 9 for defense of this conten¬ tion), then species are individuals—in some cases much “better” individuals than conventional bodies of organisms—by all vernacular criteria. Under punctuated equilibrium, species originate at points of birth with initial fuzzi¬ ness confined to an insignificant (usually unmeasurable) moment properly scaled against later existence in stasis. They experience even clearer moments of death, for nearly all species terminate by true extinction and not by trans¬ formational passage into a descendant that vernacular (non-cladistic) usage will wish to recognize with a different name (a phenomenon called “pseudoextinction” by paleontologists). And species surely maintain “sufficient sta¬ bility” during their geological lifetimes by all criteria outlined on page 602. They remain discrete by reproductive isolation (conventionally cited, ever
Species as Individuals in the Hierarchical Theory of Selection since Buffon, as the chief criterion of “specieshood”). They function as a unit and persist continuously. Above all, they do not change substantially in phe¬ notype—the crucial concept of stasis. Surely, the average species in stasis un¬ dergoes less temporal change (with less directionality) than human bodies experience in our passage from babyhood through adult vigor and into senes¬ cence. If humans retain discrete personhood through all these slings and ar¬ rows of ontogeny, then species (under punctuated equilibrium) function as equally good or even better individuals by the same criteria of vernacular definition. In describing exceptions and fuzzinesses in the application of these vernac¬ ular criteria to organisms, and acknowledging that species face the same dif¬ ficulties of definition, Hull (1976, p. 177) wrote: “However, exactly the same questions arise for both. If organisms can count as individuals in the face of such difficulties, then so can species.” But Hull assumed that these common problems plague species far more intensely than they threaten organisms. I would suggest that the opposite situation may prevail in nature: species may be even better individuated than organisms when punctuated equilibrium ap¬ plies (and we consider species at their appropriate scales of geological time). This issue unites these two chapters in a crucial link between the theory of punctuated equilibrium (Chapter 9) and the classical debate about “units” or “levels” of selection (Chapter 8)—a conjunction that underlies my views on the importance and validation of macroevolutionary theory. Interestingly, albeit through a glass darkly, Hull (1976) grasped the logical link between the phenomenology of punctuated equilibrium and the defini¬ tion of species as individuals in his first important paper on this subject—even though he had not, by this time, encountered our empirical and theoretical arguments for such a pattern (Eldredge, 1971; Gould and Eldredge, 1971; Eldredge and Gould, 1972). (In his more inclusive review of 1980, Hull then explicitly joined our particular claims to the defense of species as individuals.) Hull begins by stating the problem (1976, p. 185): “Earlier I described indi¬ viduals as reasonably discrete, spatiotemporally continuous and unitary enti¬ ties individuated on the basis of spatiotemporal location rather than similar¬ ity of some kind. But one might object that species lack these characteristics. For example, in most cases new species arise gradually.” Hull then recognized that some neontological models of speciation acceler¬ ate the rate of branching relative to the supposedly standard rate of anagene¬ sis within species—and that such an acceleration will sharpen the definability of species by the criterion of discrete birth: “But there are processes in nature which serve to narrow the boundaries between ancestral and descendant spe¬ cies . . . The end result is that the number of organisms intermediate between the ancestral and descendant species is reduced considerably” (1976, p. 185). Finally, Hull stresses the important point that all individuation, at any ap¬ propriate scale, entails some fuzziness at the boundaries—and that species therefore need not be construed as “worse” individuals than bodies (1976, p. 185). “If processes similar to those just described are common in nature, then the boundaries between ancestral and descendant species can be nar-
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THE STRUCTURE OF EVOEUTIONARY THEORY rowed considerably, though not to a one-dimensional Euclidean line. But, of course, the replication of organisms does not happen instantaneously either. If absolutely discrete boundaries are required for individuals, then there are no individuals in nature. It is only our relative size and duration which make the boundaries between organisms look so much sharper than those between >
*
species.” But, to continue the Euclidean metaphor, and using an appropriate ruler with (say) a minimally noticeable geological increment equal to 10,000 years, the boundaries of many species do become momentary under punctuated equilibrium. Stasis persists for a long run of increments. At a commonly ob¬ served duration of 5 to 10 million years for marine invertebrate species in the fossil record (Raup, 1985; Stanley, 1985), one thousand increments of stasis would represent the geometry of a species lifetime, while even a million for the much shorter average duration of terrestrial mammalian species yields 100 increments. By comparison, many (probably most) events of speciation unroll within a single increment—leading to abrupt and momentary origin at geological scales, and the right-angle convention that has become stan¬ dard for plotting the emergence of species under punctuated equilibrium (see Fig. 8-3).
Criteria for evolutionary individuality The vernacular criteria discussed above provide necessary, but obviously in¬ sufficient, conditions for identifying an entity as an evolutionary “individual” with the capacity to act as a causal agent in a process of Darwinian selection. Most unambiguous vernacular individuals cannot operate as Darwinian ac¬ tors. The earth, for example, surely merits designation as a well-defined indi¬ vidual—with a specifiable birth (perhaps attended by some initial fuzziness as a primordial fireball), sufficient stability over billions of years (including enough climatic homeostasis to provide a stage for the history of life), and a forthcoming rapid death (presumably by absorption after the sun burns out some five billion years from now, and expands in diameter at least to the orbit of Jupiter). But the earth remains “infertile” in the crucial Darwinian sense of reproductive potentiality. Planets do not have children, and therefore cannot function as Darwinian individuals. I do not cite this example to win an argument by ridicule, but rather to em¬ phasize, once again, that all definitions must be embedded within theories. Mere vernacular individuality does not suffice for identification as a causal actor in Darwinian theory. Evolutionary individuality (or, more strictly, Dar¬ winian individuality, for different theories of biological change may entail other criteria) requires an additional set of attributes rooted in two features of Darwin’s world: the genealogical basis of evolution as a branching tree, and the causal efficacy of selection as the leading process of evolutionary change. Reproduction. Darwinian individuals must be able to bear children.
Biological evolution is defined as a genealogical process. Darwinian evolution operates by the differential increase of your progeny (or whatever you pass
Species as Individuals in the Hierarchical Theory of Selection into future generations) relative to the progeny of other individuals within the larger entity of your membership. Inheritance. Your children must, on average, be more like you than
like other parents of your generation—so that evolution may proceed by the differential increase of your own heritable attributes (a requirement of Dar¬ winian systems, not of all conceivable evolutionary mechanisms). In other words, a principle of inheritance must prevail to permit the tracing of genea¬ logical patterns—so that the relative reproductive success of ancestors may be assessed. Variation. This criterion lies so deeply, and so fundamentally, within
the constitution of Darwinism as a revolutionary ontology (and not just as a theory of evolution), that we should, perhaps, not even list variation as a sep¬ arate criterion, but merely state that this conception underlies all Darwinian thinking. We can hardly imagine a more radical restructuring of the material
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THE STRUCTURE OF EVOLUTIONARY THEORY world than the Darwinian shift to variation among members of a popula¬ tion as an ultimate and irreducible reality (see Mayr, 1982b; Gould, 1996a)— a reversal of the old Platonic notion that essences (approximated empiri¬ cally by measuring mean values, or by trying to construct an abstract ideal form and then searching for a closest actual embodiment) define the nature of things, and that variation among actual individuals (organisms in pop¬ ulations, in our most relevant example) can only be construed as “acci¬ dental,” and judged by relative departure from a materially unattainable ideal. Heredity and reproduction work in concert with variation to empower Darwinian selection in genealogically recognizable lineages. The failure of any criterion debars Darwinian evolution as a genealogical process. An ab¬ sence of reproduction, for example, enforces an oddly limited form of “evolu¬ tion” restricted to rules (or vagaries) of change within one or a number of in¬ dividuals, all separately constructed at the outset. Vernacular usage, in fact, does apply the term “evolution” to some nongenealogical systems of this sort—as in the “evolution” of stars along the H-R sequence. But the causes of such systematic temporal changes, unfolding predictably under laws of nature (and not by the contingencies of variational history), differ so pro¬ foundly from Darwinian evolution that we really should insist upon different words for these maximally disparate modes of history (Gould, 2000a). (A great burden of misunderstanding, in both popular and professional cultures, must be ascribed to our confusing use of common terminology for such dif¬ ferent causes. Many interested laypeople feel that biological evolution must unfold by internal necessity just as stars follow their predictable sequences and as galaxies expand following the big bang. And many professional evolu¬ tionists, suffering from the common affliction of physics envy, and immured in the reductionistic biases of Western scientific culture, have tried to find pro¬ gressive patterns directly imposed by natural law, where Darwinian contin¬ gency actually reigns.) An absence of variation also stymies Darwinian change by eliminating the raw material or substrate for any selective mechanism. Evolution in non¬ varying populations might be treelike and genealogical, but such a process could not be Darwinian. One would have to imagine some very unearthlike way to generate change and diversity—for example, random dispersal of ini¬ tially identical creatures to varying environments, followed by a Lamarckian or directly inductive process of heritable environmental stamping upon all members of a population. Variation without heredity (that is, an absence of correlation between properties of offspring and parents) also stymies Darwinian causality. Selec¬ tion could occur in a single generation. That is, the biggest or the ugliest might outreproduce all others, or even ruthlessly murder all small and beauti¬ ful conspecifics—but to what evolutionary avail, if the offspring of survivors then reconstitute all the original variation in original proportions? If varia¬ tion occurred without correlation to parental constitution, but with inherent bias in a given direction—so that even random mortality produced a trend—
Species as Individuals in the Hierarchical Theory of Selection then evolution would occur. But we have always labeled such styles of inter¬ nally-directed change as non-Darwinian, with Lamarckism as a primary and historically most influential example. Interaction. At each level, the varying individuals of an evolving pop¬
ulation (organisms of a deme, demes of species, species of a clade) must inter¬ act with the environment in such a way that some individuals achieve rela¬ tively greater reproductive success as a causal result of heritable properties manifested by these fitter, and not manifested (or not as effectively expressed) by less fit individuals. This causal claim embodies the key feature of natural selection as an active process. In other words, we must be able to devise a testable causal scenario about why the differential possession of certain heri¬ table properties yields increased reproductive success. These statements inevitably engage the crucial issue of whether we should define selection by this causal interaction of individuals and environments, or by the product actually transmitted to future generations (see next section). The logic of Darwinism dictates that the form of heredity’s product-—how¬ ever fascinating in variety across nature’s scales—cannot specify agency of selection. Interaction with environment defines agency (Lloyd and Gould, 1993; Gould and Lloyd, 1999)—and agents must be individuals (by both ver¬ nacular and evolutionary criteria). Some interacting individuals (like genes) usually pass faithful copies to the next generation. Others (like species) pass inevitably modified copies that are still more like themselves than like any other individual at their level. Still others (like sexual organisms) disaggregate their personhood and pass hereditary pieces and particles. All these different strategies for hereditary passage permit us to recognize interacting individuals as causal agents of Darwinian selection. The special and unusual tactic of sexual organisms may seem curiously indirect (and we all know the enormous and confusing literature devoted to this subject), but disaggregation works as well as relatively faithful passage, so long as the es¬ sential Darwinian imperative remains in force: that is, so long as selectively successful individuals manage to bias the next generation with relatively more of their own hereditary material—however that material be passed or packaged. The “goal” of natural selection cannot be defined by faithful repli¬ cation, but rather by relative “plurifaction,” or “more-making.”"' The indi¬ vidual that plurifies by increasing the percentage of its contribution to the he¬ redity of the next generation (however the units or items of heredity be constituted) gains in the evolutionary game. And we call the game Darwinian if plurifaction occurs by a causal interaction between properties of the suc¬ cessful individual and its environment.
*In the late 1970’s and early 1980’s, I engaged in long and vociferous arguments with my graduate students Tony Arnold and Kurt Fristrup about the criteria of species selection. (As discussed on pp. 656-670, I now believe that they were right, and I was wrong.) In the course of these discussions, we developed this idea and name of “more-making” or pluri¬ faction. (If manufacture means, literally, making it by hand, and petrifaction means turning it into stone; then plurifaction simply means making more of it.) I do not now remember who first devised the word, or who contributed most to the concept’s codification.
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THE STRUCTURE OF EVOLUTIONARY THEORY As for the vernacular criteria previously discussed (see pp. 602-603), these specifically evolutionary criteria teach us that organisms are not the only indi¬ viduals capable of acting as units of Darwinian selection. In particular, and continuing to use species as a “type” example of individuality at higher lev¬ els, all evolutionary criteria apply to the species as a basic unit of macro¬ evolution. Species have children by branching (in our professional jargon, we even engender these offspring as “daughter species”). Speciation surely obeys principles of hereditary, for daughters, by strong constraints of homology, originate with phenotypes and genotypes closer to those of their parent than to any other species of a collateral lineage. Species certainly vary, for the defining property of reproductive isolation demands genetic differentiation from parents and collateral relatives. Finally, species interact with the envi¬ ronment in a causal way that can influence rates of birth (speciation) and death (extinction). As a further benefit for thus codifying the criteria of evolutionary individu¬ ality, we can immediately cut through the foolishness surrounding several dis¬ tressingly common, but artlessly and rather thoughtlessly contrived, claims (or, rather, loose metaphors) about the Darwinian character of large items in nature—an attractive idea for many people, particularly for romantics and “new-agers” who yearn for meaningful agency at the highest levels. We can dismiss these claims because the object hypothesized as an agent of selec¬ tion fails several crucial requirements for designation as an evolutionary in¬ dividual. As an obvious example, many proponents of the so-called Gaia hypothesis wax poetic about the earth and atmosphere as a homeostatic system robustly balanced by interaction with life to secure and stabilize the conditions required by organisms for diversification and geological persis¬ tence. Supporters often assume that such functional coherence must make the earth sufficiently like an organism to merit designation as a living entity. Some have even stated that the earth must therefore be recognized as the larg¬ est and most inclusive product of Darwinian selection—or even that the earth should, in fact, be viewed as a true Darwinian individual. This woolly notion confuses a gut feeling about functionality or adaptive “optimality” (for sup¬ port of life) with the requirements of Darwinian agency. The earth does not generate children, and did not arise by competitive prowess as the sole survi¬ vor among defeated brethren (who must have died or been expelled, I suppose, from the solar system long ago). Therefore, among a plethora of other rea¬ sons, the earth cannot be construed as a Darwinian agent or unit of selection. More plausibly, and more interestingly, communities and ecosystems have sometimes been designated as potential units of selection. In this instance, at least, a case could be conceived—for communities do maintain some func¬ tional coherence, some boundaries (however loose), and some potential for splitting off “daughter” communities with sufficient resemblance to a parent. But I can hardly imagine a set of circumstances that would allow such ecolog¬ ical units to express enough criteria of individuality to qualify for Darwinian agency. Communities are not (for the most part) genealogically constructed or filiated. They can rarely maintain sufficient coherence or persistence, for constituent species move in and out in relative independence. Williams (1992,
Species as Individuals in the Hierarchical Theory of Selection
p. 55) writes, for example: “The reason must be that communities lack the necessary high rates of reproduction and replacement and especially the high level of heritability required for effective selection. They change their make¬ up so rapidly that selection among communities must be overwhelmed by en¬ dogenous change.” But these principled exclusions leave us with a rich hierarchy of legitimate biological individuals, all related by the fascinating property of nested inclu¬ sion within evolution’s genealogical system. In appropriate circumstances, broad enough for vital agency in the evolution of life on earth, individuals at many levels—including genes, cell lineages, organisms, demes, species, and clades—can act as units of Darwinian selection. I doubt that we can defend any longer—or as any more than a convenient and parochial preference based on the happenstances of size and duration for a human body—the cen¬ tral Darwinian conviction that organisms represent the fundamental level of Darwinian individuality, with all other levels either nonexistent, impotent, entirely subservient, or operating only in odd and restricted circumstances.
The Evolutionary Definition of Selective Agency and the Fallacy of the Selfish Gene A FRUITFUL ERROR OF LOGIC
Science thrives upon the continuous correction of error. Most errors arise from inadequate knowledge of the empirical world, or (if grounded in a theo¬ retical prejudice) at least persist because we have no means (conceptual or technological) to secure their empirical refutation. For example, we once lacked the technology to prove that buried organic matter might petrify, and that wood made of stone might therefore represent the remains of ancient plants, and not the power of rocks to mimic organic design by a process anal¬ ogous to crystallization. Only rarely, however, do professions get sidetracked by pursuing an exten¬ sive and longlasting program of research initiated by an error in reasoning rather than an inadequacy of empirical knowledge. Yet I think that the genecentered approach to natural selection—based on the central contention that genes, as persistent and faithful replicators, must be fundamental (or even ex¬ clusive) units of selection—represents a purely conceptual error of this un¬ usual kind. In beginning with Williams’s manifesto (1966)—based on a mode of thinking rooted in the brilliantly consistent, if limited, worldview of R. A. Fisher (1930), but immediately inspired by the remarkable work of W. D. Hamilton (1964)—and proceeding through the codification of Dawkins (1976), to numerous works both popular (especially Cronin, 1991) and tech¬ nical (Dennett, 1995), this gene-based approach to selective agency has in¬ spired both fervent following of a quasi-religious nature (see R. Wright, 1994) , and strong opposition from many evolutionists, who tend to regard the uncompromising version as a form of Darwinian fundamentalism resur¬ gent (see Gould, 1997d), variously designated as ultradarwinism (Eldredge, 1995) or hyperdarwinism.
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THE STRUCTURE OF EVOLUTIONARY THEORY I shall show in this section that, while genes may be appropriately desig¬ nated as fundamental replicators (under a defendable but nonexclusive strat¬ egy of research), replicators simply aren’t units of selection or, for that matter, causal agents at all under our usual notions of mechanism in science. The misidentification of replicators as causal agents of selection—the foundation \
'
of the gene-centered approach—rests upon a logical error best characterized as a confusion of bookkeeping with causality. We fall into another serious fault of reasoning when we accept the common conceptual taxonomy that relegates error itself to a purely negative category of unfortunate blunder. Some errors do lead only to blind alleys and wasted time. But others, as thoughtful scientists have always recognized, serve as es¬ sential prods and directors of progress through correction. Darwin’s famous words, distinguishing harmful from salutary error, have frequently been cited in this context: “False facts are highly injurious to the progress of science, for they often endure long; but false views, if supported by some evidence, do lit¬ tle harm, for every one takes a salutary pleasure in proving their falseness” (from the Descent of Man). I prefer the stronger statement of the great Italian economist, Vilfredo Pareto: “Give me a fruitful error any time, full of seeds, bursting with its own corrections. You can keep your sterile truth for your¬ self. ” During my career in evolutionary science, no error has proven more fruit¬ ful in Pareto’s sense than the gene-centered approach to selection. The central claim, clearly expressed, forced us to reconceptualize the entire domain of evolutionary causality. The outrageous character of such an ultimate reduc¬ tion compelled us to rethink our subject by explicitly rejecting the oldest, most traditional and entirely commonsensical notions about our own bodies as agents. (Yet the reductionistic cast of the theory fit so well with conven¬ tional ideas about the goals of science that many biologists “caught the spirit” and “followed the program” despite its assault upon ordinary intu¬ ition.) Nevertheless, the theory could not work. However stubborn and he¬ roic the attempt, explanation inevitably faltered upon the central logical er¬ ror—especially when selection had clearly worked upon emergent properties of higher-level individuals, and no verbal legerdemain could recast the story in terms of genes as causal agents. If “Pareto errors” contain the seeds that burst their own boundaries, then such uncommon errors of fallacious reason (rather than absent fact) qualify best for this status. Empirical correction usu¬ ally requires a period of waiting for new technologies or new discoveries (as the sources of resolution do not he within the argument), but logical errors al¬ ways carry the seeds of correction within the fruit of their own structure.
HIERARCHICAL VS. GENIC SELECTION
The fallacy of gene selectionism, and the consequent validity of the alterna¬ tive (and opposite) hierarchical model of selection, can best be expressed in a series of seven arguments and vignettes—of different length, but all con¬ nected in a logical order, and all developed for the same import and purpose:
Species as Individuals in the Hierarchical Theory of Selection The distinction of replicators and interactors as a framework for discussion Both leading founders of modern gene selectiomsm as a general view of evo¬ lution (Williams, 1966; Dawkins, 1976) drew a crucial distinction between reproductive units of heredity, and entities that interact with the environment to bias the transmission of reproductive units into the next generation. Wil¬ liams viewed nearly all evolution as proceeding via genes as reproductive units, with adaptation of organisms (the interacting entities) construed as a result—a duality that he usually labeled (1966, p. 124 for example) as “genic selection and organic adaptation.” Dawkins (1976) agreed entirely, and drew a more colorful and explicit distinction between “replicators,” considered as units of selection and identified as genes—and “vehicles,” considered as merely passive repositories built by replicators for their own purposes, and identified as bodies of organisms. In other words, both Williams and Daw¬ kins invoked a criterion of replication to identify genes as the active and fun¬ damental agents of natural selection. In his 1980 review on “Individuality and selection,” David Hull formalized this distinction in a manner that has—quite usefully and properly in my view—organized the professional discussion on units of selection ever since. Hull (1980, p. 318) defined a replicator as “an entity that passes on its struc¬ ture directly in replication”; and an interactor as “an entity that directly in¬ teracts as a cohesive whole with its environment in such a way that repli¬ cation is differential.” Hull then defined selection with reference to both attributes: “a process in which the differential extinction and proliferation of interactors cause the differential perpetuation of the replicators that pro¬ duced them.” Hull insisted that a causal account of selection must include both concepts (1980, pp. 319-320): “Evolution of sorts could result from replication alone, but evolution through natural selection requires an interplay between replica¬ tion and interaction. Both processes are necessary. Neither process by itself is sufficient. Omitting reference to replication leaves out the mechanism by which structure is passed from one generation to the next. Omitting reference to the causal mechanisms that bias the distribution of replicators reduces the evolutionary process to the ‘gavotte of the chromosomes,1 to use Hamilton’s propitious phrase.” Later, Hull (1994, pp. 627-628) continued to espouse this view: “According to the terminology I prefer, there are no units of se¬ lection because selection is composed of two subprocesses—replication and interaction. Selection results from the interplay of these two subprocesses. Genes are certainly the primary (possibly sole) units of replication, whereas interaction can occur at a variety of levels from genes and cells through or¬ ganisms to colonies, demes, and possibly entire species.” I shall argue in this section that the causality of selection resides in interac¬ tion, not in replication, and that the hierarchical model almost automatically prevails once we accept this analysis of causality. Moreover, Hull’s intuitions ran in this direction from the start, for even while he insisted upon the “rele¬ vancy” of both replication and interaction, Hull always acknowledged that
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THE STRUCTURE OF EVOLUTIONARY THEORY the classical argument for multiple levels of selection only invokes inter¬ actors. He wrote in his original paper (1980, p. 325): “In most cases when biologists argue that entities more inclusive than single genes function in the evolutionary process, they have interaction in mind.” And Hull (1994, p. 628) directly followed his defense of duality (quoted just above) with this sentence: “The units-of-selection controversy concerns levels of interaction, not levels of replication.” 1 shall defend and develop Hull’s intuition in the rest of this section. Only interactors can be deemed causal agents in any cus¬ tomary or reasonable use of this central term. Replicators are important in evolution, but in a different role as items for bookkeeping. Replicators are not causal agents. If causality resides in interactors, and interactors at several levels rank as legitimate evolutionary individuals, then the hierarchical theory of selection becomes unassailable as a coherent logical structure, subject to the ultimate scientific test of empirical verification (or invalidation) in nature.
Faithful replication as the central criterion for the gene-centered view of evolution As noted above, both Williams and Dawkins chose to define units of selection as replicators rather than interactors. I shall explain under argument three why I am confident that they made the wrong choice—thus committing the fruitful “Pareto error” discussed at the outset of this section. Having thus de¬ cided, and correctly understanding that selection can only work on “individ¬ uals” as previously defined, what replicating individuals would Williams and Dawkins then designate as units? We all know that they chose genes as fundamental—and effectively ex¬ clusive—replicators, and therefore as the unit of selection in Darwinian the¬ ory (in maximal contrast with the hierarchical theory of multiple, simul¬ taneously-acting levels, as defended in this book). I will discuss the stated reasons for their choice, but I cannot know the deeper motivations of their philosophical and psychological preferences. I strongly suspect that they, and all defenders of strict gene selectionism, feel drawn to the traditional reductionism of science. They understood that Darwin himself went as far as he could in this direction, by breaking down the Paleyan edifice of highest-level intentionality (God himself) to the lowest level then practical—organisms struggling for reproductive success (see Chapter 2). They also recognized that this breakdown had produced revolutionary consequences for Western thought, particularly in reconceptualizing all perceived natural “benevo¬ lence” (especially the good design of organisms and the harmony of ecosys¬ tems) as a side-consequence of struggle for personal success among lowestlevel individuals, rather than as an explicit intention of a loving and omnipo¬ tent deity. I imagine that the more thoughtful gene selectionists then worked by analogy, reasoning that if they could break causality down even further, below the level of the organism, similarly interesting, and perhaps revolution¬ ary, consequences might follow. I can’t gainsay either the intuition or the am¬ bition—but I can fault the resulting argument for an erroneous choice of both category and of level.
Species as Individuals in the Hierarchical Theory of Selection If a search for ultimate reduction below the Darwinian body set the deeper motivation for choosing genes as units of selection, what particular rationales did proponents of this theory offer? Both Williams and Dawkins began by arguing that the conventional unit of Darwinian theory—bodies of organ¬ isms—cannot properly occupy this role because organisms lack a key feature that genes possess. The bodies of sexual organisms disaggregate in reproduc¬ tion, making only half an appearance (so to speak) in the genetic constitution of offspring. How can something so ephemeral be a unit of selection? But genes pass faithful copies of themselves into future generations, and therefore maintain the integrity required of an agent of natural selection in their defini¬ tion. Both Williams and Dawkins advance the same argument in three steps: (1) the unit of selection must be a replicator; (2) replicators must transmit faith¬ ful, or minimally altered, copies of themselves across generations; (3) sexual organisms disaggregate across generations and therefore cannot be units of selection, but genes qualify by faithful replication. Williams developed this argument in his first book (1966), and continues his verbal defense to this day, despite remarkable movement, as we shall see, towards the position ad¬ vocated in this volume. But Williams still employs the language of geneselectionism, particularly in the identification of genes as “units of selection” by virtue of faithful replication (so different from Hull’s pluralistic view that the definition of a unit must include both replication and interaction): “These complications are best handled by regarding individual [i.e. organismic] se¬ lection, not as a level of selection in addition to that of the gene, but as the primary mechanism of selection at the genic level. Because genotypes do not replicate themselves in sexual reproduction (cannot be modeled by dendrograms), they cannot be units of selection” (Williams, 1992, p. 16). Dawkins (1978) advances the same argument, with the same designation of genes as units of selection: “However complex and intricate the organism may be, however much we may agree that the organism is a unit of function, I still think it misleading to call it a unit of selection. Genes may interact, even ‘blend’ in their effects on embryonic development, as much as you please. But they do not blend when it comes to being passed on to future generations.” In a later book (1982, p. 91), Dawkins affirms the terminology of genes as units of selection, by making a strong link to his favorite subject of adapta¬ tion: “The whole purpose of our search for a ‘unit of selection’ is to discover a suitable actor to play the leading role in our metaphors of purpose. We look at an adaptation and want to say, ‘It is for the good of . . .’ Our quest... is for the right way to complete that sentence ... I am suggesting here that, since we must speak of adaptations as being for the good of something, the correct something is the active, germ-like (sic, but clearly a misprint for the intended ‘germ-line’) replicator.” Dawkins’s extended defense of genes as the unit of selection invokes a set of related criteria bearing unmistakable concordance with primal virtues of our culture, another extrascientific reason for the argument’s appeal—namely, faithfulness, (near) immortality, and ancestral priority. Dawkins enlarges the
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THE STRUCTURE OF EVOEUTIONARY THEORY basic argument about faithfulness—sexual organisms disaggregate across generations but genes transmit accurate copies—into a paean about genetic immortality compared with the tragic transiency of our personal lives: It does not grow senile, it is no more likely to die when it is a million years old than when it is only a hundred. It leaps from body to body down the generations, manipulating body after body in its own way and for its own ends, abandoning a succession of mortal bodies before they sink in senility and death. The genes are the immortals, or rather, they are defined as genetic entities which come close to deserving the title. We, the individual survival machines in the world, can expect to live a few more decades. But the genes in the world have an expectation of life which must be measured not in decades but in thousands and millions of years. In sexually reproducing species, the individual is too large and temporary a genetic unit to qualify as a significant unit of natural selec¬ tion. The group of individuals is an even larger unit. Genetically speak¬ ing, individuals and groups are like clouds in the sky or dust storms in the desert. They are temporary aggregations or federations. They are not stable through evolutionary time (1976, p. 36). Dawkins then commits one of the classical errors in historical reasoning by arguing that because genes preceded organisms in time, and then aggregated to form cells and organisms, genes must therefore control organisms—a con¬ fusion of historical priority with current domination (see Chapter 11, and Gould and Vrba, 1982, for a full discussion of this common fallacy). But Dawkins’s argument collapses for many reasons, most notably the issue of emergence. A higher unit may form historically by aggregation of lower units. But so long as the higher unit develops emergent properties by nonadditive interaction among parts (lower units), the higher unit becomes, by definition, an independent agent in its own right, and not the passive “slave” of control¬ ling constituents. In advancing this false argument, Dawkins closes with a statement that can only compete with some choice Haeckelian effusions for the title of purplest prose passage in the history of evolutionary writing: Replicators began not merely to exist, but to construct for themselves containers, vehicles for their continued existence. The replicators which survived were the ones which built survival machines for themselves to live in . . . Survival machines got bigger and more elaborate, and the pro¬ cess was cumulative and progressive . . . Four thousand million years on, what was to be the fate of the ancient replicators? They did not die out, for they are past masters of the survival arts. But do not look for them floating loose in the sea; they gave up that cavalier freedom long ago. Now they swarm in huge colonies, safe inside gigantic lumbering robots, sealed off from the outside world, communicating with it by tortuous in¬ direct routes, manipulating it by remote control. They are in you and me; they created us, body and mind; and their preservation is the ultimate ra¬ tionale for our existence. They have come a long way, those replicators.
Species as Individuals in the Hierarchical Theory of Selection Now they go by the name of genes, and we are their survival machines (1976, p. 21). One might dismiss this rhetorical flourish as harmless enthusiasm. But we must also recognize that, however extended the metaphors, Dawkins’s images do accurately express his false theory of selective agency—for if genes can be depicted as exclusive units of selection, then they become the causal agents of evolution; and if bodies are Darwinian ciphers both for their transiency and by their lethargy relative to the “lean and mean” genes living within, then bodies might as well be described as inert and manipulated repositories (“lumbering robots”). Dawkins writes in his introduction (1976, p. ix): “We are survival ma¬ chines—robot vehicles blindly programmed to preserve the selfish molecules known as genes. This is a truth which still fills me with astonishment. Though I have known it for years, I never seem to get fully used to it.” I can only re¬ gard this honest admission as a striking example of the triumph of false con¬ sistency over legitimate intuition.
Sieves, plurifiers, and the nature of selection: the rejection of replication as a criterion of agency The linkage of selective agency to faithful replication has been urged with such force and frequency that the argument now functions as a virtual man¬ tra for many evolutionary biologists. But when we consider the character of natural selection as a causal process, we can only wonder why so many peo¬ ple confused a need for measuring the results of natural selection by counting the differential increase of some hereditary attribute (bookkeeping) with the mechanism that produces relative reproductive success (causality). Replica¬ tors cannot be equated with causal agents (unless they also happen to be interactors, for only interactors can be agents). Units of selection must be ac¬ tors within the guts of the mechanism, not items in a calculus of results. Genes struck many people as promising units for a twofold reason that does record something of vital evolutionary importance, but bears little rela¬ tionship to the issue of selective agency. Persistence and replication do lie among the necessary (but not sufficient) criteria for calling any biological en¬ tity an evolutionary individual. Since evolution requires hereditary passage, and since genes transmit faithful copies of themselves, and also represent the smallest functional unit of physical continuity between generations of sexual organisms (the kind of individuals we know best for obvious parochial rea¬ sons), many biologists assumed that genes must therefore act as the basic (or even the only) unit of selection. This interesting error arises from two common fallacies in human rea¬ soning: The confusion of necessary with sufficient conditions.
We all agree that units of selection must be evolutionary individuals in Dar¬ winian theory—and that status as an evolutionary individual depends upon a set of criteria discussed on pages 602-613. These criteria do include heredi-
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THE STRUCTURE OF EVOEUTIONARY THEORY tary passage and sufficient persistence—the properties most strikingly exhib¬ ited by genes. But evolutionary individuals, to act as units of selection, must also display other properties that genes do not generally possess. In particu¬ lar, a unit of selection must interact “directly ... as a cohesive whole with its environment in such a way that replication is differential'’—to quote Hull’s definition once again (1980, p. 318). But in sexual organisms, and in other higher-level individuals, genes do not usually interact directly with the environment. Rather, they operate via the organisms that function as true agents in the “struggle for existence.” Organ¬ isms live, die, compete and reproduce; as a result, genes move differentially to the next generation. Of course genes influence organisms; one might even say, metaphorically to be sure, that genes act as blueprints to build organisms. But such statements do not substantiate the critically necessary claim that, therefore, genes inter¬ act directly with the environment when organisms struggle for existence. The issue before us—the venerable problem of “emergence”—is largely philo¬ sophical and logical, and only partly empirical. Genes would interact directly only if organisms developed no emergent properties—that is, if genes built or¬ ganisms in an entirely additive fashion, with no nonlinear interaction among genes at all. In such a situation, organisms would be passive repositories, and genes could be construed as units of selection—for anything done by organ¬ isms could then be causally reduced to the properties of individual genes. This aspect of the question must be decided empirically. But the issue is also quite settled (and was never really controversial): organisms are stuffed full of emergent properties; our sense of organismic functionality and intentionality largely arises from our appreciation of these emergent features. Thus, since genes interact with the environment only indirectly through selection upon organisms, and since selection on organisms operates largely upon emergent characters, genes cannot be units of selection when they function in their customary manner as faithful and differential replicators in the process of or¬ dinary natural selection among organisms. Dawkins’s metaphors of selfish genes and manipulated organisms may be colorful, but such images are also fatefully misleading because Dawkins has reversed nature’s causality: organ¬ isms are active units of selection; genes, while lending a helping hand as archi¬ tects, remain stuck within these genuine units. The theory-bound nature of concepts and definitions. We are drawn to the faithfulness of gene replication, especially when com¬ pared with the contrasting transiency of sexual organisms, who must disag¬ gregate to reach the next generation. We might therefore assume that genes become primary candidates for units of selection as a consequence of their potential immortality, while organisms fall from further consideration by the brevity of their coherent lives. “Sufficient stability” surely ranks as an important criterion for the “evolu¬ tionary individuality” required of a “unit of selection.” But, in Darwinian theory and the search for units of selection, “sufficient” stability can only be defined as enough coherence to participate as an unchanged individual in the causal process of struggle for differential reproductive success. To be causal
Species as Individuals in the Hierarchical Theory of Selection
units under this criterion, organisms need only persist for the single genera¬ tion of their lifetimes—as they do. This endurance may not strike us as a long time in some intuitively appealing psychological sense, or relative to the per¬ sistence of faithful gene replicates, or considered in comparison with geologi¬ cal scales—but these temporal frameworks are irrelevant to the question and theory at hand. Organisms last long enough to act as units of selection in a Darwinian process; they therefore possess the “sufficient stability” required of evolutionary individuals. Of course, evolutionary individuals must all be able to pass—differentially and in a heritable manner—their favorable properties into future generations. But no aspect of this requirement implies or requires that units of selection must pass copies of themselves, bodily and in their entirety, into the next gen¬ eration. The criterion of heredity only demands that units of selection be able to bias the genetic makeup of the next generation towards features that se¬ cured the differential reproductive success of parental individuals. Units of se¬ lection only need to plurify their own representation in the next generation; they need not copy themselves. Sexual organisms happen to plurify by disag¬ gregation and subsequent differential passage of genes and chromosomes. Other kinds of individuals, including genes, asexual organisms and species, plurify more coherently. This common confusion of plurifaction with faithful replication has erected a serious stumbling block to proper understanding of the hierarchical theory of selection. We can best clarify this crucial issue of the relationship between selective agency and criteria of faithful replication vs. plurifaction if we drop, for a moment, the conventional framework of replication vs. interaction, and re¬ turn instead to a different metaphor commonly invoked during 19th century debates about the nature of Darwinism and natural selection—namely sieves. We may use the classical metaphor of sieving to illustrate the inappropri¬ ateness of faithful replication as a criterion for defining units of selection. The “goal” of a unit of selection is not unitary persistence (faithful replication)— and I can’t quite figure out why so many late 20th century Darwinians ever tried to formulate the concept in this manner. The “goal” of a unit of se¬ lection is concentration by plurifaction—that is, the differential passage of “youness” into the next generation, an increase in relative representation of your heritable attributes (whether you pass yourself on as a whole, or in disaggregated form, into the future of your lineage). In the favored metaphor of Darwin’s day, selection works like a sieve laden with all the individuals of one generation. Surrounding environments shake the sieve, and particles of a certain size become concentrated, while others pass through the webbing (lost by selection). Sieving represents the causal act of selection—the interaction of the environment (shaking the sieve) with varying individuals of a population (particles on the sieve). As a result of this interaction, some individuals live (remain on the sieve), while others die (pass through the sieve)—and survival depends causally upon variation in emer¬ gent properties of the particles (in this simplest case, large particles remain, and small particles pass through to oblivion). The surviving particles need to reproduce in genealogical systems of evolu-
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THE STRUCTURE OF EVOLUTIONARY THEORY tionary individuals. They may do so by fissioning (faithful passage) or by disaggregation and reconstitution of new individuals as mixtures of heredi¬ tary parts of previous individuals. The individuals of the old generation even¬ tually die and evaporate. The individuals^ of the new generation now live on the sieve, waiting for the next shake. But this specification of the varied modes for constituting new individuals does not represent what we mean by selection. An entity must be able to re¬ produce to be defined as an evolutionary individual, but this entity need not replicate faithful copies of itself. Rather, it needs to be able to plurify—that is, to increase, relative to other individuals, the representation of its hereditary contribution to the next generation. Integral “you” may be disaggregated in the process, but so long as the next generation contains a relative increase in your contributions, and so long as you operated as an active causal agent of the Darwinian struggle while you lived, then you qualify as a unit of selection (and a winning unit in this case). An interesting episode in the history of Darwinism clarifies this concept in a striking manner. We all know that Darwin accepted the idea of “blending inheritance,” or the averaging of parental characteristics in the offspring of sexual reproduction. Now blending inheritance marks an ultimate denial of faithful replication—for the hereditary basis of any selected character be¬ comes degraded by half in breeding with an average individual. A paradox therefore arises. If units of selection must be faithful replicators, and if Dar¬ win both understood natural selection and believed in blending inheritance, then why did he ever imagine that selection could work as a mechanism? We can only resolve this conundrum by recognizing that faithful replica¬ tion is not—and never was—the defining characteristic (or even a necessary property) of a unit of selection. Darwin, even given his belief in blending in¬ heritance, could view sexual organisms as primary units of selection because he understood agency in a different way that remains valid today: units of se¬ lection are evolutionary individuals that interact with the environment and plurify as a causal result. We may return to the metaphor of sieving. Natural selection can work under blending inheritance because shaking the sieve fa¬ vors the possessors of advantageous traits in each generation—for any indi¬ vidual with a phenotype biased in the favored direction gains a better chance of remaining on the sieve. The offspring of the most favored individuals will blend substantially back to the mean, but this style of inheritance only slows the process of selection—for, as a result of differential survival and reproduc¬ tion in each generation, the mean itself still gradually moves in the favored di¬ rection.
Interaction as the proper criterion for identifying units of selection The aforementioned arguments about sieves, plurifaction, and the inappro¬ priateness of faithful replication for designating units of selection lead to a simple conclusion: we can only understand the causal nature of selection when we recognize that units of selection must be defined as interactors, not
as replicators. Hull’s distinction has great merit, but he fell into an overgener-
Species as Individuals in the Hierarchical Theory of Selection ous pluralism in arguing that identification of causal agency must include statements about both the faithfulness of replicators and the potency of inter¬ actors. Individuals need not replicate themselves faithfully to be units of se¬ lection. Rather, they must contribute to the next generation by hereditary pas¬ sage, and they must plurify their contributions relative to those of other individuals. But the contributions themselves can be wholes or parts, faithful replicates or disaggregated bits of functional heredity. Selection demands plurifaction, not faithful replication. The simple observation of plurifaction—the relative increase of an individ¬ ual’s representation in the heredity of subsequent generations—does not suf¬ fice to identify the operation of natural selection, for plurifaction can occur by nonselective means, and phenotypes can increase in frequency but then be unable to plurify. Consider the primary example of each phenomenon. First, individuals may plurify by accidents of genetic drift. Suppose that individuals fall through the sieve of selection at random, but survivors show increased frequency of certain heritable traits by accident. These surviving individuals will plurify, but they have not operated as active units of selection. Second, in¬ dividuals may increase in frequency for phenotypic reasons unrelated to he¬ redity. Suppose that large individuals remain differentially on the sieve, but that individuals grow larger than average for purely ecophenotypic reasons uncorrelated with any aspect of heredity that can pass to subsequent genera¬ tions. Large phenotypes have increased in frequency for causal reasons—but they will not be able to plurify because they cannot bias the heredity of subse¬ quent generations. So selection demands plurifaction because evolutionary individuals must maintain lineages by hereditary passage, and selection occurs by increase in relative representation. But plurifaction can only represent a necessary condi¬ tion, not a cause. We define selection as occurring when plurifaction results from a causal interaction between traits of an evolutionary individual (a unit of selection) and the environment in a manner that enhances the differential reproductive success of the individual. Thus, and finally, units of selection must, above all, be interactors. Selection is a causal process, not a calculus of results—and the causality of selection resides in interaction between evolu¬ tionary individuals and surrounding environments. The study and documen¬ tation of group and higher-level selection has been stymied and thrown into disfavor by our confusion over these issues—and especially by the blind alley of a logically false argument that identified replicators rather than interactors as units of selection, and then constructed a fallacious, reductionistic theory, precisely opposite in structure to the hierarchical model, by specifying genes (because they replicate faithfully) as ultimate or exclusive units of selection. In this context, I note with delight that group selection has risen from the ashes to receive a vigorous rehearing (Sober and Wilson, 1998, for a full treatment; Lewin, 1996, for a popular account under the title “Evolution’s new heretics”; and Gould and Lloyd, 1999, for resolution of a final logical problem). This potent revival rests upon two proposals that, as centerpieces of this book, could not gain my stronger assent: the identification of evolu-
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THE STRUCTURE OF EVOEUTIONARY THEORY tionary individuals as interactors, causal agents, and units of selection; and the validation of a hierarchical theory of natural selection based upon a prin¬ cipled understanding that evolutionary individuals exist at several levels of organization—including genes, cell lineages, organisms, demes, species, and clades. D. S. Wilson has most vigorously championed this revival (Wilson, 1980, 1983), while his collaboration with philosopher E. Sober has produced a particularly important paper and a subsequent book on the subject (Wilson and Sober, 1994, with 33 accompanying commentaries and the authors’ re¬ sponse; Sober and Wilson, 1998). Wilson and Sober anchor their argument by insisting that units of selection must be defined as interactors, not rep¬ licators. I must raise only one mild quarrel with Wilson and Sober. I agree en¬ tirely that units of selection must be defined as interactors, but I prefer a “looser” or “broader” concept of interaction that fosters the proper identi¬ fication of highest-level individuals in species and clade selection. Wilson and Sober stress the “organism-like” properties of interactors, and therefore make the confusing and regrettable linguistic decision to use “individual” for conventional bodies, and “organism” as the general name for a unit of selec¬ tion at any hierarchical level; whereas I and most biologists (see Gould and Lloyd, 1999) advocate a reversed terminology. In characterizing the evolu¬ tionary principle of interaction, I would stress the potential for a rich panoply of emergent fitnesses, and for the consequent capacity of plurifaction. Their chosen stress on “organism-like” properties leads Wilson and Sober to emphasize direct modes of interaction based on actual contact of sympatric individuals—the old vision of two gladiators duking it out to the finish. But interaction does not require physical contact. Interaction occurs between individuals and environments, not necessarily between individual and in¬ dividual. The interaction must be able to yield plurifaction for causal rea¬ sons based on properties that enhance differential reproductive success— but, again, competing individuals need not interact directly with each other. Rather, to speak of selection, competing individuals only need to plurify at different relative rates based on similar causal interactions with environ¬ ments. But the environments may be spatially separate and broadly defined. This issue does not often arise at the traditional level of Darwin’s chosen evo¬ lutionary individuals—that is, organisms. But higher-level individuals, partic¬ ularly species and clades, do often compete without contact—and our notion of units of selection must include this important mode of interaction. Several thoughtful biologists have stressed this point, and I have compiled a small file of such statements. I shall present here only the forceful argument of Williams (1992, p. 25), who has changed his view substantially since for¬ mulating the theory of gene selectionism in 1966: There are many further questions on the meaning and limits of clade se¬ lection. One issue is whether the populations that bear the gene pools need be in ecological competition with each other. I believe that this is
Species as Individuals in the Hierarchical Theory of Selection not required, any more than individuals within a population need inter¬ act ecologically to be subject to individual selection. The reproductive success or failure of a soil arthropod, with an expected lifetime dispersal of a few meters, will hardly influence prospects for a conspecific a hun¬ dred meters away. But the descendants of these two individuals might compete, and genes passed on by one may ultimately prevail over those passed on by the other. Selective elimination of one and survival of the other a hundred meters away is individual selection as long as the two arthropods can be assigned to the same population and their genes to the same gene pool. ... In the same way, two gene pools in allopatry can be subject to natural selection if, as must always be true, their descendants might be alternatives for representation in the biota . . . The ultimate prize for which all clades are in competition is representation in the biota.
The internal incoherence of gene selectionism I regard the heyday of gene selectionism as an unusual episode in the history of science—for I am convinced that the theory’s central argument is logically incoherent, whatever the attraction (and partial validity) of several tenets, and despite the value of a mental exercise that tries to reconceptualize all na¬ ture from a gene’s point of view. Close textual analysis"' of this theory’s lead¬ ing documents reveals persistent internal problems, explicitly recognized by authors and invariably met by arguments so flawed in construction that even the defenders seem embarrassed, or at least well aware of the glaring insuf¬ ficiency. I am not alone in noting this peculiar situation, and in calling for some seri¬ ous consideration by historians. Wilson and Sober (1994, p. 590) write: “The situation is so extraordinary that historians of science should study it in de¬ tail: a giant edifice is built on the foundation of genes as replicators, and therefore as the ‘fundamental’ unit of selection, which seems to obliterate the concept of groups as organisms. In truth, however, the replicator concept cannot even account for the organismic properties of individuals. Almost as
"'This kind of textual exegesis, a standard mode of scholarly work in the humanities, should be pursued more often in scientific discussion as well. Scientists tend to reject such an approach, I suppose, because we believe that forms of argument and rhetorical styles only lend a superficial patina to the “real” substance of logic and evidence, and therefore can teach us nothing of interest. I think that we have thereby missed a major source of in¬ sight about the operation of science—a source that would not only deepen our understand¬ ing of history and procedure, but would also help us to judge and analyze such contempo¬ rary issues as the logic of selectionist theory. If we locate consistent slips, foibles, jagged edges, strains, or near apologies—as presented verbally—then we can often pinpoint weak¬ nesses in logic or failure of empirical support. I show, in this section, that all major support¬ ers of gene selectionism fall into such verbal patterns at the theory’s main loci of inconsis¬ tency. In previous books, 1 have tried to use this mode of analysis to explicate such issues as the nature of geological time (Gould, 1987b), the logic of biological determinism (1981a), and the concept of evolutionary progress (1996a) and predictability (1989c).
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THE STRUCTURE OF EVOEUTIONARY THEORY an afterthought, the vehicle concept is tacked onto the edifice to reflect the harmonious organization of individuals, but it is not extended to the level of groups.” The central problem lies as deep as our definition of the key concept of “cause” in science. Aristotle proposed a broad concept of causality divided into four aspects, which he called material, efficient, formal and final (or, roughly, stuff, action, plan and purpose—that is, the bricks, the mason, the blueprint and the function, in the standard “parable of the house,” used for more than two millennia to explicate Aristotle’s concept). As many historians have noted, modern science may virtually be defined by a revision of this broad view, and a restriction of “cause,” as a concept and definition, to the aspect that Aristotle called “efficient.” (The word “efficient” derives from the Latin facere, to make or to do. Efficient causes are actual movers and shakers, the agents that apply the forces. Aristotle’s term does not engage the modern English meaning of doing something well, as opposed to doing something at all.) The Cartesian or Newtonian world view, the basis of modern science, banned final cause for physical objects (while retaining the concept of pur¬ pose for biological adaptation, so long as mechanical causes, rather than con¬ scious external agencies, could be identified—a problem solved by natural se¬ lection in the 19th century). As for Aristotle’s material and formal causes, these notions retained their relevance, but lost their status as “causes” under a mechanical world view that restricted causal status to active agents. The material and formal causes of a house continue to matter: brick or sticks fash¬ ion different kinds of buildings, while the bricks just remain in a pile, absent a plan for construction. But we no longer refer to these aspects of building as “causes.” Material and formal attributes have become background condi¬ tions or operational constraints in the logic and terminology of modern sci¬ ence. I present this apparent digression because the chief error of gene selectionism lies in a failed attempt to depict genes as efficient causes in ordinary natural selection—and the chief “textual mark” of failure can be located in tortuous and clearly discomforting (even to the authors!) arguments ad¬ vanced by all leading gene selectionists in a valiant struggle to “get through” this impediment. For no matter how an author might choose to honor genes as basic units, as carriers of heredity to the next generation, as faithful repli¬ cators, or whatever, one cannot deny a fundamental fact of nature: in or¬ dinary, garden-variety natural selection—Darwin’s observational basis and legacy—organisms, and not genes, operate as the “things out there” that live and die, reproduce or fail to propagate, in the interaction with environ¬ ments that we call “natural selection.” Organisms act as Aristotle’s efficient causes—the actors and doers—in the standard form of Darwin’s great and universal game. Gene selectionists know this, of course—so they must then struggle to con¬ struct an argument for saying that, even though organisms do the explicit work, genes may somehow still be construed as primary “units of selection,”
Species as Individuals in the Hierarchical Theory of Selection or causal agents in the Darwinian process. This misguided search arises from a legitimate intuition—that genes are vitally important in evolution, and clearly central to the process of natural selection—followed by the false infer¬ ence that genes should therefore be designated as primary causes. Needless to say, no biologist wishes to deny the centrality or importance of genes, just as this intuition holds. But genes simply cannot operate as efficient causes in Darwin’s process of organismic selection. Genes, as carriers of continuity to the next generation, may be designated as material causes in Aristotle’s aban¬ doned terminology. But we no longer refer to the material aspects of natural processes as “causes.” Organisms “struggle” as agents or efficient causes; their “reward” may be measured by greater representation of their genes, or material legacies, in future generations. Genes represent the product, not the agent—the stuff of continuity, not the cause of throughput. The standard gambit of gene selectionists, in the light of this recognized problem, invokes two arguments, both indefensible. Attempts to assign agency to genes by denying emergent properties TO organisms. Once one admits, as all gene selectionists
must and do, that genes propagate via selection on organisms as interactors, how then can one possibly ascribe direct causal agency to genes rather than to bodies? Only one logical exit from this conundrum exists: the assertion that each gene stands as an optimal product in its own place, and that bodies im¬ pose no consequences upon individual genes beyond providing a home for joint action. If such a view could be defended, then bodies would become pas¬ sive aggregates of genes—mere packaging—and selection on a body could then be read as a convenient shorthand summary for selection on all resident genes, considered individually. But such a reductionistic view can only apply if genes build bodies with¬ out nonlinear or nonadditive interactions in developmental architecture. Any nonlinearity precludes the causal decomposition of a body into genes con¬ sidered individually—for bodies then become, in the old adage, “more than the sum of their parts.” In technical parlance, nonlinearity leads to “emer¬ gent” properties and fitness at the organismic level—and when selection works upon such emergent features, then causal reduction to individual genes and their independent summations becomes logically impossible. I trust that the empirical resolution of this issue will not strike anyone as controver¬ sial, for we all understand that organisms are stuffed full of emergent fea¬ tures—an old intuition stunningly affirmed by the first fruits of mapping the human genome (see the full issues of Science and Nature in February 2001 and my own initial reaction for general audiences in Gould, 2001). What else is developmental biology but the attempt to elucidate such nonlinearities? The error of gene selectionists does not he in their stubborn assertion of pure additivity in the face of such knowledge, but rather in their concep¬ tual failure to recognize that this noncontroversial nonlinearity destroys their theory. Dawkins admits the apparent problem (1976, p. 40): “But now we seem to have a paradox. If building a baby is such an intricate venture, and if every
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THE STRUCTURE OF EVOLUTIONARY THEORY gene needs several thousands of fellow genes to complete its task, how can we reconcile this with my picture of indivisible genes, springing like immortal chamois from body to body down the ages: the free, untrammeled, and selfseekings agents of life?” Dawkins attempts a lame resolution by invoking the quintessentially Ox¬ bridge metaphor of rowing, with the nine men (eight oarsmen and a cox) as genes, and the boat as a body. Of innumerable candidate rowers, we put to¬ gether the best boat “by random shuffling of the candidates for each posi¬ tion”—and then running large numbers of trials until the finest combination emerges. Of course the rowers must cooperate in a joint task, but we generate no nonlinearities because localized optimality prevails, and the winning boat ends up with the best possible oarsman in each place. Dawkins then transfers this image back to biology and asserts his view of selection as piecemeal opti¬ mization—so that each locus (each seat in the boat) eventually houses the best candidate: “Many a good gene gets into bad company, and finds itself sharing a body with a lethal gene, which kills the body off in childhood. Then the good gene is destroyed along with the rest. But this is only one body, and replicas of the same good gene live on in other bodies which lack the lethal gene . . . Many [good genesj perish through other forms of ill luck, say when their body is struck by lightning. But by definition luck, good and bad, strikes at random, and a gene which is consistently on the losing side is not unlucky; it is a bad gene” (1976, p. 41). Such a notion of individualized genetic optimality must be rejected as empirically false; but even if true, this concept still wouldn’t support the re¬ quired claim for nonexistence of emergent organismic features. Even Daw¬ kins admits (in the quotation just above) that selection can only optimize “phenotypic consequences” (1982, p. 237)—and if phenotypes arise (as they do) by complex nonadditivity among genetic effects, then the genes in your body cannot maintain the essential property of independence represented by Dawkins’s metaphor of optimal goats, hopping happily and separately across the generations. In any case, this false view of organisms as additive consequences of indi¬ vidually optimized genes underlies the familiar metaphorical language devel¬ oped by Dawkins over the years: “I am treating a mother as a machine pro¬ grammed to do everything in its power to propagate copies of the genes which reside inside it” (1976, p. 132). Or “A monkey is a machine which pre¬ serves genes up trees; a fish is a machine which preserves genes in the water; there is even a small worm which preserves genes in German beer mats. DNA works in mysterious ways” (1976, p. 22). These colorful images misstate ac¬ tual pathways of causality. Organisms work in wondrous ways, and they op¬ erate via emergent properties that invalidate Dawkins’s concept of genes as primary agents. The ceteris paribus dodge. When the logic of an argument re¬
quires that the empirical world operate in a certain manner, and nature then refuses to cooperate, unwavering supporters often try to maintain their advo¬ cacy by employing the tactic of conjectural “as if.” That is, you admit the fail-
Species as Individuals in the Hierarchical Theory of Selection ure of complex nature to meet your theoretical needs, but then claim that you will simplify the actual circumstances “as if” the system under study operated in the required way. The classical “as if” argument goes by its Latin title of ceteris panbus, or “all other things being equal.” Ceteris paribus imposes additivity upon a system truly made of complexly interacting parts. You iso¬ late one factor and state that you will analyze its independent effects by hold¬ ing all other factors constant. Ceteris paribus ranks among the oldest of scholarly devices, an indispens¬ able tactic for any student of complex systems. I am certainly not trying to mount a general assault upon this venerable and valuable mode of exempli¬ fication. Two common circumstances define the legitimate domain of ceteris paribus: (1) as a heuristic or exploratory device for approaching systems of such complexity that you don’t even know how to think about influences of particular parts, unless you can hypothetically assign all others to a theoreti¬ cal background of constancy; and (2) as a powerful experimental tool when you can actually hold other factors constant and perturb the system by vary¬ ing your studied factor alone. But ceteris paribus becomes an illegitimate dodge, an invalid prop to make a potentially false argument unbeatable by definition, in systems dominated by nonadditivity—that is, where the very act of holding all other factors con¬ stant may make your favored factor work in a manner contrary to its actual operation in a real world of interaction. If A conquers B only when the two entities share a field alone, but usually loses to B when C also dwells on the field, and if real fields invariably include C, then we cannot crown A as abso¬ lutely superior to B on the basis of a single and artificial ceteris paribus trial that excluded C from action and consideration. The use of ceteris paribus to support gene selectionism constitutes a simi¬ lar denial of a known reality. This tactic represents a fallback position after acknowledging the impossibility of asserting a genuine claim for nonaddi¬ tivity in the translation of genes to organisms. In other words, you admit that massive nonlinearity actually exists, but then state that, for purposes of dis¬ cussion, you will counterfactually impose ceteris paribus so that genes can be equated with linear effects. For example, Dawkins explicitly invokes the key phrase (in English rather than Latin) in defending his requisite (but fal¬ lacious) notion that genes may be identified as operating “for” particular parts of phenotypes, thus creating the impression that organisms may be treated as additive aggregates rather than entities defined by nonlinear inter¬ action. For purposes of argument it will be necessary to speculate about genes “for” doing all sorts of improbable things. If I speak, for example, of a hypothetical gene “for saving companions from drowning,” and you find such a concept incredible, . . . recall that we are not talking about the gene as the sole antecedent cause of all the complex muscular con¬ tractions, sensory integrations, and even conscious decisions, which are involved in saving somebody from drowning. We are saying nothing
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THE STRUCTURE OF EVOLUTIONARY THEORY about the question of whether learning, experience, or environmental in¬ fluences enter into the development of the behavior. All you have to con¬ cede is that it is possible for a single gene, other things being equal and lots of other essential genes and environmental factors being present, to make a body more likely to save somebody from drowning than its allele would (1976, p. 66). In another passage (1976, p. 39), Dawkins unwittingly surrenders this neces¬ sary tactic by admitting that we dare not discuss the interactive web of em¬ bryonic development, lest we be unable to speak of genes “for” particular as¬ pects of organismal phenotypes: Everybody knows that wheat plants grow bigger in the presence of ni¬ trate than in its absence. But nobody would be so foolish as to claim that, on its own, nitrate can make a wheat plant. Seed, soil, sun, water, and various minerals are obviously all necessary as well. But if all these other factors are held constant, and even if they are allowed to vary within limits, addition of nitrate will make the wheat plants grow bigger. So it is with single genes in the development of an embryo. Embryonic development is controlled by an interlocking web of relationships so complex that we had best not contemplate it. As a striking demonstration that ceteris paribus cannot rescue gene selectionism from logical paradoxes and violations of ordinary linguistic usage, Dawkins (1982, p. 164) addresses the problem of how to treat a genetic dele¬ tion favored by natural selection at the organismic level, if genes represent the fundamental units of selection, and if we must be able to treat each genetic item as a Darwinian individual with a distinct and independent history. If “gene language” must prevail, and if we need to specify the selective value of such a deletion, what can we call the loss but “a replicating absence”! Should we not, at this point, admit instead that organisms are the relevant causal agents in this case, and that organisms have achieved a selective benefit by the alternate but orthodox genetic route of deletion rather than substitution? Some humans have done well with “plenty of nothing,” but I don’t think we should root our ontology in taxonomies for various kinds and forms of faith¬ fully propagating absences. Any organism that happened to experience a random deletion of part of its selfish DNA would, by definition, be a mutant organism. The deletion itself would be a mutation, and it would be favored by natural selection to the extent that organisms possessing it benefited from it, presumably because they did not suffer the economic wastage of space, materials, and time that selfish DNA brings. Mutant organisms would, other things being equal [ceteris paribus again!], reproduce at a higher rate than the loaded down “wild type” individuals, and the deletion would conse¬ quently become more common in the gene pool. Here we are recognizing that the deletion itself, the absence of the selfish DNA, is itself a replicat¬ ing entity (a replicating absence!), which can be favored by selection.
Species as Individuals in the Hierarchical Theory of Selection All major proponents of gene selectionism have unintentionally illustrated the theory’s incoherence by trying to “cash out” their system, and failing at the crucial point of assigning causal agency in natural selection. For, however these proponents may talk about genes as primary agents or units of selec¬ tion, they cannot deny that nature’s Darwinian action generally unfolds be¬ tween discrete organisms and their environments. These authors therefore ac¬ knowledge this basic fact and then tend to lapse into verbal obfuscation on the gene’s behalf. I have already noted a prime example in Williams’s claim, quoted previously in another context, that organismic selection should be re¬ garded not “as a level of selection in addition to that of the gene, but as the primary mechanism of selection at the genic level.” But what does this state¬ ment mean? Williams recognizes organismic selection as the “primary mecha¬ nism” by which genes pass differentially from one generation to the next. But primary mechanisms are efficient causes in any standard analysis of the logic of science. Williams (1992, p. 38) presents an accurate epitome of selection in the following passage: he states that selection must always operate on interactors (and he knows, as the previous quotation shows, that organisms usually constitute the relevant interactors in cases that he wishes to describe as genic selection); he also recognizes that information must pass to future generations by faithful heredity, and he seems to acknowledge that such bi¬ ased passage defines the result, not the cause, of selection. Yet he fails to take the final step of acknowledging that these statements debar gene selectionism as the mechanism of evolution. “Natural selection must always act on physi¬ cal entities (interactors) that vary in aptitude for reproduction, either because they differ in the machinery of reproduction or in that of survival and re¬ source capture on which reproduction depends. It is also necessary that there be what Darwin called ‘the strong principle of inheritance,’ so that events in the material domain can influence the codical record. Offspring must tend to resemble their own parents more than those of other offspring. Whenever these conditions are found there will be natural selection.” Over the years, Dawkins has developed a litany of similar admissions. Of course organisms must be regarded as the foci of selection, but since biased gene passage occurs as a result of this process, we may identify genes as agents of selection. (But results are not causes, although foci of action surely are): “Just as whole boats win or lose races, it is indeed individuals [organ¬ isms) who live or die, and the immediate manifestation of natural selection is nearly always the individual level. But the long-term consequences of nonrandom individual death and reproductive success are manifested in the form of changing gene frequencies in the gene pool” (1976, p. 48). Dawkins then apologizes for framing his descriptions in terms of organ¬ isms as causal actors, excusing himself for succumbing to temptations of con¬ venience. (But perhaps we find this mode “convenient” because we achieve the best description of a causal reality thereby—while the genic mode remains tortuous and uncomfortable because we sense the central error in such for¬ mulation): “In practice it is usually convenient, as an approximation, to re¬ gard the individual body as an agent ‘trying’ to increase the number of all its
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THE STRUCTURE OF EVOLUTIONARY THEORY genes in future generations. I shall use the language of convenience. Unless otherwise stated, ‘altruistic behavior’ and ‘selfish behavior’ will mean behav¬ ior directed by one animal body towards another [p. 50] . . . We shall con¬ tinue to treat the individual as a selfish machine, programmed to do whatever is best for his genes as a whole. This is the language of convenience [p. 71].” In a later book (1982, p. 4) Dawkins admits that gene replicators must be selected by proxy—that is, via organisms as causal actors: “The most impor¬ tant kind of replicator is the ‘gene’ or small genetic fragment. Replicators are not, of course, selected directly, but by proxy; they are judged by their phenotypic effects.” This argument, I think, has truly become an inadaptive meme, destined for eventual extinction, but propagated wherever gene selectionism survives, whether in technical literature or popular presentation. A major popular book on this topic holds (Cronin, 1991, p. 289): “If organisms are not repli¬ cators, what are they? The answer is that they are vehicles of replicators . . . Groups, too, are vehicles, but far less distinct, less unified ... In this weak sense, then, ‘group selection’ could occur . . . [but it] would in no way under¬ mine the status of genes as the only units of replicator selection. This does not mean that higher level entities are unimportant in evolution. They are impor¬ tant, but in a different way: as vehicles.”
Bookkeeping and causality: the fundamental error of gene selectionism The error and the incoherence of gene selectionism, as documented above, can be summarized in a single statement illustrating the fruitful, “Paretolike” character of the central fallacy: proponents of gene selectionism have confused bookkeeping with causality. This error achieves its Pareto status of substantial utility because changes recorded at the genetic level do play a fundamental part in characterizing evolution, and records of these changes (bookkeeping) do maintain an important role in evolutionary theory. But the error remains: bookkeeping * is not causality; natural selection is a causal process, and units or agents of selection must be defined as overt actors in the mechanism, not merely as preferred items for tabulating results. No one has ever stated the issue more accurately or succinctly than George Williams himself (1992, p. 13), thus increasing my puzzlement at his failure to recognize how his own formulation invalidates the gene selectionism that still wins his lip service: “For natural selection to occur and be a factor in evo¬ lution, replicators must manifest themselves in interactors, the concrete reali¬ ties that confront a biologist. The truth and usefulness of a biological theory
"'Working through the logic and problems of this vexatious issue has been pursued as a collective enterprise among many biologists and philosophers for more than 20 years. I have used the terminology of bookkeeping and causality for some time (Gould, 1994), and have developed or sharpened some of the arguments. But I do not think that I devised the labels. I believe that I first picked up the terminology of bookkeeping from arguments pre¬ sented by the University of Chicago philosopher Bill Wimsatt. Many authors have used this fruitful distinction for some time.
Species as Individuals in the Hierarchical Theory of Selection must be evaluated on the basis of its success in explaining and predicting ma¬ terial phenomena. It is equally true that replicators (codices) are a concept of great interest and usefulness and must be considered with great care for any formal theory of evolution, either cultural or biological.” Williams’s state¬ ment agrees completely with the position that I have advanced in this book— an attitude that, by general consensus, leads logically and directly to the hierarchical model of selection, and the invalidity of single-level, gene-based views. Williams allows that interactors represent the “concrete realities” con¬ fronting biologists (and chapter 4 of his 1992 book eloquently defends the concept of legitimate interactors at several hierarchical levels of increasing ge¬ nealogical inclusion). He admits that both the “truth and usefulness” of a bi¬ ological theory, natural selection in this case, depends upon the explanation of material phenomena—that is, interactors operating as agents. He does not include replicators—the basis of gene selectionism—in this category, for his last sentence grants them a separate but equal status in evolutionary theory: “It is equally true that replicators (codices) are a concept of great interest” needed “for any formal theory of evolution.” Now, if replicators are not causal agents, but are vital for any full account of evolution—then what are they? I suggest that we view gene-level replicators as basic units for keeping the books of evolutionary change—as “atoms” in the tables of recorded results. Williams did not slip or misspeak in the quotation cited above. He repeats this separation of a causal agent from a basis of hereditary transmission— with interactors as agents and replicators as transmitters—in several other passages, including (1992, p. 38) “Natural selection must always act on phys¬ ical entities (interactors) ... It is also necessary that there be what Darwin called ‘the strong principle of inheritance’ ...” Whereas Williams makes valid separations and defines proper roles, but then seems unwilling to own the theoretical consequences, Dawkins, on the other hand, seems merely confused. In discussing group selection (1982, p. 115), for example, Dawkins writes: “The end result of the selection dis¬ cussed is a change in gene frequencies, for example, an increase of ‘altruistic genes’ at the expense of ‘selfish genes.’ It is still the genes that are regarded as the replicators which actually survive (or fail to survive) as a consequence of the (vehicle) selection process." By putting the word “vehicle” in parentheses, as a reminder of selection’s intrinsic nature rather than a mere modifying adjective, Dawkins admits that interactors (vehicles in his terminology), not replicators, operate as agents of selection. He describes the differential survival of replicators as a consequen¬ tial result of this causal process—therefore as units for bookkeeping rather than agents of causality—but he then fails to disentangle these two different aspects of evolution, while continuing to grant favored status to genes. We may indeed, and legitimately as a practical measure, decide to keep track of an organism’s success in selection by counting the relative representa¬ tion of its genes in future generations. (In large part, we count at the genic level for the reason always emphasized by Williams and Dawkins—because
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THE STRUCTURE OF EVOEUTIONARY THEORY sexual organisms do not replicate faithfully and therefore cannot be traced as discrete entities across generations.) But this practical decision for counting does not deprive the organism of status as a causal agent, nor does such a procedure grant causality to the objects counted. The listing of accounts is bookkeeping—a vitally important subject in evolutionary biology, but not a form of causality. If, as I have argued (see also Wilson and Sober, 1994), the incoherence of gene selectionism denotes a rare case in science of an influential theory felled by a logical error—in this case the confusion of bookkeeping with causality— rather than a fallacious proposal about the empirical world, then we must ask why so many people fell into this error so readily, and why the fallacy did not become more quickly apparent. I suspect that three major reasons underlie not only the error of gene selectionism, but also the strong willingness, even the fervor, expressed by so many evolutionists in embracing the concept. The first two reasons may claim a social basis in traditions of scientific inquiry. But the third reason, and surely the most intriguing from both a scientific and philosophical perspective, emanates directly from the logical structure of hi¬ erarchies, the conceptual framework that must replace gene selectionism. The two reasons rooted in traditions of scientific procedure include the most general of statements and a preference peculiar to traditions of Anglophonic evolutionary biology. For the generality, I state nothing profound or original in pointing out that a decision to privilege the level of genes plays into the strongest of all preferences in Western science: our traditions of reductionism, or the desire to explain larger-scale phenomena by properties of the smallest constituent particles. The allure of reductionism encourages the following kind of error, or sloppy thinking: we correctly note that genes play a fundamental role in evo¬ lution (as preferred items for a calculus of change—the bookkeeping func¬ tion); we also recognize that genes lie at the base of a causal cascade in the de¬ velopment of organisms; finally, and most generally, we view genes as the closest biological approach to an “atom” of basic structure, and therefore as the cardinal entity of a reductionistic research program. From these state¬ ments, we easily slip into the unwarranted inference that genes must also be fundamental units or agents in natural selection, the primary causal theory of our profession—all the while forgetting the criteria of individuality and inter¬ action that define units or agents within the logic of the theory itself. The second, and more particular, reason flows from explicit traditions of the Modern Synthesis, especially from the approach favored by Fisherian population genetics (see Chapter 7). The heuristics of this field prospered greatly with models that kept track of gene frequencies without worrying much about the locus of selective action. A common fallacy in science then conflates a practical basis for success with the causal structure of nature. Jim Crow (1994, p. 616), one of the world’s most thoughtful geneticists, ex¬ pressed this point particularly well, but then also failed to distinguish book¬ keeping from causality. Writing “In praise of replicators”—and well should we praise them, but, I would argue, as excellent agents for accounting!—
Species as Individuals in the Hierarchical Theory of Selection Crow explained why our traditions have favored the genetic level of analysis (1994, p. 616): The reason, I think, is that these pioneers [Fisher and other founders of the Synthesis] and their intellectual heirs have been concerned, not with selection as an end in itself, but with selection as a way of changing gene frequencies. Selection acts in many ways: it can be stabilizing; it can be diversifying; it can be directional; it can be between organelles; it can be between individuals; it can be between groups . . . But the bottom line has always been how much selection changes allele frequencies and through these, how much it changes phenotypes. This suggests that we should judge the effectiveness of selection at different levels by its effects on gene frequencies. I could not ask for a better statement of (unconscious) support for the posi¬ tion here maintained. Again, as I noted in several other quotations from gene selectionists, Crow allows that selection, as a causal force (“selection as an end in itself,” in his words), operates on interractors at several hierarchical levels of individuality, including groups. Fie also admits that changes in gene frequencies arise as a result of such selection. He then states—and again I don’t object—that these alterations in allelic frequencies should be read as a “bottom line” in judgments about selection’s effect. Nicely said, but a bottom line for what? Crow then gives his crucial answer—for keeping the books of evolutionary change: “we should judge the effectiveness of selection at differ¬ ent levels by its effects on gene frequencies.” Altered gene frequencies are therefore results (for bookkeeping), while selection (the cause of the changes) operates upon interactors “at different levels” of individuality. This notion of a “bottom line” also provides our best entree into the third and most important reason for choosing genes as units of bookkeeping: the intrinsically asymmetrical nature of causal flow in hierarchies of inclusion. I particularly appreciate the doubly amplified utility of hierarchy theory in this example—for the hierarchical view, as I shall show, both serves as a replace¬ ment for gene selectionism, but also (in a situation not devoid of irony)"' pro¬ vides the rationale for why many biologists chose, albeit for fallacious rea¬ sons, to focus on genes in the first place! We do need to keep the books of evolutionary change, and bookkeeping does require a basic unit of accounting. Candidates for this status must obey the primary criterion always stressed by gene selectionists: faithful replica¬ tion. But genes do not exhaust the range of faithful replicators. Asexual or¬ ganisms and species also rank as sufficiently faithful. Reductionistic prefer¬ ences in general, and claims for relatively greater faithfulness of genic vs. higher-level replication, might set a preference for genes—but another crucial argument, usually unrecognized or unmentioned, seals the case. *The logic of this case recalls the celebrated example (see pp. 492-502) of Rutherford’s use of radioactivity both to impugn the theoretical basis for Kelvin’s young age for the earth and then to provide the empirical basis for measuring a revised and much older age.
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THE STRUCTURE OF EVOEUTIONARY THEORY Because bookkeeping is not the same enterprise as causality, and because we are not, in simply counting, trying to establish the causes of differential success, we want to make sure, above all, that we choose a unit better suited than any other to record all evolutionary changes, whatever their causal ba¬ sis. No single unit of bookkeeping can monitor every conceivable change, but the gene becomes our unit of choice because the nature of hierarchies dictates that genes inevitably provide the most comprehensive record of changes at all levels. (Even so, gene records will miss certain kinds of changes that we gener¬ ally call evolutionary. For example, as Wilson and Sober (1994) point out, assortative mating of organisms within a population may greatly increase the ratio of homozygotes to heterozygotes at many loci, but need not change gene frequencies in the population.) Hierarchies are allometric, not fractal (see Gould and Lloyd, 1999), and various levels translate a common set of causes to strikingly different results and frequencies. Moreover, hierarchies are directional, and therefore not in¬ different to the nature of the flow of influence. As the most important of all such asymmetries, change at a low level may or may not produce an effect at higher levels—“upward causation” in the standard terminology (see Camp¬ bell, 1974; Vrba, 1989). But change at a higher level must always sort the in¬ cluded units of all lower levels—by the analogous process of “downward causation.” If a gene increases in copy number within a genome by duplication and lat¬ eral spread (gene selection in the genuine sense), phenotypes of organisms may or may not be affected. But selection on higher-level individuals always sorts the lower-level individuals included within. If ugly organisms outcompete beautiful conspecifics, then genes for ugliness increase in the population. If stenotypic species prevail over eurytopes in species selection, then genes as¬ sociated with stenotypy increase within the lineage. If species of polychaetes eliminate species of priapulids in competition over geological time, then polychaete genes increase in the marine biota. Given this intrinsic asymmetry, what single unit would a good bookkeeper choose? Obviously not the organism, or any high-level individual, because we would then miss many changes at lower levels—and a good bookkeeper wants, as the chief desideratum of his profession, to record all changes. As noted above, low-level selection need not impose any effect upon higher lev¬ els at all. Equally obviously, our optimal bookkeeper will choose genes— not because genes are intrinsically more basic (the reductionist fallacy); not because genes are primary causal agents, or causal agents at all (the gene selectionist fallacy); and not because genes replicate faithfully (for other kinds of individuals do so as well); but, rather, because genes, as the lowestlevel individuals in a hierarchy, manifest the unique property of recording all changes. Thus, the intrinsic nature of hierarchies sets our preference for genes as units of bookkeeping—for only genes act as nearly ubiquitous re¬ corders of all evolutionary alterations, whatever their level or cause of occur¬ rence. Finally, we must note one other property that, while strongly favoring genes as units of bookkeeping, shows even more clearly why genes cannot be
Species as Individuals in the Hierarchical Theory of Selection exclusive units of selection, or causal agents. Bookkeepers must, above all, be objectivists, not sleuths or storytellers. A good bookkeeper wants an unim¬ peachable record, not a causal hypothesis (that can always be wrong). Books kept in terms of gene frequencies become the best objective records of “de¬ scent with modification” because they do not make causal attributions, but only count changes (“just the facts, ma’am,” to cite a famous detective from the early days of television). The hierarchical nature of evolutionary mechan¬ ics, and the simultaneous action of selection on individuals at several levels, implies our inability to know the causal basis of change from records of al¬ tered gene frequencies alone. Genetic change cannot, of itself, specify the causal level of sorting because selection at any higher level sorts individuals at all included lower levels as an automatic effect, and not necessarily for direct causal reasons at all. Two ba¬ sic considerations bar inferences of cause from the genetic account books alone. First, an observation of genetic sorting doesn’t specify the relevant causal level. A gene associated with strong jaws may increase in frequency within the class Polychaeta because polychaete organisms with strong jaws outcompete weaker-jawed conspecifics in organismic selection; because poly¬ chaete species with strong jaws also develop emergent populational charac¬ ters that defeat weak-jawed species in species selection; or because strongjawed polychaetes do especially well in driving out jawless priapulids by clade selection. Second, even when we can identify the level of causality for an incident of genetic sorting, we cannot know (from the increase in frequency alone) whether the gene sorted positively has prevailed by a selected effect upon the phenotype, or for a set of possibly nonadaptive reasons. Does the gene associ¬ ated with strong jaws actually promote the construction of this phenotypic basis for organismic selection, or has this gene hitchhiked to greater fre¬ quency by close linkage with another gene that does build the selected pheno¬ type? Nonadaptive possibilities only increase for selection at higher levels. If polychaetes have increased by clade selection over priapulids, does the plurified polychaete gene big-A build part of the relevant priapulid-beating phenotype, or does big-A just count as one of the myriad polychaete genes that happen to specify, by homology and a few hundred million years of evo¬ lutionary separation, the historical uniqueness of the clade? The nature of hierarchies dictates a choice of genes as optimal units of bookkeeping. The nature of hierarchies also creates a possibility—then real¬ ized in nature for fascinating reasons largely unknown, and mostly beyond the scope of this book (but see Buss, 1987)—for structuring the world of biol¬ ogy as a hierarchy of individuals, each encompassing the ones below as new levels accrete in evolution, and each capable of acting as a unit of selection, a causal agent of Darwin’s expanded theory.
Gambits of reform and retreat by gene selectionists As I have emphasized throughout this section, gene selectionism can’t be made to work as a general philosophy. The logic of the theory does not co¬ here, and the system cannot attain consistent completion. Yet the allure of the
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THE STRUCTURE OF EVOLUTIONARY THEORY gene remains powerful, largely for reasons of general preference in our cul¬ ture, rather than for any observed power or intrinsic biological status pos¬ sessed by evolutionary individuals of this lowest level. When an incoherent argument remains intriguing, and supporters cannot bear the wrench of total abandonment, a favored theory must be festooned with compromises and “howevers,” or so changed in form that only lip service remains to cover a truly altered substance. Often, given human tendencies to paint a bright face on adversity, gene selectionists have made their necessary retreats, but pre¬ sented them as refinements or elaborations of the original theory. In this clos¬ ing section, I shall show that the two most prominent “revisions” of gene selectionism—Dawkins’s extended phenotype (1982) and Williams’s codical selection (1992)—represent defeats rather than improvements as advertised. DAWKINS ON THE
“EXTENDED PHENOTYPE.” I always
admired the
chutzpah of Senator Aikens’ brilliant solution to the morass of our involve¬ ment in the Vietnamese War. At the height of our reverses and misfortunes, he advised that we should simply declare victory and get out. Richard Dawkins got in with his 1976 book, The Selfish Gene. He declared victory with The Extended Phenotype in 1982—although he had really, at least with respect to the needs and logic of his original argument, gotten out. With admirable clarity, and no ambivalence, Dawkins proclaimed the doc¬ trine of exclusive gene selectionism in 1976: “I must argue for my belief that the best way to look at evolution is in terms of selection occurring at the low¬ est level of all ... I shall argue that the fundamental unit of selection, and therefore of self-interest, is not the species, nor the group, nor even, strictly, the individual. It is the gene, the unit of heredity (1976, p. 12). So selection occurs at only one lowest level—the gene, labelled as ‘the fundamental unit of selection.’ Nothing more inclusive, not even an organism, can be called a unit of selection.” Dawkins presented his later work, The Extended Phenotype, as an ex¬ tension and elaboration of gene selectionism: “This book,” he wrote, “is in some ways the sequel to my previous book, The Selfish Gene” (1982, p. v). Dawkins had admitted, in 1976, that genes work through phenotypes of the “lumbering robots” (organisms) serving as their passive homes. But if genes are nature’s real actors, and phenotypes only their means of expression, then why limit phenotypes to bodies? Any consequence of a gene should be equally capable of carrying the gene’s interest in a process of selection. Dawkins admitted of course that most aspects of this extended phenotype— the footprint of a shorebird in the sand, for example—will be too ephemeral, or too by-the-by, to be effective in the gene’s interest. But other parts of the extended phenotype (with the beaver’s dam as Dawkins’s favorite example) do contribute to the success of beaver genes, and should be included within “the extended phenotype” that the gene—the ultimate and only unit of selec¬ tion (at least in 1976)—can manipulate in its full range of machinations for replicative success. Dawkins (1982, pp. iv-vii) therefore insisted that the viewpoint of The
Species as Individuals in the Hierarchical Theory of Selection Extended Phenotype evolved gradually and progressively from The Selfish Gene. “The present book,” he tells us, “goes further,” presumably in the same direction: This belief—that if adaptations are to be treated as “for the good of” something, that something is the gene—was the fundamental assump¬ tion of my previous book. The present book goes further. To dramatize it a bit, it attempts to free the selfish gene from the individual organism which has been its conceptional prison. The phenotypic effects of a gene are the tools by which it levers itself into the next generation, and these tools may “extend” far outside the body in which the gene sits, even reaching deep into the nervous system of other organisms. Since it is not a factual position I am advocating, but a way of seeing facts, I wanted to warn the reader not to expect “evidence” in the normal sense of the word. So genes have become even more fundamental, and bodies even more in¬ consequential: “Fundamentally, what it going on is that replicating molecules ensure their survival by means of phenotypic effects on the world. It is only incidentally true that these phenotypic effects happen to be packaged up into units called individual organisms” (Dawkins, 1982, pp. 4-5).* But now the argument begins to unravel. Just when the gene seems poised to swallow the organism entirely as just one incidental aspect of the gene’s ar¬ mamentarium (the fully extended phenotype), Dawkins turns around, and tells us that we may treat organisms as focal entitites after all, and describe evolution from the organism’s point of view just as well: “I am not saying that the selfish organism view is necessarily wrong, but my argument, in its strong form, is that it is looking at the matter the wrong way up ... I am pretty con¬ fident that to look at life in terms of genetic replicators preserving themselves by means of their extended phenotypes is at least as satisfactory as to look at it in terms of selfish organisms maximizing their inclusive fitness” (1982, pp. 6-7). Shall we then favor the gene or the organism? Dawkins claims to prefer genes and to find greater insight in this formulation. But he allows that you or I might prefer organisms—and it really doesn’t matter. In a telling analogy, Dawkins compares genes and organism to the two possible versions (different *But it is not “only incidentally true” that genes generally come packaged into organ¬ isms on our planet—and that, in full extension, organic matter coagulates into evolution¬ ary individuals at several levels of an inclusive hierarchy: genes, cell lineages, organisms, demes, species, and clades. This process of coagulation has occurred for active and interest¬ ing structural reasons only dimly understood (Buss, 1987). But how could we regard this most fundamental feature of the organic world, constituting the basis of evolutionary cau¬ sality in units of selection, as “only incidentally true”? This structure may well be contin¬ gently true—in the sense that we can imagine an alternative world composed only of naked genes—but our planet’s biological reality surely cannot be designated as incidental in the usual sense of unimportant or not fundamental. Indeed, the origin of such hierarchical structure may not even be contingently true, but broadly predictable (see Kauffman, 1993; Maynard Smith and Szathmary, 1995).
639
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THE STRUCTURE OF EVOLUTIONARY THEORY cerebral resolutions of the same visual reality) in the famous optical illusion known as the Necker Cube: After a few more seconds the mental image flips back and it continues to alternate as long as we look at the picture. The point is that neither of the two perceptions of the cube is the correct or “true” one. They are equally correct. Similarly the vision of life that I advocate, and label with the name of the extended phenotype, is not probably more correct than the orthodox view. It is a different view and I suspect that, at least in some respects, it provides a deeper understanding. But I doubt that there is any experiment that could be done to prove my claim (1982, p. 1). Moreover, we really needn’t quarrel over our choices because the issue can achieve no empirical resolution in any case. I’ll push my preference (and hope to persuade you of its greater capacity for mind stretching, its superior liter¬ ary charm, or its greater tickling of the fancy); and you can then advocate your opposite, and equally valid, version. Dawkins begins his book: “This is a work of unabashed advocacy. I want to argue in favor of a particular way of looking at animals and plants, and a particular way of wondering why they do the things that they do. What I am advocating is not a new theory, not a hypothesis which can be verified or falsified, not a model which can be judged by its predictions, ... I am not trying to convince anyone of the truth of any factual proposition” (1982, p. 1). This argument about equally valid, but quite inverse, perspectives on a common reality pervades the entire book, as in this late passage (1982, p. 232): “The whole story could have been told in . . . the language of individual manipulation. The language of extended genet¬ ics is not demonstrably more correct. It is a different way of saying the same thing. The Necker Cube has flipped. Readers must decide for themselves whether they like the new view better than the old.” Among professional philosophers, such Necker-Cube thinking goes by the name of conventionalism, an argument that frameworks of explanation can¬ not be judged as true or false, or even more or less empirically adequate—but only as equally correct, and only as more or less preferable by such nonfactual criteria as depth of insight provided or satisfaction gained in understand¬ ing. Conventionalism may offer an interesting and fruitful approach, espe¬ cially for some scientific debates that seem especially refractory to empirical resolution—and also (more generally) for teaching people that ideas and atti¬ tudes influence science; and that “naive realism,” with its assumption that improved theories arise only from observation, represents a silly and bank¬ rupt approach to the natural world. But conventionalism cannot apply to this case because an empirical resolu¬ tion exists, and the apparent Necker-cube duality of gene or organism does not denote, as Dawkins mistakenly argues, two equally valid perspectives on the same issue, but rather expresses a correct vs. a false view of the nature of causality in Darwinian theory. Dawkins has misconstrued his categories in judging gene-based and organism-based viewpoints as alternative versions of
Species as Individuals in the Hierarchical Theory of Selection the same kind of explanation. The gene-based view works best for bookkeep¬ ing, while the organism-based view represents one legitimate level of causal¬ ity—the one regarded as effectively ubiquitous and exclusive by Darwin him¬ self. In this sense, both views are valid; but they are not comparable—and genes vs. organisms do not represent alternatives on an identical playing field of common explanatory intent. Moreover, Dawkins’s shift from the selfish gene to the extended phenotype does not reflect a simple extension or elaboration of a consistent and develop¬ ing viewpoint. He tries to save face with such a portrayal, but his strategy fails. The conventionalism of The Extended Phenotype negates and denies the explicit defense of gene selectionism as an empirical reality, as presented in The Selfish Gene. Dawkins’s first book says, in no uncertain terms (see quotation on page 618), that genes are exclusive units of selection (or causal agents), and that bodies, as passive lumbering robots, cannot play such a role. The second book says that we can view evolution equally well from either the gene’s or the organism’s point of view, that Dawkins still prefers genes, but that others remain free to favor bodies with just as much claim to empirical adequacy. The disparate logic of these two formulations precludes their inter¬ pretation as developing versions of the same view of life, and one theory is not a subtler extension of the other. These two positions connote logically contrasting, and mutually exclusive, accounts of causality in evolution. I do not happen to regard either as correct, but I think we can all agree that Dawkins’s later view of the extended phenotype derails and controverts his earlier defense of gene selectionism as nature’s true way. I do not know why Dawkins altered his view so radically. But may I suggest that he simply could not—because no one can after a proper analysis of the basic logic of the case—maintain full allegiance to the fallacious argument of strict gene selectionism. Dawkins tried hard in 1976, but ultimately needed to make so many statements from the organism’s point of view that he must have begun to wonder whether he could really continue to regard such organismal language as a mere convenience, while touting the genic formula¬ tion as a unique reality. Perhaps he finally decided that if organism-based lan¬ guage seemed so stubbornly ineluctable, then organism-based causality might be equally inevitable, at least as a legitimate option. With such an admission, the selfish gene becomes an impotent meme. WILLIAMS’S CODICAL HIERARCHY. Williams’s epochal book of 1966 set
the intellectual basis for gene selectionism, and may justly be called the founding document for this ultimate version of Darwinian reductionism. But by 1992, Williams had realized that interactors, and not replicators, consti¬ tute units of selection, or causal agents in the usual sense of the term—and that hierarchy must hold because no level of interaction can be deemed exclu¬ sive, or even primary. Williams, however, did not wish to abandon his old ap¬ paratus for viewing genes as fundamental and preferred units of selection. But que faire? Genes are replicators in their only universal role (they can also
641
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THE STRUCTURE OF EVOLUTIONARY THEORY be interactors in the much more restricted status of one legitimate level in an extensive hierarchy, as discussed on pp. 689-695)—and interactors, not repli¬ cators, are units of selection in the causal sense. Williams therefore tried an interesting gambit. He admitted that interactors form a hierarchy of evolutionary individuals at several levels, and that these interactors are units of selection in our usual sense of material entities partici¬ pating in a causal process. These interactors build a material hierarchy—and gene selectionism cannot apply to this legitimate domain. Williams therefore established a different and parallel hierarchy* for abstract units of informa¬ tion (as opposed to material entities)—and he construed genes as basic “units of selection” in this alternative and parallel domain, which he called codical (the adjectival form of codices, the plural of codex, his term for a single unit "'This interesting idea of parallel hierarchies to separate the replicative and interactive criteria of evolutionary individuality originated with Eldredge (1989; see also Vrba and Eldredge, 1984), who spoke of genealogical and economic hierarchies. The scheme contin¬ ues here with Williams’s similar distinction of material and codical systems. I find the idea of dual hierarchies both interesting and challenging, but ultimately flawed and counterpro¬ ductive in the introduction of unnecessary complexity. (My rejection of this scheme defines my only major difference with my closest colleague Niles Eldredge, who has worked with me for 25 years on problems of macroevolutionary theory.) Eldredge’s “economic” and Williams’s “material” hierarchies include the interactors defined as proper units of selection in this book—and also in Wilson and Sober (1994), and (by unintended verbal admission, though not explicitly) by such gene selectionists as Dawkins and Williams, as I have shown throughout this section. (Eldredge calls this hierar¬ chy “economic” to stress the doing and dying of such entities in nature’s ecosystems.) Eldredge’s “genealogical” and Williams’s “codical” hierarchies express the concept of rep¬ lication (as nonmaterial units of information for Williams, but as an alternative hierarchy of replicating material entities for Eldredge). I find the framework of dual sequences unnecessarily complex and divisive because a sin¬ gle theme unites our search to define units of selection, and a single hierarchy expresses this theme in the best and clearest way. Units of selection must be evolutionary individuals by the criteria outlined on pages 608-613. Above all, such individuals must be interactors in order to function as units of selection in a causal process. They must also possess a mecha¬ nism of plurifaction—that is, interactors must be able to bias the heredity of subsequent generations towards more of their own contribution, however these contributions be pack¬ aged. This need for plurifaction underlies our sense that replication plays a vital role in evo¬ lutionary individuality—a role sufficiently important to be mistaken as causal and primary by gene selectionists, or at least to warrant a separate hierarchy (by Eldredge). But I raise two points to obviate the need for a separate hierarchy of replicators: (1) replication (or some other form of hereditary passage) constitutes only one of several necessary criteria for de¬ fining evolutionary individuality; and (2) this criterion of hereditary passage only demands that interactors possess a means of plurifaction; faithful replication represents one style of hereditary passage, but not a necessary mode for attribution of evolutionary individuality or designation as a unit of selection. Sexual organisms plurify by disaggregation and differ¬ ential passage of genes; other kinds of evolutionary individuals plurify by faithful passage. We should formulate a single hierarchy—call it material, genealogical, or perhaps simply evolutionary—composed of interactors with adequate modes of plurifaction. These evolu¬ tionary individuals build a hierarchy of inclusion, with each higher level encompassing the individuals beneath as parts. Most units in Eldredge’s parallel hierarchies appear in both his economic and genealogical arrays—and therefore represent the evolutionary individuals we seek for a single hierarchy—for these are the entities that possess both the interactive (economic) and hereditary (genealogical) properties required of any evolutionary indi¬ vidual.
Species as Individuals in the Hierarchical Theory of Selection of information). If genes can’t claim exclusivity (or even causal status at all) as units of selection in the usual domain of material objects, then Williams would establish a new and separate hierarchy for nonmaterial units of infor¬ mation—and here the gene could continue to reign. Williams therefore proposed a fundamental distinction between entities and information, speaking of “two mutually exclusive domains of selection, one that deals with material entities and another that deals with information and might be termed the codical domain” (1992, p. 10). But I do not think that the codical domain can claim either meaning or existence as a locus for causal units of selection, for two reasons: Odd mapping upon legitimate intuitions. Williams continues
his allegiance to the nemesis of gene selectionism, the false criterion that has always doomed the theory to incoherence: faithful replication as the defining property for a “unit of selection”—now reformatted as a unit that only exists in the newly formulated codical domain, for Williams has now admitted that replicators are not causal agents in the usual realm of material entities. Wil¬ liams promotes his old standard—faithful replication—as the primary crite¬ rion for “unithood” in his codical domain, thus leading to the following pe¬ culiar position: genes are units of selection (as the replicating consequence in the codical domain of selection upon organisms in the material domain); gene pools are also units of selection (as replicating consequences of higher-level selection upon groups to clades); but genotypes, in an intermediate category, are not units of selection (except in asexual organisms, where replication is faithful). Thus the codical domain skips a space in the hierarchy, and contains no organismic level of selection (except for asexual creatures) because the corresponding codex is impersistent. The old error of confusing bookkeeping with causality.
Williams’s complex move in devising a separate hierarchy for nonmaterial units of information (and then juxtaposing this new sequence against the conventional hierarchy of evidently material and admittedly causal units), amounts to little beyond an elaborate and superfluous effort to rescue the unsalvageable theory of gene selectionism by granting both primacy and causal status (but only linguistically) to genes as replicators. But nothing new has been added beyond some terminology. The old error remains in full force—if anything, even heightened by the counterintuitive complexities and mental manipulations required to operationalize the scheme of dual hierarchies. A parallel hierarchy for nonmaterial entities of information? What can such a claim mean? Take the idea apart; pull the codical clothing off this new em¬ peror, and whom do we find naked underneath? our old friend, the book¬ keeper. Why must he continually try to play on the field of material objects engaged in nature’s grand game of causality? Why should he be ashamed of his vital but different role? Bookkeeping is also a necessary, and entirely hon¬ orable, activity. The results of causal processes must be tabulated, and we rightly treasure the lists. We continue to stand in awe before “60” in Babe Ruth’s home run column for 1927, and “70” in Mark McGwire’s for 1998. But 70 is a record, not a cause—a summary of a great achievement, not the
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THE STRUCTURE OF EVOLUTIONARY THEORY bat itself, or the muscles in a pair of strong arms. As nonmaterial objects suited for recording, codices are units of bookkeeping. The history of gene selectionism has provided a grand intellectual ad¬ venture for evolutionary theory—from inception as a manifesto (Williams, 1966), through numerous excursions into pop culture, to valiant (though doomed) attempts to work through the logical barriers and to develop a consistent and workable theory (Dawkins, 1982; Williams, 1992). “Paretoerrors” always inspire a good race. No one really loses—though false theories like gene selectionism must eventually yield—because the resulting clarifica¬ tions can only strengthen a field, and interestingly fallacious ideas often yield important insights. Without this debate, evolutionary biologists might never have properly clarified the differing roles of replicators and interactors, items for bookkeeping and units of selection. And we might not have developed a consistent theory of hierarchical selection without the stimulus of an opposite claim that genes could function as exclusive causal agents. Some evolutionists, largely perhaps in fealty to their own pasts, continue to use the language of gene selectionism, even while their revised accounts eluci¬ date and unconsciously promote the hierarchical view (see, in particular, Wil¬ liams’s excellent fourth chapter, in his 1992 book, on selection upon multiple interactors at several levels). Williams, to use a locution of our times, may still be talking the talk of gene selectionism, but he is no longer walking the walk. Nearly all major participants in this discussion met at Ohio State Univer¬ sity in the summer of 1988. There I witnessed a wonderful little vignette that may serve as an epitome for this section. George Williams presented his new views (the substance of his 1992 book), and surprised many people with his conceptual move towards hierarchy (within his unaltered terminology). I could not imagine two more different personalities in the brief and telling interchange that followed. Marjorie Grene—the great student of Aristotle, grande dame of philosophy, one of the feistiest and toughest people I have ever known, and a supporter of the hierarchical view—looked at Williams and simply said: “You’ve changed a lot.” George Williams, one of the calmest and most laconic of men, replied: “It’s been a long time.”
Logical and Empirical Foundations for the Theory of Hierarchical Selection LOGICAL VALIDATION AND EMPIRICAL CHALLENGES
R. A. Fisher and the compelling logic of species selection R. A. Fisher added a short section entitled “the benefit of species” to the sec¬ ond edition (1958) of his founding document for the Modern Synthesis: The Genetical Theory of Natural Selection (first published in 1930). I do not know why he did so, but the result could not be more favorable for fruitful debate—for Fisher, in these few additional paragraphs, grants to the concept of species selection the two requisite properties for any healthy and contro-
Species as Individuals in the Hierarchical Theory of Selection versial theory. In presenting his argument, Fisher proclaims the logic of spe¬ cies selection unassailable, and then denies that this genuine phenomenon could have any substantial importance in the empirical record of evolution on our planet. No situation can be more propitious for useful debate about a sci¬ entific theory than validation in logic accompanied by controversy about ac¬ tual evidence! (Obviously, I do not share Fisher’s pessimism about empirical importance, and shall devote this section to explaining why.) Fisher begins this interpolated passage by stating that Natural Selection (in his upper-case letters), in its conventional Darwinian mode of action among organisms, cannot explicitly build any features for “the benefit of the spe¬ cies” (though organismic adaptation may engender such effects as side con¬ sequences). Speaking of instinctual behaviors, Fisher writes (1958, p. 50): “Natural Selection can only explain these instincts in so far as they are indi¬ vidually beneficial, and leaves entirely open the question as to whether in the aggregate they are a benefit or an injury to the species.” But Fisher then rec¬ ognizes that, in principle, selection among species could occur, and could lead to higher-level adaptations directly beneficial to species. However, lest this logical imperative derail his strict Darwinian commitments to the primacy of organismic selection, Fisher then adds that species selection—though clearly valid in logic and therefore subject to realization in nature—must be far too weak (relative to organismic selection) to have any practical effect upon evo¬ lution. I regard the following lines (Fisher, 1958, p. 50) as one of the “great quotations” in the history of evolutionary thought: There would, however, be some warrant on historical grounds for saying that the term Natural Selection should include not only the selective sur¬ vival of individuals of the same species, but of mutually competing spe¬ cies of the same genus or family. The relative unimportance of this as an evolutionary factor would seem to follow decisively from the small num¬ ber of closely related species which in fact do come into competition, as compared to the number of individuals in the same species; and from the vastly greater duration of the species compared to the individual. Fisher’s theoretical validation of the logic behind species selection has never been effectively challenged. Even the most ardent gene selectionists have granted Fisher’s point, and have then dismissed species selection from exten¬ sive consideration (as did Fisher) only for its presumed weakness relative to their favored genic level, and not because they doubt the theoretical validity, or even the empirical reality, of selection at this higher level. Dawkins (1982, pp. 106-107) has emphasized Fisher’s argument about impotence by noting that, at most, species selection might accentuate some relatively “uninterest¬ ing” linear trends (like size increase among species in a lineage), but could not possibly “put together complex [organismal] adaptations such as eyes and brains.” Dawkins continues: When we consider the species . . . the replacement cycle time is the inter¬ val from speciation event to speciation event, and may be measured in
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thousands of years, tens of thousands, hundreds of thousands. In any given period of geological time, the number of selective species extinc¬ tions that can have taken place is many orders of magnitude less than the number of selective allele replacements that can have taken place . . . We shall have to make a quantitative judgment taking into account the vastly greater cycle time between replicator deaths in the species selec¬ tion case than in the gene selection case. I strongly support Dawkins’s last statement, but will argue (see pages 703712) that, when we factor punctuated equilibria into the equation, species se¬ lection emerges as a powerful force in macroevolution (though not as an ar¬ chitect of complex organismic adaptations). Williams has also supported Fisher’s argument about the logic of higher level selection—even in his gene selectionist manifesto of 1966, where he de¬ fends the possibility, but then denies the actuality: “If a group of adequately stable populations is available, group selection can theoretically produce bi¬ otic adaptations, for the same reason that genic selection can produce organic adaptations” (1966, p. 110). In his later book, however, Williams becomes much more positive about the importance and reality of selection at several hierarchical levels: “To Darwin and most of his immediate and later follow¬ ers, the physical entities of interest for the theory of natural selection were discrete individual organisms. This restricted range of attention has never been logically defensible” (1992, p. 38). The developing literature has added three “classical” arguments against higher-level selection to supplement Fisher’s point that cycle times for species must be incomparably long relative to the lives of organisms. All these argu¬ ments share the favorable property of accepting a common logic but chal¬ lenging the empirical importance of legitimate phenomena—a good substrate for productive debate in science, in contrast with the confusion about con¬ cepts and definitions that so often reigns. In the rest of this section, I shall summarize the four classical arguments (Fisher’s original plus the three addi¬ tions); note that they can all be effectively challenged at the level for which they were devised (“group,” or interdemic, selection); and then demonstrate that none has any strong force, in principle, against the empirical importance of the still higher level of species selection.
The classical arguments against efficacy of higher-level selection The usual arguments against higher-level selection admit that such phe¬ nomena must be possible in principle, but deny any meaningful efficacy on grounds of rarity and weakness relative to ordinary natural selection upon organisms. Weakness (based on cycle time). R. A. Fisher’s classical argu¬ ment: How could species selection exert any measurable effect upon evo¬ lution? Rate and effect depend upon numbers and timings of births and deaths—to provide a sufficient population of items for differential sorting. But species endure for thousands or millions of years, and clades count their
Species as Individuals in the Hierarchical Theory of Selection
“populations” of component species in tens, or at most hundreds, and not as the millions or billions of organisms in many populations. How could species selection yield any measurable effect at all (relative to ordinary organismic se¬ lection) when, on average, billions of organismic births and deaths occur for each species origin or extinction, and when populations of organisms contain orders of magnitude more members than populations of related species in a clade? Weakness (based on variability).
Hamilton (1971, 1987), in de¬
vising arguments against interdemic selection, pointed out that variation among demic mean values for genetically relevant and selected aspects of organismic phenotypes will generally be lower than variation among or¬ ganisms within a population for the same features. Group selection cannot become a strong force if the mean phenotypes of such higher-level indi¬ viduals express such limited variation to serve as raw material for selective change. Instability, as the
desert and
in Dawkins’s clouds
metaphors
in the
sky.
of
duststorms
in
This argument has also been
most frequently advanced against interdemic selection. Demes, by definition, maintain porous borders because organisms in the same species can inter¬ breed, and members of one deme can therefore, in principle, invade and join another in a reproductive role. If such invasions become frequent and numer¬ ous, the deme ceases to function as a discrete entity, and cannot be called an evolutionary individual. Moreover, many demes lack cohesion on their own account, and not only by susceptibility to incursion. Demes may arise as en¬ tirely temporary and adventitious aggregrates of organisms, devoid of any in¬ herent mechanism for cohesion, and defined only by the transient and clumpy nature of appropriate habitats that may not even persist for a requisite gener¬ ation—as in the deme of all mice in a haystack, or all cockroaches in an urban kitchen. Invasibility from below.
from
other
more
potent
levels,
usually
This standard argument, related both to Fisher’s first point
about cycle time and to the third point about invasibility discussed just above —and classically used to question the potential evolution of altruism by interdemic selection—asks how higher-level selection could possibly become effective if its operation inherently creates a situation where more powerful, lower-level invaders can cancel any result by working in the opposite direc¬ tion. Suppose that interdemic selection, cranking along at its characteristic pace, increases the overall frequency of altruistic alleles in the entire species because demes with altruists enjoy differential success in competition against demes without altruists. This “leisurely” pace works well enough, but as soon as a selfish mutant arises in any deme with altruists, the advantage of this mutant in organismic selection against the altruistic allele should be so great that the frequency of altruistic genes must plummet within the deme, even while the deme profits in group selection from the presence of altruistic organisms. By Fisher’s argument of cycle time, organismic selection of the self-serving should trump interdemic selection for altruism.
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THE STRUCTURE OF EVOLUTIONARY THEORY Overcoming these classical arguments, in practice for interdemic selection, hut in principle for species selection Since the bulk of modern debate about higher-level selection has addressed interdemic (or so-called group) selection, J:he classical arguments have been framed mainly at the level just above our conventional focus upon organisms (though I predict that emphasis will shift to higher levels, particularly to spe¬ cies selection, as macroevolutionary theory develops). All four arguments have force, and do spell impotence for interdemic selection in many circum¬ stances. But, as full generalities, these arguments have failed either to dis¬ prove interdemic selection as a meaningful process worthy of consideration at all, or to deny the efficacy of interdemic selection in several important cir¬ cumstances. I shall not review this enormous literature here (as my primary concern rests at still higher levels of selection), but I wish to note that two classes of argument grant interdemic selection sufficient strength and presence to count as a potentially major force in evolution (see Wade, 1978; Sober and Wilson, 1998). First, much mathematical modelling (and some experimental work) have adequately shown that, under reasonable conditions of potentially fre¬ quent occurrence in nature, group selection can assert its sway against the le¬ gitimate power of the four classical objections. In the cardinal example, un¬ der several plausible models, the frequency of altruistic alleles can increase within a species, so long as the rate of differential survival and propagation of demes with altruistic members (by group selection) overcome the admitted decline in frequency of altruists within successful demes by organismic selec¬ tion. The overall frequency may rise within the species even while the fre¬ quency within each surviving deme declines. Second, some well-documented patterns in nature seem hard to explain without a strong component of interdemic selection. Female-biased sex ra¬ tios, as discussed by Wilson and Sober (1994, pp. 640-641), provide the classic example because two adjacent levels make opposite and easily tested predictions: conventional organismic selection should favor a 1:1 ratio by Fisher’s famous argument (1930); while interdemic selection should promote strongly female-biased ratios to enhance the productivity of groups. Williams (1966) accepted this framework, which he proposed as a kind of acid test for the existence of group selection. He allowed that female-biased ratios would point to group selection, but denied that any had, in fact, been documented, thus validating empirically the theoretical arguments he had developed for the impotence of group selection. Williams concluded (1966, p. 151): “Close conformity with the theory is certainly the rule, and there is no convincing ev¬ idence that sex ratios ever behave as a biotic adaptation.” But empirical ex¬ amples of female-biased ratios were soon discovered aplenty (see Colwell, 1981, and numerous references in Wilson and Sober, 1994, p. 592). Some au¬ thors (Maynard Smith, 1987, for example) tried to interpret this evidence without invoking group selection, but I think that all major participants in the discussion now admit a strong component of interdemic selection in such results—and reported cases now number in the hundreds, so this phenome-
Species as Individuals in the Hierarchical Theory of Selection
non cannot be dismissed as an odd anomaly in a tiny corner of nature. Wil¬ liams now accepts this interpretation (1992, p. 49), writing “that selection in female-biased Mendelian populations favors males, and that it is only the se¬ lection among such groups that can favor the female bias.” The primary appeal of this admirably documented example lies in the usual finding of only moderate female biases—more than organismic selec¬ tion could allow (obviously, since any bias at all would establish the point), but less than models of purely interdemic selection predict. Thus, the empiri¬ cal evidence suggests a balance between adjacent and opposing levels of selec¬ tion—with alleles for female-biased sex ratios reduced in frequency by organ¬ ismic selection within demes, but boosted above the Fisherian balance (across species as a whole) because they increase the productivity of demes in which they reside, however transiently, at high frequency. When we move to the level of species selection, the most important for macroevolutionary theory, we encounter an even more favorable situation. For interdemic selection, the classical contrary arguments had legitimate force, but could be overcome under conditions broad enough to grant the phenomenon considerable importance. For species selection, on the other hand, three of the classical arguments don’t even apply in principle—while the fourth (weakness due to cycle time) becomes irrelevant if punctuated equilibrium prevails at a dominant relative frequency. Proceeding through the classical objections in reverse order, the fourth ar¬ gument about invasibility from below has strength only in particular con¬ texts—when, in principle, a favored direction of higher-level selection will usually be opposed by stronger selection at the level immediately below. (In the classic case, selfish organismal “cheaters” derail group selection for altru¬ ism. Nonetheless, while the argument of invasibility may hold for this partic¬ ular case—and while, for contingent reasons in the history of science, this ex¬ ample became the paradigm for discussion of interdemic selection—I see no reason in principle for thinking that organismal selection must always, or even usually, oppose interdemic selection. The two levels may operate simul¬ taneously and in the same direction, or at least orthogonally—see Wade’s (1978) classic work on this subject.) In any case, no general reason has been advanced for thinking that organis¬ mic or interdemic selection should characteristically oppose species selec¬ tion—and the argument of invasibility therefore collapses. Of course, organ¬ ismic selection may operate contrary to the direction of species selection— and must frequently do so, particularly in the phenomenon that older text¬ books called “overspecialization,” or the development of narrowly focussed and complex adaptations (the peacock’s tail as a classic example) that en¬ hance the reproductive success of individual organisms, but virtually guaran¬ tee a decreased geological life span for the species. Other equally common modes of organismic selection, however, either tend to increase geological longevity (improvements in general biomechanical design, for example) or to operate orthogonally, and therefore “beneath the notice” of species selection. Since our best examples of species selection work through differential rates of
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THE STRUCTURE OF EVOLUTIONARY THEORY speciation rather than varying propensities for extinction, and since most organismal adaptations probably don’t strongly influence a population’s rate of speciation (or at least don’t manifest any bias for decreasing the rate), es¬ sential orthogonality of the two levels will^often prevail in evolution. The third argument of instability, while potent for demes, clearly does not apply to species. Sexual species are as well bounded as organisms. Just as genes and cell lineages generally do not wander from organism to organism (whereas organisms often move readily from deme to deme), neither can or¬ ganisms or demes wander from species to species. The reasons for such tight¬ ness of bounding differ between organism and species, but these two evolu¬ tionary individuals probably exceed all others in the strength of this key criterion. Species maintain and “police” their borders just as well as organ¬ isms do. The tight bounding of an organism arises from functional integration among constituent parts, including an impermeable outer covering in most cases, and often an internal immune system to keep out invaders. The tight bounding of a species (as classically defined for sexually reproducing eukary¬ otes) arises from reproductive interaction among parts (organisms), with firm exclusion of parts from any other species. Moreover, this exclusion is actively maintained, not merely passively propagated, by traits that became a favorite subject of study among founders of the Modern Synthesis, especially Dobzhansky and Mayr—so-called “isolating mechanisms.” Species may lack a literal skin, but they remain just as well-bounded as organisms in the sense required by the theory of natural selection. This discussion on the validity and centrality of species as units of selec¬ tion highlights my only major unhappiness with Wilson and Sober’s (1994) superb defense of hierarchical selection, otherwise followed closely in this book. They insist upon functional integration as the main criterion for identi¬ fying units of selection (vehicles in their terminology, interactors or evolution¬ ary individuals for others). They insist that the following question “is and always was at the heart of the group selection controversy—can groups be like individuals in the harmony and coordination of their parts” (1994, p. 591). I do not object to the invocation of functionality itself, but rather to their narrow definition, too parochially based upon the kind of functionality that organisms display. The cohesion (or “functionality”) of species does not lie in the style of interaction and homeostasis that unites organisms by the integra¬ tion of their tissues and organs. Rather, the cohesion of species lies in their ac¬ tive maintenance of distinctive properties, achieved by joining their parts (or¬ ganisms) through sexual reproduction, while excluding the parts of other species by evolution of isolating mechanisms. I much prefer and support Wilson and Sober’s more general definition (1994, p. 599): “Groups are real to the extent that they become functionally organized by natural selection at the group level.” Species meet this criterion by evolving species-level properties that maintain their cohesion as evolution¬ ary individuals. The key to a broader concept of “functionality” (that is, the
Species as Individuals in the Hierarchical Theory of Selection
ability to operate discretely as a unit of selection) lies in the evolution of ac¬ tive devices for cohesion, not in any particular style of accomplishment—ei¬ ther the reproductive barriers that maintain species, or the homeostatic mech¬ anisms that maintain organisms. The second argument of weakness based on lack of sufficient variability among group mean values also doesn’t apply to species. Demes of mice from separated but adjacent haystacks may differ so little in group properties that the survival of only one deme, with replenishment of all haystacks by mi¬ grants from this successful group, might scarcely alter either allelic frequen¬ cies across the entire species, or even average differences among demes. But new species must differ, by definition, from all others—at least to an extent that prevents the reproductive merging of members. Thus, the differential success of some species in a clade must alter—usually substantially—the aver¬ age properties of the clade (whereas, one level down, the differential success of some demes need not change the average properties of the species very much, if at all). The first argument about weakness due to long cycle time and small popu¬ lations therefore remains as the only classical objection with potential force against species selection. And, at first glance, Fisher’s argument would seem both potent and decisive. The basic observation cannot be faulted: billions of organism births usually occur for each species birth; and populations of or¬ ganisms within a species almost always vastly exceed populations of species in a clade. How then could species selection, despite its impeccable logic, maintain any measurable importance when conventional organismal selec¬ tion holds the tools for such greater strength? The logic of Fisher’s argument cannot be denied, but we must also con¬ sult the empirical world. Organismic selection must overwhelm species selec¬ tion when both processes operate steadily and towards the same adaptive “goal”—for if both levels work in the same direction, then species selection can only add the merest increment to the vastly greater power of organismic selection; whereas, if the two levels work in opposite directions, organismic selection must overwhelm and cancel the effect of species selection. But the empirical record of the great majority of well-documented fossil species affirms stasis throughout the geological range (see next chapter). The causes for observed nondirectionality within species have not been fully re¬ solved, and the phenomenon remains compatible with the continuous opera¬ tion of strong organismic selection—for two common explanations of stasis as a central component of punctuated equilibrium include general prevalence of stabilizing selection, and fluctuating directional selection with no overall linear component due to effectively random changes of relevant environments through time. In any case, however, the observation of general stasis seems well established at high relative frequency (I would say dominant, but I also must confess my partisanship). In this factual circumstance, since change does not generally accumulate through time within a species, organismic selection in the conventional grad¬ ualistic and anagenetic mode cannot contribute much to the direction of
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THE STRUCTURE OF EVOEUTIONARY THEORY trends within a clade. Change must therefore be concentrated in events of branching speciation, and trends must arise by the differential sorting of spe¬ cies with favored attributes. If new species generally arise in geological mo¬ ments, as the theory of punctuated equilibrium holds, then trends owe their explanation even more clearly to higher-level sorting among species-individu¬ als acting as discrete entities with momentary births and stable durations in geological time. Organismic selection may trump species selection in principle when both processes operate at maximal efficiency, but if change associated with speci¬ ation operates as “the only game in town,” then a weak force prevails while a potentially stronger force lies dormant. Nuclear bombs certainly make con¬ ventional firearms look risible as instruments of war, but if we choose not to employ the nukes, then bullets can be devastatmgly effective. The empirical pattern of punctuated equilibrium therefore becomes the factual “weapon” that overcomes Fisher’s strong theoretical objection to the efficacy of species selection. (This argument provides a second example for the importance of punctu¬ ated equilibrium in validating the independence of macroevolutionary theory by failure of pure extrapolationism from microevolutionary dynamics. We saw previously (pp. 604-608) that punctuated equilibrium strongly fosters the argument for species as evolutionary individuals capable of operating as units of selection. We now note that punctuated equilibrium also affirms the potential strength of species selection against a cogent theoretical claim for its impotence.) In summary, three of four classical arguments against higher-level selection do not apply to species, while the fourth loses its force in a world dominated by punctuated equilibrium. I see no barrier to the cardinal importance of spe¬ cies selection in the history of life.
EMERGENCE AND THE PROPER CRITERION FOR SPECIES SELECTION Differential proliferation or downward effect? This subject and its literature, as I have noted throughout the chapter, have been plagued to an unusual degree by conceptual confusions and disputes about basic definitions and terminology. As an important example, and as many participants have noted (see especially Damuth and Heisler, 1988; and Brandon, 1988, 1990), two quite different criteria for the definition of higherlevel selection have circulated through the literature. (In most cases, they yield the same conclusion, so this situation has not produced anarchy; but in a few cases, some crucial, they may lead to different assertions, so the situation has fostered confusion.) In the first approach, one chooses a focal level of analysis (conventionally one of the two lower levels of organism or gene), and then considers the effect of membership in a higher-level group upon fitness values of the chosen lower-level unit. If, for an identical organism, life in one kind of deme yields a
Species as Individuals in the Hierarchical Theory of Selection
fitness different from life in another kind of deme, then selection includes a group effect from the deme level. (We invoke this formulation, for example, if we argue for group selection by showing that organisms in a deme with altru¬ ists do better than identical organisms in a group lacking altruists.) In the second approach, strongly favored here, we hold firm to the classi¬ cal bare-bones Darwinian definition, but recognize that selection can work on evolutionary individuals at many hierarchical levels. Selection has tradi¬ tionally been defined as the differential reproductive success of evolutionary individuals based on the fitnesses of their traits in interaction with the en¬ vironment. Thus, we recognize higher-level selection by the differential pro¬ liferation of some higher-level individuals (demes, species, clades) over oth¬ ers—just as we define conventional natural selection by the differential reproductive success of some organisms based on phenotypic traits that con¬ fer fitness. These two approaches often yield concordant results for the obvious rea¬ son that differential proliferation of higher-level units (the second criterion) often defines the group effect that influences the fitness of lower-level individ¬ uals chosen as a focus (the first criterion). But the two criteria need not corre¬ spond, leading to situations where we v/ould identify group selection by one criterion, but deny the same process by the other. For example, under the first criterion of group effects on lower-level fitness, some higher-level properties of groups can influence lower units without causing any differential repro¬ duction of the groups themselves. On this criterion, for example, some biolo¬ gists have held that frequency dependent selection must be viewed, ipso facto, as an example of group selection—a claim simply incomprehensible under the alternative criterion of differential group proliferation. (The unresolved, and perhaps largely semantic, issue of whether kin selection should be inter¬ preted as a form of group selection, or only an extension of conventional lower-level selection, also presupposes this criterion of group effect upon lower-level fitness—see Wilson and Sober, 1994.) A predominantly sociological issue has often set preferences between these criteria. Paleontologists, and other students of species selection, myself in¬ cluded, have strongly advocated the criterion of differential reproduction for higher-level individuals as a strict and obvious analog of ordinary natural se¬ lection as conventionally understood. Neontologists and students of group selection have generally (though not always) preferred the criterion of “group effect on gene or organismal fitness,” both from fealty to Darwinian tradi¬ tions for using organisms as a primary focus, and because certain contentious issues, especially the evolution of altruism, have generally been posed in organismal terms—“why can saintly Joe be so nice if he loses reproductive success thereby?” Three major reasons lead to my strong preference for the criterion of differ¬ ential proliferation correlated with properties of relevant evolutionary indi¬ viduals that confer fitness in interaction with their environment. First, we thereby follow standard definitions of selection, which have always been based on causal plurifaction, not on mere effect. Second, why would we ever
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THE STRUCTURE OF EVOLUTIONARY THEORY prefer an elaborate and indirect definition—in terms of effects on something else at a scale far removed from the causal interaction—over a simpler ac¬ count rooted in the direct result of the causal process itself? Considered in these terms, the criterion of “group effect on organismal fitness” seems downright peculiar. We only entertain such a standard for contingent reasons of history and philosophical preference—the Darwinian tradition for focus¬ ing on organisms, and our larger scientific allegiance to reductionism. Third, we can too easily lose the force and location of causality when we study a phenomenon through indirect effects expressed elsewhere, rather than by im¬ mediate operation. True, we are supposed to assess the separate effects upon lower-level focal units—from deme membership, or species membership, for example. But since several higher levels may simultaneously affect a lower fo¬ cal unit, we may not be able to untangle the differences, and we may end up with an account of consequences, rather than causes. As an obvious example of these pitfalls, I point out that gene-selectionism, with all its fallacies, arises from an erroneous inversion in the criterion of “group effect on lower-level fitness.” One begins with the basic statement that membership in higher-level units affects the fitness of genes. So far, so good. But if one then makes the error of assuming that replicators, rather than interactors, should be units of selection—and then chooses genes as fun¬ damental replicators both by general reductionistic preference, and by alle¬ giance to faithfulness in replication as a necessary criterion—then one be¬ comes tempted to misidentify effects as causes. The gene selectionist then slides down the following slippery slope: why should I talk about higher-level interactors affecting gene fitness? why don’t I just consider higher-level inter¬ actors as one aspect of the gene’s environment? in that case, why should I talk about higher-level interactors as entities at all? environment is environment, however constituted, and whether clumped into interactors housing the genes or not? in fact, why even try to identify the environment’s forms of dumpi¬ ness? why not, instead, simply average the gene’s fitness over all aspects of environment to achieve a single measure of the gene’s evolutionary prowess? This line of argument, as its least attractive feature, relentlessly dissolves causality. We begin with the causal agents of selection—interactors at various hierarchical levels. (Even the most ardent gene selectionists, as I show on pages 631-632, cannot avoid discussing the causal process of selection in terms of these interactors.) We then represent interactors by their effects on genes. Next, we decide to consider interactors only as environments of genes. Then we lose interest in their nature and action because “environmental clumping” (the “expression” of interactors in this view) does not appear to represent an important issue. Finally, we dissolve the interactors entirely by deciding to average the fitness of genes across all aspects of the environment. And, before we notice what we have really done, causality has disappeared. In a vigorous defense of gene selection against the hierarchical view of Wil¬ son and Sober (1994), Dawkins (tongue-in-cheek to be sure) pretends to be “baffled” by “the sheer, wanton, head-in-bag perversity of the position that they champion” (commentary in Wilson and Sober, 1994, p. 617). Such a
Species as Individuals in the Hierarchical Theory of Selection
sense of strong psychological frustration must arise when you and your oppo¬ nents seem to be saying the same thing, but in such utterly different ways, and to such radically different effect. Thus, Dawkins presents his gene-selectionist reformulation of Wilson and Sober’s Weltanschauung (mine as well, by the way): Selection chooses only replicators . . . Replicators are judged by their phenotypic effect. Phenotypic effects may happen to be bundled, to¬ gether with the phenotypic effects of other replicators, in vehicles. Those vehicles often turn out to be the objects that we recognize as organisms, but this did not have to be so. . . . There did not have to be any vehicles at all. . . . The environment of a replicator includes the outside world, but it also includes, most importantly, other replicators, other genes in the same organism and in different organisms, and their phenotypic products. (Note that I did not exaggerate or caricature in my previous summary; gene selectionists do regard “clumping” into vehicles as beside the point, and they do dissolve these vehicles—the true units of selection—into “environment” considered as the sum of contexts for any gene.) Wilson and Sober (1994, p. 641) responded to Dawkins with their own frustration: Dawkins remains so near, yet so far . . . We could not ask for a better summary of the gene-centered view. The question is, are vehicles of selec¬ tion absent from this account or have they merely been reconceptualized as environments of the genes. The answer to this question is obvious at the individual [organism] level, because Dawkins acknowledged long ago that individuals [organisms] can be vehicles of selection . . . despite the fact that they are also environments of the genes. The answer is just as obvious at the group level . . . [Dawkins’s] passage does not refute the existence of vehicles, but merely assumes that the vehicle concept can be dispensed with and that natural selection can be studied entirely in terms of average genic effects. Is this brouhaha much ado about nothing? Are the two views—selection on a hierarchy of interactors, and representation of all selective forces in terms of gene fitnesses, with interactors treated as environments of genes—truly equiv¬ alent, and our decision just a matter of preference, or a question of psycho¬ logical judgment about superior sources of insight? Is this twofold choice just another manifestation of Dawkins’s old Necker Cube (see p. 640)—a flipping between two equivalent facets of reality, an example of conventionalism in philosophy? The answer, I think, must be a clear and resounding “no.” The two alterna¬ tives represent strikingly different views about the nature of reality and cau¬ sality. We all agree that we need to know causes—and natural selection is a
causal process. Gene selection confuses bookkeeping (properly done at the genic level) with causality (a question of evolutionary individuals plurifying
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THE STRUCTURE OF EVOLUTIONARY THEORY differentially, based on interaction of their phenotypes with the environ¬ ment). If we dissolve interactors into an overall “environment” of the genes, and then average a gene’s fitness across all environments—the procedure of gene selectionism—then we lose causality., Wilson and Sober (1994, p. 642) also reject the purely pluralist, or Necker Cube view: “There is no room for pluralism on these substantive empirical is¬ sues . . . Group-level adaptations can be represented at the individual [organ¬ ism] and gene level by averaging the fitness of lower level units across higher level units. Gene- and individual-level adaptations cannot be interpreted as group adaptations without committing the errors of naive group selection, but the gene’s-eye view and the individual’s-eye view cannot deny the exis¬ tence of group-level adaptations (when groups are vehicles of selection) with¬ out being just plain wrong.” Arnold and Fristrup (1982, p. 115) make the same point for the intrin¬ sic reality—and not just preferential status vs. other equivalent representa¬ tions—of species selection: “The characters that increase individual [organismic] fitness do not necessarily cause speciation or prevent extinction. Thus, it is misleading to adopt the convention of expressing all higher level trends in terms of individual [organism] level fitness.” For all these reasons, I strongly advocate that we define higher-level se¬ lection as the differential proliferation of relevant evolutionary individuals based on causal interaction of their properties with surrounding environ¬ ments—rather than by representing the effect of higher-level membership on the fitness of a designated lower-level individual. Only in this way will we avoid a set of confusions, and two pitfalls that easily follow, one after the other, with the first as a kindly delusion, and the second as an outright error: first, a falsely pluralistic belief in the equivalency of alternative representa¬ tions at different levels; and, second, the siren song of gene selection as defin¬ ing the only legitimate level of causal analysis in evolution. Only in this way will we achieve a clear and unified view that treats each level in the same manner, and approaches each evolutionary individual with the same set of questions. With this apparatus of analysis, we can attain a coherent and comprehensive theory of hierarchical selection—the most potentially fruitful, promising, and proper expansion of the Darwinian research program now before us.
Shall emergent characters or emergent fitnesses define the operation of species selection? Once we agree to define higher-level selection by differential proliferation of relevant units based on interaction between their traits and the environment, then we must (above all) develop clear criteria for the definition and recogni¬ tion of traits in the unfamiliar world of higher-level individuals. Since we en¬ counter enough trouble in trying to define and parse traits for the kind of in¬ dividuals we know best—integral, complex, and continuous organisms like ourselves—we should not be surprised that this issue becomes particularly re¬ fractory at higher levels, and thus acts as a considerable impediment to the
Species as Individuals in the Hierarchical Theory of Selection
development of a rigorous theory of hierarchical selection. In particular, what should count, for purposes of defining evolutionary interaction with the envi¬ ronment, as a trait of a species? The developing literature on this subject has featured a rich and interest¬ ing debate between two quite different approaches that, nonetheless, can be united in a coherent way to form the basis of a unified macroevolutionary theory of selection: the “emergent character” approach, as particularly cham¬ pioned by Elizabeth Vrba (1983, 1984b, 1989; Vrba and Eldredge, 1984; Vrba and Gould, 1986); and the “emergent fitness” approach inherent in the classic paper of Lewontin (1970), developed and defended in the important work of Arnold and Fristrup (1982), given further mathematical form in Damuth (1985), and Damuth and Heisler (1988), and most fully codified and expressed by Lloyd (1988—see also Lloyd and Gould, 1993; and Gould and Lloyd, 1999). Grantham (1995), in an excellent review of hierarchical theories of macro¬ evolution, has christened this discussion “The Lloyd-Vrba Debate,” so the is¬ sue has now even acquired a proper name. The codification makes me feel a bit strange, since I have written papers on the subject with both protagonists (Gould and Vrba, 1982; Vrba and Gould, 1986; Lloyd and Gould, 1993; Gould and Lloyd, 1999), and do not view the issue as dichotomous; though the two viewpoints are surely distinct, and I have changed my mind—as a for¬ mer supporter of Vrba’s “strict construction,” who became convinced that Lloyd’s more inclusive formulation forges a better match with conventional definitions of selection, and provides more promise for constructing an oper¬ ational theory. But Lloyd does not disprove Vrba; rather, Vrba’s exclusive do¬ main becomes a subset of “best cases” in Lloyd’s formulation. In this crucial sense, the theories sensibly intermesh. Vrba’s “emergent character” approach requires that a trait functioning in species selection be emergent at the species level—basically defined as origin by non-additive interaction among lower-level constituents. Since all science works within particular sociological and historical circumstances, we must understand that the greatest appeal of this strict criterion lies in its ability to “fend off” the conventional objection to species selection in a Darwinian and reductionistic world—namely, that the trait in question, although describable as characterizing a species, “really” belongs to the constituent, lower-level parts—and that the causal process therefore reduces to ordinary Darwin¬ ian natural selection on organisms or genes. For, when Vrba’s criterion of emergence holds, one can’t, in principle, ascribe the trait in question to lower levels. The trait, after all, does not exist at these lower levels. It makes a “first appearance” at the species level, for the trait arises through non-addi¬ tive interaction of component lower-level parts or influences. If one species proliferates differentially within a clade by higher rates of speciation based upon such populational traits as geographic range, or density of packing among organisms, then we cannot ascribe selection to the organismic level— for organisms, by the logic of definition, cannot possess a population density, while the geographic range of a species need not correlate at all, or in any
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THE STRUCTURE OF EVOLUTIONARY THEORY simple way, with the size of an organism’s personal territory during its lifetime. The strength of the “emergent character” criterion lies in its ability to iden¬ tify a set of hard-line, unambiguous cases for species selection. For we must speak of selection among species if the relevant trait not only doesn’t exist at any lower level, but can’t even be represented as a linear combination of lower-level parts—for the nonadditive interactions that build the populational trait only arise within the population, and make no sense outside such an aggregation. But we soon begin to worry that such a criterion may be too restrictive in eliminating a wide variety of traits that we intuitively view as features of pop¬ ulations, but that do not arise by nonlinear interaction of subparts, and do not therefore qualify as emergent by Vrba’s criterion (which also matches the standard definition of the important concept of emergence in philosophy). Species and other higher-level individuals also develop features that seem to “belong” to them as an entity, but that arise additively as “aggregate” or “sum-of-the-parts” characters. Consider the mean value of a trait? This fig¬ ure belongs to no individual and becomes, in this legitimate sense, a character of the population. But a mean value doesn’t “emerge” as a functional “or¬ gan” of the population by nonlinear interactions among organisms. A mean value represents an aggregate character, calculated by simple summation, fol¬ lowed by division. And how shall we treat variability—an even more “intuitive” candidate for a species-level character that may be important in survival and proliferation of species? An individual organism doesn’t possess a variability, so the prop¬ erty belongs to the species. But variability also represents an aggregate char¬ acter—another average of a sum-of-the-parts. Do we not want to talk about species selection when species B dies because constituent organisms show no variation for a trait that has become strongly inadaptive in the face of envi¬ ronmental change—while species A lives and later multiplies because the same trait varies widely, and includes some states that can prosper in the new circumstances? Yes, species B dies because each of its parts (organisms) ex¬ pires. In this sense, we can represent extinction as a summation of deaths for organismal reasons. But don’t we also want to say that A survived by virtue of greater variability—a trait that does not exist at the organismal level, but that surely interacted with the new environment to preserve the species? Vrba’s solution, which I greatly respect but now regard as less useful than the alternative formulation, requires that we not designate differential prolif¬ eration of species based on aggregate characters of populations as species se¬ lection—but rather that we interpret such cases as upward causation from the traditional organismal level. Vrba (1980 et seq.) has coined and developed the term “effect hypothesis” for such situations—since the differential prolifera¬ tion of species A vs. species B arises as an effect of organismal properties (of the individuals in species A that vary in the “right” direction), resulting in the survival of species A.
Species as Individuals in the Hierarchical Theory of Selection
Vrba, and (I think) all other major workers in this area, have always re¬ garded the effect hypothesis as a macroevolutionary theory because, in a heu¬ ristic and descriptive sense, one must apply the notion to species considered as items of evolutionary history. But events under the effect hypothesis are causally reducible to the traditional organismic level. (This kind of situation represents the minimal claim for an independent macroevolutionary theory— the need for descriptive engagement at the level of species, even if no distinct causality emerges at this higher level. This book defends the stronger claim for important causal uniqueness at the species level and above. Vrba, of course, also advocates this stronger version because she argues that some cases of differential species proliferation arise by the effect hypothesis, while others occur by true species selection based on emergent characters. I advo¬ cate a much larger role for causal uniqueness by defending the emergent fitness approach, a criterion that greatly expands the frequency and impor¬ tance of species selection.) To facilitate this distinction, Vrba and I developed a terminology to re¬ solve a common confusion in evolutionary theory between the simple, and purely descriptive, observation of differential reproductive success—which we named “sorting”—and the causal claim—always and properly called “se¬ lection”—that observed success arises from interaction between properties of the relevant evolutionary individual and its environment (see Vrba and Gould, 1986). Evolutionary biology needs this distinction because students of the field have often—with misplaced confidence in selection’s ubiquity and exclusivity—made a case for selection based on nothing more than an obser¬ vation of differential reproductive success (sorting), without any attempt to elucidate the cause of such sorting. A leading textbook, for example, pro¬ claimed that “selection ... is differential survival and reproduction—and no more” (Futuyma, 1979, p. 292). Under Vrba’s criterion of emergent characters, differential species prolifer¬ ation by the effect hypothesis counts only as sorting at the species level—since the characters responsible for selection belong to organisms, but transfer an effect to the species level by upward causation. On the other hand, differen¬ tial species proliferation based on emergent species characters does count as selection at the species level. However, under the broader criterion of emer¬ gent fitness, any species-level trait that imparts an irreducible fitness to spe¬ cies in their interaction with the environment defines a true process of selec¬ tion at the species level, whether the trait itself be aggregate or emergent. In the “emergent fitness” approach, we do not inquire into the history of species-level traits that interact with the environment to secure differential proliferation. We do not ask where the traits originated in a structural or tem¬ poral sense—that is, whether such traits arose by emergence at the species level, or as aggregate features by summation of properties in component or¬ ganisms or demes. We only require that these traits characterize the species and influence its differential rate of proliferation in interaction with the envi¬ ronment. In other words, we only demand that aspects of the fitness of the
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THE STRUCTURE OF EVOEUTIONARY THEORY
species be emergent and irreducible to the fitnesses of component organisms. For cases where species function as interactors, or potential units of selection, Lloyd and Gould write (1993, pp. 595-596): *
Interactors, and hence selection processes themselves, are individuated by the contributions of their traits to fitness values in evolutionary mod¬ els; the trait itself can be an emergent group property or a simple summa¬ tion of organismic properties. This definition of an entity undergoing se¬ lection is much more inclusive than in the emergent character approach, since an entity might have either aggregate or emergent characters (or both) . . . The emergent fitness approach requires only that a trait have a specified relation to fitness in order to support the claim that a selection process is occurring at that level. ... In other words, the interactor’s fitness covaries with the trait in question. In a classic example, much discussed in the literature (Arnold and Fristrup, 1982; Gould, 1982c; Lloyd and Gould, 1993; Grantham, 1995), several clades of Tertiary gastropods show trends to substantial decrease in relative frequency of species with planktotrophic larvae vs. species that brood their young. In one common explanation (by no means universally accepted), this reduction occurs by species sorting based on the lower speciation rate of planktotrophic species—an hypothesized consequence of the lower probabil¬ ity for formation of isolates in species with such widespread and promiscuous larval dispersal. The sorting clearly occurs by selection, since low speciation rate arises as a consequence of interaction between traits of interactors and their environment. But at what level does selection occur? Under the emergent character approach, the case becomes frustrating and ambiguous. Does the crucial property of “low speciation rate” in planktotrophs result from an emergent species character? In one sense, we are tempted to answer “yes.” Organisms, after all, don’t speciate; only popula¬ tions do—so mustn’t the trait be emergent at the population level? But, in an¬ other sense, low speciation rate arises as a consequence of population struc¬ tures induced by planktotrophy, a presumed adaptation at the organismal level—so perhaps the key character can be reduced to simple properties of or¬ ganisms after all. I have gone round and round this example for twenty years, often feeling confident that I have finally found a clear resolution, only to recognize that a different (and equally reasonable) formulation yields the opposite interpreta¬ tion. All other participants in this debate seem equally afflicted by frustration, so perhaps, the fault lies in the concepts, and not in ourselves that we seem to be underlings, unable to achieve closure. Ffowever, if we invoke the broader criterion of emergent fitness, the prob¬ lem gains a clear resolution in favor of species selection. A structural feature of populations, leading to a low frequency of isolation for new demes, must be treated as a character of populations in any conventional usage of lan¬ guage. As stated above, individual organisms don’t speciate; only populations
Species as Individuals in the Hierarchical Theory of Selection
do —so the character belongs to the species. However, the character may rep¬ resent an aggregate rather than an emergent feature—thus debarring species selection under the emergent character approach. But, under the emergent fitness approach, so long as the character (whether aggregate or emergent) belongs to the species, and so long as the fitness of the species covaries with the character—and no one denies the covariation in this case—we have de¬ tected an instance of species selection. Arnold and Fristrup (1982, p. 114) present this argument in a clear and forceful way: The critical characters—larval strategies—may well have arisen for rea¬ sons that can be seen as adaptive in a traditional Darwinian sense. How¬ ever, regardless of the mechanism by which they became fixed, these strategies behave as properties of species in that they result in distribu¬ tions of rates of speciation and extinction within this group ... It might be tempting to assume that there are fewer planktotrophic species be¬ cause the individuals in these species were somehow less fit than the indi¬ viduals in non-planktotrophic species. However, it is obvious that the same result could obtain even if planktotrophic and non-planktotrophic individuals [organisms] have equal fitnesses, by virtue of the population structures that are concomitants of these larval strategies. Thus, the ob¬ served distribution of species types within these gastropods is not pre¬ dicted from individuals level fitness alone, underscoring the necessity of the higher level of analysis. In other words, the relative frequency of planktotrophic species falls not because planktotrophic organisms must be less fit (they may, in fact, be more fit on average across the clade), but because a character fixed by organismic selection yields the effect of lowering the speciation rate at a higher level. The population structure produced by planktotrophy may not rank as an emer¬ gent character, but does confer an emergent fitness at the species level—a fitness irrelevant to individual organisms, which, to emphasize the obvious point one more time, do not speciate. Finally, we may seal the case by citing Grantham’s important argument (1995, p. 301) that “species selection does not require emergent traits be¬ cause higher-level selection acting on aggregate traits can oppose lower-level selection.” Vrba herself has argued (1989, p. 80) that “the acid test of a higher level selection process is whether it can in principle oppose selection at the next lower level.” Surely such an opposition can arise “in principle” (and probably in actuality) in this case—for planktotrophy could be positively se¬ lected at the organismic level, but may, through its strong effect on popula¬ tion structure, and the resulting consequences for rates of speciation, enjoin negative selection at the species level. To summarize, we all agree that an independent theory of macroevolution must identify higher-level causal processes that are not reducible to (or simple effects of) causes operating at conventional lower levels of gene and organ-
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THE STRUCTURE OF EVOLUTIONARY THEORY ism. This premise defines the theoretical salience of the debate about species selection—for if such a process exists, and can also be validated as both com¬ mon in evolution and irreducible in principle, then macroevolutionary theory has been achieved. For this reason, evolutionary biologists, who usually es¬ chew academic philosophy (as the mildly philistinistic culture of science gen¬ erally dictates), have joined in such classical philosophical debates as the meaning of reduction and emergence. Vrba’s criterion of emergent characters establishes an obvious case for irreducibility because the trait that causes species selection can claim neither existence nor representation at the conventional organismic level. Grantham writes (1995, p. 308): “When a component of species-level fitness is corre¬ lated with an emergent trait, this correlation cannot be reduced because the trait cannot be represented at the lower level.” But Lloyd’s broader criterion of emergent fitness also establishes irreducibility, even if the trait involved in the correlation between trait and fitness is reducible under the effect hypo¬ thesis. In Lloyd’s case, the fitness is irreducible (as shown practically in the previous example of gastropod lineages, where higher-level fitness based on speciation rate opposes lower-level fitness based on the same trait of larval adaptation). The technical point may be summarized in the following way: selection is defined by the correlation between a species-level trait and spe¬ cies-level fitness; therefore, the irreducibility of either component of the corre¬ lation establishes irreducibility for the selection process. Grantham notes (1995, p. 308): “Emergent traits are not, however, necessary for species selec¬ tion. If an aggregate trait affects a component of species-level fitness (e.g. rate of speciation) and this component of fitness is irreducible, then the traitfitness correlation will be irreducible.” Vrba’s emergent character approach embodies one great strength, but two disarming weaknesses. This criterion does identify the most irrefutable, and in many ways the most interesting, subset of cases for species selection—ex¬ amples based on genuine species adaptations (for an emergent character that evolved as a consequence of its value in fitness is, ipso facto, an adaptation); whereas nonemergent characters that contribute to species fitness via the ef¬ fect hypothesis are exaptations (Gould and Vrba, 1982; Gould and Lloyd, 1999), at the species level (and adaptations at the lower level of their origin). But the emergent character criterion suffers from two problems that would render the theory of species selection, if framed exclusively in its light, eter¬ nally contentious and, perhaps, relatively unimportant as well. First, by in¬ cluding only the “hardest-line” cases within the concept, we may be unduly limiting species selection to an unfairly small compass. (For example, and as an analogy, we wouldn’t want to restrict the concept of “adaptation” only to the small subset of true biomechanical optima—for most adaptations only hold the status of “better than,” not ne plus ultra). Second, emergence can of¬ ten be extremely difficult to document for characters—so, in practice, the concept may be untestable in most circumstances. To differentiate between a truly emergent species character and an effect of a lower-level character, one often needs a great density of narrative information about the actual history
Species as Individuals in the Hierarchical Theory of Selection
of the lineage in question—information only rarely available in the fossil re¬ cord, not to mention our spotty archives for living species. By contrast, the emergent fitness approach enjoys the great virtue of fully general applicability. For, when one only has to consider current circum¬ stances (the trait-fitness correlation), and need not reconstruct prior history (as the designation of emergence for a species-level character so often re¬ quires), then we can study any present reality that offers enough information for a resolution. We certainly use this most broadly applicable, nonhistorical approach in traditional studies of natural selection at the organismic level— that is, we identify current selective value whether the feature conferring dif¬ ferential reproductive success arose as an adaptation for its current contribu¬ tion to fitness, or got coopted for its present role from some other origin or utility. (In other words, both preadaptations and spandrels—features that arose as adaptations for something else, or for no adaptive purpose at all— can function just as well in a regime of current selection as true adaptations forged by the current regime.) The historical origin of characters, and their later shifts in utility, constitute a central and fascinating question in evolu¬ tionary theory—and provide a main theme for Chapter 11 of this book. But we define the process of selection ahistorically—as differential reproductive success based on current interaction between traits of evolutionary individu¬ als and their environments—that is, the concept of selection remains agnostic with respect to the historical origin of the traits involved. The emergent fitness approach presents four favorable features that estab¬ lish species selection as a central, fully operational, and vitally important sub¬ ject in evolutionary biology—thereby validating both the necessity and the distinctness of macroevolutionary theory. 1. Rather than depending upon a documentation of prior history in the narrative mode (often untestable for lack of information), we move to a fully general mathematical model that can, in principle, identify components of higher-level selection in any case where we can obtain sufficient data on the current operation of a selection process. Arnold and Fristrup (1982) expanded Price’s (1970, 1972) covariance formulae to encompass a set of nested levels, and devised an approach closely allied to analysis of covariance, considering selection at one level as a “treatment effect” upon selection at an adjacent level. Damuth and Fleisler (1988) developed a similar method, also based on covariances (or regression of fitness values on characters); this pro¬ cedure has been expanded by Lloyd (1988; Lloyd and Gould, 1993). As Lloyd and Gould (1993, p. 596) describe the method: “This is done by de¬ scribing interactors at the lower level first. If a higher-level interactor exists, the higher-level correlation of fitness and trait will appear as a residual fitness contribution at the lower level; we must then go to the higher level in order to represent the correlation between higher-level trait and higher-level fitness.” Lest this method seem to fall into the very reductionistic trap that species selection strives to overcome—because we begin at the lowest level and only move higher if we find a residual fitness—I point out that we use this proce¬ dure only as a convenient and operational research method, and decidedly
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THE STRUCTURE OF EVOLUTIONARY THEORY not with the reductionistic hope that no residuals will appear, and that the lowest level will therefore suffice for a full explanation. We may be stuck with the technical term “residual” as a common statistical usage in such circum¬ stances—but there is nothing conceptually residual about higher-level selec¬ tion. Selection at lower levels cannot be designated as more true or basic, with higher levels then superadded if necessary. The statistical “residual” of our procedure exists as a separate but equal natural reality in our fascinating world of hierarchical selection. 2. The emergent fitness approach establishes a large and general realm for the operation of species selection. Any evolutionary trend that must be de¬ scribed, at least in part, as a result of species sorting automatically becomes subject to the analytical apparatus here proposed, and therefore a candidate for identification of species selection. (And I can hardly imagine that any im¬ portant trend unfolds without a major—I would say almost always predomi¬ nant (see Chapter 9)—component of species sorting, for extensive anagene¬ sis rarely occurs in single lineages, and none can persist very long without branching in any case.) 3. The emergent fitness approach allows us to use a single, familiar, and minimalist definition of selection in the same manner at each level—differen¬ tial proliferation of evolutionary individuals based on interactions of their traits with the environment. We therefore achieve a unified theory of selection at all scales of nature. The availability of a fully operational analytical appa¬ ratus, connected with this definition, greatly enhances the scientific utility of emergent fitness as a definition of species selection. 4. As an admittedly more subjective and personal point, the emergent fitness approach allows us to encompass under the rubric of species selection several attributes of populations that many participants in this debate have intuitively wished to include within the causal compass of species acting as evolutionary individuals, but which the more restrictive emergent character approach rules out. Many of us have felt that two distinct kinds of species properties should figure in species selection because, for different reasons, such features cannot function at the lower and traditional level of organismic selection. In the first category, emergent characters of species obviously can’t operate at the organismic level because they don’t exist for organisms. These features clearly serve as criteria of species selection in either the emergent character or the emergent fitness approach. In a second category, some important aggregate characters of species can’t function in selection at the organismic level, not because they have no expres¬ sion at this lower level (for they clearly exist as organismic properties, at least in the form of traits that aggregate additively to a different expression at the species level), but because such properties do not vary among organisms, and therefore supply no raw material for selection’s necessary fuel. I speak here of a common phenomenon recognized by different jargons in various sub¬ disciplines of our field—autapomorphies for cladists, or invariant Bauplan characters for structuralists. Suppose that each species in a clade has evolved a unique state of a homologous character—and that, within each species,
Species as Individuals in the Hierarchical Theory of Selection
all organisms develop the same state of the character, without meaningful variation. In this situation, all variation for the homologous character oc¬ curs among species, and none at all within species. If a trend now develops within the clade when some species live and proliferate because they possess their unique state of the character, while others die because their equally dis¬ tinct and unvarying state has become maladaptive in a changed environment, should we call such a result species selection—for each species manifests a single attribute, and all variation occurs among species? Interestingly, de Vries originally coined the term species selection (see pages 448-451) for pre¬ cisely this situation, where no relevant variation exists within species, and all variation occurs among species. To summarize: in the first situation, the character doesn’t exist at the organismal level, and each species develops only one state of the (emergent) character because the character belongs to the species as a whole. Therefore, selection for this character can only occur among species. In the second situa¬ tion, the character doesn’t vary at the organismal level, and each species in a clade has evolved a unique and different state of the character. Again, selec¬ tion can only occur among species. In either situation, each species manifests one different and unvarying state of a feature that cannot operate in organismic selection—so selection for this feature can only occur among species. The emergent status of the character leads us to designate the first situation as species selection without any ambiguity or alternative. But we balk at des¬ ignating the second situation as species selection because the relevant specieslevel character (lack of variation) represents an aggregate, not an emergent, feature. The emergent fitness criterion rescues us from this dilemma, and forges an intuitive union between the two situations by designating both as species selection. Lack of variation—the aggregate species character—in¬ teracts with the environment to influence differential rates of proliferation among species. This character imparts an emergent fitness to the species, and therefore becomes an agent of species selection. (After all, the species doesn’t die because organism A, or B, or C, possesses a trait that has become mal¬ adaptive; the species dies because none of its parts (organisms) can develop any other form of the trait—and this lack of variation characterizes the spe¬ cies, not any of its individual organisms.) I believe that such “species selection on variability”—the title that Lloyd and I gave to our 1993 paper—will prove to be a potent style of selection at this level. (When I was struggling with the issue of whether such an aggregate character as variability could count as a property of species, I asked Egbert Leigh, a brilliant evolutionist and the leading late 20th century disciple of R. A. Fisher, whether he thought that variability could operate as a character in species selection—and he replied: “if variability isn’t clearly a character of a species, then I don’t know what is.”) To cite just one hypothetical example that I have often used to illustrate this issue and to argue for species selection on variability: Suppose that a wondrously optimal fish, a marvel of hydrodynamic perfection, lives in a pond. This species has been honed by millennia of conventional Darwinian
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THE STRUCTURE OF EVOLUTIONARY THEORY selection, based on fierce competition, to this optimal organismic state. The gills work in an exemplary fashion, but do not vary among individual organ¬ isms for any option other than breathing in well-aerated, flowing water. An¬ other species of fish—the middling specias—ekes out a marginal existence in the same pond. The gills don’t work as well, but their structure varies greatly among organisms. In particular, a few members of the species can breathe in quite stagnant and muddy waters. Organismic selection favors the optimal fish, a proud creature who has lorded it over all brethren, especially the middling fish, for ages untold. But now the pond dries up, and only a few shallow, muddy pools remain. The op¬ timal fish becomes extinct. The middling species persists because a few of its members can survive in the muddy residua. (Next decade, the deep, well-aer¬ ated waters may return, but the optimal fish no longer exists to reestablish its domination.) Can we explain the persistence of the middling species, and the death of the optimal form, only by organismic selection? I don’t think so. The middling species survives, in large part, as a result of the greater variability that al¬ lowed some members to hunker down in the muddy pools. (We may even ar¬ gue that the optimal fish always prevailed against most members of the mid¬ dling species, even at the worst time, so that most middlings died quickly when the pond dried, while the optimals hung on longer, but eventually suc¬ cumbed.) The middling species survived qua species because the gills varied among its parts (organisms), not because all its members gained advantage when the environment changed. (For most middling organisms continued to fare worse than the optimal fishes.) We may represent this story at the organismal level by discussing the gills of the few middling fishes that carried the species through the crisis. But the middling species prevailed by species selection on variability—for this greater variability imparted an emergent fitness to the interaction of the species with the changed environment. Species selection on variability also possesses the salutary property of unit¬ ing the two major themes of this book, the concepts that I regard as the most important revisions now needed to mend and strengthen the two main legs of the essential Darwinian tripod: the hierarchical theory of natural selection as a vibrant expansion of Darwin’s focus on the organismal level, and the cen¬ trality of constraint as a channeler of evolutionary direction in concert with natural selection (which can no longer maintain the exclusivity that strict Darwinians wished to impart). An important component for explaining the patterning of life’s history lies in limitations and channels imposed and re¬ tained by developmental architecture—and these constraints do much of their work at higher levels, in large part by influencing “species selection on variability.” I close this discussion with three points that validate the status of spe¬ cies selection as an irreducible macroevolutionary force, and place the pro¬ posed criteria of emergent characters and emergent fitnesses under a common rubric.
Species as Individuals in the Hierarchical Theory of Selection THE FALLACY OF “NECKER CUBING” The philosophical doctrine of con¬
ventionalism, as expressed by Dawkins (1982) in his Necker Cube metaphor (see pages 640-641), presents an important challenge to claims for an inde¬ pendent macroevolutionary theory based on higher-level selection. For if all cases of higher-level selection, however cogently defended, represent only one legitimate way to describe a process that can always be causally expressed in terms of selection at conventional lower levels as well, then why bother (ex¬ cept for the fun of it, or for the psychological insight thus provided) with the alternative higher level, when the traditional Darwinian locus invariably works just as well? I do not doubt that some evolutionary events can be alternatively expressed (and I shall mention one category under my second point below), but Necker cubing will not apply to genuine cases of irreducible species selection because the nature of the world (not the conventions of our language) regulates the lo¬ cus of causality. Two reasons debar the Necker cube for true cases of species selection. First, for Vrba’s “hardest” category of species selection based on emergent characters, no expression at conventional lower levels can be for¬ mulated because the relevant species character does not exist at the usual Darwinian locus of organisms. Second, for Lloyd’s broader category of spe¬ cies selection based on the emergent fitness associated with aggregate species characters, the “Necker cubers” commit a basic error in logic. They correctly note that the aggregate character can be represented at the organismic level— so they invoke the conventionalism of alternative and equally valid expres¬ sion. But, as discussed on page 659, the species-level fitness imparted by the aggregate character, not the character itself, denotes the irreducible feature that defines species selection on this criterion. In other words, Necker cubers commit the same error in this case that Dawkins made in his original use of the metaphor to claim that all organismal selection can also be expressed in terms of gene selection. The metaphor of the Necker cube only applies when the same thing attains equal and alter¬ native representation, not when the Cube’s two versions represent genuinely different aspects of a common phenomenon. In Dawkins’s original error, something can always be represented at the gene level—but that something
counts as bookkeeping, not as the causality of selection, which remains orgamsmal in his standard cases. Similarly, for aggregate species-level charac¬ ters involved in selection, something can always be represented at the organ¬ ismic level—but that something, in this case, only involves the composition of the character, not the causal process of selection, which occurs irreducibly at the species level as identified by emergent species-level fitnesses. A UNIFIED PICTURE OF SPECIES SELECTION In advocating such an ex¬
panded role for species selection, we must guard against the ultimate fallacy of claiming too much—for if we turn all forms of species sorting into species selection by verbal legerdemain, then we falsely “win” by definition, but ac¬ tually lose by an overly imperialistic extension that permits no distinctions
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THE STRUCTURE OF EVOLUTIONARY THEORY and therefore sacrifices all utility as an empirical proposition in science. For¬ tunately, we can unite both criteria of emergent fitnesses and characters into a unified scheme that establishes two realms of species selection, one more in¬ clusive than the other, but that also identifies a domain of species sorting lead¬ ing us to reject causation by species selection. Grantham (1995) has presented such a scheme, reproduced here as Figure 8-4. (I had independently developed the same system, almost with the same picture, in preparing to write this chapter. I mention this not to compromise Grantham’s originality or priority in any way—for priority is chronology, and his cannot be gainsaid!—but to express the firm and almost eerie satisfac¬ tion that such a ‘"multiple” formulation brings (see Merton, 1965), and to of¬ fer this example as proof that the inherent logic of a complex argument often drives independent researchers to a definite and almost ineluctable result— validating in this case the coherence of this “take” on species selection.) Grantham’s diagram circumscribes two realms of species selection, labeled as “hierarchical explanations.” The A realm contains Vrba’s firmest examples based on emergent characters, while the B realm adds Lloyd’s cases based on the emergent fitnesses associated with aggregate species-level characters. (Vrba, of course, would restrict species selection to the A realm, and ascribe the B realm to the “effect hypothesis”—but everyone seems to agree on the structure and relationships of the realms.) The A realm seems firmer because emergent characters count as adaptations of species, and maintain no expres-
hierarchical explanations
A-emergent species-level traits affect species-level fitness
B-aggregate traits affect irreducible species-level fitness
C-aggregate traits affect reducible species-level fitness
8-4. Grantham’s 1995 epitome of criteria for invoking species selection in hierar¬ chical models. The A domain includes rare best cases of species selection based on emergent species-level traits. The B domain adds aggregate traits that affect irreducible species-level fitness, and therefore also participate in species selection under the interactor approach. The aggregate and reducible traits of the C do¬ main belong only to organisms and cannot figure in arguments for species selec¬ tion.
Species as Individuals in the Hierarchical Theory of Selection
sion at lower levels. The B realm seems “looser” because these aggregate spe¬ cies characters can be represented at the organismic level, even though they may also rise by upward causation to become exaptations of species (Gould and Vrba, 1982; Vrba and Gould, 1986; Gould and Lloyd, 1999). But, in any case, the resulting species-level fitnesses are irreducible—so the B realm also represents species selection by standard definitions of selection as a causal process. The C realm includes cases of species sorting based on aggregate specieslevel characters that impart only a reducible fitness at the species level—and therefore do not count as species selection. One might add a D realm at the base for cases describable as species sorting, but not associated with any higher-level character, either aggregate or emergent, and therefore not quali¬ fying for consideration as species selection on any definition of species as evolutionary individuals and interactors. The D realm, which may be quite large, includes several categories, most obviously species sorting based on the higher-level analog of drift—or random differentials in survival and death of species within a clade (see my summary chart, pp. 718-720). As for any scientific theory, we want, most of all, to be able to make clear and testable distinctions at the crucial boundary between cases that affirm and cases that fall outside the hypothesis under consideration—in this case, between the B and C realms separating irreducible species selection from spe¬ cies sorting reducible to organismic selection. In these formative days for the theory of species selection, we have not yet developed a full set of firm crite¬ ria for making these crucial allocations. But let me suggest one guidepost at the outset. Ever since this literature began, astute workers have devel¬ oped a strong intuition that species sorting based on events of differential birth (speciation rates) will usually represent true species selection, while species sorting based upon differential death (extinction) will often be reduc¬ ible to organismic level (see Gilinsky, 1981; Arnold and Fristrup, 1982; Vrba and Eldredge, 1984; Grantham, 1995; Gould and Eldredge, 1977; Gould, 1983c). The source of this intuition—-which may turn out to be both wrong, and superficially based—arises from a sense that the extinction of a species may often be adequately explained simply as the summed deaths of all organisms, each for entirely organismal reasons and with no significant contribution from any species-level property. When the last reproductive organism dies, the species becomes extinct. But how could a new species originate without some involvement of population-level features? After all, individual organ¬ isms do not speciate; only populations do. But individual organisms die, and the extinction of a species might, at least in principle, represent no more than the summation of these deaths. Grantham expresses this common intuition particularly well (1995, pp. 309-310): The concept of “speciation rate” cannot be expressed at the organismic level because there is no simple set of organismic traits that determine speciation rate. Rather, a diverse set of organismic and population-level
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THE STRUCTURE OF EVOLUTIONARY THEORY traits (including dispersal ability, population structure, and behavioral compatibility between members of distant populations) affect gene flow and therefore affect speciation rates. Because of the large variety of fac¬ tors affecting speciation rate . . . the higher level property of “speciation rate” is, at best, extraordinarily difficult to express in organismic terms. The speciation rate of a taxon is irreducible ... A species goes extinct if and only if every individual dies. Whereas differences in speciation rates cannot be expressed in organismic terms, differences in extinction rates will often be reducible (unless population-level traits such as variation matter). Thus, I suspect that the A and B realms will be heavily populated with cases based on differential speciation, whereas the C realm will feature cases based on differential extinction. A PERSONAL ODYSSEY Many historians of science, particularly feminists like Donna Harraway (1989, 1991), have forcefully argued that scholars can strike their most effective blows against the myth of pure objectivism by being candid about the interaction of their own autobiographies with their current claims—thus exposing the inevitable (and basically welcome) cultural and psychological embeddedness of science, while opening an author’s prejudice both to his own scrutiny, and to the examination of his readers. To do so ob¬ sessively or promiscuously in a book of this sort would only clutter a text that would then become even more insufferably egocentric or idiosyncratic—so I have usually desisted (except for some parts of Chapter 1, and the dubious in¬ dulgence of my appendix to Chapter 9). But I will follow Harraway’s recom¬ mendation in this particular case, because no other subject in evolutionary theory has so engaged and confused me, throughout my career, as the defini¬ tion and elucidation of species selection. For no other problem have I made so many published mistakes, and undergone so many changes of viewpoint be¬ fore settling on what I now consider a satisfactory framework. Moreover, my basic reason for current satisfaction rests upon an interesting correction from within my own body of work—and, though I remain heartily embarrassed for not grasping both the inconsistency and the necessary resolution many years earlier, I do take some pleasure in my eventual arrival—and I do think that the story may help to illustrate the intellectual coherence of the frame¬ work now proposed in this book. I made two sequential errors of opposite import. When Niles Eldredge and I first formulated punctuated equilibrium, I was most excited by the insight that trends would need to be reconceptualized as differential success of spe¬ cies, rather than anagenesis within lineages (a theme only dimly grasped in Eldredge and Gould, 1972, but fully developed in Gould and Eldredge, 1977, after much help from Stanley, 1975, and later from Vrba, 1980). I then com¬ mitted the common fallacy of extending personal excitement too far—and I made the error (as we all did in these early days of “species selection” under punctuated equilibrium) of labelling as species selection any pattern that
Species as Individuals in the Hierarchical Theory of Selection needed to be described in terms of differential success for species treated (un¬ der punctuated equilibrium) as stable entities. In other words, we failed to distinguish selection from sorting, and used the mere existence of sorting at the species level as a criterion for identifying species selection. This definition of species selection must be rejected as clearly wrong—particularly for the in¬ valid “promotion” of several cases properly viewed as effects of causes fully reducible to conventional organismic selection. In reaction to this previous excess, I then retreated too far in the other di¬ rection, by restricting species selection too severely—i.e., only to cases based on characters emergent at the species level (Gould, 1983c; Vrba and Gould, 1986). My later work with Elizabeth Lloyd (Lloyd and Gould, 1993; Gould and Lloyd, 1999) convinced me that emergent characters, while properly identifying species selection, only identified a subset of genuine cases, and that emergent fitness, as defended in this section, provided a conceptually broader, and empirically more testable criterion. In preparing this chapter, I finally realized why I had originally erred in re¬ stricting species selection to emergent characters. The source for amending Vrba and Gould (1986) lay in an earlier paper that I had written with Elisa¬ beth Vrba (Gould and Vrba, 1982), particularly in the codification of adapta¬ tion (or the origin of a character directly for its current utility) and exaptation (or the cooptation of a preexisting character for an altered current utility) as subsets of the more inclusive phenomenon of aptation (any form of current utility, whatever the historical origin). We developed this terminology, which has now been widely accepted (see extensive discussion in Chapter 11), in order to make a crucial, but often dis¬ regarded, distinction between “reasons for historical origin” and “basis of current utility-” The common conflation of these entirely separate notions has engendered enormous confusion in evolutionary theory—a situation that we documented and tried to correct in our paper (Vrba and Gould, 1986). Hardly any principle in general historical reasoning (not only in evolutionary theory) can be more important than clear separation between the historical basis of a phenomenon and its current operation. Lor example, crucial com¬ ponents of current utility often arose nonadaptively as spandrels, or sideconsequences, of other features actively constructed or evolved (Gould and Lewontm, 1979). I felt so enlightened by this distinction, and so committed (as a paleontolo¬ gist and historian) to the special role of historical origin, that I longed to ap¬ ply this notion to the important concept of species selection. I therefore con¬ cluded that we should not speak of species selection unless the character that imparted the relevant fitness could be identified as a true adaptation at the species level—that is, as a feature belonging to the species as a higher-level Darwinian individual, and evolved directly for current utility in promoting the differential success of the species. Emergent species characters qualify as adaptations—and I therefore felt drawn to this narrow criterion for identify¬ ing species selection. In so doing, I committed a basic logical error about the nature of selection.
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THE STRUCTURE OF EVOLUTIONARY THEORY However much I may love history, selection cannot be, and has never been, defined as a historical relationship of character and result. Selection must be defined by present operation, as identified by an observable differential in re¬ productive success based on the current interaction of a trait of a Darwinian individual with its environment. This definition includes no reference to the historical origin of any relevant trait, which may be either an adaptation or an exaptation. Damuth and Heisler (1988) emphasize this crucial point, with an apt literary flourish at the end to note the irrelevancy of a trait’s “aristoc¬ racy” (depth of historical origin, or “blue-blooded” continuity) to the hierar¬ chy of selection: The historical origin of a character is irrelevant to the way that it func¬ tions in a selection process. Thus, the issue of whether a character is a group or individual “adaptation,” and whether it has been shaped for its present role by any particular process, is of no importance in the study of the selection mechanism. There may certainly be historical significance in such observations about the origin of characters. Nevertheless, selection evaluates characters in terms of their current relationship to fitness only, not in terms of their history. There is hierarchy in the world of natural se¬ lection, but no aristocracy. Once I recognized the irrelevancy of historical origin to the identification of selection—my only previous rationale for insisting that characters for species selection must be species-level adaptations, and therefore emergent at the spe¬ cies level—I understood that the “emergent character” criterion must be re¬ jected as too restrictive (while correctly identifying the firmest subset of cases for species selection), whereas the “emergent fitness” criterion must be pre¬ ferred, as not only legitimately broader in scope, but also properly formu¬ lated in terms of conventional definitions of selection. In my own preferred nomenclature, species-level characters that are exaptations rather than adap¬ tations can function perfectly well in species selection. Aggregate species-level characters originate as exaptations of species because they arise at the organismal level and pass upwards as effects to the species level. When I mistakenly thought that characters for species selection had to be species-level adapta¬ tions, I had excluded aggregate characters (as species-level exaptations), and therefore falsely rejected the emergent fitness approach (see Gould and Lloyd, 1999, for an elaboration of this argument). In the early 1980’s, my own students Tony Arnold and Kurt Fristrup had strongly urged the criterion of emergent fitness upon me, and I well remember my bitter disappointment that I could not convince them to use the restrictive criterion of emergent characters! (I had not yet developed the nomenclature of adaptation and exaptation, and therefore did not yet possess the personal tools for a conceptual resolution.) Thus, my error reflected an active commit¬ ment (not a passive consequence of inattention), maintained in the face of an available correction that I now regard as one of the finest papers ever pub¬ lished on the subject (Arnold and Fristrup, 1982). I did not grasp, for another decade, how the terminology developed by Vrba and me also derailed the cri-
Species as Individuals in the Hierarchical Theory of Selection terion that we both preferred. To sum up: selection operates on current utili¬ ties, and remains agnostic about historical origins in utilizing both adapta¬ tions and exaptations with equal facility. Emergent species-level characters will generally count as adaptations, thus clearly available for species selec¬ tion. But all aggregate species-level characters represent potential exapta¬ tions, and therefore become equally available for species selection under the proper criterion of emergent fitness. I would, however, salvage a lesson from this odyssey of errors. Vrba and I were not wrong in identifying emergent characters as especially interesting (we only erred in deeming them necessary for species selection). Emergent characters belong exclusively to the species. As adaptations, they become part of the defining cohesion that permits a species to function as an evolutionary individual. Emergent characters thus stand out in designating the style of in¬ dividuality maintained by species. Aggregate characters, on the other hand, do not clearly define a species as a functional entity (variability, for example, represents an attribute, not an “organ,” of a species)—for aggregate charac¬ ters belong as much to the component organisms, as to the entire species. Thus, emergent characters are special and fascinating (though not essential to the definition and recognition of species as legitimate Darwinian individu¬ als—see Gould and Lloyd, 1999). Emergent characters do deserve primary consideration in discussions about the structural basis of species both as nat¬ ural entities in general, and as the basic individuals of macroevolution in par¬ ticular. But we do not require emergent characters to identify a process of se¬ lection. As a final note, and as one contribution to recognizing the crucial and char¬ acteristic differences among Darwinian individuals at the six primary levels of the evolutionary hierarchy, we should suspect that species selection will emphasize exaptations, whereas organismal selection employs a higher rela¬ tive frequency of adaptations—for species, as more loosely organized in func¬ tional terms than organisms, probably possess far fewer emergent characters than organisms. But species “make up” for their relative paucity of adapta¬ tions by developing a higher frequency of exaptations. Most of these exaptations derive their raw material from adaptations at the organismal level that cascade upwards to effects at the species level. By joining fewer ad¬ aptations (emergent characters) with more exaptations (usually based on ag¬ gregate characters), species may become just as rich as organisms in features that can serve as a basis for selection. Species selection may therefore become just as strong and decisive as conventional Darwinian selection at the level of organisms—a process whose power we do not doubt, and whose range we once falsely extended to encompass all of nature.
HIERARCHY AND THE SIXFOLD WAY
A literary prologue for the two major properties of hierarchies Our vernacular language recognizes a triad of terms for the structural de¬ scription of any phenomenon that we wish to designate as a unitary item or
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THE STRUCTURE OF EVOEUTIONARY THEORY “thing.” The thing itself becomes our focus, and we call it an object, an entity, an individual, an organism, or any one of a hundred similar terms, depending on the substance and circumstance. The subunits that build the individual are then called “parts” (or units, or components, or organs, etc., depending upon the nature of the focal item); while any recognized grouping of similar indi¬ viduals becomes a “collectivity” (or aggregation, society, organization, etc.). In other words, and in epitome, individuals are made of parts and aggregate into collectivities. The hierarchical theory of selection recognizes many kinds of evolutionary individuals, banded together in a rising series of increasingly greater inclu¬ sion, one within the next—genes in cells, cells in organisms, organisms in demes, demes in species, species in clades. The focal unit of each level is an in¬
dividual, and we may choose to direct our evolutionary attention to any of the levels. Once we designate a focal level as primary for a particular study, then the unit of that level—the gene, or the organism, or the species, etc.—be¬ comes our relevant or focal individual, and its constituent units become parts, while the next higher unit becomes its collectivity. Thus, if I place my focus at the conventional organismic level, genes and cells become parts, while demes and species become collectivities. But if my study enjoins a focus on species as individuals, then organisms become parts, and clades become collectivities. In other words, the triad of part—individual—collectivity will shift, as a three¬ fold entirety, up and down the hierarchy, depending upon the chosen subjects and objects of any particular study. This dry linguistic point becomes important for a fundamental reason of psychological habit. We humans are hidebound creatures of convention, par¬ ticularly tied to the spatial and temporal scales most palpably familiar in our personal lives. Among nature’s vastly different realms of time, from the femtoseconds of some atomic phenomena to the aeons of stellar and geologi¬ cal time, we really grasp, in a visceral sense, only a small span from the sec¬ onds of our incidents to the decades of our lives. We can formulate other scales in mathematical terms; we can document their existence and the pro¬ cesses that unfold in their domains. But we experience enormous difficulty in trying to bring these alien scales into the guts of or our understanding— largely for the parochial reason of personal inexperience. We make frequent and legendary errors because we tend to extrapolate the styles and modes of our own scale into the different realms of the incompre¬ hensibly immense or tiny in size, vast or fleeting in time. Geologists, for ex¬ ample, well appreciate the enormous difficulties that most people encounter (including our professional selves, despite so many years of training and ex¬ perience) in trying to visualize or understand the meaning of any ordinary statement in “deep” or “earth time"—that a landscape took millions of years to develop, or that a lineage exhibits a trend to increasing size throughout the Cretaceous period. All of us—professionals and laypeople alike—continue to make the damnedest mistakes. I have, for example, struggled for thirty years against the conventional misreading of punctuated equilibrium as a saltational theory in the generational terms usually applied to such a concept in
Species as Individuals in the Hierarchical Theory of Selection evolutionary studies. The theory’s punctuations are only saltational on geo¬ logical scales—in the sense that most species arise during an unmeasurable geological moment (meaning, in operational terms, that all the evidence ap¬ pears on a single bedding plane). But geological moments usually include thousands of human years—more than enough time for a continuous process that we would regard as glacially slow by the measure of our lives (see Goodfriend and Gould, 1996, for an example). Thus, punctuated equilibrium represents the proper geological scaling of speciation events that may span several thousand years, not a slavish promotion of “instantaneity,” as con¬ ventionally measured in a human time frame, to the origin of species. As we misunderstand scales of time, we fail just as badly with viscerally un¬ familiar realms of size. Our bodies lie in the middle of a continuum ranging from the angstroms of atoms to the light years of galaxies. Individuality exists in all these domains, but when we try to understand the phenomenon of “thingness” at any distant scale, we easily fall under the thrall of the greatest of all parochialisms. We know one kind of individual so intimately and with such familiarity—our own bodies—that we tend to impose the characteristic properties of this level upon the very different styles of being that other scales generate. This inevitable human foible provokes endless trouble, if only be¬ cause organismal bodies represent a very peculiar kind of individual, serving as a very poor model for the comparable phenomenon at most other scales. The “feel” of individuality at other scales becomes so elusive that most of the best exploration has been accomplished by literary figures, not by scien¬ tists. The tradition extends at least as far back as Lemuel Gulliver, whose “alien” contacts did not depart greatly from our kind of body and our norm of size. This theme has best been promoted, in our generation, within the genre of science fiction. I particularly recommend two “cult” films, Fantastic
Voyage and Inner Space, both about humans reduced to cellular size and in¬ jected into the body of another unaltered conspecific. This ordinary body be¬ comes the environment of the shrunken protagonists, a “collectivity” rather than a discrete entity—while the “parts” of this body become individuals to the shrunken guests. When Raquel Welch fights a bevy of antibodies to the death in Fantastic Voyage, we understand how location along the triadic continuum of part—individual—collectivity depends upon circumstance and concern. A tiny, if crucial, part to me at about two meters tall becomes an en¬ tire and ultimately dangerous individual to Ms. Welch at a tiny fraction of a millimeter. The parochiality of time has served us badly enough, but the parochiality of bodily size has, for two reasons, placed even more imposing barriers in our path to an improved and generalized evolutionary theory—a formulation well within our grasp if we can learn how to expand the Darwinian perspec¬ tive to all levels of nature’s hierarchy. First, we know almost viscerally what our bodies do best as Darwinian agents—and we then grant universal impor¬ tance to these properties both by denying interest to the different “best” properties of individuals at other levels, and by assuming that our “bests" must, by extension, power Darwinian systems wherever they work. Our bod-
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THE STRUCTURE OF EVOLUTIONARY THEORY ies are best at developing adaptations in the complex and coordinated form that we call “organic.” Many evolutionists therefore argue, in the worst pa¬ rochialism of all, that only adaptations matter as an explanatory goal of Dar¬ winism, and that such adaptations must therefore drive evolution at all levels. 1 don’t even think that such a perspective works well for organisms—surely the locus of most promising application (Gould and Lewontin, 1979)—but this attitude will surely stymie any understanding of individuality at other levels, where complex adaptations do not figure so prominently. Evolutionists will not be able to appreciate the different individuality of species, where exaptive effects hold at least equal sway with adaptations, if they continue to regard spandrels, sequelae, and side consequences as phenomena generated by “the boring by-product theory” (Dawkins, 1982, p. 215). Second, we just don’t comprehend the scale-bound realities in other do¬ mains of size, and we err by imposing our own perceptions when we try to think about the world of a gene, or of a species. In one of the most famous statements of 20th century biology, D’Arcy Thompson (1942, p. 77) ended his chapter “On Magnitude” (in his classic work, On Growth and Form—see the first section of Chapter 11 for a general analysis of his work) by noting how badly we misread the world of smaller organisms because our large size places us in gravity’s domain (a result of low surface/volume ratios in larger creatures, but not a significant feature in other realms of size). If we encoun¬ ter so much trouble for extremes within our own level of organismic individ¬ uality, how will we grasp the even more distant worlds of other kinds of evo¬ lutionary individuals? D’Arcy Thompson wrote (1942, p. 771): Life has a range of magnitude narrow indeed compared to that with which physical science deals; but it is wide enough to include three such discrepant conditions as those in which a man, an insect, and a bacillus have their being and play their several roles. Man is ruled by gravitation, and rests on mother earth. A water-beetle finds the surface of a pool a matter of life and death, a perilous entanglement or an indispensable support. In a third world, where the bacillus lives, gravitation is forgot¬ ten, and the viscosity of the liquid, the resistance defined by Stokes’s law, the molecular shocks of the Brownian movement, doubtless also the electric charges of the ionized medium, make up the physical environ¬ ment and have their potent and immediate influence on the organism. The predominant factors are no longer those of our scale; we have come to the edge of a world of which we have no experience, and where all our preconceptions must be recast. As one example, consider the difficulty we experience, despite our prefer¬ ences for reductionism in science, when we try to visualize the world of our genes, where nucleotides function as active and substitutable evolutionary parts—and where chromosomes build a first encasement, followed by nuclei and cells, with our body now serving as an entire universe, whose death will also destroy any gene still resident within. Think of the initial resistance that most of us felt towards Kimura’s neutralist theory—largely because we falsely
Species as Individuals in the Hierarchical Theory of Selection “downloaded” our adaptationist views about organisms into this different domain, where high frequencies of neutral substitution become so reasonable once we grasp the weirdly (to us) divergent nature of life at such infinitude. And if we fare so badly for the small and immediate, supposedly so valued by our reductionist preferences, how can we comprehend an opposite extension into the longer life, the larger size, and the markedly different character of species-individuals—a world that we have usually viewed exclusively as a col¬ lectivity, an aggregation of our bodies, and not as a different kind of individ¬ ual in any sense at all? I like to play a game of “science fiction” by imagining myself as an individ¬ ual of another scale (not just as a human being shrunken or enlarged for a visit to such a terra incognita). But I do not know how far I can succeed. As organisms, we have eyes to see the world of selection and adaptation as ex¬ pressed in the good design of wings, legs, and brains. But randomness may predominate in the world of genes—and we might interpret the universe very differently if our primary vantage point resided at this lower level. We might then note a world of largely independent items, drifting in and out by the luck of the draw—but with little islands dotted about here and there, where selec¬ tion slows down the ordinary tempo and embryology ties things together. How, then, shall we comprehend the still different order of a world much larger than ourselves? If we missed the strange world of genic neutrality be¬ cause we are too big, then what passes above our gaze because we are too small? Perhaps we become stymied, like genes trying to grasp the much larger world of change in bodies, when we, as bodies, try to contemplate the do¬ main of evolution among species in the vastness of geological time? What are we missing in trying to read this world by the inappropriate scale of our small bodies and minuscule lifetimes? Once we have become mentally prepared to seek and appreciate (and not to ignore or devalue) the structural and causal differences among nature’s richly various scales, we can formulate more fruitfully the two cardinal prop¬ erties of hierarchies that make the theory of hierarchical selection both so in¬ teresting and so different from the conventional single-level Darwinism of organismal selection. The key to both properties lies in “interdependence with difference”—for the hierarchical levels of causality, while bonded in in¬ teraction, are also (for some attributes) fairly independent in modality. More¬ over, these levels invariably diverge, one from the other, despite unifying prin¬ ciples, like selection, applicable to all levels. Allometry, not pure fractality, rules among the scales of nature. 1. Selection at one level may enhance, counteract, or just be orthogonal to selection at any adjacent level. All modes of interaction prevail among levels and make prominent imprints in nature. I emphasize this crucial point because many students of the subject have fo¬ cussed so strongly on negative interaction between levels—for a sensible and practical reason—that they verge on the serious error of equating an opera¬ tional advantage with a theoretical restriction, and almost seem to deny the
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THE STRUCTURE OF EVOEUTIONARY THEORY other modes of positive (synergistic) and orthogonal (independent) interac¬ tion. Negative interaction wins primary heuristic attention because this mode provides our most cogent evidence, not merely for simultaneous action of two levels, but especially for the operation of a controversial or unsuspected level. If two levels work in synergism, then we easily miss the one we do not expect to see, and attribute the full effect to an unsuspected strength for the level we know. But if the controversial level yields an unexpected effect con¬ trary to the known direction of selection at a familiar level, then we may be able to specify and measure the disputed phenomenon. In the example cited previously, individual selection favors a balanced sex ratio, while interdemic selection leads to female bias in many circumstances. Our best evidence for the reality of interdemic selection emerges from the dis¬ covery of such biases—not so strong as purely interdemic selection would produce (for organismic selection operates simultaneously in the other direc¬ tion), but firm enough to demonstrate the existence of a controversial phe¬ nomenon. But if interdemic selection also worked towards a 1:1 ratio, we could attribute such an empirical finding exclusively to the conventional op¬ eration of organismic selection. Negative interaction, however, does yield one distinguishing consequence to highlight this mode as especially important in the revisions to evolutionary theory that the hierarchical model will engender. In conventional one-level Darwinism, stabilities generally receive interpretation as adaptive peaks or optima, thus enhancing the functionalist bias inherent in the theory. The ma¬ jor structuralist intrusion into this theme ordinarily occurs when we have been willing to allow that natural selection can’t surmount a constraint—ele¬ phants too heavy to fly even if genetic variability for wings existed; insects confined to small sizes by the inherited Bauplan of an exoskeleton that must be molted, and a respiratory system of skeletal invaginations that would be¬ come too extensive at the surface/volume ratio of large organisms. But the constraints in these cases act as passive walls, not active agents. The hierarchical theory of selection suggests a theoretically quite different and dynamic reason for many of nature’s stabilities: an achieved balance, at an intermediary point optimal for neither, between two levels of selec¬ tion working in opposite directions. Several important phenomena may be so explained: weak female bias as the negative interaction of organismal and interdemic selection (see above); restriction of multiple copy number in “selfish DNA” as a balance between positive selection at the gene level, sup¬ pressed by negative selection (based, perhaps, on energetic costs of produc¬ ing so many copies irrelevant to the phenotype) at the organismic level. I also suspect that stable and distinctive features of species and clades must repre¬ sent balances between positive organismic selection that would drive a fea¬ ture to further elaboration, and negative species selection to limit the geo¬ logical longevity of such “overspecialized” forms. In any case, a world of conceptual difference exists between stabilities read as optima of a single pro¬ cess, and stabilities interpreted as compromises between active and opposed forces. As an example of overemphasis upon negative interaction, Wilson and So-
Species as Individuals in the Hierarchical Theory of Selection ber (1994, p. 592) ask: “Why aren’t examples of within-individual [organ¬ ism] selection more common?” They mention the most familiar case of meiotic drive, and then discuss the conventional argument for rarity of such phenomena: the integrity of complex organisms implies strong balance and homeostasis among parts; therefore, any part that begins to proliferate inde¬ pendently will threaten this stability, and must therefore be disfavored by organismic selection, a force generally strong enough to eliminate such a threat from below. If selection within bodies generally opposes the organismic level, as this discussion implies, then we properly expect a low frequency for the phenome¬ non, since evolution has endowed the organismic level with a plethora of de¬ vices for resisting such dysfunctional invasion from within. Although I accept this argument for a low frequency of selection contrary to the interests of en¬ closing organisms, selection within bodies may not be so rare when we in¬ clude the other modalities of synergistic and orthogonal directions. The most interesting hypothesis for extensive selection at the gene level, the notion originally dubbed “selfish DNA” (Orgel and Crick, 1980; Doolittle and Sapienza, 1980), attributes the observed copy number of much middle-repetitive DNA to orthogonal gene-level selection initially “unnoticed” by the organ¬ ism, though eventually suppressed by negative selection from above when copies reach sufficient numbers to exact an energetic drain upon construction of the phenotype (see fuller discussion on pp. 693-695). In fact, I suspect that organismic complexity could never have evolved without extensive gene-level selection in this orthogonal (or synergistic) mode. For if we accept the com¬ mon argument that freedom to evolve new phenotypic complexity requires genetic duplication to “liberate” copies for modification in novel directions, then how could such redundancy arise if organismic selection worked with such watchdog efficiency that even a single “extra” copy, initially unneeded by the organismic phenotype, induced strong negative selection from above, and immediately got flushed out—thus, in an odd sense, making the organism a delayed Kamikaze, killing its “invader” now and, by summation of such consequences, itself later? Leo Buss (1987), in a fascinating book on the role of hierarchical selection in the phylogenetic history of development (see pp. 696-700 for further dis¬ cussion), offers a compelling case for the vital importance of both synergistic and negative selection between levels in the history of life, which he views largely as a tale of sequential addition in hierarchical levels—so that nature’s current hierarchy becomes a problem for historical explanation, not an inher¬ ent structure fully present throughout time. Buss argues that synergism must fuel the first steps in adding a new level atop a preexisting hierarchy (for ini¬ tial negativity against the previous highest level would preclude the origin of a new level). But, having once achieved a tentative foothold, the new level sta¬ bilizes best by imposing negative selection against differential proliferation of individuals at the level just below—for these individuals have now become parts of the new level’s integrity, and selection at the new level will tend to check any dysfunctional imbalance caused by differential proliferation from below.
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THE STRUCTURE OF EVOEUTIONARY THEORY 2. Each hierarchical level differs from all others in substantial and interest¬ ing ways, both in the style and frequency of patterns in change and causal modes. Nature’s hierarchy, for all the commonality of its unifying principles (selection, for example, acting at each level4), does not display fractal structure with self-similarity across levels. As the theory of hierarchical selection develops, I predict that no subject within its aegis will prove more fascinating than the varying strengths and modalities among levels. Just as the study of allometry has recorded charac¬ teristic and predictable scale-dependent differences in structure and function of organisms at strongly contrasting sizes—a prominent subject in biology ever since Galileo formulated the principle of surfaces and volumes in 1638, and so elegantly codified in D’Arcy Thompson’s masterpiece of both prose and concept, On Growth and Form (1917, second edition, 1942)—so too does individuality as a tiny gene imply substantially different properties for a unit of selection than “personhood” as a large species or an even larger clade. Allometric effects across hierarchical levels should greatly exceed the familiar (and extensive) differences between tiny and gigantic organisms for two un¬ surprising reasons (see Gould and Lloyd, 1999, for a detailed development of this argument). First, the size ranges among levels are far greater still. Second, organisms share many common properties simply by occupying a common level of evolutionary individuality despite an immense range of size; but the levels themselves differ strongly in basic modes of individuality, and therefore develop far greater disparity. But this promise also implies a corresponding danger. In some famous lines composed for a quite different, but interestingly related purpose, Alexander Pope explored the paradox of man’s intermediary status between two such disparate extremes, both so desperately needed to know and to understand (the bestial and the godly in Pope’s concern)—but both so inscrutable as so far from our own being: Placed on this isthmus of a middle state, A being darkly wise and rudely great. . . He hangs between; in doubt to act or rest; In doubt to deem himself a god, or beast. . . Created half to rise, and half to fall; Great lord of all things, yet a prey to all; Sole judge of truth, in endless error hurl’d; The glory, jest, and riddle of the world! I appreciate this image of an “isthmus of a middle state”—a narrow standing place linking two larger worlds of smaller and greater. Pope’s dilemma may pack more emotional punch in its moral meaning (since his greater and lesser worlds define questions of value rather than geometry), but our problem fea¬ tures greater intellectual depth—for, surely, a larger conceptual chasm sepa¬ rates the gene from the clade in modes of evolutionary mechanics, than the bestial from the virtuous in styles of human behavior. The problem can be summarized with another, and much older, classical quotation. “Man is,” as Protagoras wrote in his wonderfully ambiguous epi-
Species as Individuals in the Hierarchical Theory of Selection gram, “the measure of all things11—ambiguous, that is, in embodying both positive and negative meanings: positive for humanistic reasons of ubiquitous self-valuing that might lead to some form of universal brotherhood and com¬ passion; but negative because our own “measure11 can be so parochially limit¬ ing, and therefore so conducive to misunderstanding other scales if we must assess these various domains by the allometric properties of our limited es¬ tate. This issue becomes especially serious for the hierarchical theory of selec¬ tion. Humans hold status as both evolutionary individuals and organisms— yet all other “separate but equal” evolutionary individuals at other hierarchi¬ cal levels are not organisms. Unfortunately, organisms constitute a very spe¬ cial and distinctly odd kind of evolutionary individual, imbued with unique properties absent from (or much weaker in) other individuals (at other levels) that are equally potent as evolutionary agents. But if we mistakenly regard our own unique properties as indispensable traits for any kind of evolution¬ ary individual—the classic error of parochialism—then we will devalue, or even fail to identify, other individuals defined by different properties and resi¬ dent at other levels. I shall explore some of these crucial differences in the next two sections (disparate properties of the six major levels; and extensive comparison of or¬ ganisms and species as evolutionary individuals). In this introductory com¬ ment, I only wish to emphasize that the uniqueness of the organism as a unit of selection lies in securing individuality by maximal homeostatic interaction among parts, an integration that ties each subpart to the fate of all, and there¬ fore strongly discourages any “breakout” or differential proliferation (by suborganismic selection) from within. To be sure, such integration represents a powerful strategy for individuation, but this strategy does not specify the only legitimate path, and other potent evolutionary individuals use other mechanisms. For this reason, I regret that Wilson and Sober (1994) so em¬ phasize these “organic” properties of individuality in their general definition, meant to apply to all levels. This parochial focus leads them to downplay the individuality of units of selection at other levels, where different defini¬ tional criteria predominate—in species, for example, where the maintenance of boundaries by reproductive isolating mechanisms, and the mixture of sub¬ parts in replenishment (sexual reproduction), maintain cohesion and stability just as well as organisms do by the different strategies of homeostasis and functional interaction of subparts.
Redressing the tyranny of the organism: comments on characteristic features and differences among six primary levels I have little tolerance for numerical mysticism. I feel no special affinity for threes (as trinities), fours (Jung’s primal archetype), fives (for fingers or echinoderms), sevens (for notes of the musical scale, planets in the Ptolemaic sys¬ tem, and so much else), or nines (the trinity of trinities). Similarly, in recogniz¬ ing six hierarchical levels for this discussion—genes, cells, organisms, demes, species, and clades—I only utilize a device of convenience, and do not make
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THE STRUCTURE OF EVOLUTIONARY THEORY any assertion about a fixed number of units in the expanded hierarchy of Darwinian action. Any such claim of definity could only rank as both foolish and incoherent for at least two reasons. First, the hierarchy has not been set by structural or logical principles, but historically evolved in a contingent manner. Thus, be¬ fore the inventions of sexual reproduction and multicellular organisms, nei¬ ther species nor organisms (as a level distinct from cells) existed, and a quad¬ ripartite hierarchy held sway (and still does today in the dominant world of asexual unicells)—gene, cell, clone, and clade. Second, several of the levels discussed here coagulate numerous phenomena because they lie between two clear boundaries. As Buss (1987) points out, for example, we might, in cer¬ tain contexts, recognize several items that encase genes but serve as parts of multicellular organisms: chromosomes, organelles, cells, organs, etc. Before the multicellular organism evolved, and began to act with such effectiveness as a suppressor of mtraorganismic selection, we might have construed this domain of “proto-individuality” quite differently, and with finer resolution. As a second argument against granting necessary or inherent status to these six levels, I have followed nearly all students of this field in preferring a fully nested hierarchy of increasing inclusion, to other legitimate interactors that function only occasionally, transiently, or in special circumstances. This fully nested hierarchy operates with Linnaean logic in requiring that lower units amalgamate completely, and under strict genealogical constraint—so that no lower unit can belong to more than one higher unit, while no higher unit can “forage” outside its hereditary line to incorporate the lower units of other distinct evolutionary branches at the same level. Just as a genus can’t belong to two families, a species of flies cannot incorporate some onychophores and a few myriapods to construct a more versatile species-individual. We logically require this property of nesting to correlate the nonhistorical process of selection with a set of quintessential^ historical phenomena in evolutionary biology, including phylogenetics and the study of adaptation. Without such a fully inclusive hierarchy, for example, we could not use one level as a surrogate or convenient descriptor for events at other levels in the same nest—as when we choose the gene level for keeping the general books of evolution (see pp. 632-637 on the error of gene selectionism). Nature, of course, does not always obey this logical stricture, though we may appeal to the empirical success of this formulation as an indicator that nature does comply at a preponderant relative frequency. If life did not gener¬ ally work within a hierarchy of inclusion, the biotic world would present such a different appearance that our conventional ordering devices would not operate usefully, and would never have been proposed or accepted. (I am not a naive realist, and I have argued throughout this book that we impose our social preferences upon nature in constructing our theories. But nature does provide a strong input, and does impose a powerful constraint upon our formulations.) No one would ever have suggested a nested system like Linnaeus’s, if common experience proclaimed that novel taxa generally arise by distant amalgamation—if, for example, each new mammal arose by a
Species as Individuals in the Hierarchical Theory of Selection principle of “disparate thirds,” say with equal mixtures of dugong, aardvark, and howler monkey. (We all know, of course, though we rarely discuss the subject in polite company, that the Linnaean logic, which presupposes a to¬ pology of branching without amalgamation, cannot apply to groups that do show massive mixture, as in some families of plants with extensive hybridiza¬ tion, or especially in prokaryotes evolving with frequent lateral transfer—a phenomenon that, on accumulating evidence, may be common enough to truly discombobulate the Linnaean version for the pre-multicellular majority of life’s tree (see Doolittle, 1999), with practical and theoretical consequences as broad as any revolutionary discoveries in the recent history of evolutionary biology.) Similarly, we all appreciate the conceptual difficulties imposed by some prominent cases in evolution, mostly at the genic or cellular level, that do violate the hierarchy of inclusion—most notably, the origin of some or¬ ganelles as symbiotic prokaryotes. Since units of selection operate as interactors with the environment, and since entities obeying the criteria of “personhood” (see pages 602-613) do occasionally cohere by distant genealogical amalgamation, nature does pres¬ ent some exceptions to the principle of a fully nested hierarchy for evolution¬ ary individuals. But these exceptions truly function as the “rule provers” of our mottoes (in the sense of probing, or testing, our generalities), and not as falsifiers. The most widespread cases, including the origin of cellular organ¬ elles by endosymbiosis, represent “frozen” phenomena of history, not active amalgamations presently building evolutionary individuals by junction of disparate genealogical lines. (However, genic exceptions, as noted above, may be rife if lateral transport occurs as frequently as current theory and data are now beginning to suggest, especially for prokaryotes.) The most common, ac¬ tive cases involve symbiotic and coevolutionary unions tight enough to obey the Biblical rule of Naomi and Ruth: “whither thou goest, I will go.” Wilson and Sober (1989), for example, present a fascinating discussion of “phoretic associations,” or obligate carriage, by wingless insects as they move among resource patches, of various mites, nematodes, fungi, and microorganisms. In some cases, the load of these “hangers on” can disable or even kill the insect, and conventional Darwinism will then work in its usual, competitive, and organismic mode. But the phoretic associates may be limited to densities that do not affect the insect, and may also provide resources indispensable for suc¬ cessful colonization of new patches—in which case, the entire association may be evolving as a “superorganism.” With these caveats in mind—the somewhat arbitrary division of the evolu¬ tionary hierarchy into six levels, and the acknowledgment of interesting ex¬ ceptions to full nesting among nature’s various individuals—I shall try to specify some distinctive “allometric” properties of the levels and their inter¬ actions: THE GENE-INDIVIDUAL As we enter this first unfamiliar world of such
great, and literally basic, importance to evolution, we encounter an initial rung of strong difference from the organism-individuals that, if only for psy-
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THE STRUCTURE OF EVOLUTIONARY THEORY chological reasons,-must stand as prototypes for our parochial concept of how a proper Darwinian unit must function in natural selection. If we could ever truly grasp the gene’s world, with full sympathy and appreciation for rel¬ ative frequencies, hard-line selectionism would yield to a fascinating enlarge¬ ment that would actually strengthen selectionist theory by synergism with other (non-contradictory) forces—so this subject should therefore not intimi¬ date strict Darwinians. For the most part, however, the necessary acknowl¬ edgment of different gene-level processes has unfolded within the traditional perspective of organismic selection—with three basic categories of interpreta¬ tion as “good” for organisms, and acceptable on this basis; “bad” for organ¬ isms, and a destabilizing danger that must be conquered; or irrelevant to or¬ ganisms and therefore unimportant/" The implications for a hierarchical reconstruction of evolutionary theory have therefore
been missed or
downplayed. Consider the two major themes of recent literature: Motoo Kimura and the “neutral theory of molecular evolution.” Although I have called this book “the structure of evolution¬ ary theory,” I have propagated my own lamentable parochialism under a pre¬ tense of generality. For this book, despite its exuberant length, largely re¬ stricts itself to the Darwinian tradition of conventional causal explanations based on selection as a central mechanism. I do, to be sure, treat the major critiques of unbridled selectionism (constraints as channels, failure of pure extrapolationism into geological time), but I conduct this discussion within a Darwinian world, and do not adequately consider truly alternative mecha¬ nisms of change and their domains of operation. Since selection is a causal theory of change based on distinctive traits of definable individuals within specified environments (quite apart from any stochastic sources for the varia¬ tion that provides raw materials of change), the obvious first-line alternative to selection must lie in random reasons for change itself. As a basic statement in the logic of an argument, this point can hardly be denied, and therefore enjoys a long history of recognition in evolution¬ ary thought. But recognition scarcely implies acceptance. The Victorian age, basking in the triumph of an industrial and military might rooted in technol¬ ogy and mechanical engineering, granted little conceptual space to random events, so the issue barely arose in Darwin’s own time. (Darwin got into enough trouble by invoking randomness for sources of raw material; he wasn’t about to propose stochastic causes for change as well! To this day, a distressingly familiar vernacular misunderstanding of Darwinism rests upon confusing these two components (sources of raw material and causes of change)—as in the common charge that Darwinism must be wrong because human complexity couldn’t arise by purely random processes. Nineteenth
"'Anyone, like me, who grew up in America with fiercely traditional immigrant grand¬ parents from “the old country” will appreciate the humor of such limited and inappropri¬ ate reference points. My grandmother’s only concern for any cultural or historical event (all of which she followed with great interest and intensity) stood out in her single, invariant question: “Is it good for the Jews?”
Species as Individuals in the Hierarchical Theory of Selection century theories of probability also eschewed ontological randomness in fa¬ vor of causal production by interaction of so many fundamentally orthogo¬ nal mechanisms that stochastic formulations would best fit the observed results—the philosophical solution traditionally adopted by the scientific determinists who invented probability theory, most notably by Laplace him¬ self.) For these primarily societal reasons, theories of random change enjoyed lit¬ tle currency before our own century, when for both external reasons of a new cultural context (spawned by such events as the breakup of colonial empires, the devastation of World Wars, and the consequent questioning of predict¬ able progress as time’s direction), and internal prods from the mathematical apparatus of population genetics, random models of change became a major and controversial subject in evolutionary theory. I shall not review this wellknown story, centering on the life and work of Sewall Wright (see Wright’s own magnificent four-volume summing up, and Provine’s fine biography). I only need to remind readers that genetic drift (often called “the Sewall Wright effect” in early literature), while unimpeachable in theory, and therefore surely operative in nature, received very short shrift, especially as the Modern Synthesis hardened around its adaptationist core (see Chapter 7). The Synthe¬ sis did not and could not deny genetic drift; instead, supporters resorted to the classical argument for dismissal in natural history—relegation to insig¬ nificant relative frequency. I learned the argument as a near mantra in all my graduate classes during the mid 1960’s: fixation by genetic drift can only oc¬ cur in populations so tiny that most will already be on the brink of extinction. Genetic drift at the traditional organismic level enjoys far more respect and currency today, but the basic argument of the Synthesis does have merit at this hierarchical level. Sexually-reproducing, multicellular organisms gener¬ ally share two properties that greatly limit the efficacy of genetic drift: they live in populations far too large for random fixation in the face of nearly any measurable selection pressure; moreover, the style of individuality manifested by organisms, based on well-balanced functional integration among sub¬ parts, renders the traits of these interactors particularly subject to scrutiny by natural selection. Do these good reasons for demoting random change at the organismic level doom this alternative style of evolution to weakness or impotency through¬ out the hierarchy? Clearly not, as the recent history of our profession proves; moreover, we may even invert the standard hope for extrapolation from the level we know best, and assert instead that the organismic level discourages random change as a peculiarity of individuality in this realm—and that ana¬ logs of genetic drift at other levels should expect healthy, if not dominant, relative frequencies. All evolutionists also know that ideas of random change have enjoyed greatest success, based on inherent plausibility, at the genic level, where the so-called “neutral theory of molecular evolution,” most strongly associated with the great Japanese geneticist Motoo Kimura (1968, 1983, 1985, 1991a and b), but initiated and developed by others as well (Jukes, 1991), has of-
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THE STRUCTURE OF EVOEUTIONARY THEORY ten been hailed as the most interesting revision of evolutionary theory since Darwin. When we consider the two properties of organisms that depress the fre¬ quency of fixation by drift at this level, we easily spot the difference that makes randomness so important at the lower genic level. Population size, also characteristically large for gene-individuals, cannot supply the reason. But the workings of DNA establish a strong supposition for absence of selective pressure from the organismal level at a high percentage of nucleotide sites, where alternative states do not influence the phenotypes of organisms—hence the designation of drift at this level as the neutral theory of molecular evo¬ lution. Kimura’s classical categories of evidence all depend upon the observation that maximal rates of nucleotide change occur at sites that do not influence the organismal phenotype—on the reasonable assumption that organismal selection usually acts in the stabilizing mode to preserve favorable sequences, and that sites under selective influence must therefore change at less than the maximal rate. The threefold confirmation of this prediction provides power¬ ful evidence for the neutral theory—(1) for synonymous substitutions of the third nucleotide in a triplet; (2) for much higher rates of change in untrans¬ lated introns than in surrounding exons; and (3) for entirely untranslated pseudogenes, where rates at all three positions of triplets match the rapid third-position rate for translated DNA. The move from mere plausibility to the important claim for high, or even dominant, relative frequency arises both by implication from the basic theory, and from observation. The three phenomena described above, after all, in¬ clude a large percentage of all nucleotide changes—so neutralism must main¬ tain a high relative frequency at this level if we have interpreted the rates of change correctly. At the broadest scale of geological time, the (admittedly ap¬ proximate) ticking of the molecular clock in so many phylogenetic studies achieves its most plausible reading as a consequence of generally comparable rates for the high percentage of neutral substitutions. (The alternate explana¬ tion of averaging out for fluctuating selective control over sufficiently long periods of time cannot be dismissed a priori, but smacks of special pleading— whereas neutralism expects this result as the consequence of a central propo¬ sition.) Kimura has always stressed the high frequency of neutral substitutions as his main challenge to Darwinian traditions. He writes, for example (1991a, p. 367), that “in sharp contrast to the Darwinian theory of evolution by natu¬ ral selection, the neutral theory claims that the overwhelming majority of evolutionary changes at the molecular level are caused by random fixation (due to random sampling drift in finite populations) of selectively neutral (i.e., selectively equivalent) mutants under continued inputs of mutations.” At the same time, Kimura also consistently insisted—and not, I think, merely for diplomacy’s sake, or for any lack of resolve, but rather with genuine conviction (I discussed the matter several times with Kimura in person, so I will also stand as witness)—that the neutral theory did not contradict or de-
Species as Individuals in the Hierarchical Theory of Selection throne Darwinism, but should rather be integrated with natural selection into a more complete and more generous account of evolution. Most neutral changes, after all, occur “below” the level of visibility to conventional Dar¬ winian processes acting at the organismic level. Moreover, although most nu¬ cleotide changes may be neutral at their origin, the variability thus provided may then become indispensable for adaptive evolution of phenotypes if envi¬ ronmental change promotes formerly neutral substitutions to organismic visi¬ bility—-an important style of cross-level exaptation (Vrba and Gould, 1986; Gould and Lloyd, 1999) that may serve as a chief prerequisite to the evolu¬ tion of substantial phenotypic novelty. Kimura writes, for example (1985, p. 43): “Of course, Darwinian change is necessary to explain change at the phenotypic level—fish becoming man—but in terms of molecules, the vast majority of them are not like that. My view is that in every species, there is an enormous amount of molecular change. Eventually, some changes become phenotypically important; if the environment changes, some of the neutral molecules may be selected and this of course follows the Darwinian scheme.” Thus, Kimura’s statement exemplifies the central principle that the various levels of evolution’s hierarchy work in characteristically different ways—and that levels can interact fruitfully in these disparate modes. The chronological reaction of Darwinian hardliners to the neutral theory can be epitomized in a famous, if sardonic, observation about the fate of controversial theories. Tradition attributes this rueful observation to T. H. Huxley, but some form of the statement may well date to antiquity, the usual situation for such “universal” maxims. In any case, the earliest reference I know comes from the great embryologist von Baer, who attributed the line to Agassiz (von Baer, 1866, p. 63, my translation): “Agassiz says that when a new doctrine is presented, it must go through three stages. First, people say that it isn’t true, then that it is against religion, and, in the third stage, that it has long been known.” The first two stages unfolded in their conventional manner, with quizzical denial followed by principled refutation in theory (see p. 521 on Mayr’s argu¬ ment that neutralism cannot be true because we now know the ubiquity of se¬ lection). However, the third stage—still stubbornly occupied by some strict Darwinians—arose with an interesting twist, providing a cardinal illustration for this section’s major theme: the dangers of parochialism, particularly the tendency to interpret all evolution from an organismal vantage point. Instead of simply stating that neutralism has long been known (so what’s the big deal?), detractors now tend to say: “well, yes, it’s true, and let’s be generous and give Kimura and company due credit. But, after all, neutral substitutions only occur at sites without consequence for organismic phenotypes. So why focus upon such changes? Without any organismal effect, they can’t be im¬ portant in evolution. And no one can blame Darwin or Darwinian tradition for ignoring an invisible phenomenon.” This exculpation of Darwin cannot be faulted in logic, but the rest of the argument reflects a narrow and discouraging attitude. Isn’t the claim of unim¬ portance absurd prima facief How can anyone advance an argument for
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THE STRUCTURE OF EVOLUTIONARY THEORY downgrading, as marginal, a process potentially responsible for more than half of all nucleotide substitutions—the supposed basis of evolution within a scientific ethos centered on reductionist preferences? Only a lingering preju¬ dice for viewing organisms as a unique andhntrinsic focal level could possibly generate such a claim. Yes, an organism might view the world of its own compatriots as stable is¬ lands rising above an invisible sea—and choose to disregard random change within this swirling ocean of underlying, constant activity. But (if I may pur¬ sue this strained metaphor for a moment), any dynamic particle in the ocean could just as well, and perhaps with more merit, view the islands as rare and insignificant pedestals intruding into the truly fundamental substrate. May I just note the sterility of such a subjective argument, and state that any process with so strong an impact on change at any level cannot be unimportant in a world judged by relative frequencies. As an illustration of the importance (and separability) of hierarchical lev¬ els, we may invoke balances produced by negative interaction among levels as a measure for the indispensability of molecular neutrality in full explanations of evolutionary phenomena. Just as a stable balance may arise by opposite forces of selection at adjacent levels, different processes-—in this case neutral¬ ity at one level vs. selection at another—can also produce an intermediary re¬ sult testifying to the importance of both styles of change. In such cases, more¬ over, neutrality enjoys a special heuristic advantage because random models yield general, quantitative predictions, while selectionist explanations usually require knowledge of particular circumstances that are much harder to deci¬ pher, and often impossible to quantify (for lack of requisite historical infor¬ mation). For example, Spalax ehrenbergi, a blind Near Eastern mole rat, develops a rudimentary eye with an irregular lens that cannot focus an image. The eye is covered by thick skin and hair, and the animal shows no neurological re¬ sponse to powerful flashes of light (see p. 1282 for fuller discussion of this case in a different context). As expected under the neutral theory, the major lens protein, aA-crystallin, evolves much faster in S. ehrenbergi than in other murine rodents with normal vision (Hendricks et ah, 1987)—nine amino acid replacements in a sequence of 173, over 40 million years of evolutionary sep¬ aration, whereas the other nine rodents of this study show identical amino acid sequences, with no alterations at all from the ancestral state. But this rate of change for Spalax represents only 20 percent of the average for true pseu¬ dogenes, our best standard for a maximal and purely neutral pace of evolu¬ tion. At a rate of alteration too fast for stabilizing selection, but too slow for pure neutrality, the results imply a dynamic balance between molecular drift and weakened selective control at the organismic level. (Suggestions for con¬ tinued utility of a non-seeing lens include possible function in adjusting physi¬ ology to seasonal cues from changing day lengths, though we know no mech¬ anism for perceiving such fluctuations without vision, see Haim et ah, 1983; and developmental constraint based on formation of eyes as a necessary in¬ ducer of some later and fully functional feature in embryology.)
Species as Individuals in the Hierarchical Theory of Selection I close this woefully insufficient commentary by reemphasizing the point that our discomfort or disinterest in random change largely reflects the pecu¬ liarity of the individual and level that we know best—organisms—and does not record any rarity or impotence for stochastic forces as agents of phyletic change in evolution. Processes of drift probably exert least influence upon the organismic level, for the two reasons cited earlier: large population sizes, and a style of individuality that forges coherence by strict functional coordination of subparts, and therefore makes nearly every trait of the organism subject to selection strong enough to overwhelm drift. But the organism is a unique and peculiar kind of individual—and these strictures upon drift do not apply so strongly at any other level. We have seen, in this section, how structural features of DNA impose neutrality or nearneutrality upon selection at a large percentage of sites, perhaps a majority. For this reason (and not by limitation of population size), randomness be¬ comes a fundamental process of evolutionary change at the genic level, how¬ ever weak such a force may be (or, indeed, may not be!) at the organismic level. We shall see that, at the highest levels of species and clades, randomness again attains a high relative frequency—but this time mostly as a result of low N for species in clades. If such different causes grant randomness a high rela¬ tive frequency at several important levels of evolution’s hierarchy—and if we can only assert low relative frequency at one level, and for reasons rooted in the peculiar character of individuality in this realm alone—then have we not committed a great conceptual error, and seriously narrowed our general view of evolution and the history of life, by giving short shrift to this most obvious of all alternatives to selection as a cause of change? True genic selection. When future historians chronicle the interest¬
ing failure of exclusive gene selectionism (based largely on the confusion of bookkeeping with causality), and the growing acceptance of an opposite hier¬ archical model, I predict that they will identify a central irony in the embrace by gene selectionists of a special class of data, mistakenly read as crucial sup¬ port, but actually providing strong evidence of their central error. Gene selec¬ tionists have always welcomed genuine cases of a phenomenon that they then falsely generalize to all evolution—that is, differential proliferation of genes within genomes for reasons acting at the genic level, and independent of ef¬ fects introduced by downward causation from selection at any higher level. Gene selectionists have naively embraced these examples as apparent con¬ firmations of their belief that effectively all selection operates at this lowest level. If genes can work their magic even without a boost from the vehicles they usually employ as lumbering robots subject to their will, then our appre¬ ciation for their omnipotence can only increase. But such superficial admira¬ tion obscures a true distinction that actually illustrates the bankruptcy of exclusive gene selectionism. These examples do not showcase the maximal power of a ubiquitous phenomenon; rather, and quite to the contrary, they represent the only class of instances where pure and untrammeled gene selec¬ tion can operate at all! As argued previously in this chapter (pp. 613—644), when gene selectionists
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THE STRUCTURE OF EVOLUTIONARY THEORY speak of genes using, organisms as their vehicles, they commit a deep error by inverting causality and ascribing to genes (which only record the causal re¬ sult, and therefore serve as good units of bookkeeping) the agency in natural selection that really belongs to the organism—for vehicles (or interactors) op¬ erate as units of selection, or causal agents of Darwinian evolution. But when genes do not use organisms as vehicles and engage in differential proliferation on their own accord, then the genes themselves do act as vehicles—and, con¬ sequently, can become units of selection. Gene selection only exists when genes can operate as vehicles (interactors); thus, these cases illustrate the re¬ stricted range of a process that gene selectionists naively regard as optimal il¬ lustrations of a ubiquitous phenomenon. The resulting irony deserves empha¬ sis. Supposed best cases become only cases, and therefore disproofs of a generality when properly interpreted. Wilson and Sober (1994, p. 592) put the point well: “These examples have been received with great fanfare by gene-centered theorists as some sort of confirmation of their theory. However, they do not confirm the thesis that genes are replicators—all genes are repli¬ cators by definition and no documentation is needed. These examples are re¬ markable because they show that genes can sometimes be vehicles. They seem bizarre and disorienting because they violate our deeply rooted notion that individuals are organisms.” Devotees of the genic level may eventually accept the defeat of their theory of exclusivity with good grace—for the supplanting hierarchical model pro¬ vides more than enough room for true (and fascinating) examples of genuine genic selection, perhaps at quite high relative frequency once we acknowledge and learn to recognize the synergistic and orthogonal modes, as well as the better-documented examples of genic selection that harms organisms. Moreover, when we recognize that many kinds and aggregations of genetic units can function in selection, the scope of this level becomes even wider. Se¬ lection may operate at the lowest unit of the nucleotide itself, if preferential substitution arises, for example, by differential production and consequently greater availability of one nucleotide vs. alternatives (the analog of natural se¬ lection by birth biasing). Selection among entire genes and other DNA seg¬ ments of comparable length may also hold great significance in evolution—as in Dover’s important hypothesis of “molecular drive” (Dover, 1982). In fact, we may be impeding a proper recognition of the substantial fre¬ quency of selection within genomes by naming the phenomenon for only one mode among many—“gene selection.” In the early days of Watson and Crick, biologists tended to conceptualize genomes as linear arrays of functional units (tightly strung beads with no spaces between in the usual metaphor). But we now know that most genes of eukaryotes, with their structure of exons separated by introns, do not maintain strict spatial continuity. More¬ over, the functional genes of most complex metazoans represent, in any case, just a few percent of the full genome. All other kinds of genomic elements, forming an overwhelming majority of sites, can also evolve by processes of drift and selection.
Species as Individuals in the Hierarchical Theory of Selection For this reason, Brosius and Gould (1992) suggested that we use a more general term—“nuon” for nucleic acid sequence (DNA or RNA)—to recog¬ nize any stretch of nucleic acid, functional or not in organismic terms, that can evolve by differential origin or replication: Genomes do not consist only of genes. Sequences located between and also within gene boundaries, accounting for a large portion of the ge¬ nomes of higher Eucarya, are not being addressed in a similar manner, partly due to the widespread opinion that these sequences are without function . . . We propose to name all identifiable structures represented by a nucleic acid sequence (DNA or RNA) as “nuons.” A nuon can be a gene, intergenic region, exon, intron, promotor, enhancer, terminator, pseudogene, short or long interspersed element ... or any other retroelement, transposon, or telomer—in short, any unit from a few nucleo¬ tides to thousands of base pairs in length. Proceeding upwards, aggregates of genes can also function as units of se¬ lection—including, as prominent agents in evolution, chromosomes (Nei, 1987), and organelles and bacterial plasmids within cells (Eberhard, 1980, 1990). Organismic selection generally works with great effectiveness in suppress¬ ing “revolts” to organismic integrity by differential proliferation of elements from within (see Buss, 1987; Leigh, 1991). Most of the characteristic prop¬ erties of genomic organization and embryological development—from Ham¬ ilton’s “gavotte of the chromosomes” in meiosis, to such phenomena as germ line sequestration and maternal determination in embryogenesis—may have evolved largely to suppress suborganismic selection, thereby assuring the in¬ tegrity of multicellular organisms. Meiosis itself presumably evolved to place one copy of each gene “in the same [gametic] boat,” thus converting organ¬ isms, rather than genes, into a primary unit of selection by the Musketeer’s criterion of “all for one.” But, once achieved, meiosis must be actively guarded by organismal selection against destabilizing drivers and distorters— all to preserve what Leigh (1991, p. 258) calls “the genome’s common inter¬ est in honest meiosis.” Nonetheless, the evolutionary literature abounds with cases, both “classic” and new, of meiotic drivers, chromosomal segregation distorters, and other phenomena that favor the plurifaction of individual genes or sequences (in¬ cluding entire chromosomes) within the genome or population of genomes— usually with negative consequences for organismal selection above. Perhaps such cases must be relatively rare in nature, and only prominent in our litera¬ ture for their intriguing oddity and exceptional status in the light of organ¬ ismal selection’s usual power to suppress such “outlaws.” Driving genes and chromosomes use a variety of devices to increase their relative representation by suborganismal selection. Some, including the clas¬ sic ^-allele of house mice (Lewontin, 1970), cause dysfunction in sperm car¬ rying the nondriving homologue; others, like the supernumerary chromo-
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THE STRUCTURE OF EVOLUTIONARY THEORY somes of rye, segregate preferentially into functional gametes. Werren (1991, p. 393) attributes the interest generated by these cases to implications for the hierarchical model of selection: “Driving chromosomes are of general interest in population genetics as examples of ‘selfish’ or ‘parasitic’ genetic elements. Such elements challenge the concept of the individual genome as a ‘coopera¬ tive’ unit because they gain a transmission advantage relative to the rest of an individual’s genome but are often detrimental to individual organisms.” Werren (1991; Werren, Nur, and Wu, 1988; Werren and Beukeboom, 1993) has also discovered and developed one of the most elaborate and inter¬ esting cases of suborganismal selection, a testimony to the complexity of in¬ teraction among levels of selection as well. In the parasitoid wasp Nasonia vitnpennis, a supernumerary chromosome called PSR (paternal sex ratio) has evolved “an extreme and unusual form of transmission drive” (Werren, 1991, p. 392). This chromosome, carried in sperm, induces supercondensation of all male chromosomes (except itself) into a chromatin mass before the fertil¬ ized egg’s first mitotic division. These chromosomes are then eliminated, while PSR survives. Since wasps are haplodiploid, this elimination converts an egg that would have become a diploid female into a haploid male (with PSR). This procedure obviously gives PSR a selective advantage in transmis¬ sion drive because this unpaired chromosome will always be transmitted by males produced from fertilized eggs. Just as obviously, organismic selection must oppose PSR, lest the en¬ tire population become both male and extinct. Werren (1991) had modeled conditions of maximal opposition from organismal selection. Subdivision of populations will be most effective in producing increased competition among PSR males, with reduced availability of females. But the story becomes even more complicated because suborganismal competition against PSR has also evolved by at least two devices that bias the sex ratio in a female direction (Werren and Beukeboom, 1993): (1) a maternally transmitted bacterium, called son-killer, that prevents the development of unfertilized (male) eggs; and (2) a cytoplasmically inherited agent of unknown structure and origin, that induces female wasps to produce nearly 100 percent daughters (called MSR, for maternal sex ratio). The possibilities introduced by haplodiploidy surely influence this variety and complexity in competing selection among suborganismic units—so stories this elaborate may not be common in nature. Still, as students of teratology in anatomy have always argued, we test and il¬ lustrate general rules by studying such cases at the limits. But the main weight of gene selection in nature—the category that estab¬ lishes a high relative frequency for the phenomenon—probably resides in cases that are synergistic with, or orthogonal to, organismic selection, and therefore not opposed by this powerful, conventional mode. Any genetic ele¬ ment that can propagate itself within the genome, either by iteration in tan¬ dem or by duplication and transposition to other chromosomes, works thereby as a vehicle of its own relative increase—and therefore as an agent of positive Darwinian selection at the genic level. If this propagation encounters no resistance at some other level (particularly by the watchful organism)—
Species as Individuals in the Hierarchical Theory of Selection either because bodies don’t “notice” the increase (at least while the number of genic copies remains “within bounds”), or because higher-level selection also benefits from such differential genic proliferation (if, for a hypothetical exam¬ ple, an X-driving chromosome helped to generate the female bias that mterdemic selection also favored)—then genic selection can be quite rapid and powerful. This general phenomenon, perhaps of great importance in evolu¬ tion, has acquired the unfortunate name of “selfish DNA,” as designated in two seminal papers, representing independent and simultaneous discovery, and published back to back in Nature in 1980 (Orgel and Crick, 1980; Doolittle and Sapienza, 1980). These authors proposed that such genic selec¬ tion, orthogonal (at first) to organismal selection, might account for most of the middle-repetitive DNA—some 15 to 30 percent of the genome in humans and Drosophila, and usually existing as tens to a few hundred copies per se¬ quence, with copies often widely dispersed among several chromosomes. Other hypotheses might explain this phenomenon, particularly as a poten¬ tial organismic need for enhanced levels of any products ultimately made by any gene. (In a purely organismal view, all genes may be able to proliferate, but not to fix their multiple copies unless organismic selection favors the in¬ crease. However, the two levels might also act synergistically, with genic drive evolving only in some genes, and for Darwinian benefit at this basal level, but with proliferation then enhanced by positive organismic selection upon bod¬ ies carrying more copies). The “selfish DNA” hypothesis includes an attractive feature, rooted in the hierarchical theory of selection, for explaining stabilization of copy number at tens to hundreds, rather than an ultimately suicidal proliferation to inevita¬ ble death of the organism and all gene-individuals contained therein. Genic selection may begin in the orthogonal mode, as initial increases impose no consequences upon the phenotype. But organisms must eventually take no¬ tice, if only for the energetic drain, and presumed slowing of ontogenetic de¬ velopment, imposed by replication of so many unneeded copies with every cellular division. Original orthogonality must therefore eventually yield to a situation of genic selection contrary to organismal interest. At this point, neg¬ ative selection at the organismic level should stabilize and limit further in¬ crease—the presumed explanation, within the theory, for the intermediary copy number of middle-repetitive DNA. Although I regard the hypothesis of “selfish DNA” as powerful, probably correct in many cases, and therefore as our best argument for substantially important selection at the genic level, two features in its initial promotion dis¬ tress me because they embody (without conscious intent, I assume) the persis¬ tent parochialism of organismic bias, even among those who explicitly pro¬ mote the hierarchical alternative. First, consider the unfortunate choices of names. Proliferating genic elements have generally been called “outlaws,” “renegades,” or “parasites”; and the general phenomenon entered our litera¬ ture as the hypothesis of “selfish DNA.” Orgel and Crick imposed a double whammy of opprobrium in the title of their original article: “Selfish DNA: the Ultimate Parasite.” The only reason that I can imagine for such deroga-
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THE STRUCTURE OF EVOLUTIONARY THEORY tory terms resides in the unstated (and probably unconscious) notion that benefits for organisms define the ultimate goal and purpose of evolution as a general phenomenon. Thus, anything that can evolve, but either hurts the or¬ ganism actively, or even just manages to sneak past organismal scrutiny, must be designated as selfishness, nastiness, or even usurpation—as promoted by some reprobate object that would place its own propagation above the gen¬ eral good of evolution. Surely, we must reject such parochial thinking and terminology. Propa¬ gating genic elements should not be described as parasites or renegades; nor can they be defined as “selfish” in any meaningful or general sense. Rather, propagating genes follow the Darwinian imperative at their own level, and therefore act as any good Darwinian agent “should”—that is, to increase their own representation within their own environment, the genome in this case. As Darwinians, we should honor their pluck in such a difficult endeavor (for organisms do tend to be watchful and suppressive), rather than heaping derogatory terms upon them. Such genes could only be deemed “selfish,” “parasitic,” etc., from a false and limited perspective that values the organism alone as an agent of evolutionary success. After all, we don’t call a peacock selfish for evolving such a beautiful tail, and thus limiting the geological lon¬ gevity of the species. To fully embrace the hierarchical model, a concept that marks a fundamen¬ tal shift in theory, not just an interesting new wrinkle upon an unaltered con¬ cept of nature’s basic construction, we must reconceptualize all of evolution, and revise both our worldview, and our language, accordingly. Second, even in terms of our conventional focus on organisms, genic selec¬ tion may provide crucial and indispensable flexibility for evolution of any substantial organismic novelty, including features conventionally placed in our most vaunted category of “increasing complexity.” The general argument has become traditional in evolutionary theory (since the pioneering book of Ohno, 1970), and represents a solution to the following, otherwise dis¬ abling, paradox: Organismal selection on the earth’s original prokaryotic biota might have constructed an optimal cell, “mean and lean” as could be, with a single copy of each gene to make, in the best possible way, one product indispensable for cellular success and propagation. But how could such an in¬ flexible organism ever change beyond minor adjustment to altered environ¬ mental circumstances? As Ohno wrote (1970): “from a bacterium only nu¬ merous forms of bacteria would have emerged.” But duplicated copies can provide requisite redundancy, permitting one copy to manufacture the needed product, while others become free to change—and to add new functions, thus providing a potential route to increasing complexity. But if selection only works at the organismic level, and our “mean and lean” bacterial prototype has attained an optimal configuration, what pro¬ cess provides evolution with the multiple copies needed for flexible addition of functions? We gain nothing from noting that duplications provide later blessings, since evolution cannot operate for the benefit of unknown and un-
Species as Individuals in the Hierarchical Theory of Selection predictable futures, unless our basic view of scientific causality needs funda¬ mental revision, and the future can determine the present. Hierarchical selection provides the most promising exit from this substan¬ tial paradox: multiple copies cannot originate for future organismic benefit, but they can evolve by present genic selection! (Later exaptive utilization in the generation of organismal complexity illustrates the important historical principle that reasons for origin must be sharply separated from current util¬ ity—see Chapter 11 for extensive discussion. Evolution continually recycles, in different and creative ways, many structures built for radically different initial reasons.) In 1970, Ohno wrote with great prescience: “The creation of a new gene from a redundant copy of an old gene is the most important role that gene duplication played in evolution.” Thus, if duplication requires genic selection in many or most cases, then the first level of evolution’s hierarchy not only operates with respectable relative frequency, but even provides an indispensable boost for generating the summum honum of our deepest prejudices—the complex organism, with even¬ tual evolution of a single strange mammalian species endowed with a unique capacity for self-reflection, but occupying an isthmus of a middle state, a good vantage point for looking down with thanks to duplicating genes, and up with awe to a tree of life that could generate such an interesting and acci¬ dental little twig. THE CELL-INDIVIDUAL I speak here not of free-living unicells (where cell
and organism represent the same unit of evolutionary individuality), but of cells that generally house full genomes and form the environment of genes at the level below, while also serving as parts and building blocks of multi¬ cellular organisms at the level above. From our limited viewpoint as highly complex metazoans built by intricate and integrated programs of embryological development, we tend to neglect this intermediary level of differential cellular proliferation (not just to build bigger organs in the somatic environ¬ ment, for such a process yields no evolutionary reward in competition with other cells for representation in future generations, but rather to gain prefer¬ ential access to the germ line, and thus to achieve evolutionary success by positive selection at the cell level). We neglect this subject because positive se¬ lection now so rarely occurs at this level in complex metazoans—and for a reason continually emphasized in this chapter: the effectiveness of multi¬ cellular organisms in suppressing the differential propagation of subparts as a necessary strategy for maintaining functional integrity, the definitive property of individuality at the organismal level. This suppression has been so effective, while the consequences of failure re¬ main so devastating, that human organisms have coined a word for the cell lineage’s major category of escape from this constraint, a name with power to terrify stable human organisms beyond any other threat to integrity and per¬ sistence—cancer. I suspect that we would learn much more about this large class of diseases (mistakenly viewed by most of the public as a single entity) if
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THE STRUCTURE OF EVOEUTIONARY THEORY
we treated the subject in evolutionary terms as a historical result of the cell’s initial capacity, retained from its phylogenetic past as an entire organism, for differential proliferation over other cells (formerly competitors as separate organisms, not compatriots as components of other organs). Of course, mod¬ ern human cells that escape this constraint do themselves no ultimate good, for they have no access to the germ line, and their unrestrained growth even¬ tually eliminates both their own lineage and the entire surrounding organism. To this extent, the organism’s general strategies do eventually prevail, follow¬ ing an initially successful assault by a cell lineage. But what a pyrrhic victory! Nonetheless, the double effectiveness of a virulent cancerous cell lineage-— crowding out in place and distant metastasis to other locations in the body— recalls the more “benign” strategies of other successful evolutionary plurifiers within a constrained space (genic proliferation by tandem duplication and transposition; budding off of new demes and “capture” of existing demes by immigration and transformation). If selection at the level of cell lineages now plays only a minor role in most groups of multicellular organisms, we should not view this hierarchical level as intrinsically impotent, but rather as historically suppressed in “the evolu¬ tion of [multicellular] individuality,” to cite the title of Leo Buss’s seminal book on this intriguing subject (Buss, 1987). In Buss’s terminology selection upon cells must now unfold in the “somatic environment,” where suppres¬ sion reigns in the service of organismic integrity, whereas such selection once occurred in the “external environment,” where unicellular organisms could experience the full independence and competitive range of Darwin’s world. (In fact, since most organisms on earth remain unicellular—see Gould, 1996a, on the persistence of the bacterial mode throughout the history of life—this transition has never occurred for the vast majority of organisms on earth.) This cellular level therefore provides our best demonstration that the cur¬ rent evolutionary hierarchy in styles of individuality arose both historically and contingently, and not with necessity as a timeless, predictable, invari¬ ant consequence of natural law. Levels have surely been added sequentially through time, as Buss has emphasized. If life began with naked replicators at the genic or subgenic level, then these earliest times for life may have featured, uniquely for this initial interval, the property that strict Darwinians have tried so hard to impose upon our richer world of modern life—selection at one level only. The evolution of cells led to a tripartite hierarchy that charac¬ terized most of life’s 3.5 billion year history, and still regulates the majority of earthly organisms: genes, cells, and clones. The evolution of sexual reproduc¬ tion added species, while the complex processes that constructed the multi¬ cellular individual then added the organism (the body that encloses cells and cell lineages). Suppression of cell lineage selection by the multicellular organism has greatly restricted a once vibrant and multifarious level. I must confess to my own parochialism in recognizing just one unit, the cell, as a surrogate for all entities that enclose genomes and form parts of organisms. Certainly
Species as Individuals in the Hierarchical Theory of Selection
organelles (or at least the mitochondria and chloroplasts that began their evo¬ lutionary history as symbiotic prokaryotes), and sometimes tissues and mod¬ ules of embryological development, can also act (in principle) as suborganismal units of selection. I make the amalgamation because this level has been largely suppressed, and therefore doesn’t often come to our attention in stud¬ ies of modern multicellular organisms. In so doing, I feel some allegiance to the folk taxonomist who (as so often recorded for indigenous cultures) assid¬ uously names each species (much as a trained Linnaean systematist would do) for creatures important to his life, but then lumps into large categories (weeds, butterflies, bugs) the organisms of no great moment in his world. As the central premise of his fascinating and seminal book, Buss (1987) ar¬ gues that the multicellular individual arose by “the interplay between selec¬ tion at the level of the individual and selection at the level of the cell lineage” (p. 29). More specifically, he attributes the distinctive features of metazoan development to an initial competition among cell lineages, eventually tamed and regulated by organismic selection in the interests of bodily integrity. Buss writes: “The thesis developed here is that the complex interdependent pro¬ cesses which we refer to as development are reflections of ancient interactions between cell lineages in their quest for increased replication. Those variants which had a synergistic effect and those variants which acted to limit subse¬ quent conflicts are seen today as patterns in metazoan cleavage, gastrulation, mosaicism, and epigenesis” (p. 29). Clearly, such a concept becomes intelligible only under the aegis of a hier¬ archical model of selection, as defended in this book’s central thesis. Buss recognizes this conceptual link, of course, and his work becomes a strong confirmation of both the efficacy and necessity of this basic reconstruction in evolutionary theory. In terms similar to the views expressed here, Buss writes (pp. 5-6): “The logical structure of Darwin’s argument allows any unit to evolve if it replicates with high fidelity, and if selection distinguishes be¬ tween the variants. Species, populations, and lineages of individuals, cells, organelles, and gene sequences can all potentially evolve. Yet we have been largely content to attribute the whole of biological diversity to selection upon individuals [organisms]. The once comfortable cloak of the Modern Synthesis has become restrictive.” (I am also grateful to Buss for recognizing the role of my profession, particularly in the work of Eldredge, Jablonski, Stanley, Vrba, and myself, in developing the hierarchical theory of selection. He writes (p. ix): “Indeed, hierarchical perspectives on evolution are undergoing a re¬ birth among paleontologists at the moment.”) In Buss’s model of historical and sequential construction for nature’s hier¬ archy, new levels arise to enclose the individuals of older levels by a two-step process. The initial features of the nascent level must originate in synergism, or positive interaction, with selection at the level just below, which formerly stood topmost, but will now be superseded (in the literal sense of “sat upon”) by the newly-emerging style of organization. New levels must begin with such a helpful boost, for the initial tentative and unformed steps cannot yet possess enough power to suppress or regulate a well-established level beneath. But
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THE STRUCTURE OF EVOLUTIONARY THEORY stabilization of the new level, implying a power to suppress at least some *
forms of harmful proliferation from within, then requires negative interac¬ tion, once the new and higher level achieves enough coherence to act in its own right. Since we have no direct data for key transitions that occurred so long ago and left no fossil evidence (so far as we know), Buss constructs some hypo¬ thetical examples of how such a process could work. (Such entirely specula¬ tive scenarios must be understood within their acknowledged limits—that is, as hypothetical stories, “cartoons” in Buss’s words, invented to illuminate a potential mode, and not as claims about any historical actuality.) For exam¬ ple, if the first tentative multicellular organisms evolved as little more than spherical colonies of identical protists floating in the ocean, how might essen¬ tial organismic properties like cellular differentiation emerge? Suppose that a variant cell lineage arose in such a loosely-knit, hollow sphere of cells, caus¬ ing members of the new line to enter the sphere’s center, where proliferation could continue. In this way, a new cell lineage (and the beginning of cellular differentiation for the organism) could originate and proliferate by selection at the cell level. Buss then supposes that such an event might also be beneficial for the organism, and he draws an analogy to the ontogeny of some modern sponges: The origin of a variant cell line which entered the center of such a sphere to continue cell division . . . may have produced a structure which was sufficiently negatively buoyant to fall to the sea floor. Many modern sponges ... do just this. A flagellated sphere populated by amoeboid cells simply drops to the ocean bottom . . . The pelago-benthic life cycle of sponges may have arisen as a consequence of variants which, in pursu¬ ing their own replication, fortuitously presented the individual with a benthic existence and all the attendant opportunities inherent in the in¬ vasion of a new adaptive zone. This move toward a more complex and better integrated organism begins with an initial synergism between cellular and organismic selection (origin of a new cell lineage by invasion and proliferation in the organism’s hollow cen¬ ter, leading to organismal advantages through an imposed change of habitat). But later stabilization of this innovation requires the suppression of cell lin¬ eage selection by the organismic level—for if the two cell lineages (at the sphere’s periphery and center) engage in an anarchic battle for ever greater representation in cellular percentages, either the organism will lose coherence and die, or one lineage will win and the organism will return to its previous state of minimal differentiation. Moving away from speculation and towards an explanation of metazoan development, Buss interprets several defining features of many (but not all) metazoan phyla as records of successful suppression of cell-lineage selection by organismal selection from above. In particular, he views early germ-line sequestration (Weismann’s crucial criterion in his defense of Darwin against resurgent late 19th century Lamarckism, see Chapter 3), and maternal pre-
Species as Individuals in the Hierarchical Theory of Selection destination, as organismal devices evolved to set and stabilize the course of development as early in ontogeny as possible, thus greatly reducing the po¬ tential for new forms of differential cellular proliferation either to arise at all in later ontogeny, or to reach the germ line and act in cell-lineage selection even if they do manage to originate. Buss sums up his thesis: Selection at the level of the individual has opposed selection at the level of the cell lineage by acting to set the timing of terminal somatic differ¬ entiation as far back in ontogeny as possible—whenever possible into the maternal cytoplasm itself, (p. 5). . . . The release of the totipotent germinative lineage from the task of producing somatic tissues meant that the number of divisions made by the totipotent lineage could be re¬ duced and, consequently, the opportunity for variants to arise to become severely restricted (p. 100) . . . Metazoans, by the twin devices of mater¬ nal predestination and germ-line sequestration, have effectively closed their ontogenies to heritable intrusion arising in the course of that ontog¬ eny. A novel epigenetic program can only arise if a mutation of extraor¬ dinarily improbable precision and autonomy occurs in the germ cells themselves (p. 102). But nothing can be won without a price in our complex world of interact¬ ing levels, either in evolution or in human society. In stabilizing the organismic level with such effective devices to suppress cellular and other forms of suborganismic selection, organisms have greatly reduced their flexibility for future evolutionary change of more than a superficial nature. For these mech¬ anisms of development do not suppress only the forms of cell-lineage selec¬ tion that would harm the organism; rather, they impede any effective cellular selection at all, whether beneficial or harmful. These policing devices of the organism therefore close off an avenue once open for substantial change in basic designs, thus restricting maximal potency to the iteration of essentially similar species (as in such famous examples as the cichlids of African lakes, or the Galapagos finches), now representing evolution in its most vigorous con¬ temporary mode. Ou sont les neiges d’antan? “The clear implication is that evolution of cellular differentiation fueled the evolution of controls over vari¬ ants which fail to behave altruistically. The mechanisms which metazoans employ to limit the heritability of variants which fail to contribute to somatic functions are blind to the traits which a variant might express. Potentially beneficial variants are as limited as are potentially detrimental ones” (p. 103, Buss’s italics). This perspective implies a striking limitation upon the strictly Darwinian style of extrapolative and gradualistic selection that the Modern Synthesis promulgated as an adequate explanation for evolution at all scales of time and effect (see quotation from Wilson et al. on p. 583). If Buss’s views are valid, then conventional Neo-Darwinian evolution must work within stric¬ tures of essentially established ontogenies that can surely generate exuberant adaptive variations upon set themes, but may be effectively unable to con¬ struct major innovations that establish the outlines of macroevolution. Once
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THE STRUCTURE OF EVOEUTIONARY THEORY again, we grasp the need for independent macroevolutionary theory—and Buss has supplied an important piece of the general argument with his con¬ cept of a correlation between such major innovations and the origin of new hierarchical levels, a theme that obviously requires the hierarchical model and cannot be encompassed within the strict Darwinism of the Modern Syn¬ thesis. Buss concludes (p. 188, his italics): “Synergisms between the units drove the elaboration of a higher unit and conflicts arising between units were minimized by adaptations limiting further variation. This conclusion has the fascinating and crucial corollary that the major features of evolution were shaped during periods of transition between units of selection.” THE ORGANISM-INDIVIDUAL As virtually the entire history of Darwinian
thought has unfolded under the assumption that organisms act as nearly ex¬ clusive agents of selection (or at least that our interest in evolution centers upon the alterations and fates of organisms), I shall not dwell upon this ca¬ nonical individual here. I want only to reemphasize the unique and decidedly peculiar features of our kind of entity (in contrast to the characteristic prop¬ erties of individuals at other levels): maximal cohesion based on functional integration, including relatively inflexible spatial orientations of subparts (spatiotemporal if we include embryogenesis). This style of integrity enables the organism to be particularly effective in suppressing selection against its interests by potential evolutionary individuals dwelling within and forming its parts. As noted above, the virtual “extinction” of effective cell lineage se¬ lection in complex metazoan phyla occurred as a historical result of the evo¬ lutionary “invention” of the intricate organism—perhaps the only example of an “endangered level” in the entire history of evolution! As another portentous implication of individuality in this mode, organisms become chock full of adaptations as a consequence, under natural selection, of building coherence by functional integration. This local phenomenon at one level of Darwinian individuality has generated an understandable and commanding concern with adaptation, leading to doctrines of exclusivism in extreme cases (all too common, given our psychological preferences for simple and unifying worldviews—a need traditionally met theologically, but sometimes, particularly in our increasingly secular age, scientistically). If, as some strict Darwinians believe, “organized adaptive complexity” repre¬ sents both the primary result of evolution and the cause of all other patterns in the history of life, then we will fail to understand nature for two cardinal reasons: (1) because we have adopted a criterion too strict even for its organismal level of most promising application (see Chapters 10 and 11); and (2) because the criterion of “organized adaptive complexity” does not strongly characterize the nature or definition of individuality at most other levels of the hierarchy. Nature’s hierarchy is not fractal; each level, to express the point metaphori¬ cally, does some things well, and other things poorly or not at all—and the evolutionary pattern of nature features many essential things. In our mother’s house—the Earth—are many mansions. Gene selection is “good” at iterat-
Species as Individuals in the Hierarchical Theory of Selection ing elements—an important input of raw material for generating “organized adaptive complexity” at a higher level. Organisms are good at building com¬ plex adaptations. Species are good at forging temporal trends of geological duration, and their efforts largely regulate the relative diversity among phyla (why so many beetles, and so few pogonophorans). To say (as Dawkins, Wil¬ liams, and other detractors often do) that species selection must be unimpor¬ tant because such a process can’t build organismal complexity reminds me of the cook who didn’t like opera because singing couldn’t boil water. THE DEME-INDIVIDUAL This kind of individual has borne the brunt of
the general argument about higher-level selection ever since Darwin awarded the idea a strictly limited amount of conceptual space in trying to puzzle out the origins of human altruism (see pp. 133-137). The subject has been ex¬ tensively reviewed and controverted (Wynne-Edwards, 1962, vs. Williams, 1966, for an early and generally unacceptable version; Wade, 1978, 1985; D. S. Wilson, 1980, 1983, 1989; Wilson and Sober, 1994; Sober and Wilson, 1998, for reviews). I shall therefore provide only an idiosyncratic sketch here, for the terms and concepts of this discussion permeate the chapter, while my own interest as a paleontologist flows to the still higher levels that have not been extensively studied. In a curious way, the development and acceptance of hierarchy theory has been impeded because the classical treatment of this subject has been focussed so strongly, indeed almost exclusively, on this level—and demes are the hard¬ est of all individuals to validate and justify within the evolutionary hierarchy. All other individuals build better boundaries (to retain their own subparts, or lower-level individuals, and to exclude the subparts of other individuals at their level), and experience less difficulty in remaining sufficiently stable for the requisite time until reproduction. But demes are especially vulnerable to the classic objection (see p. 647) that, lacking strong internal mechanisms for coherence, their individuality may be too fleeting and subject to change by loss or invasion—as in Dawkins’s well-formulated and memorable image of dust storms in the desert or clouds in the sky. Indeed, as I argued previously (p. 648), the classic defense of interdemic selection depends upon the identi¬ fication of plausible conditions that would allow such adventitious groups to remain stable long enough to act as units of selection. The centering of the general argument for higher-level selection upon demes has, by false and un¬ fortunate implication, led to the widespread impression that any kind of supraorganismal selection must face the same difficulties—perhaps with problems growing ever more intense as individuals become more inclusive. But this argument, based on illogical assumptions about linear extrapolation, does not hold because demes (in most circumstances) are uniquely unstable in the evolutionary hierarchy. Species, for example, usually attain as much sta¬ bility and coherence as organisms, though by different mechanisms (see pp. 703-705). Group selection has traditionally been invoked under our organismic bi¬ ases as an explanation for bodily behaviors—with altruism as a paradigm,
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THE STRUCTURE OF EVOLUTIONARY THEORY ever since Darwin .himself (see Chapter 2, pp. 133-136)—that seem, prima facie, difficult to explain as beneficial to organisms, but can easily be con¬ strued as valuable for groups. But we should recognize such restricted invoca¬ tion (only for cases that trouble organismtc traditionalists) as yet another pa¬ rochial limitation, and we should acknowledge a potentially general role for interdemic selection within any species of appropriate population structure. (Under such a criterion of judgment by relative frequency, we must ask a dif¬ ferent, and quite unanswered, fundamental question: how many higher taxa generally maintain population structures that promote interdemic selection; in what environments; and with what correlations to such factors as phylo¬ genetic status, body size, behavioral complexity, etc.) If various arguments for the rarity of extensive evolution within large panmictic populations hold merit, and if Sewall Wright’s shifting balance the¬ ory applies to a high percentage of populations, then interdemic selection may become a major mechanism for evolution within species through time. However, if punctuated equilibrium generally holds (see Chapter 9 for a de¬ fense of this view), then anagenesis within species will be rare in any case (whether by transformation via organismic selection under panmixia, or by shifting balance via interdemic selection in appropriately subdivided popula¬ tions). Or perhaps, as an intermediate position, panmictic transformation is rare, but shifting balance frequent, in species that meet the criteria for appro¬ priate population structure. The high relative frequency of punctuated equi¬ librium would then measure the relative rarity of such population structures, and the few groups that show extensive gradualism within species may gener¬ ally subdivide their populations according to Wrightian criteria. This conjec¬ ture has not been tested, but could be, and with an interesting mixture of paleontological data on the history of species and neontological information on population structures within modern representatives of the same groups. In any case, even if Wright’s criteria don’t hold often enough within the central range of species during the heart of their geological life, Mayr’s peripatric model of speciation suggests that the origin of most species may occur by a process close to interdemic selection, and operating near a blurred bor¬ derline with species selection. If many species spawn large numbers of periph¬ erally isolated demes, but only a few of these demes become species; and if the small class of successful speciators possess traits at the population level that encourage full speciation in interaction with the environment; then species will arise by selection and differential preservation on a just a few “winners” within a set of populations that begin as demes of an ancestral species (as best illustrated by the probable main reason for failure of others to speciate— reincorporation of a peripherally isolated deme into the larger parental popu¬ lation). For all these reasons, I suspect that selection among deme-individuals holds an importance as yet unrealized (and perhaps occurring in modes as yet unconceptualized) within our general picture of evolution. I have, in my ca¬ reer, witnessed three examples of widespread dismissal by ridicule as part of a professional ethos: the rejection of continental drift as physically inconceiv-
Species as Individuals in the Hierarchical Theory of Selection able; the shunning of Goldschmidt’s macroevolutionary ideas as dangerous to the Darwinian consensus; and the dismissal of group selection as addlepated nonsense (see pp. 553-556). Nothing in my intellectual life has made me feel more uncomfortable. I take great pleasure in the comeuppance of the smug ridiculers in all three cases. Plate tectonics has validated continental drift to become a new para¬ digm for geology. Goldschmidt’s particular genetics win no general plaudits, but his views on a conceptual break between micro- and macroevolution now enjoy substantial support. The vindication of group selection has been slower, but now moves on apace (see Sober and Wilson, 1998)—with a vigor¬ ous professional discussion finally occurring, and with general attention now accorded, both in the popular press (Lewin, 1996), and in the commen¬ tary sections of general professional journals (Morrell, 1996). Sic semper tyrannis. THE SPECIES-INDIVIDUAL I propose, as the central proposition of macro¬
evolution, that species play the same role of fundamental individual that or¬ ganisms assume in microevolution. Species represent the basic units in theo¬ ries and mechanisms of macroevolutionary change. In this formulation, the origins and extinctions of species become strictly analogous to the births and deaths of organisms—and just as natural selection works through differential proliferation based on schedules of organismal births and deaths, so too does species selection operate upon the frequencies and timetables of origins and extinctions. The next section of this chapter—entitled “the grand analogy”— shall complete this argument by attempting to cash out this comparison in de¬ tail, with all the intriguing differences that arise when disparate individuals at two such different levels work by the same abstract mechanism. I will therefore confine this preliminary discussion to the three major objec¬ tions that have been raised against the foundational idea that species can act as important evolutionary individuals. These objections treat, in reverse or¬ der, the three words in the key phrase, “important evolutionary individuals.” The first objection holds that species cannot be construed as proper individu¬ als; the second admits that species are individuals, but argues that they can¬ not operate as interactors (as required for units of selection); while the third allows that species may be recognized as both individuals and interactors, but insists that they must remain effectively impotent in both roles. Species as individuals. The classic argument of evolutionary gradu¬ alism denies real existence to species because they can only be defined as arbitrarily delineated segments of a lineage in continual anagenesis. Both Lamarck and Darwin, despite their maximally different views about pro¬ posed evolutionary mechanisms, strongly supported the nominalistic claim that only organisms exist as natural units, and that species must therefore represent abstractions, formally designated only for human convenience. (As many historians have remarked, Darwin chose an odd title for his revolution¬ ary book—for he focusses upon the explanation of substantial change by anagenesis, and says little about speciation by branching of lineages.)
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THE STRUCTURE OF EVOEUTIONARY THEORY I presented the case for treating species as individuals in an earlier sec¬ tion of this chapter (pp. 603-608), noting that punctuated equilibrium greatly aids such a delineation, but then extending and generalizing the argu¬ ment by holding that species can be individuated under any scheme that de¬ picts their origin as an event of branching, rather than anagenetic transformation. Critics of this view, particularly Williams (who does not dispute the truly necessary claim for origin by branching), continue to raise standard ob¬ jections, especially “an absence of a decisive beginning for a species” (Wil¬ liams, 1992, p. 121). But Williams, in advancing this argument, commits the classic error of failure to appreciate proper scales. His claim for fatal fuzzi¬ ness in origins views the question from a generational perspective at the scale of human lifetimes. The great majority of species, however, arise in geological moments (thousands of years, and thus overly long only at the inappropriate scale of our personal lives)—a shorter period of ambiguity (relative to later duration as a clearly separate entity) than we note for most asexual organ¬ isms that reproduce by budding (the proper organismic analog for the origin of a new species by branching)! Most other published objections to species as individuals also express little beyond our psychological difficulty in making a transition to different crite¬ ria at unfamiliar scales. Some critics have argued, for example, that species can’t develop the requisite property of heritability, because no mechanism can be analogized with the well-known Mendelian basis for this phenomenon at the organismic level. But heritability measures the correlation between par¬ ents and offspring based on direct transmission of formative properties—and daughter species surely inherit parental characteristics by this standard route. The required correlation arises by transmission of autapomorphic characters through retained homology—the appropriate mechanism of heritability at this higher scale, and in no way “worse” than Mendelian criteria for the con¬ struction of organisms. Moreover, species heritability can be measured in the same general way, and with the same potential accuracy, as standard organis¬ mic heritability—as Jablonski (1987) has done in our best-recorded case of species selection for the evolution of marine mollusks in normal times and ep¬ isodes of mass extinction. Other critics charge that species are too spatially diffuse, or too lacking in mechanisms of internal coherence, to count as individuals. But, again, these arguments only arise from failure to conceptualize this different scale in an appropriate manner—a mental foible rooted in our parochial allegiance to the particular (and poorly-scaling) criteria of individuality for organisms. Species don’t build a physical skin, but reproductive isolating mechanisms maintain their borders just as sharply. Species don’t evolve immune systems and other forms of “policing” against outside invaders, but the constant ad¬ mixture among their parts via sexual reproduction maintains coherence with more than adequate force. Species as interactors. This more interesting and challenging argu¬
ment has unfolded among supporters of macroevolutionary theory as an “in-
Species as Individuals in the Hierarchical Theory of Selection house” debate. Most discussants, including Brandon, Gould, Jablonski, Lloyd, Stanley, and Vrba, strongly support the concept of species as units of selection; while Damuth and Eldredge grant species a role as replicators, but not as interactors, and therefore not as agents of selection. Grantham (1995) has tried to mediate these positions with a compromise that will, I suspect, satisfy neither side. Critics allow that species may be “fundamental units” of macroevolution in some sense—but, they say, only as the replicators that serve as “atoms” of cladistic phylogeny, and not as interacting units that forge macroevolutionary change by active competition in natural environments. (Eldredge, for exam¬ ple, includes species in the genealogical column of his two-hierarchy scheme —see page 642 for a critique—but not in his economic column of inter¬ actors.) A species, the critics continue, may live in too broad a range of envi¬ ronments, and over too wide a geographic range—often discontinuous to boot—to serve as an interactor, or unit of selection. Moreover, although indi¬ vidual populations of two species may compete sympatrically over a well-de¬ lineated geographic range, entire species rarely maintain sufficient overlap to interact with each other as complete units. To resolve this apparent dilemma, Damuth (1985) proposes that we define a new interactor corresponding most closely to the hierarchical level where species serve as replicators. Using a criterion of direct competition in sympatry, Damuth proposes the term “avatar” for such interactors, defined as sympatric populations in ecological competition, and therefore interpretable as alternatives subject to selection. Grantham’s (1995) “compromise” posi¬ tion maintains allegiance to Damuth’s insistence upon potential interaction in sympatry. Grantham defends species selection, and regards species as poten¬ tial interactors—but he would restrict any particular study of species selec¬ tion to members of clades living in the same broad region. He writes (1995, p. 311): “I suggest that paleontologists focus on geographically constrained portions of monophyletic clades.” I would raise three arguments against this proliferation of terms and cate¬ gories—and for the status of species as adequate interactors. 1. A standard mode of construing competition among organisms has be¬ guiled us into thinking that interaction requires sympatry. As argued in Chap¬ ter 6 (pp. 470-477), Darwin strongly asserted the predominance of biotic over abiotic competition as the only promising path for a defense of progress in evolution. This preference has passed through the Victorian fascination with overt battle as a defining mode of competition, right into our present times, with continuing Tennysonian metaphors about “nature red in tooth and claw” (see Gould, 1992a), and newspaper stories about firms engaged in Darwinian struggles to the death as they vie directly for the allegiance of a limited population of consumers. (As I revised this chapter in the summer of 2000, a new magazine for “business evolving in the information age” made its debut under the name Darwin—also available on line at www.darwinmag.com.)
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THE STRUCTURE OF EVOEUTIONARY THEORY But this focus on the biotic mode has always been indefensible as a claim for exclusivity, or even dominant relative frequency. In Darwin’s own time, Huxley ridiculed this notion as “the gladiatorial theory of existence,” while Kropotkin (1902) and others constructed alternatives based on cooperation in sympatry and the prevalence of abiotic competition in most environments (see Todes, 1988; Gould, 1991b). Darwin himself clearly favored an expan¬ sive concept of interaction with environments in natural selection—as when he insisted, in a famous passage (1859, p. 62), that “a plant on the edge of a desert” struggles for existence against the drought and other features of the physical environment just as surely as “two canine animals in a time of dearth” struggle more overtly for a limited supply of meat. This point becomes important when we try to translate this debate about organisms to a definition of higher-level interactors. Biotic competition does require sympatry for direct and literal struggle, while abiotic competition im¬ poses no such conditions, and must often occur among organisms that never encounter each other, even while living in sympatry. If we use biotic competi¬ tion as our (often unconscious) paradigm for the entire, and far broader, con¬ cept of interaction, then we too easily become unduly committed to the false restriction that interactors must be able to duke it out directly. In upward translation, this bias leads to the idea that species-individuals can’t be inter¬ actors unless they live in the same place, and thus maintain a potential for en¬ gaging in some analog of overt battle. But interaction at the canonical level of organisms doesn’t demand direct contact, or even life in the same place—and no one has denied that organisms operate as quintessential interactors, and units of selection. If abiotic compe¬ tition dominates the history of life—as many distinguished researchers insist (see references in Allmon and Ross, 1990), at least for many groups in many circumstances—then potential for direct contact cannot be invoked as a pri¬ mary criterion for defining interactors. Williams (1992) has strongly asserted the non-necessity of sympatry (and resulting potential for direct “struggle”) in defining higher-level interactors— and he uses the same analogy here advanced for asserting a similar non-neces¬ sity at the organismal level. I presented the full quote before, but repeat the operative line here (1992, p. 25): “One issue is whether the populations that bear the gene pools need be in ecological competition with each other. I believe that this is not required, any more than individuals within a popula¬ tion need interact ecologically to be subject to individual selection.” Later, Williams specifically criticizes Damuth’s definition of avatars on this basis. Speaking of populations not in direct competition, but subject to similar stresses (a common predator in their separate environments in this hypotheti¬ cal case), Williams writes (1992, p. 52): “I am inclined to recognize that clade selection is operating even here, unlike Damuth, who maintains that only sympatric avatars, populations in ecological competition, can be alternatives subject to selection. Allopatric forms may not be ecological competitors, for the inattention of a predator or anything else, but they compete for represen¬ tation in the biota, the ultimate prize in clade selection.”
Species as Individuals in the Hierarchical Theory of Selection 2. Although I recognize that some notion of a common environment must be invoked when we wish to define allopatric species as competing inter¬ actors, I do not view such a requirement as either rarely met or particularly difficult to specify. (I only mean, by the last phrase, “any more difficult to specify than for sympatric interactors.” We cannot know, in fully adequate detail, how individuals at any level react to all nuances of the environment, in all their horrendously complex and nonlinear interactions. Who can say whether two sympatric organisms, given their inevitable differences, perceive the same local change of environment—even such a linear effect as a falling temperature—in the same way? I am only arguing that we face the same dif¬ ficulty for sympatric, as for allopatric, interactors in this respect.) At least two strong arguments support the notion of adequate environmen¬ tal similarity in allopatry: (i) Environments cannot be conceptualized (or even operationalized) as ob¬ jective places or circumstances in a world fully external to the organisms in¬ volved. First of all, environments include all interactions with other organ¬ isms, both conspecific and belonging to different taxa, and not just the climates, substrates, and other more measurable properties of a surrounding physical world. Second, and more important, as Lewontin has emphasized so forcefully (1978, 2000), environments are intrinsically referential, and ac¬ tively constructed by the organisms in question. Environments, in short, are made, not found. Thus, important properties of the environment must be suf¬ ficiently comparable in a set of closely related and partly allopatric species en¬ gaged in a process of species selection. These species share key traits as autapomorphies of their clade—and since these traits help to construct the relevant environment, sufficient similarity becomes, in part, an active con¬ struction of related organisms, not only a happenstance of common exter¬ nalities. (ii) Organisms needn’t occupy the same turf in order to be impacted in sim¬ ilar ways by the kinds of broad environmental changes that seem so impor¬ tant at geological scales. To choose an extreme example, when, 65 million years ago, a large bolide struck the earth in the region now occupied by the Yucatan peninsula, I suspect that Tyrannosaurus rex in the western United States, and its recently discovered sister taxon in Africa, experienced conse¬ quences sufficiently common and negative to influence their extinction (while some small-bodied mammals, living there and elsewhere, survived as a conse¬ quence of organismal or higher-level characters that also do not require sympatry with dinosaurs for meaningful comparison). Again, Williams (1992, p. 25) explains the issue succinctly and at more immediate scales: “Suppose a climatic change causes the brown trout of the upper Rhine to die out but lets the brown trout of the upper Danube survive. Suppose further that the differ¬ ence in fate is attributable to some difference in gene frequency that causes a difference in vulnerability to the change. That is surely clade selection. The ultimate prize for which all clades are in competition is representation in the biota.” 3. In many cases of species selection, the success of one species over an-
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THE STRUCTURE OF EVOLUTIONARY THEORY other cannot be explained by competition between their sympatric popula¬ tions, but depends upon a species-level trait of the species’s full range—in other words, species selection of the whole, not of avatars or sympatric sub¬ sections. I present in Figure 8-5 a hypothetical case developed by Robert N. Brandon (personal communication, 1988, Ohio State meeting). The three species of a clade live on four adjacent volcanic islands. Species 2 can move readily across small oceanic gaps and inhabits all four islands. Spe¬ cies 1 and 3 have limited mobility and live on only one island each. (Species 2 gains no necessary advantage of the moment thereby.) The population of Spe¬ cies 1 on Island A, and of Species 3 on Island C, may each exceed the total number of organisms in Species 2 on all four islands. In fact, on any individ¬ ual island, either Species 1 or Species 3 may always fare better than Species 2. Each island maintains an active central volcano; when the volcano erupts, all life on the island dies, but the adjacent islands remain unaffected. One fine day, the volcanoes of Islands A and C erupt. As a consequence, Species 1 and
C
D
8-5. A hypothetical example of species selection based on traits that belong to entire species—in this case the full geographic range—and not to avatars or subpopulations thereof. See text for details of this verbal case developed by R. N. Brandon. Species 2 survives by virtue of its ability to spread among is¬ lands, even though any other species dominates over species 2 on any island of joint occurrence.
Species as Individuals in the Hierarchical Theory of Selection Species 3 become extinct, but Species 2 survives thanks to populations on Is¬ lands B and D—that is, only by virtue of populations allopatric with Species 1 and 3. Clearly, Species 2 has survived as a result of greater geographic range, caused by whatever organismal, deme, or species traits permitted the coloni¬ zation of all islands. Geographic range may be either an emergent or aggre¬ gate trait of successful Species 2; but, in any case, this trait exists at the species level and confers an irreducible fitness based on superior range (obvi¬ ously a property of the species, and not of any individual organism, deme, or avatar). This hypothetical case presents a potential and plausible example of species selection based on a trait of the entire species and its complete range— and explicitly not on any sympatric avatar, or any other subsection of the full entity. Species selection as potent. Two separate arguments, one empiri¬
cal and the other theoretical, have been raised against the efficacy of species selection. 7 he first, which I regard as unfair, claims that a paucity of currently recorded empirical examples must indicate the rarity of the phenomenon. I would respond, first of all, that a few excellent (and elegant) cases have been well documented, so this process cannot rank as a distant plausibility wait¬ ing for an improbable verification, as some critics have charged. Jablonski (1987), for example, performed a pioneering study on species selection in Cretaceous mollusks during the long background interval preceding the mass extinction at the period’s end. He found that species with planktotrophic lar¬ vae (defined as floating and feeding, and therefore remaining aloft for sub¬ stantial time) generally have larger geographic ranges and longer geological durations than species with nonplanktotrophic larvae (defined as either never planktonic, or floating without feeding, and therefore aloft for only a short period). Jablonski supplies good inferential evidence for the two key claims that a hypothesis of species selection requires. First, he presents a strong case that geographic range not only correlates with longevity, but helps to cause the ex¬ tended duration. Species tend to reach their maximal range soon after their origin, and to maintain this breadth thereafter as a potent hedge against ex¬ tinction. Second, he calculated a strong heritability for geographic range by assessing the parent-offspring regression for this character. Geographic range surely constitutes a character of the species, not (obviously) of individual or¬ ganisms. This trait confers an emergent fitness on species that gain increasing longevity thereby. All necessary attributes for an interpretation based on spe¬ cies selection have therefore been identified. (The case also includes interesting complexities. As mentioned previously for similar examples in Tertiary mollusks, nonplanktotrophic species gener¬ ally experience shorter longevity and maintain smaller populations in their more restricted ranges; but they also speciate more frequently, a presumed consequence of greater ease in forming isolated populations—for their evo¬ lution of larvae without extensive periods of flotation restricts gene flow among demes. Thus, the greater longevity of planktotrophic species need not
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THE STRUCTURE OF EVOEUTIONARY THEORY imply increasing dominance of such species within the clade, for this positive trait can be counterbalanced by the higher speciation rates of shorter-lived nonplanktotrophic species. Moreover, Jablonski also showed that selective forces can change radically during episodes of mass extinction. In the great dying at the end of the Cretaceous period, geographic range of species shows no correlation with survivorship through the event. But, interestingly, geo¬ graphic range of entire molluscan clades (though not of their component spe¬ cies) does correlate positively with persistence through the mass extinction— a potential example of clade selection.) I freely admit that well-documented cases of species selection do not per¬ meate the literature. But I regard this infrequency as a great opportunity, rather than a restrictive limitation or an indication that the phenomenon scarcely exists. We have barely begun to acknowledge (much less to define or operationalize) this process, and we have still not entirely agreed upon crite¬ ria for recognition. We face the tradition of a full century spent not consider¬ ing causes at this level (indeed, actively denying the existence of such levels at all). We are just learning how to look—or, to state the issue more incisively, we have just begun to recognize that we should be looking at all! We face all the promise of a rich but unploughed field—and (to commit two literary bar¬ barisms of mixed metaphors and parodied quotations at the same time), we should summon up the courage of John Paul Jones and recognize that we have not yet begun to think. I regard the second, or theoretical, objection as even more unfair in its purely traditionalist grounding in the parochialism of viewing organisms as exclusive agents of evolutionary interest or importance—more an aesthetic defense about comfort or preference than an intellectual argument about mechanisms. Several Darwinian strict constructionists, Richard Dawkins and Daniel Dennett in particular, hold that almost everything of interest in evolu¬ tionary biology either inheres in, or flows from, natural selection’s power to craft the intricate and excellent design of organisms—“organized adaptive complexity,” in Dawkins’s favorite phrase. “Biology is engineering,” Dennett tells us again and again in his narrowly focussed book (Dennett, 1995). I do not deny either the wonder, or the powerful importance, of organized adaptive complexity. I recognize that we know no mechanism for the origin of such organismal features other than conventional natural selection at the organismic level—for the sheer intricacy and elaboration of good biome¬ chanical design surely preclude either random production, or incidental ori¬ gin as a side consequence of active processes at other levels. But I decry the parochialism of basking so strongly in the wonder of organismic complexity that nothing else in evolution seems to matter. Yet many Darwinian adaptationists adopt this narrow and celebratory stance in holding, for example, that neutrality may reign at the nucleotide level, but still be “insignificant” for evolution because such changes impose no immediate effects upon organ¬ ismal phenotypes; or that species selection can regulate longstanding and ex¬ tensive trends in single characters, but still maintains no “importance” in
Species as Individuals in the Hierarchical Theory of Selection evolution because such a process can’t construct an intricate organismal phe¬ notype of numerous, developmentally correlated traits. Dawkins (1982, pp. 106-108), for example, damns species selection with faint praise in these terms: I shall argue that a belief in the power of species selection to shape simple major trends is not the same as a belief in its power to put together com¬ plex adaptations such as eyes and brains . . . The species selectionist may retreat and invoke ordinary low level natural selection to weed out illcoadapted combinations of change, so that speciation events only serve up already tried and proved combinations to the sieve of species selec¬ tion. But this “species selectionist” . . . has conceded that all the interest¬ ing evolutionary change results from inter-allele selection and not from interspecies selection, albeit it may be concentrated in brief bursts punc¬ tuating stasis . . . The theory of species selection ... is a stimulating idea which may well explain some single dimensions of quantitative change in macroevolution. I would be very surprised if it could be used to ex¬ plain the sort of complex multi-dimensional adaptation that I find so in¬ teresting. This statement commits the classic intentional fallacy of the prosecutor: attributing beliefs not held to adversaries, and then castigating them for apos¬ tasy (or praising them for good sense in recantation)—as illustrated by the paradigm for an opening thrust in a line of inquiry: “when did you stop beat¬ ing your wife?” Dawkins finds the adaptive complexity of organisms uniquely interesting. I also regard the subject as fascinating, and I would never attribute this quintessential property of organisms to selection at some other level. I fully acknowledge, as do all species selectionists, that the adap¬ tive complexity of organisms arises primarily by causal processes operating at the organismic level. But this pluralistic principle applies equally well to other levels. If adaptive complexity marks “what organisms do,” and must therefore be explained at the organismic level—then “what species do” implies a consideration of cau¬ sation at the species level. Species “do” two primary things in macroevolu¬ tion: they carry trends within clades across long geological stretches of time, and they stand as basic units (geological “atoms” if you will) for counting the waxing and waning of differential diversity through time (why does our cur¬ rent biota feature 500,000 named species of beetles, but fewer than 50 of priapulids?). As a paleontologist, I regard these two phenomena as surpass¬ ingly important, while I remain happy to grant Dawkins’s commanding inter¬ est in the adaptive complexity of organisms. But just as I try not to impose my causes (for other scales and levels) upon his material a priori, I ask him to ac¬ knowledge the importance of my favored themes within a comprehensive evolutionary theory (even if they do not engage his personal concern), and therefore to recognize the efficacy of different appropriate causes at this paleontological level. In short, Dawkins and others commit a classic psycho-
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THE STRUCTURE OF EVOEUTIONARY THEORY logical fallacy in denying status to species selection by confusing personal in¬ terest with general importance. Only one line of defense remains open to those who still wish to deny the importance of species-level processes aftar correcting this psychological fal¬ lacy, and admitting that trends and changing patterns in diversity rank as vital subjects in a complete evolutionary theory, and also represent “what species do.” Such a Darwinian stalwart must argue that all (or nearly all) phe¬ nomena at the species level find their causes in upward translation from ordi¬ nary natural selection on organisms. Thus, if current biotas feature half a mil¬ lion species of beetles, this plethora can only imply that beetle organisms maintain a particularly favorable adaptive design. And if geological trends privilege increasing body size, larger brains, more complex ammonite su¬ tures, more symmetrical crinoid cups, fewer horse toes, and a thousand other documented patterns, these features must triumph by their adaptive value to organisms. I shall make no further arguments against such a narrow perspec¬ tive here (to save my rebuttal for Chapter 9, pp. 886-893), and will only quote a great American character, Sportin’ Life in Porgy and Bess, to remind us that received wisdom does not always prevail: The things that you’re liable to read in the Bible It ain’t necessarily so. THE CLADE-INDIVIDUAL
Although a logical space must exist in our struc¬
ture of explanation for this highest level of the evolutionary hierarchy, I am not sure that clade selection plays a major role in evolution. Most clades con¬ tain so few parts (species) that their waxing and waning must often occur by processes that either operate as random inputs to the clade level, or result from selection among subparts (species selection, or lower-level selection), and therefore appear as drives at the clade level (and not as selection among entire clades treated as individuals). Secondly, while I have advocated a plu¬ rality of mechanisms for coherence of individuals at various levels in the hier¬ archy, I do have trouble in conceptualizing an adequate “glue” for clades, es¬ pecially since their parts (species) may live in such complete independence, and in such different ecologies, on distant continents. Finally, clades maintain the peculiar property (perhaps only an odd “allometric” consequence of nec¬ essary structure at this highest level, and not any compromise in efficacy) of necessarily originating as a single subpart—the founding species, and gaming definition (as a full level) only retrospectively, after adding new parts (more species) sequentially. How then, given all these difficulties, could clades compete, qua clades as discrete and integral evolutionary items, even under the broad definition (see p. 706) that does not require direct contact or even life in sympatry? Is a clade, uniquely among evolutionary individuals of the hierarchy, more a “holding firm” for subparts than a coherent entity frequently subject to selec¬ tion at its own level? One route to claiming a potential importance for clade selection remains
Species as Individuals in the Hierarchical Theory of Selection open, but I am not confident that the argument can prevail (though Williams, 1992, despite his past as an ardent gene selectionist, has become a strong advocate of this view). What do we mean, for example, when we say that di¬ nosaurs died and mammals survived, or that brachiopods dwindled to a rem¬ nant while clams continually expanded? Do these descriptive statements im¬ ply clade selection? A general argument would have to be framed in the following way: any distinct clade maintains defining autapomorphic charac¬ ters expressed by all subparts (species). If a clade survives, while another living in roughly comparable habitats, dies—and if survival can be tied to autapomorphic characters held by the persisting clade (and absent in the ex¬ tinct clade)—may we not speak of clade selection based on a range of vari¬ ability that includes the key characters in the surviving case, but precludes their expression in the extinct clade? For example, if mammals survived in part by virtue of small body sizes, and dinosaurs died for a set of consequences related to invariably (and sub¬ stantially) larger body size, couldn’t we say that mammals, as a clade, pos¬ sessed genetic determinants (shared by homology in all subparts, with homol¬ ogy as the “glue” of cladal coherence) that all dinosaurs lacked as a result of their own evolved cladal distinctions? If such a scenario can count as clade se¬ lection (rather than just clade sorting, as an obviously valid description), then selection at this highest level becomes common in nature—for many clades yield in geological time to phylogenetically distant clades that share sufficient similarity in habitat and function to rank as genuine “replacements.” I am not comfortable with this general argument, for no one has yet ar¬ ticulated firm and operational criteria for distinguishing true clade selection (based on irreducible fitness conferred by a clade-level property) from de¬ scriptive clade sorting (or differential survival as an effect of lower level prop¬ erties belonging to species or organisms, but translating upwards to success or failure of a clade as a geologically persistent entity). Some examples proba¬ bly do represent genuine clade selection—as in Jablonski’s (1987) case of clade survival (through mass extinction), correlated with geographic range of the entire clade, but not with ranges of component species. Most other exam¬ ples, however, may not invoke any genuine clade-level character (either aggre¬ gate or emergent), but only represent the death of each species, item by item (part by part in cladal terms, for this highest-level individual also maintains the peculiar property of relative immunity, especially in clades with large numbers of widely distributed subparts, to the fate of individual subparts). We may frame our best descriptions for such cases in terms of clade sorting, but do they also qualify as cases of clade selection? At a minimum, however, such arguments illustrate a need for macroevo¬ lutionary accounts at all levels, even when causality arises from lower levels and merely affects the fate of higher-level individuals. Thus, the explicit study of macroevolution would remain vital even if traditionalists had been correct in ascribing all causality to organismic selection. But we needn’t take refuge in this “minimalist” defense. Causal processes—and not only selection, as I shall demonstrate in the next section—do operate at substantial (often con-
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THE STRUCTURE OF EVOLUTIONARY THEORY trolling) relative frequency at all levels (with the possible exception of some dubiety about the importance of clade selection as expressed above, and some recognition that organismic selection has effectively squashed most cell-lin¬ eage selection in many phyla of multicellular organisms). I therefore end this section with two statements from George Williams (1992), who once rejected higher-level selection with such verve and skill (1966), but who (while properly reasserting his excellent arguments against the old form of so-called “naive group selection,” or mterdemic selection in the Wynne-Edwards modality) now strongly defends both the importance of selection at the species level (“clade selection” of lowest rank in his termi¬ nology, because he rejects species as units), and our lamentable failure to consider this vital process in our previous theorizing. Echoing my method¬ ological point that a rarity of recorded examples does not imply any actual weakness in nature, Williams writes (1992, p. 35): “Only the barest begin¬ nings have been made in searching the fossil record for evidence of clade se¬ lection. The record can be searched for statistically significant trends in diver¬ sity and abundance of particular clades ... It can also be searched for consistent selection of certain characters.” In an expansive and forceful plea for pluralism—representing the finest form of support that a paleontologist could obtain from colleagues engaged in the study of microevolution—Williams (1992, p. 31) then states that allelic change in populations cannot account for evolution because gene-pools func¬ tion in nature through their entrapment within higher-level individuals oper¬ ating and interacting as coherent and distinct entities in macroevolution. The natural selection of alternative alleles, acting largely independently at each locus, is the only force tending to maintain or improve adapta¬ tions shown by the ephemeral organisms formed by the ephemeral geno¬ types. If one could look back to the evolution of our own or any other sexually reproducing species, back to well before the Cambrian, no other fitness enhancing process of any importance would be found. Having taken that position, I must take another. The microevolutionary process that adequately describes evolution in a population is an utterly inade¬ quate account of the evolution of the earth’s biota. It is inadequate be¬ cause the evolution of the biota is more than the mutational origin and subsequent survival or extinction of genes in gene pools. Biotic evolution is also the cladogenetic origin and subsequent survival and extinction of gene pools in the biota.
The Grand Analogy: A Speciational Basis for Macroevolution PRESENTATION OF THE CHART FOR MACROEVOLUTION ARY DISTINCTIVENESS When Niles Eldredge and I first formulated the theory of punctuated equilib¬ rium in the early 1970’s (Eldredge, 1971; Gould and Eldredge, 1971; Eldredge
Species as Individuals in the Hierarchical Theory of Selection
and Gould, 1972; Gould and Eldredge, 1977), we had only the germ of an insight that its tenets could lend support to a generalized theory of macro¬ evolution, then entirely undeveloped. We did, however, dimly grasp the key notion that punctuated equilibrium might help to grant species a sufficient stability and coherence for status as what we would now call an evolutionary individual, or unit of selection. We developed this insight by groping towards an analogy that, when generalized and fully fleshed out (with apologies for another parochial organismic metaphor of common language!), sets a foun¬ dation for macroevolutionary theory. We dimly recognized, in short, that if species act as stable units of geological scales, then evolutionary trends—the fundamental phenomenon of macroevolution—could be conceptualized as results of a “higher order” selection upon a pool of speciational events that might occur at random with respect to the direction of a trend. In such a case, the role of species in a trend would become directly comparable with the clas¬ sical status of organisms as units of change within a population under natural selection. We wrote (1972, p. 112): A reconciliation of allopatric speciation with long-term trends can be formulated . . . We envision multiple . . . invasions, on a stochastic basis, of new environments by peripheral isolates. There is nothing inherently directional about these invasions. However, a subset of these new envi¬ ronments might. . . lead to new and improved efficiency . . . The overall effect would then be one of net, apparently directional change: but, as with the case of selection upon mutations, the initial variations [species] would be stochastic with respect to the change [trend]. Several paleontologists groped towards a generalization during the next few years, but Stanley (1975, 1979) made the greatest headway in appreciat¬ ing the full generality of such an analogistic procedure for macroevolutionary theory: “In this higher-level process species become analogous to individuals, and speciation replaces reproduction. The random aspects of speciation take the place of mutation. Whereas, natural selection operates upon individu¬ als within populations, a process that can be termed species selection oper¬ ates upon species within higher taxa, determining statistical trends” (Stanley, 1975, p. 648). Stanley preceded this statement with a claim that I regard as fully justified and prescient, but that became a lightning rod for unfair criticism: “Macro¬ evolution is decoupled from microevolution, and we must envision the pro¬ cess governing its course as being analogous to natural selection but operat¬ ing at a higher level of organization” (1975, p. 648). Largely on the basis of this claim about “decoupling,” Stanley, Eldredge and I, and others, were of¬ ten accused of trying to scuttle Darwinism, and to invent an entirely new (and fatuously speculative) causal apparatus for evolutionary change (meaning, and explicitly so stated in this reductionistic critique, a new genetics). We made no such claim, and the words quoted above speak for themselves. We were trying to explore the different workings of selection on individuals at levels of the evolutionary hierarchy higher than the conventional Darwin¬ ian focus upon organisms. Not only do I continue to regard this procedure as
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THE STRUCTURE OF EVOLUTIONARY THEORY fruitful and fully justified, but I would also defend such an effort as the basis for an independent macroevolutionary theory that can harmoniously expand our conventional and exclusive focus on organisms to yield a more satisfac¬ tory general account of life’s workings and history. I also continue to regard the individuality of species as the central proposi¬ tion of such an expanded theory. If organisms are the traditional units of se¬ lection in classical Darwinian microevolution within populations, then spe¬ cies operate in the same manner as basic units of macroevolutionary change. This perspective establishes an irreducible hierarchical structure in nature, precluding the smooth upward extrapolation of microevolutionary change within populations to explain evolution at all scales, particularly phenotypic trends and patterns of diversity displayed in geological time—the proposition that true devotees of microevolutionary exclusivism rightly feared. If species, as stable units and genuine evolutionary individuals, interpose themselves be¬ tween populational anagenesis and trends within clades, then the lower-level process cannot smoothly encompass the higher-level phenomenon. For this fundamental (and excellent) reason—and not because any “new” genetics or anti-Darwinian forces reign in a threatening world of macroevolution—Stan¬ ley introduced his key notion of “decoupling.” The levels become decoupled because macroevolution must employ species as “atoms,” or stable and basic units of change. Decoupling then becomes in¬ tensified because higher levels exhibit allometric properties that distinguish their phenomenology from the workings of lower levels. Thus, macroevolu¬ tion with species as individuals must differ, in deep and interesting ways, from microevolution with organisms as individuals. These differences, and not any fatuous claims about “new genetics,” express the uniqueness of macroevolution, and the validity of our argument for decoupling. An extensive analogy—“the grand analogy,” if you will (see Gould and Eldredge, 1977, p. 142)—between organismal microevolution and speciational macroevolution provides a good tool for assessing the differences im¬ posed by scaling among the levels. Stanley (1975, p. 649) and Gould and Eldredge (1977, pp.
142-145) proposed some partial and preliminary
schemes, and several others have added components along the way (Stanley, 1979; Vrba, 1980; Grantham, 1995, for example). I present this grand anal¬ ogy below, largely in the form of a chart contrasting the key features of or¬ ganic structure and evolution in their organismal and speciational manifesta¬ tions. For each major category, I list the most important differences between the levels. A fuller explication of all items on the chart follows.
THE PARTICULARS OF MACROEVOLUTION ARY EXPLANATION
The structural basis The first category of structural differences seems straightforward enough. In order to construct the analogy, we ratchet the focal level of individuality up from the organism to the species, thus redefining both lower components and higher contexts in the structural triad of part-individual-collectivity (see page
Table 8-1. The Grand Analogy Feature I
The Triad of Structure
1. 2. 3. 2a.
Individual Part Collectivity Usual result of proliferation of one part to crowd out others
II
The Criteria of Individuality
1.
Production of new individuals Elimination of individuals Sources of cohesion Stability of individual
2. 3. i)
Boundaries against invasion in) “Glue” of subparts ii)
4.
Inheritance
5.
Source of new variation in newborn individuals
Organismal Level
Species Level
Organism Gene, cell Deme, species Cancer
Species Organism, deme Clade Immediately adaptive anagenesis
Birth
Speciation
Death
Extinction
Physiological homeostasis in ontogeny Skin to delineate; immune system to police Functional integration & division of labor
Sources of stasis in punctuated equilibrium Reproductive isolating mechanisms Social structure & behavioral interaction among parts (organisms); re¬ combination in sexual reproduction to mix parts in their replication “Asexual” by budding from one individual
Asexual by budding from one individual, or sex¬ ual by mixture of two individuals Mutation
4a. Spread of new variation to other individuals in the collectivity
Recombination in sexual reproduction
5a. Frequency of new variation in replicated individuals
Very rare for any single trait
Geographic (or some other form of) isolation (a precondition); drift & selection (mecha¬ nisms), causing differ¬ ences that break repro¬ ductive integrity Generally absent except for hybridization between species in some clades Inherent in birth process and always present
III Modes of Change in the Collectivity A. Drives, or Directional Variation Within or Between Individuals 1. Fieritable ontogenetic Lamarckism—powerful if Anagenesis (gradualism change within the it occurred, but prewithin species); rare by individual ^ontogenetic eluded by nature of hepunctuated equilibrium drive redity
Table 8-1 (continued)
Feature 2.
Biased production of new individuals = reproductive drive 2a. Frequency of biased production
Organismal Level
Species Level
Mutation pressure
Directional speciation
Very low (if harmful to organism) because organismal selection effectively suppresses lower levels
Potentially common for two reasons: 1) species processes don’t strongly suppress lower-level se¬ lection; 2) new individ¬ uals must originate with change from par¬ ent
B. 1.
Selection, or Differential Proliferation Due to Traits of Interactors Species selection Natural (organismal) Name of process selection Differential speciation 2. Basis in birth Differential birth Differential death Differential extinction 3. Basis in death 2a. Reason for nonInherent in nature of No necessary reason; directionality of variation mutation as unrelated benefits of organism & as precondition of to needs of organism species frequently coin¬ selection’s power cide. No relation if im¬ mediate adaptive con¬ texts of new species uncorrelated with direc¬ tion of trend. Testable as Wright’s Rule 2b. Distinctive feature of birth Usually internal to organ¬ Usually irreducible as bias ism; need not lead to based on traits of pop¬ adaptation to environ¬ ulations, not organisms ment 3a. Distinctive feature of death Usually yields adaptation Often reducible as simple bias to local environment summation of organism deaths C. Drift, or Random Differential Proliferation 1. Within the collectivity Genetic drift 2. In founding of new Founder effect collectivities la. Frequency Rare except in special cir¬ cumstances of small populations, or neutral¬ ity of many genic sites 2a. Frequency Common; depends on N of founding population
Species drift Founder drift Common because most clades have low N; intensified by reduction of N in mass extinction Very common for two reasons: 1) necessary (and often large) differ¬ ence from ancestor at each founding; 2) greatly different poten¬ tials in allopatric re¬ gions
Table 8-1 (continued)
Feature
Organismal Level
Species Level
IV External and Internal Environments A. Competition and the External Environment 1. In direct contact Most often biotic
2.
Not in direct contact; often allopatric
la. Main feature
2a. Main feature
B. 1.
2.
More often abiotic
Major source of occa¬ sional biomechanical or general progress Adaptation to local cir¬ cumstances; no general vector
Constraint and the Internal Environment Limits on runaway change Lamarckian inheritance by directional evolution of doesn’t occur parts Structural brakes upon Design limits of Bauplan change
3.
Variational brakes
4.
Developmental brakes
5.
Positive channeling by structure
6.
Positive channeling by variation
7.
Size of exaptive pool
Rarity of new mutation allayed by recombina¬ tion in sexuals; serious in asexuals (allayed by short generations in many unicells) Von Baer’s laws of com¬ plex ontogenesis Heterochrony and pre¬ ferred ontogenetic ex¬ tensions Not important, given rar¬ ity of directional varia¬ tion High in such crucial cir¬ cumstances as genetic redundancy usable in evolution of complexity
More likely to produce an effect by differential elimination More likely to produce an effect by differential birth Often reducible to organismal level Usually irreducible
Punctuated equilibrium suppresses anagenesis by stasis Positive correlation of frequency of speciation and extinction appar¬ ently unbreakable Sufficient change per new individual, but low N of species in clades
Hold of homology Differential ease & permissibility of Bauplan modifications Frequent correlation of directional speciation with differential prolif¬ eration Generally high because lower levels not sup¬ pressed and frequently correlative
720
THE STRUCTURE OF EVOEUTIONARY THEORY 673). But this basic ratcheting already reveals some pivotal differences be¬ tween the evolution of organism-individuals and species-individuals. In Table 8-1, line I2a, for example, notes the profoundly different outcome that usu¬ ally ensues when particular parts of the individual proliferate differentially and crowd out other parts. Such a process*usually spells disaster for a com¬ plex multicellular organism—and we call the result cancer—because parts lack independent viability (and therefore harm both themselves and their col¬ lectivity, the organism, by unchecked proliferation), and because organisms build coherence (an important criterion of individuality) by functional inte¬ gration and division of labor among parts. But species achieve equal coher¬ ence by other routes. The parts of a species—that is, its component organ¬ isms—do have independent viability; moreover, their interests in proliferation often coincide with the health of the enclosing species. Thus, in a species-indi¬ vidual, differential proliferation of some parts at the expense of other parts does not lead to death of the full entity, but usually to adaptation by anagenesis.
Criteria for individuality Moving to the second category of criteria for individuality (see pp. 602-613 of this chapter), we may regard the species-level analogs of organismal birth and death (lines III—2)—speciation and extinction—as both evident and well recognized. But the different causes of cohesion (line 113) are both fascinating and portentous throughout the chart. I only remind readers that the mecha¬ nisms used by species, while not clamping down so hard on lower levels, and therefore providing substantial “play” for interaction between organismal and species selection, provide species with as much coherence and stability as the “standard” devices of morphological boundaries, internal policing and functional integration among parts, do for organisms. Important differences arise in the mode of production for novel variation in newborn individuals. Mutation supplies this attribute at the organismal level. (Following conventional usage, I consider recombination in sexual or¬ ganisms as a device for spreading variation among individuals, although I recognize, of course, that novel combinations also arise thereby. In asexual organisms, a better analog for species in any case, mutation alone supplies new variation.) Speciation itself is not the proper analog of mutation at the species level (an error previously made both by me, in Gould and Eldredge, 1977, and by Stanley, 1975). Speciation, the production of a new speciesindividual by budding, is the analog of organismal birth, particularly the birth of asexual organisms. We made this error by inadequately interpreting one of the most interesting differences between organisms and species as evo¬ lutionary individuals. The birth of a new organism, particularly in asexuals, may or may not engender any substantial difference from parental form or genetics. But the birth of a new species necessarily includes the generation of enough difference from ancestors to preclude reproductive amalgation be¬ tween the parts (organisms) of the two species. We therefore mistook a forced correlate of birth at the species level (change at speciation) with the process of
Species as Individuals in the Hierarchical Theory of Selection birth itself (speciation)—and equated the correlate at one level with the phe¬ nomenon at the other. The proper analog of mutation, a source of variation for new individuals, is the change that insures reproductive isolation between species (with geographic isolation as a usual precondition, and drift and se¬ lection as mechanisms)—see line 115. This difference underlies the two important disparities listed as lines II4a and II5a—one favoring the evolutionary capacity of organisms, the other of species. Sexual organisms can spread favorable variation to other individuals in the collectivity by recombination. But the favorable features of new species remain stuck in the species and its lineal descendants, and cannot be spread to other species in the clade—except in the infrequent circumstance of hybrid¬ ization among species in a clade of multicellular forms. (By contrast, lateral transfer seems to be common in the evolution of prokaryotic lineages.) This preclusion of lateral spread puts a strong damper upon evolution within clades. The same limitation, of course, affects asexual vs. sexual organisms— and represents a standard argument for the great advantage of sex, and its evolutionary prevalence, in complex metazoans (Williams, 1975; Maynard Smith, 1978). Species do gain advantages, on the other hand, in the necessary association of birth with change (sometimes small in extent because reproductive isola¬ tion can develop with minimal genetic change, but usually quite substantial). This input helps to offset the disadvantages of small population sizes (species in clades) for species selection. The asexual budding of a new species always yields novelty; the asexual budding of a new organism usually yields clonal identity, and only produces novelty if mutation intervenes.
Contrasting modalities of change: the basic categories The greatest interest in this analogy lies in the third category of contrasting modalities. Since individuals vary and collectivities evolve (by cumulative changes in their contained individuals) in the standard formulation, I shall first define the three major styles of change within collectivities (populations for the organismal level, and clades for the species level). Drive. This term has often been used for particular cases—meiotic drive, or molecular drive, for example—but deserves to be formalized and general¬ ized. A driving process transforms a collectivity by directionally changing its contained individuals from within. Drives should be construed as opposite or at least orthogonal to selection. Drives produce change by directional trans¬ formation of relevant individuals, not by differential proliferation of some kinds of individuals over others. Thus, in pure cases of drive, change occurs without any differential proliferation. In the paradigm cases of drive, either an individual alters in the course of ontogeny and passes these modifications to offspring, while all individuals produce the same number of offspring with the same reproductive capacities (so no selection can take place); or an indi¬ vidual produces offspring endowed with directional differences from its own constitution—but again, no differential reproduction occurs, and no selection can take place. (As one complexity—an ineluctable consequence of the hier-
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THE STRUCTURE OF EVOEUTIONARY THEORY archical perspective, but a blessing in richness rather than a nuisance in con¬ fusion—drives at one level can result from selection at a lower level. In the obvious case, anagenesis within a species—a drive at the species level—tradi¬ tionally arises from selection among organisms within the species.) Sorting (selection and drift).
This descriptive term generalizes
our usual notion of evolutionary change in a collectivity by differential prolif¬ eration of some kinds of individuals vs. others. Sorting, as previously defined (p. 659), is a causally neutral and purely descriptive term for any evolution by differential proliferation, whatever the mechanism involved (see original for¬ mulation in Vrba and Gould, 1986). Of the two major modes of sorting, se¬
lection, based on causal interaction of traits with environments, ranks as the canonical style of evolution, the essence of Darwin’s insight, and the founda¬ tion of modern theorizing. But sorting can also proceed randomly, a process termed drift. In the hierarchical model, both selection and drift can occur at all levels, under appropriate conditions. I discussed previously, for example, how selectively-based sorting of species can occur either by upward causation from selection at the organismic level (“the effect hypothesis” of Vrba, 1980, also called “effect macroevolution”), or by selection based on irreducible fitness of species-level traits in their interaction with environments (true spe¬ cies selection)—see pp. 652-670.
Ontogenetic drive: the analogy of Lamarckism and anagenesis The two categories of drive present some of the most consequential and counterintuitive pairings in the entire table (at least they stimulated my own thoughts substantially). In a first category, line IIIA, we must acknowledge as an instance of “drive” any consistently directional change that occurs during the ontogeny of an individual, and then passes by inheritance to offspring. We do not usually include such a process in our standard account of evolu¬ tion for an interesting reason based on the history of evolutionary thought and the nature of Mendelian genetics: We generally focus our causal ac¬ counts exclusively on organisms in the Darwinian tradition; at the organismic level, a drive of this character would validate the most anathematized and fallacious of alternatives to Darwinism—namely Lamarckism, with “soft” inheritance of acquired characters (see Chapter 3 on Weismann’s use of hier¬ archical thinking to counteract Lamarckism). Thus, ontogenetic drives based on phenotypic changes that are generated by organic activity and then passed to offspring, probably don’t exist at the organismal level due to the nature of DNA and the mechanics of heredity. The defeat of Lamarckism—ontogenetic drive in this context—marks one of the great episodes in the history of evolu¬ tionary thought. If evolution did proceed in the Lamarckian mode, the geo¬ logical history of life would assume an entirely different appearance, primar¬ ily by enormously accelerated rates of change, and suppleness of adaptive modification. I doubt, for example, that we would find any stable higher-level entities like species in a Lamarckian world. (Human cultural change com¬ pares so poorly with Darwinian evolution primarily because our customs and technologies do evolve in this vastly more rapid and flexible Lamarckian
Species as Individuals in the Hierarchical Theory of Selection mode. Whatever we invent in one generation, we pass directly to the next by emulation and instruction.) The proper species-level analog for ontogenetic drive, or Lamarckian evo¬ lution, sounds a bit bizarre at first—but probably only for the irrelevant psy¬ chological reason that we have so firmly rejected the orgamsmic example, while promoting the species-level version as a standard mode of change. With species as individuals and organisms as parts, the gradual transformation (without branching) of an entire species by organismal selection—the stan¬ dard, canonical description of “evolution” itself—becomes the legitimate an¬ alog, at the species level, of heritable ontogenetic alteration, or Lamarckian change, at the organismal level! If, as tradition used to hold, such ontogenetic drive dominates macroevolution, then we must record this striking difference in pattern between levels. I would argue, however (and under my admittedly partisan commitment to punctuated equilibrium), that this standard impression is fictional, and that ontogenetic drive occurs only rarely at the species level. Differences in frequency will, of course, persist—for mechanisms of inheritance preclude ontogenetic drive in theory at the organismic level, while the analogous pro¬ cess remains possible in principle, though rare in fact, given the nature of populations and their modes of change, at the species level. Thus, small im¬ portance remains a common theme at both levels. Most species originate in a geological moment, and persist in stasis thereafter (with, at most, mild fluc¬ tuation about an unvarying mean, but no directional change, as the concept of drive requires—see Chapter 9). I would also venture an analogy to the organismal level in support of inher¬ ent reasonableness for the rarity of anagenesis (ontogenetic drive) at the spe¬ cies level. As argued above, the Lamarckian mode works with extraordi¬ nary rapidity and efficiency: if organisms changed in this way, we could not fail to notice, because evolution would then operate so differently. I would suggest that we approach macroevolution at the species level in the same way. If species changed gradually most of the time, the pageant of life’s his¬ tory, as shown by the fossil record, would present an entirely different ap¬ pearance. The most extensive transformations would occur in a few million years at most. (Many hypothetical calculations have been made to illustrate this point—for example, that a small, four-footed, terrestrial mammal can evolve into the largest whale in a fraction of Tertiary time, so long as a sin¬ gle population in transformation maintains the smallest effectively measur¬ able selection coefficient, unabated and without change in direction.)* Stable
*In my favorite more specific example, Williams (1992, p. 129)—who is, to say the least, no general champion of punctuated equilibrium or detractor of anagenesis—points out that mean morphological changes in some North American populations of English spar¬ rows during their century of residence on our side of the Atlantic reach a maximum of about 5% increase for the lengths of long bones of the wings and legs. This anagenetic in¬ crement, so small that “no birdwatchers will notice in their old age that the bird looks any different from what they remember from childhood” (Williams, 1992, p. 129), would, nonetheless, if maintained only for the geologically trivial interval of one million years, be
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THE STRUCTURE OF EVOEUTIONARY THEORY clades could not dominate the history of life, as they manifestly do, particu¬ larly in marine invertebrates (clams, snails, horseshoe crabs, brachiopods, all from the Paleozoic to the present); nor, among more rapidly changing terres¬ trial clades, could dinosaurs (not to mention the more stable insects) persist A-
and rule for so long in a world where most species evolved continually by the analog of ontogenetic drive in the Lamarckian mode. Of course, we could posit other reasons for braking the rapidity and ef¬ ficiency of change by ontogenetic drive in macroevolution—disruption of trends by mass extinction; high frequency of trends that benefit organisms but harm species (peacock’s tails), for example. But I suspect that the simplest of all reasons will explain the evident pattern: the species-level analog of ontogenetic drive—gradual transformation within a species—just doesn’t oc¬ cur very often. Finally, I note that R. A. Fisher’s classic argument for the impotence of spe¬ cies selection rests on the standard assumption that this mode of driving does prevail in evolution. For, if most species, most of the time, changed gradually from within (see pp. 644-646), then selection among species would be, as Fisher rightly noted, an operative but impotent process, capable of generating only an insignificant amount of change relative to the dominant and ubiqui¬ tous drives of anagenesis. But if anagenesis rarely occurs, Fisher’s argument collapses. I wonder if Fisher ever explicitly realized that anagenesis would trump species-selection because anagensis is Lamarckian at the species level, while species selection is Darwinian at the same level—for Lamarckian pro¬ cesses can always overwhelm the much weaker force of Darwinian change if both operate generally and in an unimpeded manner at the same level.
Reproductive drive: directional speciation as an important and irreducible macroevolutionary mode separate from species selection Thus, the first category of ontogenetic drive illustrated interesting differences in style between levels, but little variation in effect—for I conclude that this mode has scant impact upon evolution at either level. But when we consider the second category of reproductive drive (biased production of offspring that vary in a given direction from parents), we encounter one of the chart’s most striking disparities—a crucial, yet almost entirely unrecognized and un¬ explored difference in basic pattern between micro- and macroevolution. To choose a hypothetical example of simplified form but maximal clarity: Sup¬ pose that each collectivity (a population for the organismal level, or a clade for the species level) contains ten individuals (organisms or species, for the two levels). Each individual gives birth to a single offspring, and all offspring have identical life spans and reproductive capacities. Thus, no selection at all
“capable of turning sparrow-size into ostrich-size bones, and back again, about 54 times.” Clearly, anagenesis at virtually any rate high enough to stand out above measurement error over a human lifetime cannot be sustained in a unidirectional manner for meaningful inter¬ vals in geological time.
Species as Individuals in the Hierarchical Theory of Selection can take place. Now suppose that a strong bias exists in production of off¬ spring. so that 80 percent arise with smaller bodies than their parent (lower weight for the offspring organisms, lower average body weight for the off¬ spring species). Suppose also that this pattern continues from generation to generation. This driving process would generate a strong trend to smaller bodies in the collectivities at both levels—a gradual trend to decreased body size in the population at the organismal level; and to species with smaller av¬ erage body sizes within the clade at the species level. As discussed previously (p. 691), reproductive drives of this kind can occur at the organismal level, and a variety of names for such processes exist, in¬ cluding mutation pressure and meiotic drive. But the Darwinian tradition has always regarded such phenomena as insignificant as a consequence of their rarity. Indeed, these processes must be rare in a fully Darwinian world, be¬ cause reproductive drives violate the necessary precondition of undirected variability for natural selection (see pp. 144-146). Darwinians did not win this debate by simple logic or evident factuality, but only by a great intellec¬ tual struggle marking a crucial episode in the history of evolutionary thought. The classical debate about orthogenesis, for example (see Chapter 5), cen¬ tered upon the Darwinian denial of such reproductive drives, which, as the competing orthogeneticists all realized, would overwhelm selection by higher efficacy—if they existed. Perhaps such reproductive drives rarely occur at this level in nature because, having no known basis for inherent adaptivity, they have been actively suppressed by organismal selection—another potential ex¬ ample of the most distinctive feature of organismic individuality: the power evolved by functional integrity to suppress lower-level selection from within. However, when we move to the species level, the analogous driving phe¬ nomenon of directional speciation suffers no constraint or suppression—and may represent one of the most common modes of macroevolution. Two ma¬ jor reasons underlie the high potential frequency for directional speciation (as opposed to the rarity of its analog at the organismal level—see line III2a on the chart). First, as noted in several other contexts, the species-individual does not maintain integrity (as the organism does) by suppressing differential proliferation of some parts over others. Since drives at an upper level arise by differential proliferation of lower-level units, this absence of suppression leaves a large open field for driving processes to operate at the birth of new species. Second, since new species-individuals must arise with sufficient heri¬ table novelty to win reproductive isolation from their parent (whereas chil¬ dren of asexual organisms may be clonally identical with parents), all species births include genetic change as an automatic consequence. Any statistical directionality in such changes among species in a clade will produce a trend by drive.51' *At the risk of an unwarranted metaphorical excursion into anthropomorphic imagery, one might contrast limited change at organismal birth with necessary change at species birth in the following manner: New metazoan organisms arise by a process of complex de¬ velopment, which must discourage change for reasons recognized ever since von Baer for¬ mulated his laws of embryology (1828). At the organismal level, the new individual sepa-
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THE STRUCTURE OF EVOLUTIONARY THEORY We may postulate any number of plausible circumstances that would gen¬ erate directional biases in the origin of new species—thus producing a cladal trend without any contribution from species selection. Moreover, potential causes for directional bias exist at all levels—organismic, demic, or species— thus greatly expanding the scope of the phenomenon. As a central theoretical point, directional speciation, when based on irreducible species-level proper¬ ties, represents a style of independent and causal macroevolution not based on species selection. Thus, the claim for an independent body of macroevo¬ lutionary theory does not depend upon the validity and high relative fre¬ quency of the Darwinian analog most often discussed as a paradigm case, namely species selection. Directional speciation, when based on irreducible species-level traits or processes, designates another category of intrinsically macroevolutionary change. To continue in the hypothetical mode with the example cited previously, one can easily imagine how a cladal trend, attributable entirely to reproduc¬ tive drive (and not at all to selection), and leading to decreasing average (organismal) body size, might be caused at either the organismal, demic, or species level. At the conventional organismic level, a pervasive environmental change over the entire region of a clade’s occupancy might favor natural selection for smaller bodies. (Perhaps, to choose a somewhat cardboard ex¬ ample, a temperate region has become tropical, and smaller organisms now gain advantages within each species of a clade by the adaptive correlates of Bergmann’s Rule.) Each species produces a single daughter species and then dies out—so no selection can occur at the species level. But if most species, in the new climatic regime, originate at smaller average body size because natu¬ ral selection favors this trait among the organisms in each species, then the cladal trend arises by directional speciation with a cause based on selection at the organismal level—the classic case of a drive at a higher level produced by directional selection among contained parts at a lower level. For a hypothetical case based on interdemic selection, suppose that each species in a clade develops ten small and isolated demes at the periphery of the parental range. Suppose that average body size in these peripheral isolates varies randomly around the parental mean. Suppose further that, for each species, only one of the ten peripheral demes survives, intensifies its differ¬ ences, and eventually becomes a new species—while the parent and the other nine peripheral isolates all die. Again, no species selection can take place, for rates intrinsically from the parent; how then, may this offspring be kept sufficiently like the parent to preserve the collectivity of the population? An opposite problem attends the birth of species. At the species level, new individuals are born by speciation, which enhances change. But species do not separate intrinsically from their parents. They are born in fuzzy continuity. Their separation may be difficult. They must be cast out, or they will reinte¬ grate. Necessary change at speciation enhances this defining process of casting out from the parent. The newly born species faces a structural problem opposite from the neonatal or¬ ganism’s dilemma: how may the new species-individual become sufficiently unlike the par¬ ent to be cast out, thus enhancing the collectivity of the clade by adding another part? In short, the new metazoan organism forms outside the parent: how can it be kept close? The new species separates with difficulty from the parent: how can it he cast out?
Species as bidividuals in the Hierarchical Theory of Selection each parent spawns one and only one daughter. (I realize, of course, that these strictures sound absurd if construed as actual and coordinated occurrences in nature; I am only following the time-honored heuristic method in science of constructing “pure” end-member hypotheticals to help clarify our thoughts.) Finally, let us say that, in general, the surviving (and ultimately speciating) deme lies at or near the smaller-bodied end of the random distribution (around the parental mean) of average body size. Let us also posit that the smaller average body size of new species arises as a consequence of a demelevel property conveying differential success in interdemic selection among the ten peripheral isolates initially spun off from each species. An obvious (and not implausible) reason might be found in a strong correlation between small bodies and larger N in any population (the “more ants than elephants” principle, albeit in a more restricted range). The surviving deme might owe its success to generally larger population size in a tough peripheral environment. The cladal trend to smaller body size among species would then arise by a drive of directional speciation (new species biased to originate with smallerbodied organisms than those of their ancestors, in a situation where no spe¬ cies selection can occur). The cause of the trend, in this hypothetical case, will be interdemic selection—for the ten peripheral isolates arise as demes of the parental species. Selection among these demes favors those with smaller aver¬ age body size, based on correlation with the causally controlling deme-level property of larger population size. This deme-level property confers an irre¬ ducible fitness upon demes in their interaction with the environment. (In an extreme, albeit improbable, case, interdemic selection based on larger popu¬ lation size could even outweigh negative organismic selection against small bodies.) Again, a drive at the species level arises by selection among lowerlevel parts, in this case demes rather than organisms. Finally, a drive at the species level may be caused by an irreducible specieslevel character. Suppose that each species spins off only one peripherally iso¬ lated population and that, invariably, the parental population dies while the peripheral isolate becomes a new species. Suppose that the single peripheral deme, generated by each species, generally features organisms with a smaller average body size than the organisms of the parental population. Suppose that this directional bias arises as a result of a species-level trait in the paren¬ tal population. Perhaps, for example, the social structuring or territorial sys¬ tem of the parental population preferentially excludes smaller organisms of both sexes, and that these smaller organisms therefore tend to migrate to the species border, where they aggregate to form the isolated population that will generate a new species. (Again, the case merely requires conceivability for purposes of illustration, not plausibility.) In this situation, no selection can occur at the species level because each parental species produces one daughter species and then dies. The cladal trend to species with smaller average body size arises by the driving process of directional speciation—and the cause lies in a species-level trait of the parental population. As stated above, we here en¬ counter a case of irreducible macroevolution not based on species selection. Examples of this kind illustrate that the domain of independent macroevo-
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THE STRUCTURE OF EVOEUTIONARY THEORY lutionary theory extends well beyond the phenomenology of Darwinian ana¬ logs in species selection. As an additional argument for the importance of directional speciation as a driving force in evolution—and as an example of interesting complexity en¬ gendered by the hierarchical model (and of differences in the character of ex¬ planation between this hierarchical reformulation, and the traditional onelevel world of Darwinian evolution)—note what often happens when causes at one level correlate with emergent properties involved in causes at a higher level; for we then encounter the fascinating situation of disparate theoretical meanings for inexorably linked phenomena at two levels. (We have already discussed one common example in the causation of higher-level drives by lower-level selection.) Another important example, potentially encompassing one of the dominant phenomena of macroevolution, translates the results of ordinary selection at the organismal level into strong constraints acting as causes of directional speciation at the species level. In this sense, when consid¬ ered at the appropriate higher level, macroevolutionary pattern results much more from immediate constraint, and less from the traditional selection¬ ist mode, than we have generally been willing to allow—thus suggesting an¬ other potentially important reform and expansion of Darwinian thinking (see Chapter 10 for a fuller discussion). Consider two cases of cladal trends produced by the driving cause of direc¬ tional speciation. Figure 8-6 depicts the common pattern in both examples. At a starting point, the clade contains two kinds of species in equal num¬ bers—those bearing trait A, and those bearing trait B. Every reproducing spe¬ cies generates two daughters and no variation exists for differences in species birth rates among those species that have offspring—so no species selection can occur. Evolution proceeds rapidly by directional speciation because ASpecies can only produce A-Daughters, while B-Species produce 50 percent
8-6. A cladal trend produced entirely by directional speciation with no species selection. A species can only produce A daughters, while B species produce 50% A daughters and 50% B daughters. Under these conditions of strongly direc¬ tional speciation, a powerful trend towards A leading to quick disappearance of B from the clade, will arise, even under a regime of random mortality among species.
Species as Individuals in the Hierarchical Theory of Selection
A-Daughters and 50 percent B-Daughters. (If we posit random mortality of a given percentage of species before they split into their daughters, as in Fig. 8-6, then B-Species will eventually be entirely eliminated, and A-Species will become fixed in the clade.) A first kind of example would not disturb the tranquility of any committed adaptationist, for a functionalist theme translates well across the levels. Sup¬ pose that Cope’s Rule were true in the classical sense—it is not, by the way (Stanley, 1973; Jablonski, 1987, 1997; McShea, 1994; Gould, 1988b, 1997b, and pp. 902-905 of this book)—and that organismal selection always fa¬ vored size increase because big organisms prevail in competition. A-Species are large and B-Species are small; A’s only give rise to other A’s, while B’s give rise either to A’s (given the pervasive advantage of increasing size), or to B’s at equal frequency (for small size may still be favored in some habitats of the clade). The strong Cope’s-Rule trend in the clade occurs by directional speciation. The adaptationist theme prevails at both levels. Average organisms in the clade become larger because bigger is better; and species increase in aver¬ age body size because their parts (organisms) do better at larger size. (No spe¬ cies-level trait exists in regulating this trend, and the entire phenomenon arises by conventional organismal selection based on advantages of increased body size.) But a second kind of example—undoubtedly quite common in evolution— would perturb a strict adaptationist by translating selection at the organismic level to regulation of the cladal trend by constraint. Suppose now—and such an explanation has been urged as an alternative to species selection for the increase of nonplanktotrophic species within Tertiary clades of gastropods (Strathmann, 1978, 1988)—that a molluscan clade begins with an equal number of species of nonplanktotrophs (A-Species) and plankotrophs (B-Spe¬ cies). Planktotrophic larvae stay aloft through the motion of complex ciliary bands that beat in concert. Selection pressures for nonplanktotrophy lead to loss of these bands, and consequent benthic development of a maternallyprotected brood. Plankotrophs can always, in principle, convert to non¬ planktotrophy because the bands can be lost; but the transition cannot pro¬ ceed in the other direction because ciliary bands can’t be reconstituted once they have disappeared in evolution (see Gould, 1970b, on the proper mean¬ ing of Dollo’s Law of irreversibility in evolution). The origin of each species may be governed entirely by the conventional route of adaptation based on natural selection of organisms. But a structural limitation in possible directions of change produces the cladal trend by direc¬ tional speciation towards increasing frequency of nonplanktotrophic spe¬ cies—for a planktotrophic parent species can generate either planktotrophic or nonplanktotrophic daughters, while a nonplanktotrophic parent can only produce nonplanktotrophic daughters. The numerical situation corresponds exactly with Figure 8-6 and the previous example based on Cope’s Rule (with A-Species now read as nonplanktotrophs, and B-Species as planktotrophs), but the explanation at the cladal level differs crucially—for the trend arises by structural constraint upon possible directions of change, not from
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THE STRUCTURE OF EVOEUTIONARY THEORY any general or global advantage for nonplanktotrophic organisms. (In fact, planktotrophic species might hold a small advantage in species selection for longevity, and the trend to nonplanktotrophy might still arise by directional speciation under this potent constraint.) I strongly suspect that trends driven by structural constraints within large systems, and not by adaptational advantages to organisms, pervade evolu¬ tion, but have been missed because we focus on means or extremes in a distri¬ bution and not on the full range of variation as a more telling '‘reality” (see Gould, 1996a, for an entire book on this subject, written for popular readers; and Gould, 1988b, for a technical account). The vaunted trend to increasing complexity in the history of life, for example, only records the small and ex¬ tending tail of an increasingly right-skewed distribution through time—but with a strong and persistent bacterial mode that has never altered during life’s entire 3.5 billion year history, leaving this planet now, as always, in the Age of Bacteria (see pp. 897-901 for a further development of this example). This extending right tail may record little more than the constraint of life’s origin right next to the lower bound of preservable complexity in the fossil record. Only one direction—towards greater complexity—remained open to “inva¬ sion,” and a small number of species dribble in that direction through time, thus extending the right tail of the skewed distribution.51' But no evidence now exists to support an argument that higher complexity should be construed as a “good thing in general” (in adaptive terms, or otherwise), either at the orgamsmal or species level. In fact, the few studies based on patterns of speciation in clades where founding members lie far from any upper or lower structural boundary, and therefore impose no constraint upon either decreas¬ ing or increasing complexity, show no trend at all towards increasing com¬ plexity. Approximately equal numbers of species arise with less complex and with more complex phenotypes than their ancestor (see McShea, 1993,
"'Examples of this sort illustrate the important point that drives of directional speciation do not necessarily require a differential number of speciation events along the route of the trend. A directional bias may also arise if numbers of speciation events occur with equal frequency in either direction, but the average phenotypic magnitude of the trending half ex¬ ceeds the amount of change in the half oriented away from the trend. Such cases may be common when a founding lineage lies near a boundary, and amounts of change become se¬ verely constrained in one direction. Thus, for the bacterial mode of life, for example, we may easily imagine (data for an adequate test do not exist, so far as I know) that as many speciation events yield a less complex as a more complex descendant. But so little room exists between the mode and the lower limit that changes to reduced complexity can¬ not depart far from the ancestral state, while an open range to the right of the mode permits a far greater magnitude of change in the direction of greater complexity. For an actual ex¬ ample, Wagner (1996) documented a general trend to increasing spire height in Paleozoic gastropods, but found an equal frequency of speciation events towards lower-spired and higher-spired daughters. The trend, however, records a bias in amounts of change. For some reason, gastropods that become high spired also experience a marked reduction in the amount of change per speciation event, even though they continue to produce equal num¬ bers of daughters in both directions—whereas low-spired ancestors generate much higher average change per speciation event. The mean spire height of the entire clade therefore in¬ creases.
Species as Individuals in the Hierarchical Theory of Selection
on mammalian vertebral columns; McShea, Hall, Grimsson and Gingerich, 1995, on mammalian teeth; and Boyajian and Lutz, 1992, on ammonite sutures).
Species selection, Wright’s Rule, and the power of interaction with directional speciation I have long regarded species selection as the most challenging and interesting of macroevolutionary phenomena, and the most promising centerpiece for macroevolutionary theory. While I continue to espouse this view, my rethink¬ ing for this chapter has led me to appreciate the significant power of two other species-level processes: drives of directional speciation as just discussed (see also Gould, 1982c), and species drift, the higher-level analog of genetic drift. I would now argue that the interaction of these three processes sets the distinctive character of macroevolution. As for natural selection at the organismic level, the two major modes of species selection operate by differential rates of generating daughter species (the analog of birth biases in natural selection) and differential geological lon¬ gevity before extinction (the analog of death biases in natural selection). At the species level, however, the difference between these two modes does not rest upon the same basis that distinguishes their analogs at the organismic level. At the orgamsmal level, natural selection by birth bias works mainly upon such “internal” traits as reproductive rate and brood size, and often doesn’t increase adaptation in the conventional sense of phenotypic molding to bet¬ ter biomechanical design for local environments. For example, an organism gains a large selective advantage merely by breeding a bit earlier, though nothing else about the phenotype need alter (Gould and Lewontin, 1979, re¬ ferred to this mode as “selection without adaptation”). But natural selection by death bias among organisms usually yields phenotypic adaptation for better fit to the ambient environment. At the species level, however, our main concern moves to an interesting dif¬ ference in causal locus. Most cases of selection by differential speciation oper¬ ate by the interaction of an irreducible species-level character—some feature of population structure—with the environment, and therefore represent gen¬ uine species selection. After all, and as stated before, organisms don’t speciate; only populations do. But for selection by differential extinction, a higher frequency of cases can probably be explained as the simple summation of organismal deaths, and may therefore be causally rendered at this conven¬ tional lower level—for both organisms and species die. Thus, students of spe¬ cies selection have rightly focussed on differential speciation as their most promising category (see Gilinsky, 1981, for both theoretical arguments and empirical examples). However, the most interesting of all differences between organismal and species selection may lie not in the phenomena themselves, but rather in the character of their interaction with the two other primary modes of evolution¬ ary change: drives, and drift (I shall discuss drift in the next section). Our
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THE STRUCTURE OF EVOLUTIONARY THEORY sense of the commanding potency of organismal selection rests upon the con¬ formity of Mendeliari genetics to one of the cardinal prerequisites of Darwin¬ ian systems (see Chapter 2): that the variation serving as raw material for nat¬ ural selection be “random” (with an operational meaning of “undirected towards adaptive states,” not “equally likely in all directions”)—so that se¬ lection, rather than biases inherent ill variation, can become the “creative” force in evolutionary change (see p. 144 for further discussion in a related context). This crucial condition can be validated at the organismic level—not because mutations (and other sources of genetic variation) are truly random in the mathematical sense, but because mutation represents a process so dif¬ ferent from natural selection, and operating on material (the structure of DNA) so disparate from the bodies of organisms (integrated tissues and or¬ gans), that we cannot postulate a reason why favored directions of mutation should correspond in any way to the needs of organisms. But no comparable argument exists for any a priori expectation that the analogous variation (among species within a clade) made available for species selection should also be random with respect to the direction of a trend. Spe¬ cies do not discourage drives among their parts (organisms), while organisms usually do suppress directional variation at lower levels (because the prolifer¬ ative “interests” of individual genes and cell lineages generally run counter to the adaptive needs of organisms). Moreover, the adaptive features of organ¬ isms often confer benefits upon their species as well—as when species live longer because their well-designed organisms prevail in competition. There¬ fore, we cannot defend an a priori basis for asserting randomness in the varia¬ tion that serves as raw material for species selection. This situation creates both a problem and a challenge for the analog of Darwinism at the species level—for maximal efficiency of species selection does demand undirected variability, and by the same classical argument origi¬ nally devised for the organismic level. The randomness of species-level varia¬ tion with respect to the direction of a trend therefore becomes a matter for empirical testing, rather than a claim predictably flowing from the nature of materials and processes. Such a test should also receive high priority for any¬ one interested in discovering the frequency and strength of species selection in the explanation of evolutionary trends. For these reasons, Gould and Eldredge (1977) formulated such a test under the name of “Wright’s Rule.” We took our cue from a prescient statement by Sewall Wright (1967) that the direction of speciation might be random with respect to the origin of higher taxa, just as we consider mutation to be ran¬ dom relative to the direction of natural selection. Wright’s Rule, in our for¬ mulation, therefore asserts either that drives of directional speciation do not exist at all in a given situation (the strong version), or at least that any exist¬ ing directional bias not occur along the vector of an established trend (a weaker version, but fully adequate for assertions of species selection). If Wright’s Rule holds, then trends must be attributed to differential prolifera¬ tion of certain kinds of species (by selection or drift), and not to any drives from within based on directional variation arising from lower-level processes.
Species as Individuals in the Hierarchical Theory of Selection Wright’s Rule represents a strong test for putative species selection, but I now realize that its failure does not eliminate species selection from consider¬ ation. When Wright’s Rule holds, a trend must be attributed to species sort¬ ing, for no directional component exists at the lower level of variation among units of sorting. But if Wright’s Rule fails in any particular case, then species selection cannot forge the trend exclusively—although species selection may still operate as one contribution in a hierarchical system. A speciational drive may act synergistically with species selection to intensify a trend. (Since drives tend to be more potent than selection, a powerful drive, with strong violation of Wright’s Rule, will probably relegate species selection to an insignificant role. But small departures from Wright’s Rule permit a substantial intensi¬ fication of the trend by species selection. In any case, and in situations of un¬ usually complete paleontological data, we should be able to measure the rela¬ tive strengths of drive and sorting when the two modes act synergistically.) Wright’s Rule has been tested in some cases, but not often enough—and the subject remains ripe for future research, including several Ph.D. theses! Gould and Eldredge (1977) found Wright’s Rule validated for Gingerich’s data on early Tertiary Hyopsodus. MacFadden (1986) failed to confirm Wright’s Rule in the evolution of horses, where a directional bias exists for descendant species to arise at larger body sizes than their ancestors. Arnold, Kelly, and Parker (1995) validated Wright’s Rule for a remarkably complete data set of 342 ancestral-descendant pairs in Cenozoic planktonic foraminifera. An equal number of species arose at larger and at smaller sizes than ancestors; see Figure 8-7. In a pioneering study, notable for completeness and density of data (and a consequent capacity to distinguish among all the vari¬ ous modes of evolutionary change), Wagner (1996) documented three general and speciational trends in the evolution of gastropods during the lower Paleo¬ zoic (Cambrian through Silurian): towards higher spires, more inclined aper¬ tures and narrower sinuses. For 276 ancestor-descendant pairs over the en¬ tire clade, Wagner confirmed Wright’s Rule for spire height and inclination, where as many species differed from ancestors in a direction away from the general trend, as along the ultimately favored route. But data for sinus width, where a statistically significant bias exists for speciation in the direction of
MAGNITUDE OF SIZE CHANGE AT SPECIATION
8-7. Validation of Wright’s Rule in a study by Arnold et al. (1995) of 342 ancestral-descendant pairs of Cenozoic plank¬ tonic foraminifera. Descendant species origi¬ nate, with equal fre¬ quency, at larger and smaller sizes than their ancestors.
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THE STRUCTURE OF EVOEUTIONARY THEORY
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8-8. Top: Clear speciational trends in Lower Paleozoic gastropods towards higher spires (A—measured as shell torque), more inclined apertures (B) and nar¬ rower sinuses (C). The bottom diagram demonstrates that the first two trends obey Wright’s Rule in showing no bias in species origins in the direction of the trend. However, the trend for sinus width does show a bias for new species to originate in the direction of narrower sinuses—thus yielding a complex trend, partly produced by directional speciation, and not entirely by species selection. From Wagner, 1996.
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Species as Individuals in the Hierarchical Theory of Selection
narrower sinuses, falsified Wright’s Rule (see Fig. 8-8) and documented a drive of directional speciation. Wagner further demonstrated (1996, p. 1000) that “this bias is distributed throughout the entire clade,” for three major subclades all display the drive. Nonetheless—and showing the power of such data to identify and tease apart the different components of a trend into their relative quantitative strengths—Wagner also documented a component of species sorting in the overall trend to narrower sinuses, for “species with wide sinuses were significantly less likely to survive the end-Ordovician mass ex¬ tinction” (1996, p. 990). I would go further and suggest that synergisms of drive and sorting (as Wagner has documented for the trend to narrower sinuses in Paleozoic gas¬ tropods) should be common in the history of many clades, and probably mark a powerful mode of macroevolution distinct from conventional micro¬ evolution, where such synergism must be rare. Good a priori reasons exist for supposing that features biasing the directionality of speciation might also fa¬ vor sorting towards the same end. Such synergism should be most evident when the causes of both bias and sorting work at the same (usually organismic) level—as when, for example, a trait under strongly positive organismic selection (like large body size) arises preferentially in speciation events, and then promotes the greater longevity of species so originating. But such syner¬ gisms may also be common when causes differ in level—as when, for exam¬ ple, a drive occurs by organismal selection, and species-selection then causes sorting in the same direction. For, unlike the situation at the next lower pair¬ ing of levels (where genic and cell lineage selection so often run counter to the interest of organismal selection, and consequently become suppressed), selec¬ tion at the organismal level does not conflict in principle with selection at the species level. Selection at these two levels should, therefore, be synergistic as often as opposed. Such synergisms should therefore be frequent and powerful in macroevolution.
Species-level drifts as more powerful than the analogous phenomena in microevolution At the organismal level, the second major mode of sorting—drift by random processes—operates in two ways that should be distinguished both for poten¬ tially different roles and frequencies at this level, and because the species-level analogs diverge even more clearly. We may distinguish random shift within the collectivity—called genetic drift at the conventional organismal level— from random effects introduced at the founding of new demes or species by small numbers of organisms. Mayr (1942) introduced the term “founder ef¬ fect” to distinguish this second category (though the basic mechanism does not differ from ordinary genetic drift), and to emphasize that the differences initiating a new species need not arise entirely by natural selection, but may be significantly enhanced by random effects at the outset, because a small number of founders will, for stochastic reasons, surely not begin a new popu¬ lation with the same gene frequencies as the ancestral population, while some
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THE STRUCTURE OF EVOLUTIONARY THEORY alleles (even if favorable) will be lost by random non-inclusion in all founding organisms. Although both genetic drift and founder effects obviously occur at the organismal level, our traditions have tended to downplay the role of random processes vs. selection as sources of sorting—so the phenomena generally re¬ ceive short shrift. Some conventional arguments for genuine rarity at the organismal level may be valid, particularly given the requirement for either small populations or effective neutrality of drifting sites. (The initiating crite¬ rion of low N may, however, be quite generally met if Mayr’s theory of peripatric speciation holds, hence his emphasis on the “founder principle.” Similarly, if bottlenecking to very small numbers typically occurs during the history of many species, then genetic drift also becomes important in ana¬ genesis. The argument for effective neutrality, as discussed previously (pp. 684-689), works best at the genic level, where drift may predominate by Kimura’s neutral theory of molecular evolution.) However, at the species level, these traditional objections to high frequency for drift become invalid, and we should anticipate a major role for this sec¬ ond cause of sorting. Low population size (number of species in a clade) pro¬ vides the enabling criterion for important drift in both categories at the spe¬ cies level. The analog of genetic drift—which I shall call “species drift”— must act both frequently and powerfully in macroevolution. Most clades do not contain large numbers of species. Therefore, trends may often originate for effectively random reasons. Consider a trend produced by random deaths (a comparable argument can easily be made for random birth differentials), based on Raup’s “field of bullets” model (1991 and Chapter 12). Suppose, for example, that each of the ten species of a clade lives in a small area, with each species allopatric to all others. Over a certain period of time, a bolide (or some gentler environmental change with power to drive a local species to ex¬ tinction) strikes half the areas at random and eliminates the resident species of the clade while each of the species in the five safe areas branches off a daughter, thus restoring the cladal population of 10 species. At an N this low, some trends (and perhaps a substantial number) will inevitably arise by this mode of random removal. Perhaps, for example, four of the five species with mean body size below the cladal average will happen to die. A substantial random trend to increased body size then occurs within the clade. When we move from the homogenous “field of bullets” model to a scaling of effects in the real world, and consider the consequences of infrequent, but severe, mass extinction on a global scale, the potential role of random trends by elimination only increases—for random effects based on small numbers will be greatly intensified. (The reduction of species number in mass extinc¬ tion may be conventionally causal, but the final death of the clade, after re¬ duction to less than a handful of species, may then be effectively random. For example, so few trilobite species still lived when the great Permian extinction occurred that Pm not sure we need to seek a “trilobite specific” cause for the final elimination of this previously dominant group.)
Species as Individuals in the Hierarchical Theory of Selection
When we move to the second category of random results achieved by sort¬ ing in the colonization of new places—the analog of the founder effect—then comparison with the organismal version becomes less straightforward, al¬ though we may be confident that the species-level version holds potential for great importance in evolution. The species-level analog, which I will call ‘"founder drift” (see lines IIIC2 and IIIC2a), does not work through a sim¬ ple phenotypic difference between a colonizing species and the parental stayat-home—for all species differ by definition, and disparities arise by the usual combination of selective and random effects, usually expressed at the organismic level. The stochastic analog to Mayr’s “founder effect” at the organismal level lies in random aspects of the differential capacity for prolif¬ eration of new species in allopatric regions of a clade’s full range. A hypothetical example will illustrate this unfamiliar concept. Suppose that a clade contains only two species, living in adjacent islands with simi¬ lar environments. The islands, however, lie on different oceanic plates, and movements of plate tectonics cause the coalescence of one island with a large neighboring continent, while leaving the other island in the midst of the ocean. The species on the continent proliferates into a large subclade of new species, while the species on the island, lacking any room for expansion, re¬ mains as the only species of its subclade. Because the process of speciation yields phenotypic disparity intrinsically, the founding continental species will differ from its insular sister species. Therefore, the clade will show a strong trend in the direction of autapomorphic traits possessed by the continental founder. But such a trend will often be entirely random with respect to the plurified traits of the continental founder. That is, these spreading traits may be completely neutral in the crucial sense that if the other (insular) species had colonized the island that coalesced with the continent instead, its autapomorphies would have proliferated, and the cladal trend would have pro¬ ceeded with the same force, but in the opposite direction. Only the luck of residence on one island rather than the other (and not any preferential inter¬ action of some traits vs. others with the environment) leads to the differential proliferation of one species’s traits over those of the sister species. Situations of this sort must be common, if not virtually canonical, in evolu¬ tion. Almost any two geographic regions must maintain differential capacity to house species of a given clade. If both regions are colonized by founding species, and, many million years later, one region holds substantially more species than the other, the random component of spatial and ecological op¬ portunity must often play a greater role in differential speciation than the se¬ lective force of greater capacity for differential proliferation in one subclade vs. the other based on interactions of traits with environments. I use the term
random in a special, but surely legitimate, sense. Suppose that a large and ecologically diverse Region 1 can accommodate 50 species of a subclade, while smaller and more homogeneous Region 2 can only maintain 10 (I real¬ ize that species create their own environments, and that regions don’t main¬ tain fixed numbers of available addresses, but I invoke this simplification for
737
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THE STRUCTURE OF EVOEUTIONARY THEORY the sake of argument). Subclade A invades Region 1, while Subclade B be¬ gins in Region 2. The resulting strong cladal trend toward the autapomorphic characters of Subclade A cannot be called accidental in the global sense—for Region 1 does predictably accommodate more species. But the trend may be accidental in the sense that Subclade A, rather than Subclade B, happened to invade the more prosperous region—and that if Subclade B had been the colonizer, its progeny would have done equally well, and would have dominated the cladal trend with the same force actually shown by Subclade A. In this case, we call the trend random because A’s success does not arise from any superiority of an interacting trait (vs. B’s phenotype), but only from the accident of colonizing a more propitious place (see Eble, 1999, and Chap¬ ter 11 of this book for a discussion of this evolutionary meaning of “ran¬ dom”). As with the relationship between directional speciation and species selec¬ tion, these two forms of species-level drift must often interact with the other main cause of sorting—i.e., selection—to produce a trend (as when Subclade A, in the example just above, increases both by the good fortune of greater opportunity, and by selective benefits conferred by its traits). The organismic level may experience a higher relative frequency of domination by selective forces, but the world of species evolves by complex interactions among the processes of drive, selection, and drift.
The scaling of external and internal environments I have not tried to develop an exhaustive comparison between levels for influ¬ ences of external and internal environments upon the modes of change dis¬ cussed in previous sections. But I offer a few sketchy comments to encourage further work in this area. For environmental factors that induce competition among individuals and therefore establish selection pressures (line IVA of the chart), I contrast modes that involve direct contact among individuals with those that can proceed in allopatry. At the organismic level, this contrast exposes a strong correlation between prevalence of biotic factors in direct contact and abiotic factors in allopatry. At the species level, a different correlation may dominate: the asso¬ ciation of selection by differential elimination with direct contact, and selec¬ tion by differential birth with allopatry (lines IVA1 and IVA2). This contrast also leads to different implications at the two levels. At the organismic level, as Darwin himself argued in his primary justification for progress in the history of life (see Chapter 6), the biotic mode correlates more often with adaptation by general biomechanical improvement, and the abiotic mode with adaptation to local circumstances of the physical envi¬ ronment, with no vectorial component as environments fluctuate randomly through time. At the species level, we may expect to find a strong correlation of selection by differential elimination with potential reduction to the organismal level, while selection by differential birth represents the most promising domain for true and irreducible species selection.
Species as Individuals in the Hierarchical Theory of Selection
For constraints of internal environments (line IVB), I make a distinction between negative factors that limit amounts and directions of change, and positive properties that channel change in certain directions, or provide par¬ ticular opportunities for evolutionary novelties and breakthroughs. (I also base Chapter 10, this book’s major discussion of constraint, on the same dis¬ tinction.) The operation of these constraints often differs in interesting ways at the two levels. For some of the important limits, line IVB1 specifies a major shaping force of life’s structure, a factor not often explicitly acknowledged. Why does the world contain stable individuals at all, and at any level? Why doesn’t evo¬ lution work as continuous flux at all scales, rather than primarily by selec¬ tion upon individuals stable enough to persist, at least through one round of differential sorting? Comparable reasons can be stated at both the organismic and species levels, thus giving evolution its primary shape or structure: Lamarckian inheritance does not occur at the organismal level, thus stabiliz¬ ing the ontogeny of heritable variability. At the species level, punctuated equi¬ librium suppresses anagenesis by maintaining species-individuals in stasis. When we explore the structural brakes that limit amounts of change in most trends (line IVB2), several factors could be mentioned, but I just list, as an example, the single property that I consider most important. For organ¬ isms, those paragons of individuality by the criterion of structural and func¬ tional integrity, design limits of the Bauplan (both internally by structural constraint, and externally by adaptive possibilities) place strong brakes upon almost any evolutionary trend. Contrary to the themes of several popular films, elephants will never fly, and insects will not reach elephantine propor¬ tions and engulf our cities as a plague of megalocusts. At the species level, Stanley (1979) made an important observation that has not been sufficiently appreciated for its defining force in limiting the possibili¬ ties of species selection. If we consider the two major modes of positive spe¬ cies selection—enhancing the rate of production for new species, and extend¬ ing the geological longevity of existing species—why shouldn’t some lineages be able to maximize both properties simultaneously, thus becoming gigantic megaclades, dominating the earth’s biota? (Perhaps, of course, a few clades have been able to approach this ideal—thus explaining the great success of beetles and nematodes.) In other words, why don’t clades ratchet themselves towards this pinnacle by species selection—by working both ends of the game, and evolving species of extraordinary durability and fantastic rates of branching, superspecies that live for several geological periods and spawn large numbers of daughters all along the way? Stanley (1979) argues, with extensive data in support, that the nature of speciation as a process, and the general rules of ecology, engender a strong, and effectively unbreakable, negative correlation between speciation and ex¬ tinction rates. Unfortunately for ambitious species with dreams of megacladal domination (but happily for any ideal of a richly varied biota), the ma¬ jor factors that boost speciation rates also raise the probability of extinction;
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THE STRUCTURE OE EVOLUTIONARY THEORY
while features that enhance longevity also suppress the rate of speciation. For example, small populations in stressful environments are especially prone to both speciating and dying; while large, global populations of marked stability
and great mobility (like Homo sapiens and Rattus rattus) are remarkably re¬ sistant to extinction (unless, like one of the above, they evolve an odd capac¬ ity for potential self-destruction), but ill-equipped to form the isolated popu¬ lations that can generate new species. For a third limiting constraint of brakes on the amount of available varia¬ tion for selectional processes (line IVB3), infrequency of new mutation may play an important role at the organismal level (not so often in sexual forms, where recombination greatly boosts the amount of variability among individ¬ uals, but usually a defining limit in asexuals, and perhaps the major reason for the rarity and marginal status of asexuals among complex Metazoa, but not in unicells with short generations). At the species level, variation per indi¬ vidual may be more than adequate (given the forced correlation of birth with change), but many clades contain too few individuals, giving birth too rarely, for very efficient selection (Fisher’s argument—see page 645). For a final factor among limiting constraints (in this abbreviated list), brakes on development act strongly at both levels (line IVB4). Ever since the inception of modern embryology, von Baer’s (1828) laws have defined the hold placed by ontogenetic intricacy upon potentials for change in complex Metazoa. At the species level, the hold of homology (as expressed in all the factors, genetic and otherwise, that limit the amount of change per speciation event) functions as a developmental constraint in the same basic manner— that is, by limiting the difference that can separate a parent and its immediate offspring. All these sources of limitation also contribute to the more important posi¬ tive aspects of constraint as channeling or enhancing preferred directions for change. In the category of positive channeling by structure (line IVB5), onto¬ genetic pathways already established in the lives of organisms provide by extension, or by relative shuffling of rates among components (see Gould, 1977b), the classic mode of constrained and substantial change in organismic evolution—thus explaining the importance of heterochrony as a morpho¬ genetic phenomenon (Jones and Gould, 1999; McNamara and McKinney, 1991). At the upper level of speciational trends within clades, structural rules and differential ease of modifiability among parts and correlations of Bauplane play the same role of directing and accelerating change along cer¬
tain preferred pathways. Liem (1973), for example, showed how a set of small and accessible changes in a jaw muscle, the fourth levator externi, could greatly alter the adaptive feeding devices of cichlid fishes (but not of other re¬ lated groups), thus helping to explain the rapidly evolved species flocks of this clade in several African lakes. In a second category of positive channeling by directed variability from lev¬ els below, the organismic level experiences no important effect because such drives will generally be suppressed by organismic selection. But the driving
Species as Individuals in the Hierarchical Theory of Selection
force of directional speciation can greatly enhance and channel cladal trends by working synergistically with such species-level modes of change as species selection. Perhaps the most important positive constraint, acting similarly at both levels, lies in the large size of “exaptive pools” (see full discussion in Chapter 11), or nonrandom variation made available through evolutionary processes acting on other features or at other levels, but later exploitable by organisms or species for their own exaptive benefits. The redundancy supplied by ge¬ netic duplication for organismal flexibility serves as the classic illustration of this phenomenon at the traditional level of natural selection. The exaptive pool of the species level may become even larger because species do not sup¬ press lower levels of change, while these genic and organismal directionalities frequently act in synergism with advantages at the species level (see Gould and Lloyd, 1999, for a detailed development of this argument).
Summary comments on the strengths of species selection and its interaction with other macroevolutionary causes of change Species selection, the Darwinian analog at this higher level, but by no means the only irreducible force of macroevolutionary change, differs from conven¬ tional natural selection at the organismic level both in character and in gen¬ eral strength. The major aspect of character—as I have emphasized through¬ out this chapter in stressing the non-fractality of hierarchical levels—lies in the potency of species selection for governing “what species do.” Species se¬ lection does not, and cannot, build the complex adaptive phenotypes of or¬ ganisms, but this common statement only recognizes the general nature of hi¬ erarchical organization and does not represent a fair criticism of the efficacy of species selection, despite the claims of Dawkins (1982) and others (see p. 711 for a discussion of this point). The primary force of species selection lies in its power to promote trends within clades, and to regulate the waxing and waning of differential spe¬ cies diversity within and among clades through time. The influence of species selection upon trends will also be enhanced because this process not only builds trends in species-level characters directly, but also establishes corre¬ lated trends in any character of the organismal phenotype that either helps to determine the species-level property, or merely hitchhikes upon the trend by linkages of homology within the phylogenetic structure of evolutionary trees—a very common phenomenon, as Raup and Gould, 1974, showed in theory and practice. This insight about trends, which I shall explore more thoroughly in the next chapter (pp. 886-893), may provide a key for explain¬ ing one of the most puzzling phenomena in paleontology—persistent and pervasive cladal trends (such as decreasing stipe number in graptolites, or in¬ creasing symmetry of crinoidal cups) that have defied all attempts at explana¬ tion in traditional terms of biomechanical advantages to organisms. As for general strength, species selection (in primary comparison with the traditional level of Darwinian natural selection on organisms) includes cer-
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THE STRUCTURE OF EVOLUTIONARY THEORY tain features that diminish its influence, and others that enhance its power. Among factors that weaken the potential of species selection, we may men¬ tion: (1) The generally low population size of species in clades, and the generally long life of species-individuals, both factors limiting the amount of variation usually available for a process of selection. (2) Unlike the organism, the species-individual does not actively suppress selection at lower levels within itself. Since the individuals of lower levels, by their shorter cycle times, present much more variation for selection (per given unit of time), this unsuppressed lower-level selection may overwhelm the op¬ eration of species selection. (3) Species selection, as the analog of asexual reproduction at the organismal level, becomes subject to the same important limit that favorable traits arising in one individual cannot be transferred laterally (for mixing and matching) to other individuals, but only vertically to direct descendants. (4) Species selection is limited by particular structural constraints, encoun¬ tered only at this higher level, most notably the apparently unbreakable cor¬ relation between origination and extinction rates, thus tying together by neg¬ ative interaction the very two phenomena that, if positively associated—that is, high speciation with low extinction—could so powerfully accelerate any trend produced by species selection. But these negative forces and limits will be counteracted by several features that grant potential strength to species selection: (1) Species selection may be theoretically weak relative to the power of transformation by continuous selection of lower-level individuals (organisms in this case) within species. But, in fact, such transformation by anagenesis rarely occurs in nature, as the great majority of species exhibit stasis during their geological lifetimes. With general anagenesis usually weak or inopera¬ tive, and with effective organismal selection concentrated at the origin of new species and their differentia (and thus also limited to the cycle time of species themselves), species selection can become a predominant process. (2) The population size of species in clades may be low, but each event of speciation must produce difference from parental traits (at least enough to yield reproductive isolation)—whereas events of organismal birth need not add any new variation to the population. The amount of change per speciational event may be large, even providing a potential macroevolutionary ana¬ log to macromutation. (At such a point, however, we must also allow some possible weakening of selection’s power as well, for macromutation, by pro¬ ducing a completed form of change in one step, deprives selection of its cre¬ ative role in building adaptation gradually—see Chapter 2.) (3) At the species level, not only does each birth of a new individual include novel variation that may be substantial, but the variation also arises in an adaptive context (whereas mutation, the source of variation at the organismic level, will usually be detrimental to the organism). Of course, the adaptive component in the production of a new species-individual need only exist at the level of its own causal origin—often the organismal level, rather than the
Species as Individuals in the Hierarchical Theory of Selection
species level itself. But the new variation will often be adaptive at the species level for two reasons: first, because species-level rather than organismic pro¬ cesses often underlie the genesis of the variation; and second, because var¬ iation caused at the organismic level will often be synergistic with species advantages, whereas mutational variation rarely enjoys synergism with the benefits of organisms. (4) The common synergism of organismal with species advantages pro¬ duces a powerful acceleration of macroevolution (Gould and Lloyd, 1999). Drives of directional speciation (often based on organismal adaptation) fre¬ quently foster species selection along the same pathway by accelerating the speciation rate or, perhaps more commonly, by enhancing the longevity of species arising in the direction of the drive. On the other hand, when or¬ ganismal selection runs counter to the interests of species, negative species se¬ lection may provide the only effective higher-level force that can act as a gov¬ ernor to slow or stymie the trend—probably a common feature in phylogeny, and previously given (in textbooks of my student days, but now rarely used) the unfelicitous and unfortunate name of “overspecialization.” As a final point and guide to understanding the essential role of the speciesindividual in macroevolution, we must remind ourselves of the highly un¬ usual character of the individual conventionally (and usually unthinkingly) taken as a paradigm for all evolutionary causality—the organism. If we view evolutionary change as tripartite in causal nature—with drive, selection, and drift as the three major modes—then we may say that the organism allows se¬ lection to reign nearly supreme by “clearing out” the surrounding space of the other two processes. Drives do not seem important at the organismal level, because drives emerge from below, and organisms, as repeatedly em¬ phasized in this chapter, work so effectively as suppressors of lower-level se¬ lection. At the same time, drift produces limited impact at the organismal level because population sizes tend to be too large in most circumstances, and because the high degree of functional integration within organisms grants a selective significance to nearly every part, thus lowering the relative frequency of substantial neutrality in potential sites for drift. Therefore, selection based on organismal properties reigns at this canonical level—thus engendering the two great parochial prejudices of the strict Darwinian world view: the adaptationist program as a guide to nearly all evolutionary phenomena, and the virtual restriction of causality to natural selection working at the single level of organisms (two of the three legs of Darwin’s tripod in the terms of this book). But when we turn to the species level, we find an interesting partnership among the three causal forces of drive, selection and drift. Selection at the species level does not “clear out” these surrounding forces. Drives from be¬ low exert great influence in the phenomenon of directional speciation. Drift maintains similar impact in both its major manifestations: as species drift for the transformation of collectivities (clades); and as founder drift in differen¬ tial proliferation or reduction of subclades by accidents of propitious or limit¬ ing colonization. This absence of “clearing out” denotes no failure or weak-
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THE STRUCTURE OF EVOEUTIONARY THEORY
ness of selection at the species level, but should rather be viewed as a different “strategy” for the distinct and effective world of macroevolution. Higherlevel selection does not bestride this larger world like the colossus of its ana¬ log at the organismal level. But higher-level ^selection gains a different kind of strength and interest in its fascinating and fruitful synergism (and opposition) with drives from below and with drift in the collegiality of its own domain.
CHAPTER NINE
Punctuated Equilibrium and the Validation of Macroevolutionary Theory
What Every Paleontologist Knows AN INTRODUCTORY EXAMPLE If Hugh Falconer (1808-1865) had not died before writing his major and synthetic works, he might be remembered today as perhaps the greatest verte¬ brate paleontologist of the late 19th century. Falconer went to India in 1830 as a surgeon for the East India Company, but spent most of his time as a natu¬ ralist in two very different realms. In 1832, he became superintendent of the botanical garden at Saharanpur, at the base of the Siwaliks, a “foothill” range of the Himalayas. There he played a major role in fostering the cultivation of Indian tea, but he also collected and described one of the most famous and important of all fossil faunas, the Tertiary mammalian remains of the Siwalik Hills. Broken health forced a return to England in 1842, where he worked for several years on the collection of Indian fossils at the British Museum. He then returned to India, this time as professor of botany at Calcutta Medical College, but declining health forced his permanent repatriation to England in 1855. During the last decade of his life, Falconer studied the late Tertiary and Quaternary mammals of Europe and North America, particularly the history of fossil elephants. Colleagues revered Falconer for his prodigious memory, his gargantuan ca¬ pacity for work, and his inexhaustible attention to the minutest details. Dar¬ win, as discussed in Chapter 1 (pp. 1-6), held immense respect for Falconer, and invested much hope and trepidation in the prospect that such a master of detail might be persuaded about the probable truth of evolution. Among all his observations and general conclusions, Falconer took greatest interest in the stability he observed in species of fossil vertebrates, often through long geological periods, and across such maximal changes of envi¬ ronment as the recent glacial ages. Falconer, of course, began with the usual assumption that such stability implied creation and permanence of species. Darwin included him among the great paleontologists who supported such a view. Noting the strength of this opposition to evolution, Darwin wrote
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THE STRUCTURE OF EVOLUTIONARY THEORY (1859, p. 310): “We see this in the plainest manner by the fact that all the most eminent paleontologists, namely Cuvier, Agassiz, Barrande, Falconer, E. Forbes, etc. . . . have unanimously, often vehemently, maintained the immuta¬ bility of species.” Darwin sent Falconer a copy of the first edition of the Origin of Species, preceded by the following note (letter of November 11, 1859): “Lord, how savage you will be, if you read it, and how you will long to crucify me alive! I fear it will produce no other effect on you; but if it should stagger you in ever so slight a degree, in this case, I am fully convinced that you will become, year after year, less fixed in your belief in the immutability of species. With this au¬ dacious and presumptuous conviction, I remain, my dear Falconer, Yours most truly, Charles Darwin.” (Several years before, Darwin had chosen Fal¬ coner as one of the very few scientists to whom he confided his beliefs about evolution. Falconer had not, to say the least, reacted positively. In a letter to Hooker on October 13, 1858, Darwin had written of Falconer’s jocular, but entirely serious, response: “. . . dear old Falconer, who some few years ago once told me that I should do more harm than any ten other naturalists would do good, [and] that I had half-spoiled you already!”) Falconer wrote to Darwin on June 23, 1861, expressing his great respect (and that of so many others) for the Origin, though not his agreement: “My dear Darwin, I have been rambling through the north of Italy, and Germany lately. Everywhere have I heard your views and your admirable essay can¬ vassed—the views of course often dissented from, according to the special bias of the speaker—but the work, its honesty of purpose, grandeur of con¬ ception, felicity of illustration, and courageous exposition, always referred to in terms of the highest admiration. And among your warmest friends no one rejoiced more heartily in the just appreciation of Charles Darwin than did, Yours very truly, H. Falconer.” Darwin, greatly relieved, replied the next day: “I shall keep your note amongst a very few precious letters. Your kindness has quite touched me.” Hugh Falconer did reassess his worldview, and did accept the principle of evolution (though not causality by natural selection)—but only within the context of the one overarching phenomenon that so strongly governed the nature of the fossil record according to his extensive and meticulous observa¬ tions: the longterm stability of fossil species, even through major environ¬ mental changes. Falconer published his reassessment in an 1863 monograph entitled: “On the American fossil elephant of the regions bordering the Gulf of Mexico (E. columbi, Falc.); with general observations on the living and ex¬ tinct species.” But he first sent a copy of the manuscript to Darwin (on Sep¬ tember 24, 1862), in eager anticipation of Darwin’s reaction to his new views. In the first paragraph of his letter, Falconer reemphasized the stability of spe¬ cies through great climatic changes, arguing that any evolutionary account must deal with this primary fact of paleontology: Do not be frightened at the enclosure. I wish to set myself right by you before I go to press. I am bringing out a heavy memoir on elephants—an
omnium gatherum affair, with observations on the fossil and recent spe-
Punctuated Equilibrium and the Validation of Macroevolutionary Theory cies. One section is devoted to the persistence in time of the specific char¬ acters of the mammoth. I trace him from before the Glacial period, through it and after it, unchangeable and unchanged as far as the organs of digestion (teeth) and locomotion are concerned. Now, the Glacial pe¬ riod was no joke: it would have made ducks and drakes of your dear pi¬ geons and doves. Darwin, of course, was delighted. He wrote to Lyell on October 1, 1862: “I found here a short and very kind note of Falconer, with some pages of his ‘El¬ ephant Memoir,’ which will be published, in which he treats admirably on long persistence of type. I thought he was going to make a good and crush¬ ing attack on me, but, to my great satisfaction, he ends by pointing out a loophole, and adds, ‘. . . The most rational view seems to be that they [Mam¬ moths] are the modified descendants of earlier progenitors, etc.’ This is cap¬ ital. There will not be soon one good paleontologist who believes in immuta¬ bility.” If we turn to the key section of Falconer’s 1863 monograph, entitled “per¬ sistence in time of the distinctive characters of the European fossil elephants,” we can trace the development of an important evolutionary argument (I am quoting from the posthumous two-volume 1868 collection of Falconer’s com¬ plete works). Falconer begins with his basic claim about the constancy of spe¬ cies: “If there is one fact, which is impressed on the conviction of the observer with more force than any other, it is the persistence and uniformity of the characters of the molar teeth in the earliest known Mammoth, and his most modern successor” (p. 252). Falconer then extends his observations from this single species to the entire clade of European fossil elephants: “Taking the group of four European fossil species ... do they show any signs, in the suc¬ cessive deposits of a transition from the one form into the other? Here again, the result of my observation, in so far as it has extended over the European area, is, that the specific characters of the molars are constant in each, within a moderate range of variation, and that we nowhere meet with intermediate forms” (p. 253). Falconer finds this constancy all the more significant, given the extreme climatic variation of the glacial ages: “If we cast a glance back on the long vista of physical changes which our planet has undergone since the Neozoic Epoch, we can nowhere detect signs of a revolution more sudden and pro¬ nounced, or more important in its results, than the intercalation and subse¬ quent disappearance of the Glacial period. Yet the ‘dicyclotherian’ Mammoth lived before it, and passed through the ordeal of all the hard extremities which it involved, bearing his organs of locomotion and digestion all but un¬ changed” (pp. 252-253). But Falconer then declines to use these observations of stability and sudden geological appearance without intermediates as evidence for special creation. He proclaims himself satisfied with Darwin’s basic evolutionary premise, and draws the obvious inference that new species of elephants did not evolve by transformation of older European species, but must have emerged from other stocks:
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THE STRUCTURE OF EVOLUTIONARY THEORY The inferences which I draw from these facts are not opposed to one of the leading propositions of Darwin’s theory. With him I have no faith in the opinion that the Mammoth and other extinct Elephants made their appearance suddenly, after the type in which their fossil remains are pre¬ sented to us. The most rational view seems to be, that they are in some shape the modified descendants of earlier progenitors. But if the asserted facts be correct, they seem clearly to indicate that the older Elephants of Europe . . . were not the stocks from which the later species . . . sprung, and that we must look elsewhere for their origin (pp. 253-254). Falconer thus anticipates a primary inference of punctuated equilibrium— that a local pattern of abrupt replacement does not signify macromutational transformation in situ, but an origin of the later species from an ancestral population living elsewhere, followed by migration into the local region. Fal¬ coner suggests that the ancestry of later European species may be sought among Miocene species in India: “The nearest affinity, and that a very close one ... is with the Miocene ... of India” (p. 254). Falconer then summarizes the puzzles that such stability—of such longlasting, widespread forms in such variable environments—raises for evolu¬ tionary theory: “The whole range of the Mammalia, fossil and recent, cannot furnish a species which has had a wider geographical distribution, and at the same time passed through a longer term of time, and through more extreme changes of climatal (sic) conditions, than the Mammoth. If species are so un¬ stable, and so susceptible of mutation through such influences, why does that extinct form stand out so signally, a monument of stability?” (p. 254). Darwin’s reaction to these famous pages in the history of paleontology make fascinating reading, especially in the light of persistence (or reemer¬ gence) of all major issues in our modern debate about punctuated equilib¬ rium. First, with his usual insight into the mechanics of his own theory, Darwin expresses special surprise that teeth should be so stable within spe¬ cies—for the same features vary so greatly among species. As many modern evolutionists have remarked—though Darwin did not use the same terminol¬ ogy—natural selection works by converting variation within populations to differences among populations: a primary expression of the extrapolationist principle in Darwinian logic. But the stasis of species challenges such continuationism. (Darwin included his remarks in a long letter to Falconer, writ¬ ten on October 1, 1862, as a response to the manuscript on elephants that Falconer had sent Darwin, and that would become the 1863 publication quoted above): “Your case seems the most striking one which I have met with of the persistence of specific characters. It is very much the more striking as it relates to the molar teeth, which differ so much in the species of the genus, and in which consequently I should have expected variation.” Darwin then searches for ways to mitigate the surprise of such stasis in the face of environmental changes that should have altered selective pressures. He suggests, first, that the global fluctuations of ice-age climates might not have seemed so extensive to elephants. Perhaps they migrated with a favored climatic belt, therefore experiencing little fluctuation, and perhaps no major
Punctuated Equilibrium and the Validation of Macroevolutionary Theory
selective pressures for change: “You speak of these animals as having being exposed to a vast range of climatal changes from before to after the Glacial period. I should have thought, from analogy of sea-shells, that by migration (or local extinction when migration is not possible) these animals might and would have kept under nearly the same climate.” Searching for another way to explain the absence of anticipated (and grad¬ ual) change, Darwin then argued that altering climates may generally imply evolutionary modification, but that groups in serious decline, including ele¬ phants, often become stalled in their capacity to vary, and especially to form new taxa: “A rather more important consideration, as it seems to me, is that the whole proboscidean group may, I presume, be looked at as verging to¬ wards extinction . . . Numerous considerations and facts have led me in the
Origin to conclude that it is the flourishing or dominant members of each or¬ der which generally give rise to new races, sub-species, and species; and under this point of view I am not at all surprised at the constancy of your species.” But if Darwin had not been surprised, or at least disturbed, why did he try so hard to reconcile this unexpected phenomenon with his general theory? Fal¬ coner, in any case, replied that elephants remained in vigor, and could not be considered as a group on the verge of elimination. I recount this story at some length, as an introduction to punctuated equi¬ librium, both because Falconer and Darwin presage in such a striking man¬ ner, the main positions of supporters and opponents (respectively) of punctu¬ ated equilibrium in our generation, and because the tale itself illustrates the central fact of the fossil record so well—-geologically abrupt origin and subse¬ quent extended stasis of most species. Falconer, especially, illustrates the tran¬ sition from too easy a false resolution under creationist premises, to recogniz¬ ing a puzzle (and proposing some interesting solutions) within the new world of evolutionary explanation. Most importantly, this tale exemplifies what may be called the cardinal and dominant fact of the fossil record, something that professional paleontologists learned as soon as they developed tools for an adequate stratigraphic tracing of fossils through time: the great majority of species appear with geological abruptness in the fossil record and then per¬ sist in stasis until their extinction. Anatomy may fluctuate through time, but the last remnants of a species usually look pretty much like the first represen¬ tatives. In proposing punctuated equilibrium, Eldredge and I did not discover, or even rediscover, this fundamental fact of the fossil record. Paleontologists have always recognized the longterm stability of most species, but we had be¬ come more than a bit ashamed by this strong and literal signal, for the domi¬ nant theory of our scientific culture told us to look for the opposite result of gradualism as the primary empirical expression of every biologist’s favorite subject—evolution itself.
TESTIMONIALS TO COMMON KNOWLEDGE
The common knowledge of a profession often goes unrecorded in technical literature for two reasons: one need not preach commonplaces to the initi¬ ated; and one should not attempt to inform the uninitiated in publications
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THE STRUCTURE OF EVOEUTIONARY THEORY they do not read. The longterm stasis, following a geologically abrupt origin, of most fossil morphospecies, has always been recognized by professional pa¬ leontologists, as the previous story of Hugh Falconer testifies. This fact, as discussed on the next page, established a basis for bistratigraphic practice, the primary professional role for paleontology during most of its history. But another reason, beyond tacitly shared knowledge, soon arose to drive stasis more actively into textual silence. Darwinian evolution became the great intellectual novelty of the later 19th century, and paleontology held the archives of life’s history. Darwin proclaimed insensibly gradual transition as the canonical expectation for evolution’s expression in the fossil record. He knew, of course, that the detailed histories of species rarely show such a pat¬ tern, so he explained the literal appearance of stasis and abrupt replacement as an artifact of a woefully imperfect fossil record. Thus, paleontologists could be good Darwinians and still acknowledge the primary fact of their profession—but only at the price of sheepishness or embarrassment. No one can take great comfort when the primary observation of their discipline be¬ comes an artifact of limited evidence rather than an expression of nature’s ways. Thus, once gradualism emerged as the expected pattern for document¬ ing evolution—with an evident implication that the fossil record’s dominant signal of stasis and abrupt replacement can only be a sign of evidentiary pov¬ erty—paleontologists became cowed or puzzled, and even less likely to show¬ case their primary datum. But this puzzlement did sometimes break through to overt statement. For example, in 1903, H. F. Cleland, a paleontologist’s paleontologist—that is, a respected expert on local minutiae, but not a general theorist—wrote of the famous Devonian Hamilton section in New York State (which has since be¬ come the “type” for an important potential extension of punctuated equilib¬ rium to the integrated behavior of entire faunas, the hypothesis of “coordi¬ nated stasis”—see pp. 916-922): In a section such as that of the Hamilton formation at Cayuga Lake ... if the statement natura non facit saltum is granted, one should, with some confidence, expect to find many—at least some—evidences of evolution. A careful examination of the fossils of all the zones, from the lowest to the highest, failed to reveal any evolutional changes, with the possible exception of Ambocoelia praeumbona fa brachiopod]. The species are as distinct or as variable in one portion of the section as in another. Species varied in shape, in size, and in surface markings, but these changes were not progressive. The conclusion must be that. . . the evolution of brachiopods, gastropods, and pelecypods either does not take place at all or takes place very seldom, and that it makes little difference how much time elapses so long as the conditions of environment remain unchanged (quoted in Brett, Ivany, and Schopf, 1996, p. 2). But far better than such explicit testimonies—and following various gastronomical metaphors about the primacy of practice (knowing by fruits, proofs of the pudding, etc.)—the most persuasive testimony about dominant
Punctuated Equilibrium and the Validation of Macroevolutionary Theory
stasis and abrupt appearance inheres, without conscious intent or formula¬ tion, in methods developed by the people who use fossils in their daily, prac¬ tical work. Evolutionary theory may be a wonderful intellectual frill, but workaday paleontology, until very recently, used fossils primarily in the im¬ mensely useful activity (in mining, mapping, finding oil, etc.) of dating rocks and determining their stratigraphic sequence. These practical paleontologists dared not be wrong in setting their criteria for designating ages and environ¬ ments. They had to develop the most precise system that empirical recogni¬ tion could supply for specifying the age of a stratum; they could not let theory dictate a fancy expectation unsupported by observation. Whom would you hire if you wanted to build a bridge across your local stream—the mason with a hundred spans to his credit, or the abstract geometer who has never left his ivory tower? When in doubt, trust the practitioner. If most fossil species changed gradually during their geological lifetimes, biostratigraphers would have codified “stage of evolution” as the primary criterion for dating by fossils. In a world dominated by gradualism, maximal resolution would be obtained by specifying a precise stratigraphic position within a continuum of steady change, and much information would be lost by listing only the general name of a species rather than its immediate state within a smooth transition. But, in fact, biostratigraphers treat species as sta¬ ble entities throughout their documented ranges—because the vast majority so appear in the empirical record. Finer resolution can then be obtained by two major strategies: first, by identifying species with unusually short dura¬ tions, but wide geographic spread (so-called “index fossils”); and, second, by documenting the differing ranges of many species within a fauna and then us¬ ing the principle of “overlapping range zones” to designate geological mo¬ ments of joint occurrence for several taxa (see Fig. 9-1).
H C B
9-1. Knowledge by working bio¬ stratigraphers of stasis observed in the vast majority of fossil species led these field scientists, for practical and not for theoretical reasons, to use the criterion of “overlapping range zones” for maximal precision in stratigraphic correlation. If most species changed gradualistically within their geological lifetime, stage of evolution within individual species would provide a better criterion for correlation.
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THE STRUCTURE OF EVOLUTIONARY THEORY
This peculiar situation of discordance between the knowledge of practical experts and the expectation of theorists impressed Eldredge and me deeply when we formulated punctuated equilibrium. We therefore made the follow¬ ing remarks in closing our first paper on the application of our model to biostratigraphy (Eldredge and Gould, 1977): \
'
[We] wondered why evolutionary paleontologists have continued to seek, for over a century and almost always in vain, the “insensibly graded series” that Darwin told us to find. Biostratigraphers have known for years that morphological stability, particularly in characters that al¬ low us to recognize species-level taxa, is the rule, not the exception. It is time for evolutionary theory to catch up with empirical paleontology, to confront the phenomenon of evolutionary non-change, and to incorpo¬ rate it into our theory, rather than simply explain it away . . . We believe that, unconsciously, biostratigraphic methodology has been evolutionarily based all along, since biostratigraphers have always treated their data as if species do not change much during their [residence in any lo¬ cal section], are tolerably distinguishable from their nearest relatives, and do not grade insensibly into their close relatives in adjacent strati¬ graphic horizons . . . Biostratigraphers, thankfully, have ignored theories of speciation, since the only one traditionally available to them has not made much sense. To date, evolutionary theory owes more to biostra¬ tigraphy than vice versa. Perhaps in the future evolutionary theory can begin to repay its debt. Finally, the witness of experts engaged in a lifelong study of particular groups and times provides especially persuasive testimony because, as I have emphasized throughout this book, natural history is a science of relative fre¬ quencies, not of unique cases, however well documented. We* have never doubted that examples of both gradualism and punctuation can be found in the history of almost any group. The debate about punctuated equilibrium rests upon our claim for a dominant relative frequency, not for mere occur¬ rence. The summed experiences of long and distinguished careers therefore provide a good basis for proper assessment. The paleontological literature, particularly in the “summing up” articles of dedicated specialists, abounds in testimony for predominant stasis, often viewed as surprising, anomalous, or even a bit embarrassing, because such experts had been trained to expect gradualism, particularly as the reward of diligent study. To choose some examples in just three prominent fossil groups representing the full span of conventional “complexity” in the invertebrate record, most microorganisms seem to show predominant stasis—despite the excellent documentation of a few “best cases” of gradualism in Cenozoic planktonic Foraminifera (see pp. 803-810). For example, MacGillavry *1 may be an arrogant man, but I would never be so pompous as to use the ‘royal’ we. I cannot separate my views on punctuated equilibrium from those of my colleague and part¬ ner in this venture from the start, Niles Eldredge. When I write ‘we’ in this section, I mean ‘Eldredge and Gould.’
Punctuated Equilibrium and the Validation of Macroevolutionary Theory (1968, p. 70) wrote from long practical experience: “During my work as an oil paleontologist, I had the opportunity to study sections meeting these rigid requirements [of continuous sedimentation and sufficient span of time]. As an ardent student of evolution, moreover, I was continually on the watch for evidence of evolutionary change . . . The great majority of species do not show any appreciable evolutionary change at all. These species appear in the section (first occurrence) without obvious ancestors in underlying beds, are stable once established, and disappear higher up without leaving any descen¬ dants.” Echoing the hopes and disappointments of many paleontologists (including both Eldredge and me), who trained themselves in statistical methods primar¬ ily to find the “subtle” cases of gradualism that had eluded traditional, sub¬ jective observation, Reyment (1975, p. 665) wrote: “The occurrences of long sequences within species are common in boreholes and it is possible to exploit the statistical properties of such sequences in detailed biostratigraphy. It is noteworthy that gradual, directed transitions from one species to another do not seem to exist in borehole samples of microorganisms.” Moving to a metazoan group generally regarded as relatively “simple” in form, and especially prominent in the fossil record, particularly in Paleozoic strata, Roberts (1981, p. 123) concluded from many years of studying Aus¬ tralian Carboniferous brachiopods: “There is no evidence of ‘gradualistic’ evolutionary processes affecting brachiopod species either within or between zones, and the succession of faunas can be regarded as ‘punctuated.’” Johnson (1975), inspired by Ziegler’s (1966) documentation of one puta¬ tively gradualistic sequence in the brachiopod Eocoelia, decided to search for others—and found only examples of punctuation and stasis throughout the Paleozoic record. He wrote (1975, p. 657): After completion of Ziegler’s paper we talked a number of times about the possibilities of duplicating his efforts with other fossils and in other times. It was a heady prospect ... In subsequent years many workers have attempted to seek out and define lineages of brachiopod species and other megafossils in the lower and middle Paleozoic with little success. My conclusion, subjective in many ways, is that speciation of brachio¬ pods in the mid-Paleozoic via a phyletic mode has been rare. Rather, it is probable that most new brachiopod species of this age originated by allopatric speciation. Derek Ager, the world leader in studying later Mesozoic brachiopods, summed up his lifelong effort in several papers towards the end of his career. He wrote (1973, p. 20): “In twenty years work on the Mesozoic Brachiopods, I have found plenty of relationships, but few if any evolving lineages . . . What it seems to mean is that evolution did not normally proceed by a process of gradual change of one species into another over long periods of time.” Ten years later (1983, p. 563), Ager reiterated: “The general picture seems to fit in with the Gouldian doctrine of ‘hardly ever’ [that is, documentation of gradu¬ alism only very rarely]. Certainly there is no evidence in the group as a whole
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THE STRUCTURE OE EVOLUTIONARY THEORY of phyletic gradualism happening throughout a species at any one moment in time. Species A never changes into species B everywhere simultaneously and gradually.” When we consider trilobites, the exerpplars of Paleozoic invertebrate “complexity,” Robison (1975, p. 220) concluded from extensive study of Middle Cambrian agnostid trilobites in Western North America: “I have found a conspicuous lack of intergradation in species-specific characters, and I have also found little or no change in these characters throughout the ob¬ served stratigraphic ranges of most species.” Fortey (1985) spent many years studying a particularly favorable sequence for fine-scale temporal resolution from the early Ordovician of Spitzbergen. He examined 111 trilobite and 56 graptoloid species, finding a predominance of punctuated equilibrium in both groups—with gradualism in “less than 10 percent of the total” for trilobites, and, for graptoloid species, with punctuational origins “at least four times as important as gradualistic ones” (1985, p. 27). Fortey’s case becomes particularly convincing because he could calibrate punctuational sequences against rarer cases of gradualism in the same strata—and therefore be confident that punctuations do not merely rep¬ resent the missing strata of conventional gradualistic rates. In a later paper, Fortey, who is, by the way, no partisan of punctuated equilibrium, reaches the following general conclusion, and also affirms our point about respect for the age-old knowledge of biostratigraphic practitioners: “Many invertebrate pa¬ leontologists would agree that the fossil record of species of their groups is dominated by lack of change—by stasis—and that where phylogenies have been worked out then the evidence direct from the rocks shows punctuated lineages in a majority of cases. For reasons I have explained, it is likely that stratigraphic paleontologists would always have maintained such a view, but the difference is that now this would be accepted by paleobiologists as well” (1988, p. 13). Moving to a different arthropod group from another time, Coope’s famous studies of Late Cenozoic fossil beetles (summarized in Coope, 1979) provide one of our best cases for dominance of the punctuational mode. Unusually good preservation greatly increases the power of this example. Coope dis¬ cusses his best case (for beetles extracted from the carcasses of woolly rhinos in the western Ukraine), but then extends his argument to most examples: Here the complete beetles were preserved down to the tarsal and anten¬ nal joints; when the elytra were raised, the wings could be unfolded and mounted; and parasitic mites, both larvae and adults, were found under¬ neath the wings. Although this was quite exceptional preservation, it is common to find intact abdomens from which the genitalia can be dis¬ sected; the frequently transparent integument often reveals detailed structures of the internal sclerites. Preservation is frequently adequate to enable details of the microstructure of the surface of the hairs and scales to be examined with scanning electron microscopy (1979, p. 248).
Punctuated Equilibrium and the Validation of Macroevolutionary Theory
Coope concluded that most species showed extensive stasis, even with such detail available for observation: “The early Pleistocene fossils, probably dat¬ ing from over a million years ago, are referable to living species and some ex¬ isting species extend well back into the late Tertiary” (1979, p. 250). In what I regard as the most fascinating and revealing comment of all, George Gaylord Simpson, the greatest and most biologically astute paleontol¬ ogist of the 20th century (and a strong opponent of punctuated equilibrium in his later years), acknowledged the literal appearance of stasis and geologi¬ cally abrupt origin as the outstanding general fact of the fossil record, and as a pattern that would “pose one of the most important theoretical problems in the whole history of life” if Darwin’s argument for artifactual status failed. Simpson stated at the 1959 Chicago centennial celebration for the Origin of Species (in Tax, 1960, p. 149): It is a feature of the known fossil record that most taxa appear abruptly. They are not, as a rule, led up to by a sequence of almost imperceptibly changing forerunners such as Darwin believed should be usual in evolu¬ tion. A great many sequences of two or a few temporally intergrading species are known, but even at this level most species appear without known intermediate ancestors, and really, perfectly complete sequences of numerous species are exceedingly rare. . . . These peculiarities of the record pose one of the most important theoretical problems in the whole history of life: is the sudden appearance ... a phenomenon of evolution or of the record only, due to sampling bias and other inadequacies? Such a discordance between theoretical expectation and actual observation surely falls within the category of troubling “anomalies” that, in Kuhn’s cele¬ brated view of scientific change (1962), often spur a major reformulation.
DARWINIAN SOLUTIONS AND PARADOXES
Only one chapter of the Origin of Species bears an apologetic title—ironi¬ cally, for the subject that should have provided the crown of direct evidence for evolution in the large: the archive of life’s actual history as displayed in the fossil record. Yet Darwin entitled Chapter 9 “On the Imperfection of the Geological Record.” In Chapter 2 (pp. 146-155), I discussed Darwin’s convictions about gradu¬ alism, and the crucial link between his defense of natural selection and one of the three major and disparate claims subsumed within this complex concept: the insensibility of intermediacy. The theory of punctuated equilibrium does not engage this important meaning for two reasons: first, our theory does not question the operation of natural selection at its conventional organismic level; second, as a theory about the deployment of speciation events in macro¬ evolutionary time, punctuated equilibrium explains how the insensible inter¬ mediacy of human timescales can yield a punctuational pattern in geological perspective—thus requiring the treatment of species as evolutionary individu-
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THE STRUCTURE OF EVOLUTIONARY THEORY als, and precluding the explanation of trends and other macroevolutionary patterns as extrapolations of anagenesis within populations. Rather, punctuated equilibrium refutes the third and most general mean¬ ing of Darwinian gradualism, designated in Chapter 2 (see pp. 152-155) as “slowness and smoothness (but not constancy) of rate.” Natural selection does not require or imply this degree of geological sloth and smoothness, though Darwin frequently, and falsely, linked the two concepts—as Huxley tried so forcefully to advise him, though in vain, with his famous warning: “you have loaded yourself with an unnecessary difficulty in adopting Natura non facit saltum so unreservedly.” The crucial error of Dawkins (1986) and several other critics lies in their failure to recognize the theoretical importance of this third meaning, the domain that punctuated equilibrium does chal¬ lenge. Dawkins correctly notes that we do not question the second meaning of insensible intermediacy. But since his extrapolatiomst view leads him to re¬ gard only this second meaning as vital to the rule of natural selection, he dis¬ misses the third meaning—which we do confute—as trivial. Since Dawkins rejects the hierarchical model of selection, he does not grant himself the con¬ ceptual space for weighing the claim that punctuated equilibrium’s critique of the third meaning undermines the crucial Darwinian strategy for rendering all scales of evolution by smooth extrapolation from the organismic level. For this refutation of extrapolation by punctuated equilibrium validates the treat¬ ment of species as evolutionary individuals, and establishes the level of spe¬ cies selection as a potentially important contributor to macroevolutionary pattern. This broadest third meaning of gradualism may not be required for natural selection at the organismic level, but gradualism as slowness and smoothness of rate (not just as insensible intermediacy between endpoints of a transition) forms the centerpiece of Darwin’s larger worldview, indeed of his entire on¬ tology—as illustrated (again, see Chapter 2) in the crucial role played by this style of gradualism throughout the corpus of his works—from his first book on the origin of coral atolls (1842) to his last on the formation of topsoil by the action of worms (1881). Test anyone doubt that Darwin strongly advocated this most inclusive form of gradualism as slowness and smoothness (in addition to the less com¬ prehensive claim for insensible intermediacy of transitions), I shall cite a few examples from the full documentation of Chapter 2—cases where Darwin clearly meant “slow and steady over geological scales,” not just “insensibly intermediate at whatever rate.” For example Darwin argues that species may arise so slowly that the pro¬ cess generally takes longer than the entire duration of a geological formation (usually several million years)—thus explaining apparent stasis within a for¬ mation as gradual evolution over insufficient time to record visible change! Darwin writes (1859, p. 293): “Although each formation may mark a very long lapse of years, each perhaps is short compared with the period requisite to change one species into another.” Darwin even argued that the pace of evo¬ lutionary change might be sufficiently steady to serve as a rough geological
Punctuated Equilibrium and the Validation of Macroevolutionary Theory clock: “The amount of organic change in the fossils of consecutive forma¬ tions probably serves as a fair measure of the lapse of actual time” (1859, p. 488). I also show in Chapter 2 that Darwin’s conviction about extreme slowness and steadiness of change can be grasped, perhaps best of all, as the common source of his major errors—particularly his fivefold overestimate for the denudation of the Weald, and his conjecture that complex metazoan life of modern form must have undergone an unrecorded Precambrian history as long, or longer, than its known Phanerozoic duration. Despite this strong belief in geological gradualism, Darwin knew perfectly well—as all paleontologists always have—that stasis and abrupt appearance represent a norm for the observed history of most species. I needn’t rehearse Darwin’s solution to this dilemma, for his familiar argument represents more than a twice-told tale. Following the lead of his mentor, Charles Lyell, Dar¬ win attributed this striking discordance between theoretical expectation and actual observation to the extreme imperfection of the fossil record. (As discussed more fully on pages 479-484, this argument served as the centerpiece for Lyell’s system, and for the entire uniformitarian school. But then, what alternative could they embrace? The literal appearance of the geo¬ logical record so often suggested catastrophe, or at least “moments” of sub¬ stantial change, especially in faunal turnover. To assert a gradualism of geo¬ logical rate against this sensory evidence, one had to declare the evidence illusory by advancing the general claim—quite legitimate as a philosophical proposition—that science must often work by probing “behind appearance” to impose the expectations of a valid theory upon an empirical record that, for one reason or another, cannot directly express the actual mechanisms of nature. Moreover, the “argument from imperfection” holds substantial merit and cannot be dismissed as “special pleading.” Like most chronicles of his¬ tory, and far more so than many others, the geological record is extremely spotty. To cite Lyell’s famous metaphor once again, if Vesuvius erupted again and buried a modern Italian city atop Pompeii, later stratigraphers might find a sequence of Roman ruins capped by layers of volcanic ash and followed by the debris of modern Italy. Taken literally, this sequence would suggest a cata¬ strophic end to Rome followed by a saltation, linguistically and technologi¬ cally, to the industrial age—an artifact of nearly 2000 years of missing data that would have recorded the evolution of Italian from Latin and a gradual passage from walled cities to traffic jams.) To quote the two most famous statements on this subject from the Origin of Species, Darwin summarizes his entire argument by closing Chapter 9 with Lyell’s metaphor of the book (1859, pp. 310-311): For my part, following out Lyell’s metaphor, I look at the natural geolog¬ ical record, as a history of the world imperfectly kept, and written in a changing dialect; of this history we possess the last volume alone, relat¬ ing only to two or three countries. Of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines. Each word of the slowly-changing language, in which the his-
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THE STRUCTURE OF EVOLUTIONARY THEORY tory is supposed to be written, being more or less different in the inter¬ rupted succession of chapters, may represent the apparently abruptly changed forms of life, entombed in our consecutive, but widely sepa¬ rated, formations. In epitomizing both geological chapters, Darwin begins with a long list of reasons for such an imperfect record, and then concludes with his characteris¬ tic honesty (1859, p. 342): “All these causes taken conjointly, must have tended to make the geological record extremely imperfect, and will to a large extent explain why we do not find interminable varieties, connecting together all the extinct and existing forms of life by the finest graduated steps. He who rejects these views on the nature of the geological record, will rightly reject my whole theory.” (Huxley must have been thinking of this line when he issued his warning that Darwin’s unswerving support of natura non facit saltum represented “an unnecessary difficulty.” Darwin’s “whole theory”— the mechanism of natural selection—does not require, as Huxley pointed out, this geological style of gradualism in rate.) The paradoxes set by Darwin’s solution for the current practice of paleon¬ tology and macroevolutionary theory receive their clearest expression in an¬ other remarkable statement from the Origin of Species (1859, p. 302), a testi¬ mony to Darwin’s sophisticated understanding that nature’s “facts” do not stand before us in pristine objectivity, but must be embedded within theories to make any sense, or even to be “seen” at all. Darwin acknowledges that he only understood the extreme imperfection of the geological record when paleontological evidence of stasis and abrupt appearance threatened to con¬ fute the gradualism that he “knew” to be true: “But I do not pretend that I should ever have suspected how poor a record of the mutations of life, the best preserved geological section presented, had not the difficulty of our not discovering innumerable transitional links between the species which ap¬ peared at the commencement and close of each formation, pressed so hardly on my theory.”
The paradox of insulation from disproof The “argument from imperfection” (with its preposition purposefully chosen by analogy to the “argument from design”) works adequately as a device to save gradualism in the face of an empirical signal of quite stunning contrari¬ ness when read at face value. But if we adopt openness to empirical falsifica¬ tion as a criterion for strong and active theories in science, consider the empty protection awarded to gradualism by Darwin’s strategy. For the data that should, prima facie, rank as the most basic empirical counterweight to gradu¬ alism—namely the catalog of cases, and the resulting relative frequency, for observed stasis and geologically abrupt appearances of fossil morphospecies—receive a priori interpretation as signs of an inadequate empirical re¬ cord. How then could gradualism be refuted from within? The situation became even more insidious in subtle practice than a bald statement of the dilemma might suggest. Abrupt appearance (the punctu-
Punctuated Equilibrium and the Validation of Macroevolutionary Theory ations of punctuated equilibrium) might well be attributed to the admittedly gross imperfection of our geological archives. The argument makes logical sense, must certainly be true in many instances, and can be tested in a variety of ways on a case by case basis (particularly when we can obtain independent evidence about rates of sedimentation). But how can imperfection possibly explain away stasis (the equilibrium of punctuated equilibrium)? Abrupt appearance may record an absence of infor¬ mation, but stasis is data. Eldredge and I became so frustrated by the failure of many colleagues to grasp this evident point—though a quarter century of subsequent debate has finally propelled our claim to general acceptance (while much else about punctuated equilibrium remains controversial)—that we urged the incorporation of this little phrase as a mantra or motto. Say it ten times before breakfast every day for a week, and the argument will surely seep in by osmosis: “stasis is data; stasis is data ...” The fossil record may, after all, be 99 percent imperfect, but if you can, nonetheless, sample a species at a large number of horizons well spread over several million years, and if these samples record no net change, with begin¬ ning and end points substantially the same, and with only mild and errant fluctuation among the numerous collections in between, then a conclusion of stasis rests on the presence of data, not on absence! In such cases, we must limit our lament about imperfection to a wry observation that nature, rather than human design, has established a sampling scheme by providing only oc¬ casional snapshots over a full interval. We might have preferred a more even temporal spacing of these snapshots, but so long as our samples span the tem¬ poral range of a species, with reasonable representation throughout, why grouse at nature’s failure to match optimal experimental design—when she has, in fact, been very kind to us in supplying abundant information. Stasis is data. So if stasis could not be explained away as missing information, how could gradualism face this most prominent signal from the fossil record? The most negative of all strategies—a quite unconscious conspiracy of silence—dic¬ tated the canonical response of paleontologists to their observations of stasis. Again, a “culprit” may be identified in the ineluctable embedding of observa¬ tion within theory. Facts have no independent existence in science, or in any human endeavor; theories grant differing weights, values, and descriptions, even to the most empirical and undeniable of observations. Darwin’s expecta¬ tions defined evolution as gradual change. Generations of paleontologists learned to equate the potential documentation of evolution with the discov¬ ery of insensible intermediacy in a sequence of fossils. In this context, stasis can only record sorrow and disappointment. Paleontologists therefore came to view stasis as just another failure to doc¬ ument evolution. Stasis existed in overwhelming abundance, as every paleon¬ tologist always knew. But this primary signal of the fossil record, defined as an absence of data for evolution, only highlighted our frustration—and cer¬ tainly did not represent anything worth publishing. Paleontology therefore fell into a literally absurd vicious circle. No one ventured to document or
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THE STRUCTURE OF EVOLUTIONARY THEORY quantify—indeed, hardly anyone even bothered to mention or publish at all—the most common pattern in the fossil record: the stasis of most morphospecies throughout their geological duration. All paleontologists recognized the phenomenon, but few scientists write papers about failure to document a desired result. As a consequence, most nonpaleontologists never learned about the predominance of stasis, and sim¬ ply assumed that gradualism must prevail, as illustrated by the exceedingly few cases that became textbook “classics’': the coiling of Gryphaea, the in¬ creasing body size of horses, etc. (Interestingly, nearly all these “classics” have since been disproved, thus providing another testimony for the tem¬ porary triumph of hope and expectation over evidence—see Gould, 1972.) Thus, when punctuated equilibrium finally granted theoretical space and im¬ portance to stasis, and this fundamental phenomenon finally emerged from the closet, nonpaleontologists were often astounded and incredulous. Mayr (1992, p. 32) wrote, for example: “Of all the claims made in the punctuationalist theory of Eldredge and Gould, the one that encountered the greatest opposition was the observation of ‘pronounced stasis as the usual fate of most species,’ after having completed the phase of origination ... 1 agree with Gould that the frequency of stasis in fossil species revealed by the recent anal¬ ysis was unexpected by most evolutionary biologists.” (To cite a personal incident that engraved this paradox upon my conscious¬ ness early in my career, John Imbrie served as one of my Ph.D. advisors at Co¬ lumbia University. This distinguished paleoclimatologist began his career as an evolutionary paleontologist. He accepted the canonical equation of evolu¬ tion with gradualism, but conjectured that our documentary failures had arisen from the subtlety of gradual change, and the consequent need for sta¬ tistical analysis in a field still dominated by an “old-fashioned” style of verbal description. He schooled himself in quantitative methods and applied this ap¬ paratus, then so exciting and novel, to the classic sequence of Devonian brachiopods from the Michigan Basin—where rates of sedimentation had been sufficiently slow and continuous to record any hypothetical gradualism. He studied more than 30 species in this novel and rigorous way—and found that all but one had remained stable throughout the interval, while the single exception exhibited an ambiguous pattern. But Imbrie did not publish a tri¬ umphant paper documenting the important phenomenon of stasis. Instead, he just become disappointed at such “negative” results after so much effort. He buried his data in a technical taxonomic monograph that no working bi¬ ologist would ever encounter (and that made no evolutionary claims at all)— and eventually left the profession for something more “productive.”) Paradoxes of this sort can only be resolved by input from outside—for gradualism, having defined contrary data either as marks of imperfection or documents of disappointment, could not be refuted from within. Reassess¬ ment required a different theory that respected stasis as a potentially fascinat¬ ing phenomenon worthy of rigorous documentation, not merely as a failure to find “evolution." Eldredge and I proposed punctuated equilibrium in this explicit context—as a framework and different theory that, if true, could vali-
Punctuated Equilibrium and the Validation of Macroevolutionary Theory date the primary signal of the fossil record as valuable information rather than frustrating failure. We therefore began our original article (Eldredge and Gould, 1972) with a philosophical discussion, based on work of Kuhn (1962) and Hanson (1961), on the necessary interbedding of fact and theory. We ended this introductory section by writing (1972, p. 86): The inductivist view forces us into a vicious circle. A theory often com¬ pels us to see the world in its light and support. Yet we think we see ob¬ jectively and therefore interpret each new datum as an independent con¬ firmation of our theory. Although our theory may be wrong, we cannot confute it. To extract ourselves from this dilemma, we must bring in a more adequate theory; it will not arise from facts collected in the old way . . . Science progresses more by the introduction of new world-views or “pictures” than by the steady accumulation of information . . . We be¬ lieve that an inadequate picture has been guiding our thoughts on speciation for 100 years. We hold that its influence has been all the more te¬ nacious because paleontologists, in claiming that they see objectively, have not recognized its guiding sway. We contend that a notion devel¬ oped elsewhere, the theory of allopatric speciation, supplies a more satis¬ factory picture for the ordering of paleontological data.
The paradox of stymied practice This second paradox cascades from the first. If a theory—geologically insen¬ sible gradualism as the anticipated expression of evolution in the fossil re¬ cord, in this case—can insulate itself against disproof from within by defining contrary data as artifactual, then proper assessments of relative frequencies can never be achieved—for how many scientists will devote a large chunk of a limited career to documenting a phenomenon that they view as a cardinal re¬ striction recording a poverty of available information? Paleontological studies of evolution therefore became warped in a lamen¬ table way that precluded any proper use of the fossil record, but seemed entirely honorable at the time. We practitioners of historical sciences, as em¬ phasized throughout this book, work in fields that decide key issues by assess¬ ment of relative frequencies among numerous possible outcomes, and only rarely by the more “classical” technique of “crucial experiments” to validate universal phenomena. Therefore, any method that grossly distorts a relative frequency by excluding a common and genuine pattern from consideration must seriously stymie our work. When traditional paleontologists eliminated examples of abrupt appearance and stasis from the documentation of evolu¬ tion, they only followed a conventional precept—for they believed that both patterns recorded an artifact of imperfect data, thus debarring such cases from consideration. The relative distributions of evolutionary rates would therefore emerge only from cases of gradualism—the sole examples judged as sufficiently data-rich to record the process of evolution in adequate empirical detail. But this project could not even succeed in its own terms, for gradualism oc-
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THE STRUCTURE OF EVOLUTIONARY THEORY curs too rarely to generate enough cases for calculating a distribution of rates. Instead, paleontologists worked by the false method of exemplification: vali¬ dation by a “textbook case” or two, provided that the chosen instances be sufficiently persuasive. And even here, at this utterly minimal level of docu¬ mentation, the method failed. A few examples did enter the literature (see Fig. 9-2 for comparison of an original claim with a secondary textbook version)— where they replicated by endless republication in the time-honored fashion of textbook copying (see Gould, 1988a). But, in a final irony, almost all these fa¬ mous exemplars turned out to be false on rigorous restudy—see Hallam, 1968, and Gould, 1972, for stasis rather than gradual increase in coiling in the Liassic oyster Grypbaea; Prothero and Shubin, 1989, on stasis within all documented species of fossil horses, and with frequent overlap between an¬ cestors and descendants, indicating branching by punctuational speciation rather than anagenetic gradualism; and Gould, 1974, on complete absence of data for the common impression that the enormous antlers of Megaloceros
A
EVOLUTION OF GRYPHAEA INCURVA (After Trueman)
9-2. Truemans original claim for phyletic gradualism in the increased coiling of Grypbaea in Lower Jurassic rocks of England (left). To the right, a textbook smoothing and simplification of the same figure. Trueman’s claim has been inval¬ idated for two reasons: first, Grypbaea did not evolve from Ostrea; and, second, subsequent studies have not validated any increase of coiling within Grypbaea, despite Trueman’s graphs. Nonetheless, once such figures become ensconced in textbooks, they tend to persist even when their empirical justification has long been refuted in professional literature.
Punctuated Equilibrium and the Validation of Macro evolutionary Theory (the “Irish Elk”) increased gradually in phylogeny, with positive allometry as body size enlarged. Traditional paleontology therefore placed itself into a straightjacket that made the practice of science effectively impossible: only a tiny percentage of cases passed muster for study at all, while the stories generated for this minuscule minority rested so precariously upon hope for finding a rare phe¬ nomenon—and received such limited definition by the primitive statistical methods then available (or, more commonly, remained unidentified by any statistical practice at all)—that even these textbook exemplars collapsed upon restudy with proper quantitative procedures. But consider what might have occurred, if only paleontologists had recognized that stasis is data (I will grant some validity to the standard rationale for regarding the second phe¬ nomenon of punctuation as an artifact of an imperfect record). As Hallam said to me many years ago, after he had disproved the classical story of grad¬ ualism in Gryphaea: more than 100 other species of mollusks, many with rec¬ ords as rich as Gryphaea's, occur in the same Liassic rocks, yet no one ever documented the stratigraphic history of even a single one in any study of evo¬ lution, for all demonstrate stasis. Scientists picked out the only species that seemed to illustrate gradualism, and even this case failed. Despite the widespread use of proper quantitative methods today, and despite increasing attention to the validity of stasis as an evolutionary phe¬ nomenon, this bias still persists. I do not doubt that several species of Cenozoic planktonic Foraminifera display gradual transitions (see pp. 803-810), but I know that these examples have been extracted for study from a much larger potential sample of species never documented in detail because their apparent stasis seems “boring” to students of evolution. An eager young stat¬ istician goes to a lifelong expert and says: I want to devote my doctoral thesis to a statistical study of evolution in a species of foram (the most promising of major taxa, thanks to a hyperabundance of specimens and excellent strati¬ graphic data in oceanic cores); which species shall I choose? And the expert advises: why not study Graduloconoides gradualississima; I know that this species shows interesting changes during the upper Miocene in cores A through Z. Meanwhile, poor old boring Stasigerina punctiphora, just as abun¬ dant in the same cores, and just as worthy of study, gets bypassed in silence. I find this situation particularly frustrating as paleontology’s primary ex¬ ample of an insidious phenomenon in science that simply has not been recog¬ nized for the serious and distorting results perpetrated under its aegis. Most scientists do not even recognize the problem—though some do, particularly in the medical and social sciences, where the error has been named “publica¬ tion bias,” and has inspired a small but important literature (Begg and Berlin, 1988). In publication bias, prejudices arising from hope, cultural expectation, or the definitions of a particular scientific theory dictate that only certain kinds of data will be viewed as worthy of publication, or even of documenta¬ tion at all. Publication bias bears no relationship whatever with the simply immoral practice of fraud; but, paradoxically, publication bias may exert a far more serious effect (largely because the phenomenon must be so much
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THE STRUCTURE OF EVOEUTIONARY THEORY more common than fraud)—for scientists affected by publication bias do not recognize their errors (and their bias may be widely shared among col¬ leagues), while a perpetrator of fraud operates with conscious intent, and the wrath of a colleague will be tremendous upon any discovery. Begg and Berlin (1988) cite several documented cases of publication bias. We can hardly doubt, for example, that a correlation exists between socioeco¬ nomic status and academic achievement, but the strength and nature of this association can provide important information, for both political practice and social theory. White (1982, cited in Begg and Berlin) found a progres¬ sively increasing intensity of correlation with prestige and permanence of published source. Studies published in books reported an average correlation coefficient of 0.51 between academic achievement and socioeconomic status; articles in journals gave an average of 0.34, while unpublished studies yielded a value of 0.24. Similarly, Coursol and Wagner (1986, cited in Begg and Berlin) found publication bias both in the decision to submit an article at all, and in the probability for acceptance. In a survey of outcomes in psychother¬ apy, they noted that 82 percent of studies with positive outcomes led to sub¬ mission of papers to a journal, while only 43 percent of negative outcomes provoked an attempt at publication. Of papers submitted, 80 percent that re¬ port positive outcomes were accepted for publication, but the figure fell to 50 percent for papers claiming negative results. In my favorite study of publication bias, Fausto-Sterling (1985) tabulated claims in the literature for consistent differences in cognitive and emotional styles between men and women. She does not deny that genuine differences often exist, and in the direction conventionally reported. But she then, so to speak, surveys her colleagues’ file drawers for studies not published, or for negative results published and then ignored, and often finds that a great ma¬ jority report either a smaller and insignificant disparity between sexes, or no differences at all. When she collated all studies, rather than only those pub¬ lished, the much-vaunted differences often dissolved into statistical insig¬ nificance or triviality. For example, a recent favorite theme of pop psychology attributed dif¬ ferent cognitive styles in men and women to the less lateralized brains of women. Some studies have indeed reported a small effect of greater male lateralization; none has found more lateralized brains in women. But most experiments, as Fausto-Sterling shows, detected no measurable differences in lateralization at all and this dominant relative frequency (even in published literature) should be prominently reported in the press and in popular books, but tends to be ignored as “no story.” Paleontology’s primary example of publication bias—the nonreporting of stasis under the false belief that such stability represents “no data” for evolu¬ tion—illustrates a particularly potent form of the general phenomenon, a cat¬ egory that I have called “Cordelia’s dilemma” (Gould, 1995) to memorialize the plight of King Lear’s honest but rejected daughter. When asked by Lear for a fulsome protestation of love in order to secure her inheritance, Cordelia, disgusted by the false and exaggerated speeches of her sisters Goneril and
Punctuated Equilibrium and the Validation of Macroevolutionary Theory Regan, chose to say nothing, for she knew that “my love’s more ponderous than my tongue.” But Lear mistook her silence for hatred or indifference, and cut her off entirely (with tragic consequences later manifested in his own madness, blindness, and death), in proclaiming that “nothing will come of nothing.” Cordelia’s dilemma arises in science when an important (and often pre¬ dominant) signal from nature isn’t seen or reported at all because scientists read the pattern as “no data,” literally as nothing at all. This odd status of “hidden in plain sight” had been the fate of stasis in fossil morphospecies until punctuated equilibrium gave this primary signal some theoretical space for existence. Apparent silence—the overt nothing that actually records the strongest something—can embody the deepest and most vital meaning of all. What, in western history, has been more eloquent than the silence of Jesus be¬ fore Pilate, or Saint Thomas More’s date with the headsman because he ac¬ knowledged that fealty forbade criticism of Henry VIII’s marriage to Anne Boleyn, but maintained, literally to the death, his right to remain silent, and not to approve? In summary, the potentially reformative role of punctuated equilibrium re¬ sides in an unusual property among scientific innovations. Most new theories in science arise from fresh information that cannot be accommodated under an old explanatory rubric. But punctuated equilibrium merely honored the firmest and oldest of all paleontological observations—the documentable sta¬ sis of most fossil morphospecies—by promoting this pattern to central recog¬ nition as an expected result of evolution’s proper expression at the scale of geological time. This reformulation cast a bright light upon stasis, a preemi¬ nent fact that had formerly been mired in Cordelia’s dilemma as a grand dis¬ appointment, and therefore as “no data” at all, a pattern fit only for silence in a profession that accepted Darwin’s argument for gradualism as the canonical expression of evolution in the fossil record.
The Primary Claims of Punctuated Equilibrium BATA AND DEFINITIONS
First of all, the theory of punctuated equilibrium treats a particular level of structural analysis tied to a particular temporal frame. G. K. Chesterton (1874-1936), the famous English author and essayist, wrote that all art is limitation, for the essence of any painting lies in its frame. The same principle operates in science, where claiming too much, or too broad a scope of appli¬ cation, often condemns a good idea to mushy indefiniteness and consequent vacuity. Punctuated equilibrium is not a theory about all forms of rapidity, at any scale or level, in biology. Punctuated equilibrium addresses the origin and deployment of species in geological time. Punctuational styles of change characterize other phenomena at other scales as well (see Section V of this chapter)—catastrophic mass extinction triggered by bolide impacts, for ex-
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THE STRUCTURE OF EVOLUTIONARY THEORY ample—and proponents of punctuated equilibrium would become dull spe¬ cialists if they did not take an interest in the different mechanisms respon¬ sible for similarities in the general features of stability and change across nature’s varied domains, for science has always sought unity in this form of abstraction. But punctuated equilibrium—a particular punctuational theory of change and stability for one central phenomenon of evolution—does not directly address the potentially coordinated history of faunas, or the limits of viable mutational change between a parental organism and its offspring in the next generation. The theory of punctuated equilibrium attempts to explain the macroevolu¬ tionary role of species and speciation as expressed in geological time. Its statements about rapidity and stability describe the history of individual spe¬ cies; and its claims about rates and styles of change treat the mapping of these individual histories into the unfamiliar domain of “deep” or geological time—where the span of a human life passes beneath all possible notice, and the entire history of human civilization stands to the duration of primate phylogeny as an eyeblink to a human lifetime. The claims of punctuated equilib¬ rium presuppose the proper scaling of microevolutionary processes into this geological immensity—the central point that Darwin missed when he falsely assumed that “slowness” of modification in domesticated animals or crop plants, as measured in ordinary human time (where all of our history, and so many human generations, have witnessed substantial change within popula¬ tions, but no origin of new species), would translate into geological time as the continuity and slowness of phyletic gradualism. Once we recognize that definitions for the two key concepts of stasis and punctuation describe the history of individual species scaled into geological time, we can establish sensible and operational criteria. As a central proposi¬ tion, punctuated equilibrium holds that the great majority of species, as evi¬ denced by their anatomical and geographical histories in the fossil record, originate in geological moments (punctuations) and then persist in stasis throughout their long durations (Sepkoski, 1997, gives a low estimate of 4 million years for the average duration of fossil species; mean values vary widely across groups and times, with terrestrial vertebrates at lesser durations and most marine invertebrates in the higher ranges; in any case, geological longevity achieves its primary measure in millions of years, not thousands). As the primary macroevolutionary implication of this pattern, species meet all definitional criteria for operating as Darwinian individuals (see pp. 602613) in the domain of macroevolution. This central proposition embodies three concepts requiring definite opera¬ tional meanings: stasis, punctuation, and dominant relative frequency. (I am not forgetting the thorny problems associated with the definition of species from fossil data, where anatomy prevails as a major criterion and reproduc¬ tive isolation can almost never be assessed directly—and also with the puta¬ tive correspondence of morphological “packages” that paleontologists desig¬ nate as species with the concept as understood and practiced by students of modern populations of sexually reproducing organisms. I shall treat these is¬ sues on pages 784-796.)
Punctuated Equilibrium and the Validation of Macroevolutionary Theory Stasis does not mean “rock stability” or utter invariance of average values for all traits through time. In the macroevolutionary context of punctuated equilibrium, we need to know, above all, whether or not morphological change tends to accumulate through the geological lifetime of a species and, if so, what part of the average difference between an ancestral and descendant species can be attributed to incremental change of the ancestor during its anagenetic history. Punctuated equilibrium makes the strong claim that, in most cases, effectively no change accumulates at all. A species, at its last appearance before extinction, does not differ systematically from the anat¬ omy of its initial entry into the fossil record, usually several million years before. Of course we recognize that mean values will fluctuate through time. After all, measured means would vary even if true population values remained utterly constant—which they do not. And, with enough samples in a vertical sequence, some must include mean values (for some characters) outside con¬ ventional bounds of statistically insignificant difference from means for the oldest sample. Such fluctuation also implies that the final population will not be identical with the initial sample. In operational terms, therefore, we need to set criteria for permissible fluc¬ tuation in average values through time. Two issues must be resolved: the amount of allowable difference between beginning and ending samples of a species, and the range of permissible fluctuation through time. Since we wish to test a hypothesis that little or no change accumulates by anagenesis during the history of most species, and since we have no statistical right to expect that (under this hypothesis) the last samples will be identical with the first, we should predict either that (i) the final samples shall not differ statistically, by some conventionally chosen criterion, from the initial forms; and at very least (ii) that the final samples shall not generally lie outside the range of fluctua¬ tion observed during the history of the species. (If final samples tend to he outside the envelope of fluctuation for most of the species’s history, then anagenesis has occurred.) For the permissible range of fluctuation, we should, ideally, look to the ex¬ tent of geographic variation among contemporary populations within the species, or its closest living relative. If the temporal range of variation stays within the spatial range for any one time, then the species has remained in stasis. Obviously, we cannot apply this optimal criterion for groups long ex¬ tinct, but a variety of proxies should be available, including comparison of a full temporal range with the known geographic variation of a well-docu¬ mented and widespread nearest living relative. Studies of stasis in Neogene species can often use the optimal criterion because the actual species, or at least some very close relatives, are often still extant. In the most elegant docu¬ mentation of stasis for an entire fauna of molluscan species, Stanley and Yang (1987) used this best criterion to find that temporal fluctuation remained within the range of modern geographic variation for the same species. They could therefore affirm stasis in the most biologically convincing manner. Since stasis is data, but punctuation generally records an unresolvable tran¬ sition when assessed by the usual expression of fossil data in geological time,
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THE STRUCTURE OF EVOLUTIONARY THEORY we need to formulate an appropriate definition of rapidity. (Punctuated equi¬ librium makes no claim about the possibility of substantial change at rates that would be called rapid by measuring rods of a human lifetime. Therefore, and especially, punctuated equilibrium provides no insight into the old and contentious issue of saltational or macromutational speciation.) As a first ap¬ proach, the duration of a bedding plane represents the practical limit of geo¬ logical resolution. Any event of speciation that occurs within the span of time recorded by most bedding planes will rarely be resolvable because evidence for the entire transition will be compressed onto a single stratigraphic layer, or “geological moment.” However, the limits of stratigraphic resolution vary widely, with bedding planes representing years or seasons in rare and optimal cases of varved sedi¬ ments, but several thousand years in most circumstances. We therefore can¬ not formulate a definition equating punctuation with “bedding plane simul¬ taneity.” (After all, such a definition would, almost perversely, preclude the “dissection” of a punctuation in admittedly rare, but precious, cases of sedi¬ mentation so complete and so rapid that an event of speciation will not be compressed, as usual, onto a single bedding plane, but will “spread out” over a sufficient stratigraphic interval to permit the documentation of its rapid history.) Punctuations must, instead, be defined relative to the subsequent duration of the derived species in stasis—for punctuated equilibrium, as a theory of relative timing, holds that species develop their distinctive features effectively “at birth,” and then retain them in stasis for geologically long lifetimes. (These timings play an important role in the recognition of species as Darwin¬ ian individuals—see discussion on “vernacular” criteria of definable birth, death, and sufficient stability for individuation—Chapter 8, pp. 602-608). I know no rigorous way to transcend the arbitrary in trying to define the permissible interval for punctuational origin. Since definitions must be the¬ ory-bound, and since the possibility of recognizing species as Darwinian indi¬ viduals in macroevolution marks the major theoretical interest of punctuated equilibrium, an analogy between speciation and gestation of an organism may not be ill conceived. As the gestation time of a human being represents 1-2 percent of an ordinary lifetime, perhaps we should permit the same gen¬ eral range for punctuational speciation relative to later duration in stasis. At an average species lifetime of 4 million years, a 1-percent criterion allows 40,000 years for speciation. When we recognize that such a span of time would be viewed as gradualistic—and extremely slow paced at that—by any conventional microevolutionary scaling in human time; and when we also ac¬ knowledge that the same span represents the resolvable moment of a single bedding plane in a great majority of geological circumstances; then we can understand why the punctuations of punctuated equilibrium do not represent de Vriesian saltations, but rather denote the proper scaling of ordinary speci¬ ation into geological time. Punctuation does suffer the disadvantage of frequently compressed record¬ ing on a single bedding plane (so that the temporal pattern of the full event
Punctuated Equilibrium and the Validation of Macroevolutionary Theory cannot be dissected); moreover, an observed punctuation often represents the even less desirable circumstance of missing record (Darwin’s classic argument from imperfection), or only partial pattern (as when a punctuation in a single geological section marks the first influx by migration of a species that origi¬ nated earlier and elsewhere). Since stasis, on the other hand, provides an ac¬ tive (and often excellent) record of stability, empirical defenses of punctuated equilibrium have understandably focussed on the more easily documentable claims for equilibrium, and less frequently on more elusive predictions about punctuation. But we must not conclude, as some authors have suggested, that punctuation therefore becomes untestable or even impervious to documenta¬ tion—and that the thesis of punctuated equilibrium must therefore depend for its empirical support only upon the partial data of stasis. The documenta¬ tion of punctuation may be both more difficult and less frequently possible, but many good cases have been affirmed and several methods of rigorous testing have been developed. In the first of two general methods, one may document the reality of a punctuation (as opposed to interpretation as a Darwinian artifact based on gaps in sedimentation) by finding cases of gradualism within a stratigraphic sequence (which must then be sufficiently complete to record such an anagenetic transition), and then documenting punctuational origins for other species in the same strata. Using this technique for Ordovician trilobites from Spitzbergen, Fortey (1985) found a ratio of about 10:1 for cases of punctua¬ tion compared with gradualism. In a second, and more frequently employed, method, one searches ex¬ plicitly for rare stratigraphic situations, where sedimentation has been suf¬ ficiently rapid and continuous to spread the usual results of a single bedding plane into a vertical sequence of strata. Williamson (1981), for example, pub¬ lished a famous series of studies on speciation of freshwater mollusks in Afri¬ can Pleistocene lakes. (These articles provoked considerable debate (Fryer, Greenwood and Peake, 1983), and Williamson died young before he could complete his work. However, in my admittedly partisan judgment, William¬ son more than adequately rebutted his critics (1985, 1987).) These African lakes form in rift valleys, where sedimentation rates are un¬ usually high because the rift-block foundations of the lake sink continuously, and sediments can therefore accumulate above, without interruption. Thus, the thousand-year duration of a speciation event may span several layers of foundering sediment. With this unusual degree of resolution, Williamson was even able to demonstrate a remarkable phenomenon in change of variability within a speciating population—a pattern that appeared over and over again in several events of speciation, and may therefore be viewed as potentially general (see Fig. 9-3): Williamson found limited variation around parental mean values in the oldest samples; intermediacy of mean values within spe¬ ciating samples, but accompanied by a greatly expanded range of variation (though still normal in distribution); and subsequent “settling down" of vari¬ ation to the reduced level of the ancestral population, but now distributed around the altered mean value of the derived species.
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THE STRUCTURE OF EVOLUTIONARY THEORY
PCP1
770
9-3A. The dissection of a punctuation made possible by unusually high sedimen¬ tation rates. Williamson’s analysis of variation and central tendency during a punctuation in the B. unicolor lineage of Pleistocene fresh water pulmonate snails from the African rift valley. Each diagram shows all the specimens from the entire sequence, with only those specimens for the relevant interval depicted in black. A. Parental form before the punctuation with multivariate modal mor¬ phology concentrated to the left of the range. B. Expanded variation throughout the range during the time of the punctuation itself. C. Restricted variation again, but settling down upon the morphology of a new taxon following the punctua¬ tion, as seen in the reduction of variation with change in modal position towards the right side of the array. From Williamson, 1981.
If this kind of unusual circumstance spreads a punctuational event of speciation through a sufficient stratigraphic interval for resolution, another strategy of research will sometimes permit the dissection of a punctuation in conventional cases of full representation on a single bedding plane. Goodfriend and Gould (1996) documented such a case because they could estab-
Punctuated Equilibrium and the Validation of Macroevolutionary Theory
9-3B. Relative timings of punctuational events throughout Williamson’s entire series. From Williamson, 1981.
lish absolute dates for the individual shells on a single bedding plane. (Admit¬ tedly, this technique cannot be generally applied—especially to sediments of appreciable age, where errors of measurement for any method of dating must greatly exceed the full span of the bedding plane. But this method can be used for late Pleistocene and Holocene samples.) On a single mud flat (a modern “bedding plane,” if you will) on the island of Great Inagua, we found a complete morphological transition between the extinct fossil pulmonate species Cerion excelsior and the modern species Cerion ruhicundum. Many lines of evidence indicate that this transition oc¬ curred by hybridization, as C. ruhicundum migrated to an island previously inhabited only by C. excelsior among large species of Cerion. Ordinarily, we would find such a complete morphological transition on a single bedding
771
772
THE STRUCTURE OF EVOEUTIONARY THEORY plane, but be unable to perform any fine scale analysis in the absence of meth¬ ods for dating individual shells. That is, we would be unable to discover whether the unusual morphological range represented a temporal transition or a standing population with enhanced variation. But Goodfriend and I could date the individual shells by amino acid racemization for all specimens, keyed to radiocarbon dates for a smaller set of marker shells. We found an ex¬ cellent correlation between measured age and multivariate morphometric po¬ sition on the continuum between ancestral C. excelsior and descendant C. rubicundum (see Fig. 9-4). The transition lasted between 15,000 and 20,000 years—a good average value for a punctuational event, and a fact that we could ascertain only because the individual specimens of a single bedding plane could be chemically dated independently of their morphology. We can therefore define stasis and punctuation in operational terms, with stasis available for test in almost any species with a good fossil record, but punctuation requiring an unusual density of information, and therefore not routinely testable, but requiring a search for appropriate cases (not an un¬ usual situation in sciences of natural history, where nature sets the experi¬ ments, and scientists must therefore seek cases with adequate data). The third key issue of relative frequency may be easier to operationalize—as one need only tabulate cases pro and con within well-documented faunas—but re¬ mains harder to define. As the most important ground rule, the theory of punctuated equilibrium makes a claim about dominating pattern, or relative frequency, not just an as-
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9-4. Another way to dissect a punctuation by obtaining absolute age dates for all specimens on a bedding plane, and thus obtaining temporal distinctions within the compression. The ancestral and high spired Cerion excelsior, over no more than 15,000 to 20,000 years (well within the range of punctuational dynamics), hybridizes with invading Cerion rubicundum, with gradual fading out of all morphometric influence from the unusually shaped ancestor.
Punctuated Equilibrium and the Validation of Macroevolutionary Theory
sertion for the existence of a phenomenon. Such issues cannot be resolved by anecdote, or the documentation, however elegant, of individual cases. If any¬ one ever doubted that punctuated equilibrium exists as a phenomenon, then this issue, at least, has been put to rest by two decades of study following the presentation of our theory, and by clear and copious documentation of many cases (see pp. 822-874). Nonetheless, as pleased as Eldredge and I have been both by the extent of this research and the frequency of its success, the “ideal case study’’ method cannot validate our theory. Punctuated equilibrium does not merely assert the existence of a phe¬ nomenon, but ventures a stronger claim for a dominant role as a macroevo¬ lutionary pattern in geological time. But how can this vernacular notion of “dominant” be translated into a quantitative prediction for testing? At this point in the argument, we encounter the difficult (and pervasive) method¬ ological issue of assessing relative frequency in sciences of natural history. If species were like identical beans in the beanbag of classical thought experi¬ ments in probability, then we could devise a sampling scheme based on enumerative induction. Enough randomly selected cases could establish a pattern at a desired level of statistical resolution. But species are irreducibly unique, and the set of all species does not exhibit a distribution consistent with requirement of standard statistical procedures. It matters crucially whether we study a clam or a mammal, a Cambrian or a Tertiary taxon, a species in the stable tropics, or at volatile high latitudes. Moreover—and es¬ pecially—the “ideal case study” method has often failed, and led to paro¬ chialisms and false generalities, precisely because we tend to select unusual cases and ignore, often quite unconsciously, a dominant pattern. Indeed, proclamations for the supposed “truth” of gradualism—asserted against ev¬ ery working paleontologist’s knowledge of its rarity—emerged largely from such a restriction of attention to exceedingly rare cases under the false belief that they alone provided a record of evolution at all! The falsification of most “textbook classics” upon restudy only accentuates the fallacy of the “case study” method, and its root in prior expectation rather than objective reading of the fossil record. Punctuated equilibrium must therefore be tested by relative frequencies among all taxa (or in a truly randomized subset) in a particular fauna, a par¬ ticular clade, a particular place and time, etc. If we can say, as Ager did (see p. 753) that all but one Mesozoic brachiopod species displays stasis, or as Imbrie did (see p. 760) that all but one Devonian species from the Michigan Basin shows no change, then we have specified a dominant pattern, at least within a particular, well-defined and evolutionarily meaningful package. I cannot give a firm percentage for what constitutes a “dominant” relative fre¬ quency—for, again, we encounter a theory-bound claim, where “dominant” specifies a weight, beyond which the morphological history of a clade must be explicated primarily by the differential success of species treated as stable en¬ tities, or Darwinian individuals in macroevolution—and not by anagenetic change within species. More research must be done, largely in the testing of mathematical models under realistic circumstances, to learn the relative fre-
773
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THE STRUCTURE OF EVOEUTIONARY THEORY
quencies and rates required to impart such dominance to species-individuals in the course of macroevolution. For now, and for empirically minded pale¬ ontologists, the study of relative frequencies in entire faunas, rather than the extraction of apparently idealized cases, should be pursued as a primary strategy of research. >
‘
Critics have sometimes stated that punctuated equilibrium rests upon dec¬ laration rather than documentation. (Maynard Smith once compared the the¬ ory to “Aunt Jobisca’s” maxim about ancient verities “known” by folk wis¬ dom a priori.) We do indeed assert that working paleontologists know the fact of dominant stasis in their bones—but this claim represents a fair con¬ sensus about the history of a field, and does underscore a paradox of non¬ concordance between deep practical knowledge and imposed theoretical ex¬ pectation. We have never tried to argue that such a “professional feeling” constitutes documentation for punctuated equilibrium. As with all scientific theories, punctuated equilibrium will live or die by concrete and quantifiable evidence. As with any good hypothesis, punctuated equilibrium becomes op¬ erational when workable definitions can be provided for key claims and ex¬ pectations—in this case, for stasis, punctuation, and relative frequency. Con¬ trary to the impression of some critics who have not followed the primary literature of paleobiology during the last 25 years, punctuated equilibrium has proven its fruitfulness and operational worth by being tested—and usu¬ ally confirmed, but sometimes confuted—in a voluminous literature of richly documented cases (see pp. 822-874).
Microevolutionary links Eldredge and I coined the term punctuated equilibrium in a paper first presented (Gould and Eldredge, 1971) at a symposium entitled “Models in Paleobiology” at the 1971 Annual Meeting of the Geological Society of America. T. J. M. Schopf, the organizer of the symposium, conceived the en¬ terprise as a tutorial in modern evolutionary theory for professional inverte¬ brate paleontologists. By accidents of history, invertebrate paleontologists generally receive their advanced academic degrees from geology departments, not from biology. Fossils became primary tools for stratigraphic correlation long before the development of evolutionary theory, and even before all scien¬ tists had accepted them as remains of ancient organisms! Given traditions of narrowness in postgraduate education—particularly in Europe, where stu¬ dents often attend no formal courses at all, and certainly no courses for credit, outside the department that will grant their degree—most paleontolo¬ gists, before the present generation, did not receive any explicit training in evolutionary biology, and could not articulate the basic concepts of popula¬ tion genetics or theories of speciation. In paleontological usage, “evolution” designated little more than the inferred pathway of phylogeny. This “little learning'1 often became the “dangerous thing” of Alexander Pope’s classic couplet, as paleontologists derived their understanding of evolution from memories of old textbooks, or from shared impressions amounting to little more than the blind leading the blind. This situation has now changed dra-
Punctuated Equilibrium and the Validation of Macroevolutionary Theory matically—and Eldredge and I do take pride in the role played by punctuated equilibrium in encouraging this shift of interest—as a profession of paleo¬ biology, supported by several new journals dedicated to the subject (Paleobi¬ ology, Historical Biology, Lethaea, Palaios, and Palaeogeography Palaeochmatology Palaeoecology (or P-cubed to aficionados), for example), has arisen to accommodate burgeoning research in the application of evolution¬ ary theory to the fossil record, and in enlarging and revising the theory in the light of novel macroevolutionary data. In any case, Schopf’s symposium featured a series of presentations, each suggesting how one aspect of paleontological work might be enlightened by modern microevolutionary theory, particularly as expressed in the applica¬ tion of models, preferably quantitative in nature. Eldredge and I drew the topic of species and speciation—-and our original article on punctuated equi¬ librium (Eldredge and Gould, 1972) emerged as a result. (As I have often stated, the basic idea had been presented in Eldredge, 1971. We had been graduate students together at the American Museum of Natural History, un¬ der the tutelage of Norman D. Newell. We had discussed these issues often and intensely throughout our graduate years. We had been particularly frus¬ trated—for we had both struggled to master statistical and other quantitative methods—with the difficulty of locating gradualistic sequences for applying these techniques, and therefore for documenting “evolution11 as paleonto¬ logical tradition then defined the term and activity. When I received Schopf’s invitation to talk on models of speciation, I felt that Eldredge’s 1971 publica¬ tion had presented the only new and interesting ideas on paleontological im¬ plications of the subject—so I asked Schopf if we could present the paper jointly. I wrote most of our 1972 paper, and I did coin the term punctuated equilibrium—but the basic structure of the theory belongs to Eldredge, with priority established in his 1971 paper.) I mention this background to clarify the original context and continuing focus of the theory of punctuated equilibrium—a notion rooted in the explicit goal that Eldredge and I set for ourselves: to apply microevolutionary ideas about speciation to the data of the fossil record and the scale of geological time. Before we proposed the theory of punctuated equilibrium, most paleon¬ tologists assumed that the bulk of evolutionary change proceeded in the anagenetic mode—that is, by continuous transformation of a unitary popula¬ tion through time (see Fig. 9-5). In this context, most paleontological discus¬ sion about species centered itself upon a contentious issue that constantly cir¬ culated throughout our literature (see Imbrie, 1957; Weller, 1961; McAlester, 1962; Shaw, 1969) and even generated entire symposia dedicated to potential solutions (see Sylvester-Bradley, 1956): the so-called species problem in pale¬ ontology. This supposed problem—more philosophical and definitional than empiri¬ cal (once one accepts the underlying assumptions about anagenesis as a domi¬ nant factual reality)—arises because a true continuum cannot be unambigu¬ ously divided into segments with discrete names. If population A changes so extensively by anagenesis that we feel impelled to provide the resulting popu-
775
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THE STRUCTURE OF EVOLUTIONARY THEORY
9-5. Typical textbook illustration of evolution by continuous anagenetic trans¬ formation of an unbranched population through time. This textbook labels the figure explicitly and exclusively as its icon of “evolution” itself, not of gradual¬ ism or any other subcategory of evolutionary change. From the standard paleontological textbook of my student generation, Moore, Lalicker, and Fischer, 1953.
lation with a new Linnaean name (as species B), then where should we place the breakpoint between A and B? Any boundary must be arbitrary—if only by the illogic of the unavoidable implication that the last parental generation of species A could not, in principle, breed with its own immediate offspring in species B. (We may abhor human incest for social reasons, but we can scarcely deny the biological possibility—hence the perceived societal need for a taboo.) This problem generated a large, tedious, and fruitless literature, pri¬ marily because the issue always remained available, unresolved and therefore ripe for yet another go-round whenever a paleontologist needed to deliver a general address and couldn’t think of anything else to say. Punctuated equilibrium took a radically different approach by admitting unresolvabihty under the stated assumptions, but then denying the focal em¬ pirical premise that new species usually (or even often) arise by gradualistic anagenesis. Instead, Eldredge and I argued that the vast majority of species originate by splitting, and that the standard tempo of speciation, when ex¬ pressed in geological time, features origin in a geological moment followed by long persistence in stasis. Thus, the classic and endlessly-fretted “species problem in paleontology” disappears because species act as well-defined Dar¬ winian individuals, not as arbitrary subdivisions of a continuum. Species then gain definability because they almost always arise by speciation (that is, by splitting, or geographic isolation of a daughter population followed by genetic differentiation from the parental population), not by anagenesis (or transformation of the entire mass of an ancestral species). To be sure, a new species must pass through a short period of ambiguity during its initial differentiation from an ancestral population, but, in the proper scaling of macroevolutionary time, this period passes so quickly (almost always in the
Punctuated Equilibrium and the Validation of Macroevolutionary Theory unresolvable geological moment of a single bedding plane), that operational definability encounters no threat. Of course, gradualists did not deny that speciation often occurs by branch¬ ing. They just didn’t grant this process of splitting any formative role in the accumulation of macroevolutionary change for three reasons. First, they con¬ ceived speciation only as an engine for generating diversity, not as an agent for changing average form within a clade (that is, for the key macroevolu¬ tionary phenomenon of trends—see quotes of Ffuxley and Ayala, and Mayr’s response, on p. 563). Trends arose by anagenesis (see Fig. 9-6), and speciation only served the subsidiary (if essential) function of iterating a favorable fea¬ ture, initially evolved by anagenesis, into more than one taxon-—thus provid¬ ing a hedge against extinction. Second, they granted little quantitative weight to the role of speciation (splitting as opposed to anagenesis) in the totality of evolutionary change. In a famous estimate that became canonical, Simpson (1944) stated that about 10 percent of evolutionary change occurred by speciation, and 90 percent by anagenesis. Third, when gradualists portrayed speciation at all (see Fig. 9-7), they depicted the process as two events of anagenesis proceeding at characteristi¬ cally slow rates. Thus, they identified nothing distinctively different about change by speciation. Some contingency of history, they argued, splits a pop¬ ulation into two separate units, and each proceeds along its ordinary anagenetic way. Punctuated equilibrium, on the other hand, proposes that the geological tempo of speciation differs radically from gradualistic anagenesis. (We also argue, of course, that such anagenesis rarely occurs at all!) The theory of punctuated equilibrium therefore began as a faithful re¬ sponse to Schopf’s original charge to Eldredge and me: to show how standard
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9-6. A standard illustration from Simpson (1944), showing that all trends, and all stability for that matter, originate primarily in the anagenetic mode—that is, by change during the lifetime of individual species, with branching serving pri¬ marily to diversify and iterate the favorable designs originated by anagenesis, and thus to prevent extinction of the lineage.
777
778
THE STRUCTURE OF EVOLUTIONARY THEORY
9-7. Another illustra¬ tion from the standard student paleontological textbook of the 1950’s, with speciation de¬ picted merely as two events of gradualistic change, following a separation of lineages. From Moore, Lalicker, and Fischer, 1953.
microevolutionary views about speciation, then unfamiliar to the great ma¬ jority of working paleontologists, might help our profession to interpret the history of life more adequately. (As a best testimony to this unfamiliarity, I note that most paleontologists didn’t even recognize the conceptual and ter¬ minological distinction between “speciation” defined as a process of splitting, and the accumulation of enough change by anagenesis to provoke the coining of a new Linnaean name for an unbranched single population.) In this crucial sense, the theory of punctuated equilibrium adopts a very conservative position. The theory asserts no novel claim about modes or mechanisms of speciation; punctuated equilibrium merely takes a standard microevolutionary model and elucidates its expected expression when prop¬ erly scaled into geological time. This scaling, however, did provoke a radical reinterpretation of paleontological data—for we argued that the literal ap¬ pearance of the fossil record, though conventionally dismissed as an artifact of imperfect evidence, may actually be recording the workings of evolution as understood by neontologists.* This empowering switch enabled paleon¬ tologists to cherish their basic data as adequate and revealing, rather than pitifully fragmentary and inevitably obfuscating. Paleontology could emerge from the intellectual sloth of debarment from theoretical insight imposed by poor data—a self-generated torpor that had confined the field to a descriptive role in documenting the actual pathways of life’s history. Paleontology could now take a deserved and active place among the evolutionary sciences. The major and persisting misunderstanding of punctuated equilibrium among neontologists—a great frustration for us, and one that we have tried
"'All professions maintain their parochialisms, and I trust that nonpaleontological read¬ ers will forgive our major manifestation. We are paleontologists, so we need a name to con¬ trast ourselves with all you folks who study modern organisms in human or ecological time. You therefore become neontologists. We do recognize the unbalanced and parochial nature of this dichotomous division—much like my grandmother’s parsing of Homo sapi¬ ens into the two categories of ‘Jews’ and ‘non-Jews.’
Punctuated Equilibrium and the Validation of Macroevolutionary Theory to explicate and resolve again and again (Gould and Eldredge, 1977, 1993; Gould, 1982c, 1989e), though without conspicuous success—involves the false assumption that if we are really saying something radical, we must be staking a claim for a novel mechanism of speciation, or for a different (read non-Darwinian) style of genetic change. When our critics then join this false assumption to our terminology of “unresolvable geological moments” or "punctuations,” they begin to fear that the dreaded specter of saltationism must be lurking just around the corner, trying yet again to raise its ugly head after such a well-deserved burial. Vituperation then trumps logic, angry as¬ sumption precludes careful reading, and punctuated equilibrium becomes a loathed doctrine of ignorant and grandstanding paleontologists who ought to stay in their own limited bailiwick, and get on with the job of documenting large scale patterns generated by mechanisms that can be recognized and comprehended only by neontologists. But punctuated equilibrium makes no iconoclastic claim about speciation at all. The radicalism of punctuated equilibrium lies in the extensive conse¬ quences of its key implication that conventional mechanisms of speciation scale into geological time as the observed punctuations and stasis of most spe¬ cies, and not as the elusive gradualism that a century of largely fruitless paleontological effort had sought as the only true expression of evolution in the fossil record. The central intellectual strategy of our original 1972 paper rests upon this premise. We took Mayr’s allopatric theory (as expressed in his classic treatise of 1963, deemed “magisterial” by Huxley), and tried to eluci¬ date its implied expression when scaled into geological time. We did not select this theory to fit a paleontological pattern that we wished to validate. We choose Mayr’s formulation because his allopatric theory represented the most orthodox and conventional view of speciation then available in neontological literature—and we had been given the task of applying standard evolutionary views to the fossil record. I recognize, with 30 years of hindsight, that our original assessment both of Mayr’s theory and of professional consensus may have been both naive and overly dichotomous, but we could not have stated our intent more clearly—the reform of paleontological practice by the para¬ doxical route of applying a fully conventional apparatus of neontological the¬ ory. We wrote (1972, p. 94): “During the past thirty years, the allopatric the¬ ory has grown in popularity to become, for the vast majority of biologists, the theory of speciation. Its only serious challenger is the sympatric theory. Here we discuss only the implications of the allopatric theory for interpreting the fossil record of sexually-reproducing metazoans. We do this simply because it is the allopatric, rather than the sympatric, theory that is preferred by biol¬ ogists.” Mayr’s version of allopatry fit the paleontological pattern of punctuation and stasis particularly well. If most new species arise from small populations peripherally isolated at the edges of a parental range, then we cannot expect to document a gradual transition by analyzing the stratigraphic sequence of samples for a common species. For we will usually be collecting from the population’s central range during its period of stability. Daughter species
779
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THE STRUCTURE OF EVOEUTIONARY THEORY
originate in three circumstances that virtually guarantee a punctuational ex¬ pression in the fossil record: (1) they arise rapidly (usually instantaneously) in geological time, and they originate both (2) in a small geographic region (the peripheral isolate), and (3) elsewhere (beyond the borders of the parental range that provides the exclusive source for standard paleontological collec¬ tions). The “sudden” entrance of a daughter species into strata previously oc¬ cupied by parents usually represents the inward migration of a peripheral iso¬ late, now “promoted” by reproductive isolation to full separation, not the origin of a new species in situ. Eldredge and I have often been asked what we think of sympatric speciation, or of various models, like polyploidy, for rapid origin even in hu¬ man time. We do not mean to be evasive or obscure in our assertions of ag¬ nosticism. (I am intensely interested in the literature on speciation, and I would love to know the relative frequencies of these other models vs. classical Mayrian peripatry. But this important issue does not strongly impact punctu¬ ated equilibrium, and surely cannot be resolved by paleontological data.) Punctuated equilibrium simply requires that any asserted mechanism of spe¬ ciation, whatever its mode or style, be sufficiently rapid and localized to ap¬ pear as a punctuation when scaled into geological time. If I understand them correctly, most alternative models to peripatry generally operate even more rapidly than the conventional Mayrian mode that we invoked to anchor our theory—as obviously true for polyploidy, and also for most versions of sym¬ patric speciation (if only because the constant threat of dilution by gene flow from surrounding parentals can best be overcome by rapid achievement of re¬ productive isolation in ecological time). Therefore, punctuated equilibrium can only gain strength if these alternative mechanisms become validated at meaningful relative frequencies. (The faster the better, one might say.) But punctuated equilibrium does not require this boost—and we therefore remain agnostic—because the most conventional form of Mayrian peripatry already yields the full set of phenomena predicted by punctuated equilibrium when properly scaled into the immensity of geological time. (Punctuated equilib¬ rium, on the other hand, does not maintain a similar agnosticism towards any putative mechanism of speciation that conceives the process of splitting as no more rapid than imagined rates for the gradual anagenesis of large central populations. Some models of so-called “dumbbell allopatry”—or the split¬ ting of a parental population into two effectively equal moieties, with subse¬ quent anagenesis in each—do construe speciation as consequently slow in geological expression, and therefore do threaten punctuated equilibrium. But I do not think that such models enjoy much support among biologists, espe¬ cially for operation at a high relative frequency.) Geological time can be both a wonder and a snare because we grasp the idea in our heads (all scientists know how many zeroes follow the one in ex¬ pressing millions or billions), but we face a primal, and fundamentally psy¬ chological, difficulty in trying to incorporate this central concept into the guts of our intuition. We can lose information in upward scaling when glacial slowness in human history becomes a passing and unresolvable geological
Punctuated Equilibrium and the Validation of Macroevolutionary Theory
moment. But we can also gain when operational invisibility at our scale (in¬ ability to distinguish a small effect from measurement error) becomes palpa¬ ble and prominent in the large, or when the almost inconceivable rarity of an event that averages one expression in ten thousand years achieves guaranteed repetition across millions.
MACROEVOLUTIONARY IMPLICATIONS If punctuated equilibrium has broader utility beyond the reform of paleonto¬ logical practice, then we must look to potential implications for macroevolu¬ tionary theory, and for consequent enrichment in our general understanding of mechanisms that regulate the history of life. I have linked my treatments of punctuated equilibrium and the hierarchical theory of natural selection to form the longest section of this book (presented as two chapters, 8 and 9) be¬ cause I believe that punctuated equilibrium supplies the central argument for viewing species as effective Darwinian individuals at a relative frequency high enough to be regarded as general—thereby validating the level of species as a domain of evolutionary causality, and establishing the effectiveness and inde¬ pendence of macroevolution by two of the three criteria featured throughout this book as indispensable foundations of Darwinism. First, punctuated equilibrium secures the hierarchical expansion of selec¬ tionist theory to the level of species, thus moving beyond Darwin’s preference for restricting causality effectively to the organismic realm alone (leg one on the essential tripod). Second, by defining species as the basic units or atoms of macroevolution—as stable “things” (Darwinian individuals) rather than as arbitrary segments of continua—punctuated equilibrium precludes the ex¬ planation of all evolutionary patterns by extrapolation from mechanisms operating on local populations, at human timescales, and at organismic and lower levels (leg three on the tripod of Darwinian essentials). Thus, as em¬ phasized in the last section, punctuated equilibrium presents no radical pro¬ posal in the domain of microevolutionary mechanics—in particular (and as so often misunderstood), the theory advances no defenses for saltational models of speciation, and no claims for novel genetic processes. Moreover, punctuated equilibrium does not attempt to specify or criticize the conven¬ tional mechanisms of microevolution at all (for punctuated equilibrium emerges as the
anticipated expression,
by proper scaling,
of micro¬
evolutionary theories about speciation into the radically different domain of “deep,” or geological time). But punctuated equilibrium does maintain, as the kernel of its potential novelty for biological theory, that these unrevised microevolutionary mechanisms do not hold exclusive sway in evolutionary explanation, and that their domain of action must be restricted (or at least shared) at the level of macroevolutionary pattern over geological scales— for punctuated equilibrium ratifies an effective realm of macroevolutionary mechanics based on recognizing species as Darwinian individuals. In other words, punctuated equilibrium makes its major contribution to evolutionary theory, not by revising microevolutionary mechanics, but by individuating
781
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THE STRUCTURE OF EVOEUTIONARY THEORY
species (and thereby establishing the basis for an independent theoretical do¬ main of macro evolution). As discussed in Chapter 8 (see pp. 648-652), punctuated equilibrium wins this role by refuting Fisher’s otherwise decisive argument for the impotence (despite the undeniable existence) of species selection. So long as most new species arise by branching (speciation) rather than by transformation (ana¬ genesis), species can be individuated by their uniquely personal duration, bounded by birth in branching and death by extinction. But if anagenesis, fueled by Darwinian organismic selection, operates to substantial effect dur¬ ing the lifetimes of most species, then, by Fisher’s argument, such micro¬ evolutionary transformation must overwhelm species selection in building the overall pattern of macroevolutionary change—for the number of organism-births must exceed species-births by several orders of magnitude, and if every event of birthing, at each level, supplies effective variation for evo¬ lutionary transformation, then the level of species can contribute virtually nothing to the totality of change. But if stasis rules and anagenesis rarely oc¬ curs, then speciation becomes the more effective level of evolutionary varia¬ tion. And if speciation unfolds in geological moments, then species in geologi¬ cal time match organisms on our ordinary yearly scales in both distinctness and discreteness. Thus, the pattern of punctuated equilibrium establishes spe¬ cies as effective individuals and potential Darwinian agents in the mecha¬ nisms of macroevolution. In summary, G. G. Simpson gave a singularly appropriate title to his ep¬ ochal 1944 book that defined the potential of paleontology to devise insights about evolutionary mechanisms: Tempo and Mode in Evolution. If we accept Simpson’s focus on tempo and mode as primary subjects, then punctuated equilibrium has provoked substantial revisions of macroevolutionary theory and practice in both domains.
Tempo and the significance of stasis For tempo, punctuated equilibrium reverses our basic perspective. We must abandon our concept of constant change operating within a sensible, stately range of rates as the normal condition of an evolving entity. We must then reformulate evolutionary change as a set of rare episodes, short in duration relative to periods of stasis between. Stability becomes the normal state of a lineage, with change recast as an infrequent and concentrated event that, nonetheless, renders phylogeny as a set of summed episodes through time. The implications of this fundamental shift resonate afar by impacting a set of issues ranging from the most immediately practical to the most broadly philo¬ sophical (including, in the latter category, an interesting consonance with the atomism and quantization invoked to define the general intellectual move¬ ment known as “modernism”—as expressed in disparate disciplines from Seurat’s pointillism in art, to Schonberg’s serial style in music; and as opposed to the smooth continuationism favored by earlier mechanistic views of cau¬ sality). In a theme more immediately relevant to biology, the same shift in¬ eluctably places much greater emphasis upon chance and contingency, rather
Punctuated Equilibrium and the Validation of Macroevolutionary Theory than predictability by extrapolation—for the ordinary condition of stasis provides little insight into when and how the next punctuation will occur, whereas the fractal character of gradualism suggests that causes of change at any moment will, by extrapolation, predict and explain the larger effects ac¬ cumulated through longer times. On the practical side, punctuated equilibrium’s formulation of tempo has validated the study of stasis—paleontology’s prevalent pattern within spe¬ cies—as a source of insight about evolution, rather than a cause of chagrin best bypassed and ignored as a testimony to an embarrassing poverty of evi¬ dence. Punctuated equilibrium has broken “Cordelia’s Dilemma” of silence about the supposed “nothing” of stasis, and has established a burgeoning subfield of research in the documentation of stability at several levels. In pur¬ suing and valuing this documentation, scientists then feel compelled to postu¬ late explanations for the puzzling frequency of this previously “invisible” phenomenon—and theoretical inquiry about the “why” of stasis has also flourished following the prod from punctuated equilibrium (see pp. 877-885 for fuller discussion).
Mode and the speciational foundation of macroevolution For mode, as discussed throughout this chapter, punctuated equilibrium has established a speciational basis for macroevolution. By supplying crucial data and arguments for defining species as effective Darwinian individuals—that is, as basic units for describing macroevolution in Darwinian terms as an out¬ come of patterns in differential birth and death of species treated as stable in¬ dividuals, just as microevolution works by the same process applied to births and deaths of organisms—punctuated equilibrium validates the hierarchical theory of selection. This hierarchical theory (explicated in Chapter 8) es¬ tablishes the independence of macroevolution as a theoretical subject (not just as a domain of description for accumulated microevolutionary mechan¬ ics), thereby precluding the full explanation of evolution by extrapolation of microevolutionary processes to all scales and times. In practical terms, the implications of punctuated equilibrium for evolu¬ tionary mode have strongly impacted two prominent subjects, heretofore al¬ most always rendered by extrapolation as consequences of adaptation within populations writ large: evolutionary trends within clades, and relative wax¬ ing and waning of diversity within supposedly competing clades through time. Punctuated equilibrium suggests novel, and irreducibly macroevolu¬ tionary, explanations for both phenomena (see pp. 885-916). Finally, the role of punctuated equilibrium in establishing an independent field of macroevolution includes both a weak and a strong version. The first, undoubtedly valid as a generality, “uncouples” macro from microevolution as a descriptive necessity, while not establishing independent causal principles of macroevolution. The second clearly regulates many cases, but has not yet been validated as commanding a high relative frequency; this second, or strong, version establishes irreducible causal principles of macroevolution. The weak version, based on “species sorting” rather than “species selec-
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THE STRUCTURE OF EVOEUTIONARY THEORY tion,” holds that evolution must be described as differential success in birth and death of stable species, but allows that the causality behind reasons for differential success might emerge from the conventional Darwinian level of struggling organisms within successful populations—the effect hypothesis of Vrba (see p. 658). In this version, we need a descriptive, but not a causal, ac¬ count of macroevolution based on species as individuals. However, in the strong version, based on true species selection, the differ¬ ential success of species arises from irreducible fitness defined by the interac¬ tion of species-individuals with their environments. Chapter 8 presents an ex¬ tensive argument for the efficacy of true species selection at high relative frequency. Validation of this argument would establish a genuinely causal and irreducible theory of macroevolution. This difficult issue stands far from resolution, but represents the most exciting potential for punctuated equilib¬ rium as an impetus in formulating a revised structure for evolutionary theory.
The Scientific Debate on Punctuated Equilibrium: Critiques and Responses CRITIQUES BASED ON THE DEFINABILITY OF PALEONTOLOGICAL SPECIES
Empirical affirmation The issue of whether true biospecies (or entities operationally close enough to biospecies) can be recognized in fossils has prompted long and intense debate in paleontology (see Sylvester-Bradley, 1956, and other references previously cited), and does not represent a new or special difficulty raised by punctuated equilibrium. But given the reliance of punctuated equilibrium on speciation as the mechanism behind the pattern, this old problem does legitimately as¬ sume a central place in debates about our theory (as emphasized in all nega¬ tive commentary, particularly clearly by Turner, 1986, and in the book-length critiques of Levinton, 1988, and Hoffman, 1989). At least we may begin by exposing the canonical issue of the older litera¬ ture as a Scheinproblem (literally an “appearance problem” with no real content): the logical impossibility of defining a species boundary within a gradualistic continuum (see my previous discussion on p. 775). I think we may now accept that the punctuational pattern exists at high relative fre¬ quency, and that few gradualistic and anagenetic continua have been docu¬ mented between fossil species. Turner’s (1986) sharp critique, for example (and I do accept his formulation, though not his resolution), depicts the chief claims of punctuated equilibrium as a three-pronged fork. He accepts the first tine—the existence of the punctuational pattern itself—as sufficiently demon¬ strated by enough empirical cases in the fossil record. He regards the third tine—macroevolutionary invocation of the theory to explain trends by spe¬ cies sorting—as “an important extension of evolutionary theory into a hith¬ erto little explored territory” (1986, p. 206). But he then rejects the second
Punctuated, Equilibrium and the Validation of Macroevolutionary Theory tine as both unlikely and too difficult to test in any case—explanation of the punctuational pattern as a consequence of speciation scaled into geological time. If we accept that temporal sequences of fossils generally don’t appear in the geological record as unbreakable contmua, but usually as morphologi¬ cal “packages” with reasonably defined boundaries and sufficient stability within an extended duration, how can we assert that these packages represent biospecies, or at least that they approximate these neontologically defined units with sufficient closeness to bear comparison? After all, we cannot apply conventional tests of observed ecological interaction or interbreeding to fos¬ sils—and, whereas biospecies may be recognized by morphological differ¬ entia in everyday practice, they are not supposed to be so defined. Can the temporally extended “morphospecies” of paleontology really be equated with the “nondimensional species concept” (Mayr’s words) of neontology? I certainly accept the centrality and difficulty of these issues, but I do not regard them as insuperable, and I do not view the species concept as untestable with fossils. After all, the overwhelming majority of modern species in our literature and museum drawers have also been phenotypically, not ecologically, defined. Once we accept that no special paleontological riddles arise from the Scheinproblem of temporal continua, then most paleospecies have been no worse characterized than the majority of neospecies. Still, I will not advance this excuse as exculpatory for the fossil record, for a neontologist could reply, with impeccable logic, that neospecies so defined should also be regarded as uncertain, if not vacuous, and that no paleontological de¬ fense can be mounted by arguing that ordinary practice with fossils follows the worst habits (majoritarian though they may be) of neontological tax¬ onomy. But a best defense of phenotypically defined neospecies would follow from demonstrations that taxa so established usually do match true biospecies upon proper behavioral and ecological study—a line of research often pur¬ sued with success (see references in Jackson and Cheetham, 1994, and in Jablonski, 1999). Similarly, my main source for confidence about paleo¬ species arises from proven correspondences with true biospecies in favorable cases providing sufficient information for such a test (particularly for extant species with lengthy fossil records). I do not, of course, argue that all named paleospecies are true biospecies, or that I can even estimate the percentage properly so defined (any more than we know the relative frequency of mod¬ ern taxa that represent true biospecies). But I do not see why the probability that well-defined paleospecies, based on good collections from many times and places, might represent proper biospecies should be any lower than the corresponding figure for equally well documented, but entirely morphologi¬ cally defined, modern taxa. (In fact, one might argue that well-documented paleospecies probably maintain a higher probability for representing bio¬ species, because we know their phenotypes, and have measured their stability, across long periods of time and wide ranges of environment—whereas mod¬ ern “morphospecies” may arise as ecophenotypic expressions of a single time
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THE STRUCTURE OF EVOLUTIONARY THEORY and place, therefore ranking only as local populations, rather than true spe¬ cies.) When well-defined paleospecies have been tested for their correspondence with modern biospecies, such status has often been persuasively affirmed. Two recent studies seem particularly convincing. Michaux (1989) studied four living species of the marine gastropod genus Amalda from New Zea¬ land. Fossils of this genus date to the upper Eocene of this region, while all four species extend at least to the Miocene-Pliocene boundary. The four taxa represent good biospecies, based on absence of hybrids in sympatry, and on extensive electrophoretic study (Michaux, 1987) showing distinct separa¬ tion among species and “no detectable cryptic groupings” (Michaux, 1989, p. 241) within any species. Michaux then used canonical discriminant analy¬ sis to achieve clear morphometric distinction among the species based on 10 shell measurements for each of 671 live specimens. He then made the same measurements on 662 fossil specimens from three of the species (the fourth did not yield enough shells for adequate character¬ ization). Mean values, in multivariate expression based on all 10 variables, fluctuated mildly through time (see Fig. 9-8), but never departed from the range of variation within extant populations—an excellent demonstration of stasis as dynamic maintenance within well-defined biospecies through several million years. Michaux concluded (1989, pp. 246-248): “Fossil members of three biologically distinct species fall within the range of variation that is ex¬ hibited by extant members of these species. The phenotypic trajectory of each species is shown to oscillate around the modern mean through the time pe¬ riod under consideration. This pattern demonstrates oscillatory change in phenotype within prescribed limits, that is, phenotypic stasis.” Jackson and Cheetham’s (1990, 1994) extensive studies of cheilostome bryozoan species provide even more gratifying affirmation, especially since these “simple” sessile and colonial forms potentially express all the attributes of extensive ecophenotypic variation (especially in molding of colonies to substrates, and in effects of crowding) and morphological simplicity (lack of enough complex skeletal characters for good definition of taxa) generally re¬ garded as rendering the identification of biospecies hazardous, if not effec¬ tively impossible, in1 fossils. Moreover, Cheetham had begun his paleonto¬ logical studies (see discussion on pp. 867-870) under the assumption that careful work would reveal predominant gradualism and refute the “new” hy¬ pothesis of punctuated equilibrium—so the conclusions eventually reached were not favored by any a priori preference! In a first study—devoted to determining whether biospecies could be recog¬ nized from skeletal characters (of the sort used to define fossil taxa) in sev¬ eral species within three genera of extant Caribbean cheilostomes—Jackson and Cheetham (1990) examined heritability for skeletal characters in seven species. In a “common garden” experiment (under effectively identical condi¬ tions at a single experimental site), they grew F, and F2 generations from em¬ bryos derived from known maternal colonies collected in disparate environ-
Punctuated Equilibrium and the Validation of Macroevolutionary Theory
ments and places. Multivariate discriminant analysis assigned all but 9 of 507 offspring into the same morphospecies as their maternal parent. The authors then used electrophoretic methods to study enzyme variation in 402 colonies representing 8 species in the three genera. They found clear and complete cor¬ respondence between genetic and morphometric clusterings, and also deter¬ mined (p. 581) that “genetic distances between morphospecies are consis¬ tently much higher than between populations of the same morphospecies”; moreover, they found no evidence for any cryptic division (potential “sibling species”) within skeletally defined morphospecies. In a concluding and gratifying observation—indicating that paleontolo-
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0.99, and no evidence that intraspecific rates of morphological change can ac¬ count for differences between species.” For Stylopoma, where fossil evidence had not previously been analyzed morphometrically, results also affirmed punctuated equilibrium throughout (1994, p. 420): “The excellent agreement between morphologically and ge¬ netically defined species used in this taxonomy suggests that morphological stasis reflects genuine species survival over millions of years, rather than a se¬ ries of morphologically cryptic species. Morover, eleven of the 19 species originate fully formed at p > 0.9, with no evidence of morphologically inter¬ mediate forms, and all ancestral species but one survived unchanged all with their descendants.” In a concluding paragraph about both genera, Jackson and Cheetham wrote (p. 407): “Stratigraphically rooted trees suggest that most well-sam¬ pled Metrarabdotos and Stylopoma species originated fully differentiated morphologically and persisted unchanged for > 1 to > 16 m.y., typically
Punctuated Equilibrium and the Validation of Macroevolutionary Theory alongside their putative ancestors. Moreover, the tight correlation between phenetic, cladistic, and genetic distances among living Stylopoma species sug¬ gests that changes in all three variables occurred together during speciation. All of these observations support the punctuated equilibrium model of speciation.” Despite the encouragement provided by these and other cases, problems continue to surround the definition of paleontological species—a subject of central importance to punctuated equilibrium, given our invocation of speci¬ ation as the quantum of change for life’s macroevolutionary history, and the source of raw material for higher-level selection and sorting. These prob¬ lems center upon three main issues (in both the inherent logic of the case and by recorded debate in the literature): the first untroubling, the second poten¬ tially serious, and the third largely resolved in empirical terms. All three is¬ sues raise the possibility that paleospecies systematically misrepresent the na¬ ture and number of actual biospecies. (If paleospecies don’t correspond with biospecies in all cases—an undeniable proposition of course—but if these dis¬ crepancies show no pattern and produce no systematic bias, then we need not be troubled unless the relative frequency of noncorrespondence becomes overwhelmingly high, an unlikely situation given the excellent alignments found in the few studies explicitly done to investigate this problem, as dis¬ cussed just above.) The following three subsections treat these three remain¬ ing issues seriatim.
Reasons for a potential systematic underestimation of biospecies by paleospecies Might we be missing a high percentage of actual speciation events because paleontologists can only recognize a cladogenetic branch with clear pheno¬ typic consequences (for characters preserved as fossils), whereas many new species arise without substantial morphological divergence from their ances¬ tors? In the clearest case, paleontologists (obviously) cannot detect sibling species, a common phenomenon in evolution (see Mayr, 1963, for the classic statement). Moreover, we may also miss subtle changes in phenotype, or substantial alterations (of color, for example) in features that are often im¬ portant in recognizing species, but do not achieve expression in the fossil rec¬ ord. Our harshest critics have urged this point as particularly telling against punctuated equilibrium. Levinton, for example (1988, p. 182), holds that “the vast majority of speciation events probably beget no significant change.” He then views the consequences as effectively fatal for punctuated equilib¬ rium (1988, p. 211): “The punctuated equilibrium model argues that mor¬ phological change is associated with speciation and that species are static during their history due to some internal stabilizing mechanism. There is no evidence coming from living species to support this. If anything, recent re¬ search has demonstrated that speciation occurs typically with little or no morphological change; hence the large-scale occurrence of sibling species.”
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THE STRUCTURE OF EVOLUTIONARY THEORY Hoffman (1989, p. 115) invokes this argument to assert the untestability, hence the nonscientihc status, of punctuated equilibrium: Long-term evolutionary stasis of species, however, simply cannot be tested in the fossil record. Paleontological data consist solely of a small sample of phenotypic traits—little more than morphology of the skeletal parts—which does not allow us to make any inference about changes in a species’s genetic pool or even about changes of the frequency distribu¬ tion of phenotypes in a phyletic lineage. The non-preserved portion of the phenotype of each fossil species is so extensive that it may always un¬ dergo considerable evolutionary changes that remain undetectable by the paleontologist. What appears then to the paleontologist as a species in complete evolutionary stasis may in fact represent a succession of fos¬ sil species or perhaps a whole cluster of species, a phylogenetic tree with a sizable number of branching points, or speciation events. While I freely admit all these arguments for underrepresentation of true species in fossil data, I do not comprehend how punctuated equilibrium could be thus rendered untestable, or even seriously compromised (see further argu¬ ments in Gould, 1982c and 1989e; and Gould and Eldredge, 1993). I base my argument on two logical and methodological principles, not on the probable empirical record (where I largely agree with our critics). The proper study of macroevolution.
By consensus, and ac¬
cepting a criterion of testability, science does not include, within its compass of inquiry, fascinating questions that cannot be answered (even if they ad¬ dress potentially empirical subjects). For example, and for the moment at least, we know no way to ask a scientific quesiton about what happened be¬ fore the big bang, for compression of universal matter to a single point of ori¬ gin wipes out all traces of any previous history. (Perhaps we will eventually devise a way to obtain such data, or perhaps the big bang theory will be dis¬ carded. The question might then become scientifically tractable.) Similarly, we know that many kinds of evolutionary events leave no empirical record— and that we therefore cannot formulate scientific questions about them. (For example, I doubt that we will be able to resolve the origins of human lan¬ guage, unless written expression occurred far earlier than current belief and evidence now indicate.) The nature of the fossil record leads us to define macroevolution as the study of phenotypic change (and any inferable correlates or sequelae) in lin¬ eages and clades throughout geological time. Punctuated equilibrium pro¬ poses that such changes generally occur in discrete units or quanta in geologi¬ cal time, and that these quanta represent events of branching speciation. Thus, we do identify speciation as the source of raw material for macroevolu¬ tionary change in lineages. But we do not, and cannot, argue (or attempt to adjudicate at all) the quite different proposition that all speciation events pro¬ duce measurable quanta of macroevolutionary change. The statement—our proposition—that nearly all macroevolutionary change occurs in increments of speciation carries no implications for the unrelated claim, often imputed to
Punctuated Equilibrium and the Validation of Macroevolutionary Theory punctuated equilibrium by our critics (but largely irrelevant to our theory), that nearly all events of speciation produce an increment of macroevolution¬ ary change. This conclusion flows from elementary logic, not from empirical science. The argument that all B comes from A does not imply that all A leads to B. All human births (at least before modern interventions of medical tech¬ nology) derived from acts of sexual intercourse, but all acts of intercourse don't lead to births. To draw a more relevant analogy: in the strict version of Mayr’s peripatric theory of speciation, nearly all new species arise from small populations iso¬ lated at the periphery of the parental range. But the vast majority of periph¬ eral isolates never form new species; for they either die out or reamalgamate with the parental population. Similarly, most new species may never be re¬ corded in the fossil record; but, if the theory of punctuated equilibrium holds, when changes do appear in lineages of fossils, speciation provides the source of input in a great majority of cases. Thus, most speciation could be cryptic (and unknowable from fossil evidence), while effectively all macroevolution¬ ary change still arises from the minority of speciation events with phenotypic consequences. Just as peripheral isolates might represent “the only game in town” for forming new species (though few isolates ever speciate), cladogenetic speciation may be “the only game in town” for inputting phenotypic change into macroevolution (though few new species exhibit such change). The treatment of ineluctable natural bias in science. In
an ideal world—the one we try to construct in controlled laboratory experi¬ ments—no systematic bias distorts the relative frequency of potential results. But the real world of nature meets us on her own terms, and we must accept any distortions of actual frequencies that directional biases of recording or preservation inflict upon the archives of our evidence. At best, we may be able to correct such biases if we can make a quantitative estimate of their strength. (This general procedure, for example, has been widely followed to correct the systematic undermeasurement of geological ranges imposed by the evident fact that observed first and last occurrences of a fossil species can only pro¬ vide a minimal estimate for actual origins and extinctions, for the observed geological range of a species must be shorter (and at least cannot be longer) than the actual duration. Studies of “waiting times” between sequential sam¬ ples within the observed range, combined with mathematical models for con¬ structing error bars around first and last occurrences, have been widely used to treat this important problem—see Sadler, 1981; Schindel, 1982; Marshall, 1994.) Often, however, we can specify the direction of a bias, but do not know how to make a quantitative correction. In such cases, the sciences of natural history must follow a cardinal rule: if the direction of bias coincides with the predicted effect of the theory under test, then researchers face a serious, per¬ haps insurmountable, problem; but if a systematic bias works against a the¬ ory, then researchers encounter an acceptable impediment—for if the theory can still be affirmed in the face of unmeasurable biases working against a fa¬ vored explanation, then the case for the theory gains strength.
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THE STRUCTURE OF EVOEUTIONARY THEORY For proponents of punctuated equilibrium, speciation represents the pri¬ mary source for morphological changes that, by summation of increments, build trends in the history of lineages. If a systematic bias in the nature of paleontological evidence leads us to underestimate the number of speciation events, and if we can still explain trends by this observed number (necessarily less than the actual frequency), then-the case for punctuated equilibrium be¬ comes stronger by affirmation in the face of a bias working against full ex¬ pression of the theory’s effect. Thus, although we regret the existence of any bias that we cannot correct, a systematic underrepresentation of speciation events does not subvert punctuated equilibrium because such a natural skew¬ ing of evidence makes the hypothesis even more difficult to affirm—and sup¬ port for punctuated equilibrium therefore emerges in a context even more challenging than the unbiassed world of controlled experimentation. Moreover, one might even stress the bright side and recognize that such bi¬ ases may exist for interesting reasons in themselves—reasons that might even enhance the importance of punctuated equilibrium and its implications. I doubt that Levinton (1988, p. 379) intended the following passage in such a positive light, but I would suggest such a reading: “One cannot rule out the possibility that speciation is rampant, but morphological evolution only oc¬ curs occasionally when a population is forced into a marginal environment and subjected to rapid directional selection. What then becomes interesting is why the character complexes evolved in the daughter species remain con¬ stant. This is, again, the issue of stasis, which I believe to be the legitimate problem spawned by the punctuated equilibrium model.” Finally, I am not sure that fossil species do strongly and generally underesti¬ mate the frequency of true biospeciation—although I do accept that a bias, if present at all, probably operates in this direction. The most rigorous empiri¬ cal studies on correspondence between well-defined paleospecies and true biospecies—the works of Michaux and of Jackson and Cheetham discussed above—affirm a one-to-one link between paleontological morphospecies and extant, genetically-defined biospecies.
Reasons for a potential systematic overestimation of biospecies by paleospecies If a bias did exist in this opposite direction, the consequences for punctuated equilibrium would be troubling (as implied in the previous section on accept¬ able and unacceptable forms of unavoidable natural biasing). For if we sys¬ tematically name too many species by paleontological criteria, then we might be affirming punctuated equilibrium by skewing data in the direction of our favored theory, rather than by genuine evidence from the fossil record. How¬ ever, I doubt that such a problem exists for punctuated equilibrium, especially since all experts—both strong advocates and fierce critics alike (as the preced¬ ing discussion documented)—seem to agree that if any systematic bias exists, the probable direction lies in the acceptable opposite claim for underestima¬ tion of biospecies by paleospecies. I don’t doubt, of course, that past taxonomic practice, often favoring the erection of a species name for every recognizable morphological variant (even
Punctuated Equilibrium and the Validation of Macroevolutionary Theory for odd individuals rather than populations), has greatly inflated the roster of legitimate names in many cases, particularly for fossil groups last mono¬ graphed several generations ago. (Our literature even recognizes the halffacetious term “monographic burst” for peaks of diversity thus artificially created. But this problem of past oversplitting cannot be construed as either uniquely or even especially paleontological, for neontological systematics then followed the same practices as well.) The grossly uneven, and often greatly oversplit, construction of species-level taxonomy in paleontology has acted as a strong impediment for the entire research program of the promi¬ nent school of “taxon-counting” (Raup, 1975, 1985). For this reason, the ge¬ nus has traditionally been regarded as the lowest unit of rough comparability in paleontological data (see Newell, 1949). Sepkoski (1982) therefore com¬ piled his two great compendia—the basis for so much research in the his¬ tory of life’s fluctuating diversity—at the family, and then at the genus, level (but explicitly not at the species level in recognition of frequent oversplitting and extreme imbalance in practice of research among specialists on various groups). Although this problem has proved far more serious for taxon-counters than for proponents of punctuated equilibrium, a potential bias towards overrepresentation also poses a threat for our theory, as Levinton (1988, p. 364) rightly recognizes: “The problem is not very new. Meyer (1878) claimed that the ability to recognize gradual evolutionary change in Micraster [a famous sequence of Cretaceous echinoids] was obscured by the rampant naming of separate species by previous taxonomists.” This issue would cause me serious concern—for the claim of overestima¬ tion does, after all, fall into the worrisome category of biases favoring a pre¬ ferred hypothesis under test—if two arguments and realities did not obvi¬ ate the danger. First, if supporters of punctuated equilibrium did try to affirm their hypothesis by using names recorded in the literature as primary data for judging the strength and effect of speciation upon evolutionary trends, then we would face a serious difficulty. But I cannot think of any study that utilized this invalid approach—for paleontologists recognize and generally avoid the dangers of this well-known directional bias. Punctuated equilib¬ rium, to my knowledge, has never been defended by taxon counting at the species level. All confirmatory studies employ measured morphometric pat¬ terns, not the geological ranges of names recorded in literature. Second, as stated above, all students of this subject seem to agree that if a systematic bias exists in relative numbers of paleospecies and biospecies, fos¬ sil data should be skewed in the opposite direction of recognizing fewer paleospecies than biospecies—an acceptable bias operating against the confir¬ mation of punctuated equilibrium.
Reasons why an observed punctuational pattern might not represent speciation Suppose that we have empirical evidence for a punctuational event separating two distinct morphological packages regarded as both different enough to be designated as separate paleospecies by any standard criterion, and also genea-
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THE STRUCTURE OF EVOLUTIONARY THEORY logically close enough to support a hypothesis of direct ancestry and descent. What more do we need? Does this situation not affirm punctuated equilib¬ rium ipso facto? But critics charge (and I must agree) that such evidence cannot be persua¬ sive by itself, because punctuated equilibrium explicitly links punctuational patterns to events of branching speciation. Therefore, recorded punctuations produced for other reasons do not affirm punctuated equilibrium—and may even challenge the theory if their frequency be high and, especially, if they cannot be distinguished in principle (or frequently enough in practice) from events of cladogenetic branching. Punctuational patterns often originate (at all scales in evolutionary hierar¬ chies of levels and times) for reasons other than geologically instantaneous speciation—and I welcome such evidence as an affirmation of pervasive im¬ portance (see p. 922 et seq.) for a general style of nongradualistic change, with punctuated equilibrium as its usual mode of expression at the speciational scale under consideration in this chapter. But testable, and generally applicable, criteria have been formulated for distinguishing punctuated equi¬ librium from other reasons for punctuational patterns—and available evi¬ dence amply confirms the importance and high relative frequency of punctu¬ ated equilibrium. Of the two major reasons for punctuational patterns not due to speciation, Darwin’s own classic argument of imperfection—geological gradualism that appears punctuational because most steps of a continuum have not been pre¬ served in the fossil record—retains pride of place by venerable ancestry. I have already presented my reasons for regarding this argument as inconse¬ quential (see pp. 765-774). I do not, of course, deny that many (or most) breaks in geological sequences only reflect missing evidence. But proponents of punctuated equilibrium do not base their claims on such inadequate exam¬ ples that cannot be decided in either direction. The test cases of our best liter¬ ature—whether their outcomes be punctuational or gradualistic—have been generated from stratigraphic situations where temporal resolution and den¬ sity of sampling can make appropriate distinctions by recorded evidence, not conjectures about missing data. The second reason has been highlighted by some critics, but unfairly I think, because punctuated equilibrium has always recognized the argument and has, moreover, enunciated and explicitly tested proper criteria for mak¬ ing the necessary distinctions. To state the supposed problem: what can we conclude when we document a truly punctuational sequence that cannot be attributed to imperfections of the fossil record? How do we know that such a pattern records an event of branching speciation, as the theory of punctuated equilibrium requires? When ancestral Species A abruptly yields to descendant Species B in a vertical sequence of strata, we may only be witnessing an anagenetic transformation through a population bottleneck, or perhaps an event of migration, where Species B, having evolved gradualistically from Species A in another region, invades the geographic range, and abruptly wipes out its ancestor.
Punctuated Equilibrium and the Validation of Macroevolutionary Theory But an appropriate and non-arbitrary criterion exists—and has been fully enunciated, featured as crucial, and subjected to frequent test, from the early days of punctuated equilibrium. We can distinguish the punctuations of rapid anagenesis from those of branching speciation by invoking the eminently test¬ able criterion of ancestral survival following the origin of a descendant spe¬ cies. If the ancestor survives, then the new species has arisen by branching. If the ancestor does not survive, then we must count the case either as indeci¬ sive, or as good evidence for rapid anagenesis—but, in any instance, certainly not as evidence for punctuated equilibrium. Moreover, by using this criterion, we obey the methodological requirement that existing biases must work against a theory under test. When ancestors do not survive following the first appearance of descendants, the pattern may still be recording an event of branching speciation—hence affirmation for punctuated equilibrium. But we cannot count such cases in our favor, for the plausible alternative of rapid anagenesis cannot be disproven. By restricting affirmations to cases where ancestors demonstrably survive, we accept only a subset of events actually caused by speciation. Thus, we underestimate the frequency of punctuated equilibrium—as we must do in the face of an unresolvable bias affecting a hypothesis under test. In our first papers, we did not recognize or articulate the importance of tabulating cases of ancestral survival following punctuational origin of a de¬ scendant as a criterion for distinguishing punctuated equilibrium from other forms of punctuational change. (Both of our original examples in Eldredge and Gould, 1972, did feature—and prominently discuss—ancestral survival as an important aspect of the total pattern. We had a proper “gut feeling” about best cases, but we did not formalize the criterion.) But, beginning in 1982, and continuing thereafter, we have stressed the centrality of this crite¬ rion in claims for speciation as the mechanism of punctuated equilibrium. Contrasting the difference in paleontological expression between Wright’s shifting balance and punctuated equilibrium by speciation, for example, I wrote (Gould, 1982c, p. 100): “Since punctuational events can occur in the phyletic mode under shifting balance, but by branching speciation under punctuated equilibrium, the persistence of ancestors following the abrupt ap¬ pearance of a descendant is the surest sign of punctuated equilibrium.” This criterion has been actively applied, in an increasingly routine man¬ ner (as researchers recognize its importance), in the expanding literature on empirical study of evolutionary tempos and modes in well-documented fos¬ sil sequences. Cases of probable anagenetic transformation have been doc¬ umented (no ancestral survival when good stratigraphic resolution should have recorded such persistence, had it occurred), especially in planktic marine Foraminifera, where long oceanic cores often provide unusually complete evi¬ dence (Banner and Lowry, 1985; Malmgren and Kennett, 1981, who coined the appropriate term “punctuated anagenesis” for this phenomenon). Flowever, abundant cases of ancestral survival, and consequent punctua¬ tional origin of descendant taxa by branching speciation, have also been affirmed as illustrations of punctuated equilibrium. These examples span
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THE STRUCTURE OF EVOEUTIONARY THEORY the gamut of taxonomies and ecologies, ranging from marine microfossils (Cronin, 1985, on ostracodes); to “standard” macroscopic marine inverte¬ brates (with Cheetham’s famous studies of bryozoans, 1986 and 1987, as classic and multiply documented examples), to freshwater invertebrates (Williamson’s 1981 work on multiple events of speciation in African lake mollusks, where ancestral species reinvade upon coalescence of lakes following periods of isolation that provided conditions for speciation); to terrestrial vertebrates (Flynn, 1986, on rodents; Prothero and Shubin, 1989, on horses). I shall discuss this important issue in more detail within the forthcoming sec¬ tion on evidence for punctuated equilibrium (see pp. 822-874), but I have been particularly (if parochially) gratified by the increasing application of punctuated equilibrium to the resolution of hominid phylogeny. The criterion of ancestral survival has been prominently featured in this literature, as by McHenry (1994), who notes that “ancestral species overlap in time with de¬ scendants in most cases in hominid evolution, which is not what would be ex¬ pected from gradual transformations by anagenesis.” In any case, punctuated equilibrium can be adequately and generally recog¬ nized by firm evidence linking observed punctuational patterns to branching speciation as a cause. The theory of punctuated equilibrium is eminently test¬ able and has, indeed, passed such trials in cases now so numerous that a high relative frequency for this important evolutionary phenomenon can no longer be denied (see Gould and Eldredge, 1993).
CRITIQUES BASED ON DENYING EVENTS OF SPECIATION AS THE PRIMARY LOCUS OF CHANGE Once we overcome the problem of definability for species in the fossil record, punctuated equilibrium still faces a major issue rooted in the crucial subject of speciation. Punctuated equilibrium affirms, as a primary statement, that ordinary biological speciation, when properly scaled into geological time, produces the characteristic punctuational pattern of our fossil record. We must therefore be able to defend the central implication that morphological change should be preferentially associated with events of branching specia¬ tion. Our critics have strongly argued that such a proposition cannot be justi¬ fied by our best understanding of evolutionary processes and mechanisms. I believe that our critics have been correct in this argument, and that Eldredge and I made a major error by advocating, in the original formulation of our theory, a direct acceleration of evolutionary rate by the processes of speciation. This claim, I now think, represents one of the two most important errors that we committed in advocating punctuated equilibrium during the past 25 years. (The other error, as discussed and corrected on pages 670-673, lay first in our failure to recognize the phenomenon of species selection as dis¬ tinct (by hierarchical reasoning) from classical Darwinian organismic selec¬ tion, and then (see Gould and Eldredge, 1977) in our decision to advocate an overly broad and purely descriptive definition rather than a properly limited meaning based on emergent characters or fitnesses—see pages 656-670.)
Punctuated Equilibrium and the Validation of Macroevolutionary Theory We did not urge this correlation between speciation events and morpholog¬ ical change in a self-serving and circular manner—i.eonly because the pat¬ tern of punctuated equilibrium could be best defended thereby. We did, of course, recognize the logical link, as in the following statement from Gould, 1982c, p. 87 (see also Gould and Eldredge, 1977, p. 137): “Reproductive iso¬ lation and the morphological gaps that define species for paleontologists are not equivalent. Punctuated equilibrium requires either that most morphologi¬ cal change arise in coincidence with speciation itself, or that the morphologi¬ cal adaptations made possible by reproductive isolation arise rapidly thereaf¬ ter.” But we based our defense of this proposition upon a large, and then quite standard, literature advocating a strong negative correlation between capacity for rapid evolutionary change and population size. Small popula¬ tions, under these models, maintained maximal prospects for rapid transfor¬ mation based on several factors, including potentially rapid fixation of favor¬ able variants, and enhancement of differences from ancestral populations by interaction of intense selection with stochastic reasons for change (particu¬ larly the founder effect) that can only occur with such effective speed in small populations. Large and stable populations, by the converse of these argu¬ ments, should be sluggish and resistant to change. This literature culminated in Mayr’s spirited defense for “genetic revolu¬ tion” as a common component of speciation (first proposed in a famous 1954 article, and then defended in extenso in the 1963 book that served as the clos¬ est analog to a “bible” for graduate students of my generation). Since Mayr (who coined the name “founder effect” in this context) also linked his con¬ cept of “genetic revolution” to the small, peripherally isolated populations that served as “incipient species” in his influential theory of peripatric spe¬ ciation—and since we had invoked this theory in our original formulation of punctuated equilibrium (Eldredge and Gould, 1972)—our defense of a link between speciation and concentrated episodes of genetic (and phenotypic) change flowed logically from the evolutionary views we had embraced. Thus, we correlated punctuations with the extensive changes that often occurred during events of speciation in small, peripherally isolated populations; and we linked stasis with the expected stability of large and successful popu¬ lations following their more volatile and punctuational origins as small iso¬ lates. I can claim no expertise in this aspect of neontological evolutionary theory, but I certainly acknowledge, and must therefore provisionally accept, the re¬ vised consensus of the past twenty years that has challenged this body of thought, and rejected any general rationale for equating the bulk of evolu¬ tionary change with events of speciation in small populations, or with small populations in any sense. As I read the current literature, most evolutionists now view large populations as equally prone to evolutionary transformation, and also find no reason to equate times of speciation—the attainment of re¬ productive isolation—with acceleration in general rates of genetic or pheno¬ typic change (see, for example, Ridley, 1993; and Williams, 1992). (I do, however, continue to wonder whether the Mayrian viewpoint might still hold
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THE STRUCTURE OF EVOLUTIONARY THEORY some validity, and might now be subject to overly curt and confident dis¬ missal.) This situation creates a paradox for our theory. The pattern of punctuated equilibrium has been well documented and shown to predominate in many situations (see pp. 822-874), but its most obvious theoretical rationale has now fallen under strong skepticism. So either punctuated equilibrium is wrong—a proposition that this partisan views as unlikely (although obvi¬ ously possible), especially in the face of such strong documentation—or we must identify another reason for the prominence of punctuated equilibrium as a pattern in the history of life. In our article on the “majority” (21st birth¬ day!) of punctuated equilibrium, Eldredge and I expressed this dilemma in the following manner (Gould and Eldredge, 1993, p. 226): “The pattern of punctuated equilibrium exists (at predominant relative frequency, we would argue) and is robust. Eppur non si muove; but why then? For the association of morphological change with speciation remains as a major pattern in the fossil record.” (Our Italian parody, missed by many readers of the original ar¬ ticle, alters Galileo’s famous, but almost surely legendary, rebuke to the In¬ quisition, delivered secretly and sotto voce after he had been forced to recant his Copernican views in public: Eppur si muove—nevertheless it does move. Our parody says “nevertheless it does not move”-—a reference to the over¬ whelming evidence for predominant stasis in the history of species, even if our original evolutionary rationale, based on population size, must be reas¬ sessed.) This paradox permits several approaches, including the following two that I would not favor. One might simply argue that the pattern of punctuated equilibrium demonstrably exists, so the task falls to evolutionary theorists to find a proper explanation. The current absence of a satisfactory account does not threaten the empirical record, but rather directs inquiry by posing a prob¬ lem. Or one might doubt that any single explanation can render the phenom¬ enon, and suspect that many rationales will yield the observed pattern (in¬ cluding Mayrian genetic revolutions, even if we now regard their relative frequency as low). Thus, we need to identify a set of enabling criteria from evolutionary theory, and then argue that their combination may render the observed phenomena of the fossil record. Most researchers would regard a third approach as preferable in science: an alternate general explanation of different form from the previous, but now rejected, leading candidate. I believe that such a resolution has been provided by Douglas Futuyma (1986, 1988a and b, but especially 1987),* although his *Futuyma remains quite skeptical of punctuated equilibrium in general, and I would place him more among our critics than our supporters. But he does accept the empirical pattern, and he is an expert on speciation. Thus, when he developed an original way to re¬ solve the paradox of why punctuations might correlate with events of speciation, even if processes of speciation don’t accelerate the rate of evolution, he published his ideas as a constructive contribution to the general debate. Even though Futuyma disagrees with our claims for the general importance of punctuated equilibrium (while he, obviously, does not deny the phenomenon), he has granted us serious attention and has acknowledged the in¬ tellectual interest of the debate we provoked—and no one could ask for more from a good critic. Futuyma wrote (1988, p. 225), in stressing the need to integrate “synchronic” ap-
Punctuated Equilibrium and the Validation of Macroevolutionary Theory
simple, yet profound, argument has not infused the consciousness of evolu¬ tionists because the implied and required hierarchical style of thinking re¬ mains so unfamiliar and elusive to most of us. (In fact, and with some shame, I am chagrined that I never recognized this evident and elegant resolution my¬ self. After all, I am supposedly steeped in this alternative hierarchical mode of thinking—and I certainly have a strong stake in the problems of punctuated equilibrium.) In short, Futuyma argues that we have been running on the wrong track, and thinking at the wrong level, in trying to locate the reason for a correla¬ tion between paleontological punctuations and events of speciation in a di¬ rect mechanism of accelerated change promoted by the process of speciation itself. Yet Futuyma does agree that a strong correlation exists (and has been demonstrated, in large part by research and literature generated by debate about punctuated equilibrium). Since we all understand (but do not always put into practice!) the important logical principle that correlation does not imply causality (the post hoc fallacy), an acknowledgement of the genuine link doesn’t commit us to any particular causal scheme—especially, in this case, to the apparently false claim that mechanisms of speciation inherently enhance evolutionary rates. Futuyma begins by arguing that morphological change may accumulate anywhere along the temporal trajectory of a species, and not exclusively (or even preferentially) during the geological moment of its origin. What then could produce such a strong correlation between events of branching specia¬ tion and morphological change from an ancestral phenotype to the subse¬ quent stasis of an altered descendant? Futuyma—and I am somewhat re¬ phrasing and extending his argument here—draws an insightful and original analogy between macroevolution and the conventional Darwinism of natural selection in populations. The operation of natural selection requires that Darwinian individuals in¬ teract with environments in such a manner that distinct features of these indi¬ viduals bias their reproductive success relative to others in the population. As a defining criterion of Darwinian individuality, entities that interact with the environment must show “sufficient stability” (see discussion on pp. 611— 613)—defined in terms of the theory and mechanism under discussion as enough coherence to perform as an interactor in the process of natural selec¬ tion. Darwin recognized that organisms operate as fundamental interactors for
proaches as pursued by neontologists interested in evolutionary mechanisms with the “his¬ torical” themes favored by systematists and paleontologists—all (to borrow a line from elsewhere) “in order to form a more perfect union.” We need to identify and to define rigorously questions to which both synchronic and historical evolution can make truly indispensable contributions. Some such questions have already been posed, so we now find systematists and population geneticists con¬ verging on the analysis of macromolecular sequences, geneticists publishing in Paleo¬ biology (thanks to the healthy stimulus of punctuated equilibrium), systematists and students of adaptation finding a rapprochement in the use of phylogenetic informa¬ tion to test hypotheses of behavioral, physiological, and other adaptations.
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THE STRUCTURE OF EVOLUTIONARY THEORY microevolution within populations. (Gene selectionists make a crucial error in arguing that sexual organisms are not stable enough to be regarded as units of selection because they must disaggregate in forming the next generation. But units of selection are interactors, and the “sufficient stability” required by *■
the theory only demands persistence through one episode (generational at this level) of selective interaction to 'bias reproductive success—as organisms do in the classical Darwinian “struggle for existence,” see full discussion on pages 619-625.) Organisms achieve this stability through ordinary mecha¬ nisms of bodily coherence (a protective skin, functional integration of parts, a regulated developmental program, etc.). What, then, produces a corresponding stability for units of macroevolu¬ tion? Species-individuals are constructed as complex units, composed of nu¬ merous local populations, each potentially separate (at any moment) due to limited gene flow, and each capable of adaptation to unique and immediate environments. Thus, in principle, substantial evolution can occur in any local population at any time during the geological trajectory of a species. A large and developing literature, much beloved by popular sources (media and text¬ books) for illustrating the efficacy of evolution in the flesh of immediacy (that is, within a time frame viscerally understood by human beings), has docu¬ mented these rapid and adaptive changes in isolated local populations—sub¬ stantial evolution of body size in guppies (Reznick et ah, 1997), or of leg length in anolid lizards (Losos et ah, 1997), for example (see Gould, 1997f). But these changes in local populations cannot gain any sustained macro¬ evolutionary expression unless they become “locked up” in a Darwinian indi¬ vidual with sufficient stability to act as a unit of selection in geological time. Local populations—as a primary feature of their definition—do not maintain such coherence. They can in principle—and do, in the fullness of geological time, almost invariably in practice—interbreed with other local populations of their species. The distinctively evolved adaptations of local populations must therefore be ephemeral in geological terms, unless these features can be stabilized by individuation—that is, by protection against amalgamation with other Darwinian individuals. Speciation—as the core of its macroevo¬ lutionary meaning—provides such individuation by “locking up” evolved changes in reproductively isolated populations that can, thereafter, no longer amalgamate with others. The Darwinian individuation of organisms occurs by bodily coherence for structural and functional reasons. The Darwinian in¬ dividuation of species occurs by reproductive coherence among parts (organ¬ isms), and by prevention of intermingling between these parts and the parts of other macroevolutionary individuals (that is, organisms of other species). Rapid evolution in local population of guppies and anoles illustrates a fas¬ cinating phenomenon that teaches us many important lessons about the gen¬ eral process of evolution. But such changes can only be ephemeral unless they then become stabilized in coherent higher-level Darwinian individuals with sufficient stability to participate in macroevolutionary selection. These local populations usually strut and fret their short hour on the geological stage, and then disappear by death or amalgamation. They produce the ubiquitous
Punctuated Equilibrium and the Validation of Macroevolutionary Theory
and geologically momentary fluctuations that characterize and embellish the long-term stasis of species. They are, to use Mandelbrot’s famous metaphor for fractals, the squiggles and jiggles on the coastline of Maine depicted at a scale that measures the distance around every boulder on every beach along the shore, and not at the resolution properly enjoined when the entire state appears on a single page in an atlas. Macroevolution represents the page of the atlas. The distance around each boulder (marking substantial but ephem¬ eral changes in local populations of guppies and lizards)—however important in the immediacy of an ecological moment—becomes invisible and irrelevant (as the transient fluctuations of stasis) in the domain of sustained macroevo¬ lutionary change (Fig. 9-9). In other words, morphological change correlates so strongly with speciation not because cladogenesis accelerates evolutionary rates, but rather be¬ cause such changes, which can occur at any time in the life of a local popula¬ tion, cannot be retained (and sufficiently stabilized to participate in selection) without the protection provided by individuation—and speciation, via repro¬ ductive isolation, represents nature’s preeminent mechanism for generating macroevolutionary individuals. Speciation does not necessarily promote evo¬ lutionary change; rather, speciation “gathers in” and guards evolutionary change by locking and stabilization for sufficient geological time within a Darwinian individual of the appropriate scale. If a change in a local popula¬ tion does not gain such protection, it becomes—to borrow Dawkins’s meta¬ phor at a macroevolutionary scale—a transient duststorm in the desert of time, a passing cloud without borders, integrity, or even the capacity to act as a unit of selection, in the panorama of life’s phylogeny. To cite Futuyma’s summary of his powerful idea (1987, p. 465): “I propose that because the spatial locations of habitats shift in time, extinction of and interbreeding among local populations makes much of the geographic differ¬ entiation of populations ephemeral, whereas reproductive isolation confers sufficient permanence on morphological changes for them to be discerned in
9-9. Stasis does not imply absolute stability, but rather directionless fluctuation that generally does not stray beyond the boundaries of geographic variation within simi¬ lar species and, particularly, does not trend in any given direction, especially towards the modal morphology of descendant forms. This figure shows that, when a small segment in geological stasis becomes magnified so that change may be visualized on a genera¬ tional scale, the natural fluctua¬ tions within local populations become more visible—but still do not, at the proper geological focus, exceed the bounds of stasis within the species.
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THE STRUCTURE OF EVOLUTIONARY THEORY the fossil record.” Futuyma directly follows this statement with the key impli¬ cation of punctuated equilibrium for the explanation of evolutionary trends: “Long-term anagenetic change in some characters is then the consequence of a succession of speciation events.” Later in his article, Futuyma (p. 467) explicitly links speciation with suf¬ ficient stability (individuation) for macroevolutionary expression: “In the ab¬ sence of reproductive isolation, differentiation is broken down by recombina¬ tion. Given reproductive isolation, however, a species can retain its distinctive complex of characters as its spatial distribution changes along with that of its habitat or niche . . . Although speciation does not accelerate evolution within populations, it provides morphological changes with enough permanence to be registered in the fossil record. Thus, it is plausible to expect many evolu¬ tionary changes in the fossil record to be associated with speciation.” And, at the end of his article, Futuyma (p. 470) notes the crucial link between punctu¬ ated equilibrium and the possibility of sustained evolutionary trends: “Each step has had a more than ephemeral existence only because reproductive iso¬ lation prevented the slippage consequent on interbreeding with other popula¬ tions . . . Speciation may facilitate anagenesis by retaining, stepwise, the ad¬ vances made in any one direction . . . Successive speciation events are the pitons affixed to the slopes of an adaptive peak.” I hope that Futuyma’s simple yet profound insight may help to heal the re¬ maining rifts, thereby promoting the integration of punctuated equilibrium into an evolutionary theory hierarchically enriched in its light.
CRITIQUES BASED UPON SUPPOSED FAILURES OF EMPIRICAL RESULTS TO AFFIRM PREDICTIONS OF PUNCTUATED EQUILIBRIUM
I shall treat the specifics of this topic primarily in the next section on “the data of punctuated equilibrium.” But the logic of this chapter’s development also requires that I state the major arguments and my responses in this ac¬ count of principal critiques directed at the theory—for the totality of at¬ tempted rebuttals has not only posited theoretical objections in an effort to undermine the theory’s logic or testability (as discussed in the first two parts of this section), but has also proceeded by accepting the theory’s program of research as valid, and then arguing that the bulk of data thus accumulated re¬ futes punctuated equilibrium empirically. I shall summarize discussion on the two major strategies pursued under this rubric: refutation by accumulation of important cases, and rejection by failure of actual data to fit models for pre¬ dicted phylogenetic patterns.
Claims for empirical refutation by cases PHENOTYPES.
Despite some early misunderstandings, long since resolved
by all parties to the discussion, we recognize that no individual case for or against punctuated equilibrium, however elegantly documented, can serve as a “crucial experiment” for questions in natural history that must be decided
Punctuated Equilibrium and the Validation of Macroevolutionary Theory by relative frequencies. No exquisite case of punctuated equilibrium—and many have been documented—can “prove” our theory; while no beautiful example of gradualism—and such have been discovered as well—can refute us. The key question has never been “whether,” but rather “how often,” “with what range of variation in what circumstances of time, taxon, and en¬ vironment,” and especially, “to what degree of control over patterns in phylogeny?” A single good case can only validate the reality of the phenome¬ non—and the simple claim for existence has not, surely, been an issue for more than 20 years. Similarly, an opposite case of gradualism can only prove that punctuated equilibrium lacks universal validity, and neither we nor any¬ one else ever made such a foolish and vainglorious claim in the first place. The empirical debate about punctuated equilibrium has always, and properly, focussed upon issues of relative frequency. I shall present the empirical arguments for asserting dominant relative frequency, rather than mere occurrence, for punctuated equilibrium on pages 854-874. If we ask, by contrast, whether strong evidence for predomi¬ nant gradualism has been asserted for any major taxon, time or environment, one case stands out as a potentially general refutation of punctuated equilib¬ rium in one important domain at least: the claim for anagenetic gradualism as a primary phylogenetic pattern in the evolution of Cenozoic planktonic Foraminifera. This case gains potential power and generality from the unusually favor¬ able stratigraphic context, and the consequent nature of sampling, in such studies. The data come from deep oceanic cores, with stratigraphic records presumably unmatched in general completeness, for these environments re¬ ceive a continuous supply of sediment (including foraminiferal tests) from the water column above. Moreover, these microscopic organisms can usually be extracted in large and closely spaced samples (sieved from disaggregated sedi¬ ments), even from the restricted volume of a single oceanic core. Thus, forams in oceanic cores should provide our most consistently satisfactory informa¬ tion—in terms of large samples with good stratigraphic resolution—for the study of phylogenetic pattern. If gradualistic anagenesis prevails in such situ¬ ations of maximal information—even if punctuated equilibrium predomi¬ nates in the conventional fossil record of marine invertebrates from shallow water sediments—shouldn't we then conclude that Darwin’s old argument must be valid after all; that punctuational patterns represent an artifact of missing data; and that more complete information will affirm genuine gradu¬ alism as the characteristic signal of phylogeny? I acknowledge the highest relative frequency of recorded gradualism for foraminiferal data of this type, and I also admire the procedural rigor and informational richness in several of these studies. But I do not regard this case as a general argument against punctuated equilibrium—and neither, I think, do most of my paleontological colleagues, whatever their overall opin¬ ion about our theory, for the following reasons based upon well-known features of the fossil record in general, and the biology of forams in par¬ ticular.
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THE STRUCTURE OE EVOLUTIONARY THEORY 1. As emphasized in my previous discussion of publication bias (see p. 763), I remain unconvinced that a predominant relative frequency for gradual¬ ism—as opposed to genuine documentation of several convincing cases—has been established, even for this maximally-promising taxon. No one has ever compiled an adequately random, or even an adequately numerous, sample of planktonic species drawn from the entire clade. Gradualistic lineages have been highlighted for study as a consequence of their greater “interest” under conventional views, while putatively stable lineages have tended to remain in unexamined limbo as supposedly uninformative, or even dull. Thus, the fact that gradualism prevails in a high percentage of published studies tells us little about the relative frequency of gradualism in the clade as a whole. A telling analogy may be drawn with a crucial episode in the history of ge¬ netics. With classical techniques based on the Mendelian analysis of pedi¬ grees, only variable genes could be identified. (If every Drosophila individual had red eyes, earlier researchers could legitimately assume some genetic bases for the invariance, but no genes could be specified because traits could not be traced through pedigrees. But once a white-eyed mutant fly appeared in the population, geneticists gained a necessary tool for identifying relevant genes by crossbreeding the two forms and tracing the alternate phenotypes through successive generations. In other words, genes had to vary before they could be specified at all.) Therefore, under these methodological constraints (which prevailed during most of the 20th century history of genetics), a dominant measured frequency for variable genes taught us nothing about the actual frequency of variable genes across an entire genome—for we knew no way to generate a random or unbiased sample by selecting genes for study prior to any knowledge about whether or not they varied. The fact of variation in all known genes only recorded a methodological limitation that precluded the identification of nonvariable genes. I don’t, of course, claim that methodological strictures on paleontological lineages have ever been so strong—that is, we could always have selected sta¬ ble lineages for study, had we chosen to do so. But, in practice, I’m not sure that the actual procedural bias has operated with much less force in paleon¬ tology than in genetics, so long as researchers confined their attention to lin¬ eages that appeared (by initial qualitative impression) to evolve by gradual anagenesis. Just as all known genes might be variable (while variable genes actually represent only a few percent of the total complement, because the re¬ maining 95 percent of invariant genes could not be recognized at all), most studied species might illustrate gradual trends (while gradualistic species rep¬ resent a small minority of all lineages because no one chooses to study stable species). Genetics resolved this problem by inventing techniques—with electropho¬ resis as the first and historically most important—for identifying genes prior to any knowledge about whether or not they varied. This methodological ad¬ vance permitted the resolution of several old and troubling questions, most notably the calculation of average genetic differences among human races.
Punctuated Equilibrium and the Validation of Macroevolutionary Theory This central problem of early Mendelian genetics could never be addressed— even to counter the worst abuses of biological determinism and social Dar¬ winism—because biologists could not generate random samples of genes, and could therefore only overestimate average distances by ignoring the unknow¬ able invariant genes among races, while studying the (potentially small) frac¬ tion capable of recording differences among groups. With electrophoretic techniques, and the attendant generation of a random sample with respect to potential variability, geneticists soon calculated the average genetic differ¬ ences among races as remarkably small and insignificant—a conclusion of no mean practical importance in a xenophobic world. Similarly, a truly random sample (with respect to the distribution of anagenetic rates) might show a predominance for stasis, even if previous studies (with their strong bias for preselection of variable species) had generally affirmed gradualism. I am encouraged to accept the probable validity of this argument by the im¬ portant study of Wagner and Erwin (1995), who used the different and com¬ prehensive technique of compiling full cladograms for two prominent Neo¬ gene families of planktonic forams: Globigerinidae and Globorotaliidae. In applying a set of methods for inferring probable evolutionary mode from cladistic topology (see full discussion and details on pp. 820-822), they found that, in both families, branching speciation in the mode favored by punc¬ tuated equilibrium (divergence of descendants with survival of ancestors in stasis) vastly predominated over the origin of new species by anagenetic transformation. Thus, the literature’s apparent preference for anagenesis in tabulated studies of individual lineages may only record an artifact of biased selection in material for research. 2. Even if gradualism truly does prevail in planktonic forams, we could not infer that the observed predominance of punctuated equilibrium in marine Metazoa must therefore reflect the artifact of an imperfect geological record. The difference might record a characteristic disparity between the taxa, not a general distinction in quality of geological evidence between deep oceanic cores and conventional continental sequences—a proposition defended in the third argument, just below. The deep oceanic record may usually be more complete, but the subset of best cases from conventional sequences surely matches the foram data in quality—and convincing studies of punctuated equilibrium and gradualism generally use these best records. Thus, the subset of most adequate metazoan examples should match, in quality of evidence, the usual records of forams from oceanic cores. 3. A third argument completes the trio of logical possibilities (all partially valid, I suspect, though I would grant most weight to this third point) for de¬ nying that a currently recorded maximal frequency of gradualism for plank¬ tonic foraminiferal lineages casts doubt on the general importance of punc¬ tuated equilibrium in evolution. The first argument attributes an apparent high frequency to biased sampling in the preselection, for rigorous study, of lineages already highlighted by taxonomic experts for suspected grad¬ ual change. The second and third arguments, on the other hand, hold that if high frequency truly characterizes this group, no general rebuttal of punctu-
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THE STRUCTURE OF EVOEUTIONARY THEORY ated equilibrium follows thereby. The second argument denies the common assumption that high-frequency records uniquely complete geological evi¬ dence—and that gradualism will therefore prevail whenever the fossil record becomes good enough to preserve its true-domination (with a high frequency for punctuated equilibrium then construed, by Darwin’s original argument, as the artifact of a gappy record). This second argument maintains that, while foram data may be more complete on average, the best metazoan examples of punctuated equilibrium have been validated with excellent samples from ad¬ mittedly rarer but equally complete geological sequences, thus precluding the explanation of punctuated equilibrium as artifactual. The third argument also grants the reality of higher-relative frequency for gradualism in forams, but argues against extrapolation to larger multicellular organisms on grounds of genuine difference in evolutionary mode, based on important biological distinctions between these single-celled creatures of the oceanic plankton and sexually reproducing metazoan species that, however parochially, have served as the basis for most of our evolutionary theory and, in any case, form the bulk of the known fossil record. This third argument should not be viewed as special pleading by partisans, but as a positive opportunity for developing hypotheses about the importance (or insignificance) of punctuated equilibrium based on the correlation be¬ tween differences in frequency and distinctive biological properties of various taxonomic groups—particularly in features related to speciation, the pre¬ sumed evolutionary basis of punctuated equilibrium. Planktonic forams, with their asexuality, their small size and rapid turnover of generations, their unicellularity, their vast populations, and their geographic links to water masses, display maximal difference from most metazoans, and may therefore be especially suited for helping us to understand, by contrast, the prevailing mechanisms of evolution in multicellular and sexually reproducing organ¬ isms. The general nature of these differences does indeed point to a set of factors tied to the definition and division of populations, therefore grant¬ ing plausibility to the claim that so-called “species” of planktonic forams should show more gradualism than metazoan taxa, while punctuated equi¬ librium may prevail in sexually reproducing multicellular species. The sub¬ ject deserves much more attention and rigor, but to sketch a few suggested factors: (1) Population characteristics. We conventionally name Linnaean species of asexual protistans, but even if adequately stable “packages” of form or ge¬ netic distinctness exist in sufficiently extended domains of space and time to merit a vernacular designation as “populations,” what comparison do such entities bear with species of sexually reproducing multicellular organisms? (Needless to say, I raise no new issue here, but only recycle the perennial ques¬ tion of “the species problem” in asexual organisms.) Punctuated equilibrium posits a link of observed evolutionary rates to properties of branching specia¬ tion in populations. I don’t even know how to think about such issues in planktonic forams, where vast populations may be coextensive with entire oceanic water masses, and where numbers must run into untold billions
Punctuated Equilibrium and the Validation of Macroevolutionary Theory of organisms for every tiny subsection of a geographic range. How do new populations become isolated? How do favorable (or, for that matter, neu¬ tral) traits ever spread through populations so extensive in both space and number? (2) Morphology and definition. If metazoan stasis can be attributed, at least in part, to developmental buffering, what (if any) corresponding phe¬ nomenon can keep the phenotypes of simple unicells stable? Perhaps foraminiferal phenotypes manifest substantial plasticity for shaping by forces of temperature, salinity, etc., in surrounding water masses (see Greiner, 1974)— as D’Arcy Thompson (1917, 1942) proposed for most of nature in his won¬ derfully iconoclastic classic, On Growth and Form—see pp. 1179-1208. (Thompson’s claim that physical forces shape organisms directly holds lim¬ ited validity for complex and internally buffered multicellular forms, but his views may not be so implausible for several features of simpler unicells.) Could many examples of foraminiferal gradualism (compared with metazoan stasis in similar circumstances) reflect the plasticity of these protists in the face of gradual changes in the physical properties of enveloping oceanic water masses through time? If so, such gradual trends would not be recording evo¬ lutionary change in the usual genetic sense. (3) Most interestingly (as a potential illustration of the main theoretical concern of this book), we must consider the potential for strongly allometric scaling of effects from a defined locus of change to other levels of an evo¬ lutionary hierarchy. To reiterate a claim that runs, almost like a mantra, throughout this text: punctuated equilibrium is a particular theory about a definite level of organization at a specified scale of time: the origin and de¬ ployment of species in geological perspective. The punctuational character of such change does not imply—and may even, in certain extrapolations to other scales, explicitly deny—a pervasive punctuational style for all change at any level or scale. In particular, punctuated equilibrium posits that tolerably gradual trends in the overall history of phenotypes within major lineages and clades (including such traditional tales as augmenting body size in hominids, increasing sutural complexity in ammonoids, or symmetry of the cup in crinoids) should reveal a punctuational fine texture when placed “under the mi¬ croscope” of dissection to visualize the individual (speciational) “building blocks” of the totality—what we have long called the “climbing up a stair¬ case” rather than the “rolling a ball up an inclined plane” model of fine struc¬ ture for trends. Similarly, in asking about evolutionary causality under selective models (see Chapter 8), we need to identify the primary locus of Darwinian individu¬ ality for the causal agents of any particular process—for only properly de¬ fined Darwinian individuals can operate as “interactors” in a selective pro¬ cess: that is, can interact with environments in such a way that their own genetic material becomes plurified in future generations because certain dis¬ tinctive properties confer emergent fitness upon the individual in its “struggle for existence” (see pp. 656-667). Punctuated equilibrium maintains that spe¬ cies, as well-defined Darwinian individuals, hold this causal status as irreduc-
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THE STRUCTURE OF EVOEUTIONARY THEORY ible components, or “atoms,” of evolutionary trends in clades. The apparatus of punctuated equilibrium then explains why trends, when necessarily de¬ scribed as speciational, display a punctuated pattern at geological scales (as expressed in the theory’s basic components of stasis and geologically abrupt appearance). In a larger sense, punctuational accounts of trends propose a similar allometric model for any relevant scale—that is, any microscope placed over higher-level smoothness may reveal an underlying “stair-step” pattern among constituent causal individuals acting as Darwinian agents of the trend. In sexually reproducing metazoa, species clearly play this role as causal in¬ dividuals (see Chapter 8). The theoretical validity of punctuated equilibrium depends upon such a claim and model. But when we turn to such asexually reproducing unicells as planktonic forams, designated “species” cannot be construed as proper Darwinian individuals, and therefore cannot be primary causal agents (or interactors) in evolutionary trends. To locate the proper agent, the legitimate analog of the metazoan species, we must move “down” a level to the clone—to what Janzen (1977), in a seminal paper, called the El, or “evolutionary individual.” When we execute this conceptual downshift in levels to locate the focal evolutionary individual in asexual and unicellular lineages, we recognize that the foram “species” acts as an analog to the metazoan lineage or clade, not to the metazoan species. The foram “species” represents a temporal collectivity whose evolutionary pattern arises as a summed history of the Darwinian indi¬ viduals—clones in this case—acting as primary causal agents. We can now shift the entire causal apparatus one level down to posit a dif¬ ferent locus of punctuational change in planktonic forams. Just as the punc¬ tuational history of species generates smooth trends in the collectivity of a lin¬ eage or clade in Metazoa, so too might the punctuational history of clones yield gradualism in the collectivities so dubiously designated as “species” in asexual unicells. In other words, foram “species” may exhibit gradualism be¬ cause these supposed entities are really results or collectivities, not proper Darwinian individuals or causal agents. Eldredge and I first presented this ar¬ gument in our initial commentary on the debate about punctuated equilib¬ rium (Gould and Eldredge, 1977, p. 142, and Fig. 9-10): We predict more gradualism in asexual forms on biological grounds. Their history should be, in terms of their own unit, as punctuational as the history of sexual Metazoa. But their unit is a clone, not a species. Their evolutionary mode is probably intermediate between natural selec¬ tion in populations, and species selection in clades: variability arises via new clones produced rapidly (in this case, truly suddenly) by mutation. The phenotypic distribution of these new clones may be random with re¬ spect to selection within an asexual lineage (usually termed a “species,” but not truly analogous with sexual species composed of interacting individuals). Evolution proceeds by selecting subsets within the group of competing clones. If we could enter the protists’ world, we would view this process of “clone selection” as punctuational. But we study
Punctuated Equilibrium and the Validation of Macroevolutionary Theory
their evolution from our own biased perspective of species, and see their gradualism as truly phyletic—while it is really the clonal analog of a gradual evolutionary trend produced by punctuated equilibria and spe¬ cies selection. Lenski’s remarkable studies on controlled evolution of bacteria under labo¬ ratory conditions of replication provide striking evidence for this claim (see full discussion of this work on pp. 931-936). Lenski and colleagues (Lenski and Travisano, 1994; Papadopoulos et al., 1999) monitored average cell size for 10,000 generations in 12 lineages of E. coli. Cell size increased asymptoti¬ cally in each lineage, steadily for the first 3000 generations or so, but remain¬ ing relatively stable thereafter. The fine structure of increase, however, pro¬ ceeded in a punctuational manner in each lineage—a step-like pattern of stability in average cell size, followed by rapid ratcheting of the full popula¬ tion up to larger dimensions. This punctuational pattern presumably oc¬ curred because clones act as primary Darwinian individuals in this system. The full lineage must “wait” for sudden introduction of favorable variation in the form of occasional mutations, initiating novel clones that can then sweep through the entire lineage to yield a punctuational step in the overall phylogeny (at a scale of 10,000 generations in phenotypic history). Predict¬ able, replicable size increase occurs by punctuational clone selection in each case (see Fig. 9-11). Lenski’s powerful result does not illustrate a case of punc¬ tuated equilibrium, sensu stricto, but he does provide a challenging and in-
9-10. The supposed gradualism noted in many foram species may represent a view, from too high a level, of an overall trend within a phyletic sequence prop¬ erly analyzed in terms of punctuational events at the level of clone selection—the appropriate mode in such asexual forms. 1 shows a conventional metazoan lin¬ eage in punctuated equilibrium. 2 shows the apparent gradualism in a foram lin¬ eage. 3 shows a gradualistic segment between B and C magnified so that the ap¬ propriate process of punctuational clone selection becomes visible. From Gould and Eldredge, 1977.
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THE STRUCTURE OF EVOLUTIONARY THEORY structive argument for considering the validity of punctuational change at all levels. Just as the careful watchdog at any scientific meeting will unhesitatingly call out “what’s the scale” when a colleague fails to include a measurement bar on a slide of any important object, we must always ask “what’s the level” when we analyze the causal basis of any evolutionary pattern. Punctuational clone selection can yield gradualism within collectivities conventionally (if dubiously) called “species,” just as punctuated equilibrium, acting on species as Darwinian individuals, can produce gradual trends in the overall history of lineages and clades. GENOTYPES.
Punctuated equilibrium is a theory about the evolution of phe¬
notypes (both in concept and in operational testability for paleontological hy¬ potheses), and correlations with genotypic patterns provide neither a crucial test nor even any necessary prediction. For example, critics of punctuated equilibrium have often argued that the apparently cumulative character of overall genetic distances among members of an evolving clade, expressed as a high correlation between measured disparities and independently derived times since divergence from a common ancestor—the kind of information that, in idealized (but rarely encountered) situations, yields a rough “molecu¬ lar clock”—should argue strongly against punctuational styles of evolution, while affirming anagenetic gradualism. But, leaving aside the highly questionable empirical status of these claims, the hypothesis of punctuated equilibrium would not be affected by positive outcomes, even at much higher relative frequency than the known history of life apparently validates. In supposing that “molecular clocks” tick against the requirements of punctuated equilibrium, we fall into two bad habits of thinking that impede macroevolutionary theory in general, and therefore rank as important conceptual barriers against the theses of this book. First, reductionistic biases often lead us to seek an “underlying” genetic basis for any overt phenomenon at any scale, and then to view data at this level as a fundamental locus for proper evolutionary explanation. (But consider only two among many rebuttals of such a position: (1) a genetic pattern may be
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9-11. From Lenski and Travisan 1994. At too broad a scale, the 5 increases within Lenski’s bacteri lineages seem gradual. But a fine scale analysis shows a stepwise punctuational pattern of clone selection with stasis recording tl waiting time between favorable mutations, and punctuation cau by rapid sweep of these rarely favorable mutants through the population.
Punctuated Equilibrium and the Validation of Macroevolutionary Theory non-causally correlated with coincident evolutionary expressions at other scales; and (2) in principle, genetic expressions of a common causal structure do not rank as intrinsically more “deep,” “real,” “fundamental,” or “basic” than other manifestations in different forms and at different levels; causal rel¬ evance depends upon the questions we ask and the processes that organisms undergo.) Second, the “allometric” effects of scaling either render the same process in a very different manner at various scales, or (perhaps more frequently, and primarily in this case at least) generate the distinctive patterns of different scales by independent processes, acting simultaneously, but with each process primarily responsible for results at its own appropriate level. If I could affirm, as may well often be the case, that punctuated equilib¬ rium regulated the phenotypic pattern of evolution in a given clade, while genotypic distances conformed closely to a “molecular clock,” I would not conclude that punctuated equilibrium had therefore been downgraded, or exposed as incorrect, superficial, or illusory—with genetic continuity as a physically underlying (and conceptually overarching) reality. Rather, I would regard each result as true and appropriate for its own scale and realm—with the full pattern of legitimate difference standing as an intriguing example of resolvable complexity in evolutionary scaling and causality. Moreover, this particular pattern might easily result from a highly plausible scenario of com¬ plex and multileveled causation—namely, that neutral substitutions at the nucleotide level impart a signal sufficiently like a genomic metronome to dominate the molecular results, while ordinary speciation both regulates the phenotypic history of populations, and works by the expected pattern of punctuated equilibrium. The genomic results, in principle, need not extrapo¬ late to encompass the pattern of speciational (macroevolutionary) change. After all, we do understand that gene trees do not entirely match organism trees in phylogeny! In this way—as in the foregoing example of predictable differences be¬ tween asexual unicells and sexually reproducing metazoa—punctuated equi¬ librium proves its value primarily by hypothesizing sensible distinctions: that is, by operating at scales and biological conditions where cladogenetic speci¬ ation plausibly sets evolutionary pattern. Punctuated equilibrium should not prevail where species cannot exist as Darwinian individuals, or where contin¬ uously occurring, and largely nonadaptive, substitution of nucleotides proba¬ bly regulates the bulk of genomic change. In this crucial sense, punctuated equilibrium becomes a valuable hypothesis by delineating such testable dis¬ tinctions, rather than allowing evolution to be conceptualized monistically as a single style of alteration, or a single kind of process either flowing from, or applicable to, all scales of change. The question of consistency between observed genetic patterns in living species, and the relative frequency of punctuated equilibrium in their phylog¬ eny, shall be treated in the next section on the correspondence of punctuated equilibrium with predictions of evolutionary modeling. But one genetic issue has been widely discussed in the literature, and should be included in this sec-
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THE STRUCTURE OF EVOLUTIONARY THEORY
tion on empirical results. Several researchers have noted that punctuated equilibrium implies a primary prediction about patterns of genetic differences among species: if most change accumulates at ruptures of stasis during events of speciation, and not continuously along^the anagenetic history of a popula¬ tion, then overall genetic differences between pairs of species should correlate more closely with the estimated nunlber of speciation events separating them, than with chronological time since divergence from common ancestry. (This prediction might be clouded by several factors, including the foregoing dis¬ cussion on attributing the bulk of genomic change to continuity at a lower level, and a number of potential reasons for discordance between phenotypic effect and extent of responsible genetic change. But I certainly will not quib¬ ble, and I do allow that punctuated equilibrium suggests the broad generality of such a result.) In the early days of debate about punctuated equilibrium, Avise (1977) per¬ formed an interesting and widely discussed test. In comparing genetic and morphological differences among species in two fish clades of apparently equal age but markedly different frequencies of speciation, Avise found a higher correlation of distances with age than with frequency of branching, and therefore favored gradualism over punctuated equilibrium as an explana¬ tion of his results. But Mayden (1986) then showed that Avise’s test did not apply well to his chosen case (primarily because we cannot be sure of roughly equal antiquity for the two clades). He then argued, as several supporters of cladistic methodology had urged, that such tests should be applied only to well-confirmed cladistic sister groups—for, in such cases, even if paleonto¬ logical data permit no certainty about the actual time of joint origin from common ancestry, at least we can be confident that the two clades are equally old! Mindel et al. (1989) then performed such a properly constituted test on the reptilian genus Sceloporus, and more loosely on allozymic data in gen¬ eral, and found a positive correlation between evolutionary distance and fre¬ quency of speciation—thus validating the primary prediction of punctuated equilibrium.
Empirical tests of conformity with models Limitations of the fossil record restrict prospects for testing punctuated equi¬ librium by inductive enumeration of individual species and lineages. Cases with sufficient resolution may not be common enough to establish a robust relative frequency; or systematic biases based on imperfections in the fossil record may lead to artifactual preferences for punctuated equilibrium—thus making the data unusable as a fair test for a minimal frequency. (I do not re¬ gard these problems as particularly serious, and I will provide several exam¬ ples of adequate resolution in the next section of this chapter. But we should, in the light of these difficulties, also be exploring other ways of testing punc¬ tuated equilibrium, as considered below.) In another strategy that has been pursued by some researchers, but could (and should) be exploited to a much wider and more varied extent, we might characterize, in quantitative fashion, broader patterns in the deployment of
Punctuated Equilibrium and the Validation of Macroevolutionary Theory diversity through time and space in major taxonomic groups—and then de¬ vise tests to distinguish among contrasting causes: anagenetic vs. cladogenetic; gradual vs. punctuational. If certain well defined patterns can only be generated, say, by branching speciation rather than by anagenetic transfor¬ mation (or vice versa, of course), then we can use the fit of broad results with distinctive models, rather than minute documentation on a case-by-base ba¬ sis, to establish the relative frequency of punctuated equilibrium. In an important study, for example, Lemen and Freeman (1989) investi¬ gated “the properties of cladistic data sets from small monophyletic groups (6-12 species) . . . using computer simulations of macroevolution” (p. 1538). They contrasted the differing outcomes of data generated under anagenetic gradualism vs. punctuated equilibrium, and then examined cladograms of ex¬ tant monophyletic groups for consistency with these “abstract, end-member” alternatives. They claimed better support for gradualism, but several flaws in their logic and data render their conclusions moot, much as their pioneering approach may be applauded and recommended for further study. Lemen and Freeman tested actual data against three modelled differences between cladograms generated by gradualism vs. punctuated equilibrium. 1. Punctuated equilibrium should produce a strongly positive correlation between number of branching points and apomorphies of species—because change occurs at speciation and does not further correlate with passage of time per se. Lemen and Freeman’s models did affirm this expected result, and real data, somewhat ironically, revealed “higher correlations of apomorphies and branch points than could be explained by either mode of macroevo¬ lution” (p. 1549). But the authors rejected an interpretation of this infor¬ mation as favorable to punctuated equilibrium because anagenesis, under certain conditions, also yields positive correlations, and because high cor¬ relations can also arise artificially by errors in establishing cladograms: “a consistent error in polarity can profoundly affect the correlation of total apomorphies and branch points” (p. 1551). Fair enough, but Lemen and Freeman are not nearly so circumspect when equally flawed data seem to fa¬ vor their preferred alternative of gradualism. 2. The modal (but not the mean or median) number of autapomorphies will always be zero under strict punctuated equilibrium. This odd-sounding situation arises because, in the cladograms, an event of branching produces a daughter species with some autapomorphies and a persisting parental species remaining in stasis with none. With no change except at branching points, the value of zero autapomorphies must remain most common across all species on the cladogram. Under gradualism, autapomorphies simply accumulate through time, whatever the pattern of branching, so zero should not mark a preferred or particularly common value. Lemen and Freeman never found a mode of zero autapomorphies in real data, and therefore rejected punctuated equilibrium as a predominant style of evolution. But had they pursued explanations based on artifacts (as they did so assiduously when the data seemed to favor punctuated equilibrium), they would have realized that taxonomic practice precludes the definition (or even
813
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THE STRUCTURE OF EVOEUTIONARY THEORY the recognition) of species without autapomorphies. Such species arise fre¬ quently in the modelled system as a necessary consequence of the chosen rules of generation and the general logic of cladistic analysis. But, in neontological practices of naming, a species without autapomorphies represents an oxymoronic concept, and such taxa could never be designated at all. Lemen and Freeman recognize this point in writing about their various forms of gradu¬ alistic modelling (p. 1551): “When distinctness of species is demanded the lack of autapomorphies may not be the most expected condition.” 3. Under punctuated equilibrium, “as the number of characters used in the analysis increases, the distribution of the number of autapomorphies per spe¬ cies becomes bimodal. Under gradualism, the distribution of autapomorphies remains ummodal under all conditions” (1538). This situation, a spinoff from their second criterion, arises because each branch, in an event of punc¬ tuated equilibrium, produces one changed descendant and one persisting an¬ cestor—and the more characters you measure, the more you pick up the differences between stasis on one branch and change on the other. Under gradualism, total change correlates only with elapsed time, so accumulating autapomorphies should form a unimodal distribution so long as species dura¬ tion remains unimodal as well. Lemen and Freeman found no bimodal distributions in real data, and therefore concluded again in favor of gradualism. But, once more, the differ¬ ences between idealized modeling and data from real organisms scuttles this conclusion. In the models, we know for sure that long arms without branch¬ ing are truly so constituted, for we have perfect information of all simulated events. These unbranched arms, under punctuated equilibrium, should accu¬ mulate no autapomorphies—and the low mode of the bimodal distribution arises thereby. But, in real data of cladograms based on living organisms, long unbranched arms usually (I would say, virtually always) record our ignorance of numerous and transient speciational branchings that quickly became ex¬ tinct and left no fossil record. (Moreover, since Lemen and Freeman’s clado¬ grams only include living organisms, even if successful and well-represented fossil species existed, they would not be included.) When we note a long arm without branches on a modern cladogram, and then assume (as Lemen and Freeman did) that accumulated autapomorphies between node and terminus must have arisen gradually and anagenetically, we commit a major blunder. We have no idea how many unrecorded speciation events separate node and terminus, and we cannot assert that recorded autapomorphies did not occur at these (probably frequent) branchings. In other words, Lemen and Free¬ man’s bimodality test assumes that unbranched arms of their cladograms truly feature no speciation events along their routes, whereas numerous tran¬ sient and extinct species must populate effectively all of these pathways. Other applications of this method—modeling of alternative outcomes and testing of contrasting predictions against patterns of real data—have yielded results favorable to punctuated equilibrium. In a pathbreaking paper, Stanley (1975—see elaboration in Stanley, 1979 and 1982) first proposed this style of testing and developed four putative criteria, all affirming punctuated equilib-
Punctuated Equilibrium and the Validation of Macroevolutionary Theory rium. (Stanley’s tests may be reduced to three, as his second “test of the Pontian cockles” represents a particular instance of his first “test of adaptive radiations.” Stanley argued:
Test of adaptive radiation. After calculating average species du¬ rations from the fossil record, one can affirm that pure anagenetic gradualism (or temporal stacking of species end-to-end) cannot account for the magni¬ tude of recorded adaptive radiations in the time available—so rapid cladogenesis must be invoked.
Test of living fossils. Punctuated equilibrium associates realized amounts of change primarily with frequency of speciation, anagenetic grad¬ ualism primarily with elapsed time. If so-called “living fossils”—ancient groups with little recorded change—also show unvarying low diversity through time, then we can affirm the primarily prediction of punctuated equi¬ librium, and refute the corresponding expectation of gradualism (for these groups are ancient). Stanley then documented such a correlation between clades identified as “living fossils” and persistently low diversity in these clades.
Test of generation time. Under gradualism, amounts of realized evolution should correlate strongly with generation time—for the time that should mark accumulated evolutionary change does not tick by an abstract Newtonian clock, but by number of elapsed generations, representing the number of opportunities for natural selection to operate. But, under punctu¬ ated equilibrium, amount of change correlates primarily with frequency of speciation—a property with no known relationship to generation time. Stan¬ ley then cited the well-documented lack of correlation between evolutionary rate and generation time as evidence for the prevalence of punctuated equilib¬ rium (fast-evolving elephants vs. stable invertebrates with short generations). Much as I regard Stanley’s arguments as suggestive, I cannot accept them as conclusive for two basic reasons. First, other plausible explanations exist for the patterns noted. For example, many reasons other than the prevalence of punctuated equilibrium might explain a lack of correlation between realized evolution and generation time, even in a world of anagenetic gradualism. The correlation might simply be weak or too easily overwhelmed (and therefore rendered invisible) by such other systematic factors as variation in the inten¬ sity of selection. (Maybe elephants, on average, experience selection pressures higher by an order of magnitude than those affecting short-lived inverte¬ brates; maybe population size overwhelms the factor of generation time.) Second, most of Stanley’s tests (particularly his key claim about adaptive radiation) don’t really oppose punctuated equilibrium to gradualism, but rather contrast a more general claim about the speciational basis of change (whatever the mode of speciation) with anagenesis. Moreover, the tests em¬ ploy a somewhat unfairly caricatured concept of gradualism. I doubt that the most committed gradualist ever tried to encompass the maximal change be¬ tween ancestor and any descendant in an adaptive radiation by stacking spe¬ cies end to end, and then calculating whether the full effect could arise in the allotted time. A committed gradualist might fairly say of an adaptive radia-
815
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THE STRUCTURE OF EVOEUTIONARY THEORY tion: “of course the magnitude of change in both form and diversity corre¬ lates with number of branching events (what else could a ‘radiation’ mean). But adaptive radiations only accelerate the frequency of branching in re¬ sponse to ecological opportunity (‘open’ environments just invaded or just cleared out by extinction); they do not affect the modality of change. I will al¬ low that, in adaptive radiations, m-ost new species arise in less time than usual, but still gradualistically. If full speciation takes half the average time (one million rather than a modal two million years, for example), but still oc¬ curs imperceptibly and still occupies a large percentage of an average species’s lifetime, then gradualism encounters no threat in adaptive radiation.” However, in another crucial sense, at least one of Stanley’s tests does illus¬ trate the most salutary potential role for punctuated equilibrium: its capacity to act as a prod for expansive thought and new hypotheses, whatever the out¬ come of the empirical debate about relative frequency. Paleontologists had been truly stymied in their thinking about the important and contentious topic of “living fossils.” Neither of the two conventional explanations could claim any real plausibility. Every textbook that I ever consulted as a student dutifully repeated the old saw that living fossils had probably achieved opti¬ mal adaptation to their environment. Therefore, no alternative construction could selectively replace an ideal form achieved so long ago. But no one ever presented any even vaguely plausible evidence for such a confident assertion. Why should horseshoe crabs lie closer to optimality than any other arthro¬ pod? What works so well in the design of lingulid vs. other brachiopods? What superiority can a lungfish assert over a marlin or tuna? In fact, since liv¬ ing fossils also (by traditional depiction) present such a “primitive” or “ar¬ chaic” look, the claim for optimality seemed specially puzzling. The other obvious explanation, in a gradualistic and anagenetic world ruled by conventional selection, held that living fossils had stagnated because they lacked genetic variation, and therefore presented insufficient fuel for Darwinian change. This more plausible idea seemed sufficiently intriguing that Selander et al. (1970), in the early days of electrophoresis as a novel method for measuring overall genetic variation, immediately applied the tech¬ nique to Eimulus, the horseshoe crab—and found no lowering of genetic variability relative to known levels for other arthropods. This negative pat¬ tern has held, and no standard lineage of living fossils exhibits depauperate levels of genetic variability. But punctuated equilibrium suggests another, remarkably simple, explana¬ tion once you begin to think in this alternative mode—an insight that ranks in the exhilarating, yet frustrating, category of obvious “scales falling from eyes” propositions, once one grasps the new phrasing of a basic question. If evolutionary rate correlates primarily with frequency of speciation—the car¬ dinal prediction of punctuated equilibrium—then living fossils may simply represent those groups at the left tail of the distribution for numbers of speciation events through time. In other words, living fossils may be groups that have persisted through geological time at consistently and unvaryingly low species diversity. (Average species longevity need not be particularly high,
Punctuated Equilibrium and the Validation of Macro evolutionary Theory for low species numbers, if consistently maintained without geological bursts of radiation, will yield the full effect.) Such groups cannot be common—for consistently low diversity makes a taxon maximally subject to extinction in our contingent world of unpredictable fortune, where spread and number represent the best hedges against disappearance, especially in episodes of mass extinction—but every bell curve has a left tail. This explanation holds remarkably well, and probably provides a basic ex¬ planation of “living fossils.” Such groups are neither mysteriously optimal, nor unfortunately devoid of variability. They simply represent the few higher taxa of life’s history that have persisted for a long time at consistently low species number—and have therefore never experienced substantial opportu¬ nity for extensive change in modal morphology because species provide the raw material for change at this level, and these groups have never contained many species. Westoll (1949), for example, published a classic study, summarized again and again in treatises and textbooks (Fig. 9-12), showing that lunghshes evolved very rapidly during their early history, but have stagnated ever since. The literature abounds in hypothesized explanations based on adaptation and ecological opportunity in an anagenetic world. The obvious alternative stares us in the face, but rises to consciousness only when theories like punc¬ tuated equilibrium encourage us to reconceptualize macroevolution in speciational terms: in their early period of rapid evolution, lunghshes maintained high species diversity, and could therefore change quickly in modal mor¬ phology. Their epoch of later stagnation correlates perfectly with a sharp re¬ duction of diversity to very low levels (only three genera living today, for example) with little temporal fluctuation in numbers—thus depriving macro¬ evolution of fuel for selection (at the species level), and relegating lunghshes to the category of living fossils. A breakthrough in the application of quantitative modelling to cladistic patterns of evolution directly recorded in the fossil record has been achieved by Wagner (1995 and 1999) and Wagner and Erwin (1995). These authors show, hrst of all, the pitfalls of working only with cladistic information from living organisms, and they illustrate the benehts of incorporating stratophenetic data from the fossil record into any complete analysis (see Wagner and Erwin, 1995, pp. 96-98, in a section entitled “why cladistic topology is insufficient for discerning patterns of speciation”). They then build models based on three alternative modes of evolution, and characterize the differ¬ ences in cladistic pattern expected from each: anagenetic gradualism, specia¬ tion by “bifurcation” (where, after branching, the two descendant species both accumulate differences from an ancestor then recorded as extinct), and speciation by “cladogenesis” (where one daughter species arises with autapomorphic differences, but the ancestral species persists in stasis). Cladogenesis is usually defined—both in this book and in the evolutionary literature in general—as any style of evolution by branching of lineages rather than by transformation of a single lineage (anagenesis). Wagner and Erwin restrict the term “cladogenesis” to the mode of speciation predicted by punctuated equi-
81 7
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THE STRUCTURE OF EVOEUTIONARY THEORY
9-12A. The famous figure from Westoll (1949) showing rapid morphological change early in the history of lungfishes, followed by prolonged stagnation thereafter.
librium—branching off of a descendant, leaving a persisting and unaltered ancestor. They contrast this mode with bifurcation—the style of speciation predicted by gradualism: splitting of an ancestral population into two descen¬ dant species, both diverging steadily from the ancestor (which becomes ex¬ tinct). I follow Wagner and Erwin’s restricted use of “cladogenesis” only in discussing their work, and use the broader definition throughout the rest of this book. The last two modes of bifurcation and cladogenesis both depict branching speciation in the definitional sense that two species emerge, where only one existed before. But note the crucial difference: bifurcation represents the op¬ eration of speciation in a gradualistic world, where an event of branching may be considered equivalent to two cases of gradualism following a separa¬ tion of populations, and where the separation itself need not correlate with any acceleration in rate of evolutionary transformation. Cladogenesis, on the other hand, represents the predictions and expectations of punctuated equi¬ librium. Therefore, if we can model the differences between bifurcation and cladogenesis, and test these distinctive expectations against real patterns in nature, we may achieve our best and fairest potential evaluation for the rela¬ tive frequency of punctuated equilibrium—for punctuated equilibrium can-
Punctuated Equilibrium and the Validation of Macroevolutionary Theory
9-12B. Redrawing and simplification of these data in the excellent paleonto¬ logical textbook of Raup and Stanley (1971). The bottom icon, showing an early mode and a right skew, has become canonical in textbooks. The data are firm and fascinating, but the interpretation has general been faulty as a result of gradualistic and anagenetic assumptions. Lineages did not stagnate in any anagenetic sense; rather, species diversity became so dramatically lowered (and has always stayed so—only three genera of lungfishes remain extant today) that speciational processes have never again had enough fuel to power further exten¬ sive phyletic change.
not be affirmed merely by showing that realized evolutionary patterns must record speciation and cannot be rendered by anagenetic, end-to-end stacking. Even the most committed anagenetic gradualists never denied the importance and prevalence of speciation. They hold, rather, that speciation generally oc¬ curs in the gradualistic mode—as two cases of divergence at characteristic rates for unbranching lineages—and not, as supporters of punctuated equilib¬ rium maintain, as geologically momentary bursts representing the budding of descendant populations from unchanged, and usually persisting, ancestral species in stasis. Thus, the best possible test for punctuated equilibrium must distinguish between the expectations of bifurcating vs. cladogenetic models of speciation. I am embarrassed to say that neither I nor my colleagues working on the validation of punctuated equilibrium ever conceptualized the simple and ob-
819
820
THE STRUCTURE OF EVOLUTIONARY THEORY vious best test for distinguishing the bifurcating model of speciational gradu¬ alism from the cladogenetic model of punctuated equilibrium. In this case, the impediment may be clear, but I can offer no legitimate excuse for my opac¬ ity—and I congratulate Wagner and Erwin^on their formulation. The solution lies in the distribution and frequency of “hard” polytomies in cladogenetic topologies. I failed to appreciate the following point: under punctuated equilibrium, new species branch off from unchanged and persist¬ ing ancestors. The successful ancestor remains in stasis and may live for a long time. Therefore, these “stem” species may generate numerous descen¬ dants during their geological tenure, while remaining unchanged themselves. Now what cladistic pattern must emerge from such a situation? A group of species branching at different times from an unchanged ancestor must yield a cladistic polytomy. Cladograms cannot distinguish different times of ori¬ gin from an unaltered ancestor, and can therefore only record the phenetic constancy of the common and unchanging ancestor as a polytomy, for all branches emerge from an invariant source. Bifurcation, on the other hand, can produce a range of cladistic topologies (Wagner and Erwin, 1995, p. 92), but not domination of the overall pattern by polytomies. Thus, gradualistic
vs. punctuational models of speciation should be distinguishable by distribu¬ tions of polytomies in the resulting cladogram. I suspect that many of us never recognized this point because we have been trained to view polytomies negatively as an expression of insufficient data to resolve a true set of ordered dichotomies. (Shades of our profession’s former failure to conceptualize punctuated equilibrium because we had been trained to view geologically rapid appearances as artifacts of an imperfect fossil re¬ cord!) Thus, we never recognized that polytomies might also be denoting a positive and resolvable pattern—multiple branching through time of several species from an unaltered ancestral source. Of course—and, again, just as with punctuated equilibrium itself—polytomy can also result from imperfec¬ tion, and we need criteria to separate “real” polytomies representing a signal from the history of life from polytomies that only record artifacts of an im¬ perfect record. Wagner and Erwin (1995) develop such a criterion by distin¬ guishing between “hard” polytomies that include the persisting ancestor and “soft” polytomies that arise from an inability to resolve true sets of ordered dichotomies. Wagner and Erwin’s modelling demonstrates the translation of punctua¬ tional speciation to a cladistic pattern of predominant polytomies. (Wagner and Erwin used my own model of punctuational phylogenies, done with D. M. Raup in the 1970’s (Raup and Gould, 1974), to show this mapping of punctuational phylogeny to a polytomous cladogram—see Figure 9-13—but I had never made the connection myself.) Wagner and Erwin then applied their modelled differences to cladograms for two well-resolved, but maximally different (in taxon and time) specieslevel phylogenies in the fossil record: two Neogene clades of planktonic foraminifers (Globigerinidae and Globorotalndae), and Ordovician representa¬ tives of the gastropod family Lophospiridae. In both cases, the cladograms in-
Punctuated Equilibrium and the Validation of Macroevolutionary Theory dicated an overwhelming predominance of speciation by cladogenesis as a cause of phylogenetic patterning—thus affirming the predictions of punctu¬ ated equilibrium. For globigermids, the cladistic topology revealed 40 specia¬ tion events, 5 probably anagenetic, 35 cladogenetic, and none bifurcating. Wagner and Erwin did not present full tabulations for the globorotaliids, but stated (p. 105): “The results are not presented here, but they were similar to those found for globigerinids: cladogenesis is significantly more common than anagenesis, a positive association exists between having long temporal ranges and leaving cladogenetic descendants, and no such association exists for anagenetic ancestors.” For the lophospirid gastropods, they write (p. 106): “Our preferred cladogram for lophospirids is rife with polytomies. Of the eleven polytomies, only
9-13. To my embarrassment, Wagner and Erwin (1995)—for I had not seen the obvious implication that would have enormously helped my argument—showed how phylogenies based upon iteration of several species from an unchanged par¬ ent stock (as Raup and Gould, 1974, had generated, and Wagner and Erwin re¬ produced, at the top of this figure) must yield, in cladistic representation, a polytomy. Thus, polytomies may provide evidence for punctuated equilibrium and do not necessarily represent the “signature” of missing data needed to re¬ solve the system into dichotomies. If the ancestral form doesn’t change through¬ out its geological range, all descendants must in principle arise at a polytomous junction of a cladogram.
821
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THE STRUCTURE OF EVOLUTIONARY THEORY two do not include plesiomorphic species. Thus, nine may represent hard polytomies.” Of 42 implied speciation events, a maximum of six may have been anagenetic, while only one may represent a bifurcation. Again, cladogenetic speciation, the expectation of punctuated equilibrium, dominates the phylogenetic pattern. Wagner and Erwin’s overall conclusion accords fully with patterns ex¬ pected in phylogenies built primarily—one might say overwhelmingly—by punctuated equilibrium (Wagner and Erwin, p. 110): ♦ Cladogenesis is significantly more common than anagenesis. ♦ Species with longer temporal and geographic ranges are more likely to leave descendants via cladogenesis or the factors contributing to wider temporal and geographic ranges also contribute to the likelihood of cladogenetic evolution. ♦ If anagenesis occurs, it only applies to species with restricted temporal and geographic ranges. ♦ Bifurcation accounts for a negligible amount of speciation. We cannot often obtain well-resolved species level phylogenies from pale¬ ontological data, and inferences from higher taxa will probably remain too murky and insecure to permit general use of such models for testing hypothe¬ ses explicitly based on the evolutionary behavior of species. Still, other data sets do exist in fair absolute abundance (while representing a low percentage of the total number of potential lineages in life’s history). Studies like Wagner and Erwin’s can be replicated and extended for many taxa—and such a strat¬ egy can provide powerful tests for the relative importance of punctuated equilibrium in the history of life and the generation of phylogenetic patterns. The first tests have been highly favorable, but we have scarcely any idea what an extended effort might teach us about the basic modalities of macro¬ evolution.
Sources of Data for Testing Punctuated Equilibrium PREAMBLE Punctuated equilibrium has generated a fruitful and far ranging, if sometimes acrimonious, debate within evolutionary theory (see appendix to this chap¬ ter). While we feel much pride (mixed with occasional frustration) for the role that punctuated equilibrium has played in instigating such extensive rethinking about the definitions and causes of macroevolution, we take even more pleasure in the volume of empirical study provoked by the theory of punctuated equilibrium, and pursued by paleontologists throughout the world. These carefully documented case studies (both pro and con) build a framework of proof for the value of punctuated equilibrium, as illustrated by the most important of all scientific criteria—operational utility. Such cases have been featured in numerous symposia and books dedicated to the em-
Punctuated Equilibrium and the Validation of Macroevolutionary Theory pirical basis of punctuated equilibrium. This literature includes: the 1982 symposium in Dijon, France, entitled Modalites, rythmes, mechanismes de revolution biologique: gradualisms phyletique ou equilibres ponctues and published
as
Chaline,
1983;
the
1983
Swansea
symposium
of
the
Palaeontological Association (United Kingdom) on “Evolutionary case histo¬ ries from the fossil record” and published as Cope and Skelton (1985); the book The Dynamics of Evolution: The Punctuated Equilibrium Debate in the Natural and Social Sciences (Somit and Peterson, 1992) that began as a sym¬ posium for the annual meeting of the American Association for the Advance¬ ment of Science, and then appeared as a special issue (1989) of the Journal of Biological and Social Structures; the 1992 symposium of the Geological Soci¬ ety of America on “Speciation in the Fossil Record,” held to celebrate the 20th anniversary of punctuated equilibrium, and published in book form as Erwin and Anstey (1995); and the 1994 Geological Society of America sym¬ posium on coordinated stasis, published in a special issue of the journal Palaeogeography, Palaeoclimatology, Palaeoecology in 1996 (volume 120, with Ivany and Schopf as editors). Several other unpublished symposia, in¬ cluding the notorious Chicago macroevolution meeting of 1980 (see pages 981-984), focused upon the topic of punctuated equilibrium. Finally, several books have treated punctuated equilibrium as an exclusive or major topic, including the favorable accounts of Stanley (1979), Eldredge (1985, 1995), and Vrba (1985a), and the strongly negative reactions of Dawkins (1986), Dennett (1995), Hoffman (1989), and Levinton (1988). As emphasized throughout this book, most general hypotheses in natural history, with punctuated equilibrium as a typical example, cannot be tested with any single “crucial experiment” (that is, by saying “yea” or “nay” to a generality after resolving a case with impeccable documentation), but must stand or fall by an assessment of relative frequency. Moreover, we can’t estab¬ lish a decisive relative frequency by simple enumerative induction (as in clas¬ sical “beans in a bag” tests of probability)—for individual species cannot be treated as random samples drawn from a totality with a normal (or any other kind of simple) distribution, but represent unique items built by long, com¬ plex and contingent histories. Time, taxon, environment and many other fac¬ tors strongly “matter,” and no global evaluation can be made by counting all cases equally. We may, however, be able to reach robust solutions for full pop¬ ulations within each factor—for planktonic forams, terrestrial mammals, De¬ vonian brachiopods, or species of the Cambrian explosion, for example. Part C of this section reports several such studies, nearly all Ending a predominant relative frequency for punctuated equilibrium. Nonetheless, hundreds of individual cases have been documented since we proposed punctuated equilibrium in 1972. I do not think that most authors pursue such work under any illusion that they might thus resolve the general debate, but rather for the usual, and excellent reasons of ordinary scientific practice. Researchers pursue such studies in order to apply promising general concepts to cases of special interest that draw upon their unique skills and ex-
823
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THE STRUCTURE OF EVOLUTIONARY THEORY pertise. Such studies are pursued, in other words, to resolve patterns within Australopithecus afarensis, or among species in the genus Miohippus, not to adjudicate general issues in evolutionary theory. Nonetheless, compendia of such studies do provide a “feel” for generalities of data in admittedly non-randomized samples, and they do establish ar¬ chives of intriguing and well-documented cases both for pedagogical illustra¬ tion, and simply for the general delight that all naturalists take in cases well treated and conclusively resolved. I shall therefore discuss this mode of docu¬ mentation as practiced for two categories central to punctuated equilibrium: patterns of gradualism or stasis within unbranched taxa (part B of this sec¬ tion), and tempos and modes of branching events in the fossil record (part C). Part D will then treat the more decisive theme of relative frequencies.
THE EQUILIBRIUM IN PUNCTUATED EQUILIBRIUM: QUANTITATIVELY DOCUMENTED PATTERNS OF STASIS IN UNBRANCHED SEGMENTS OF LINEAGES
As previously discussed (see pp. 758-765), the main contribution of punctu¬ ated equilibrium to this topic lies in constructing the theoretical space that made such research a valid and recognized subject at all. When paleontolo¬ gists equated evolution with gradual change, the well-known stasis of most lineages only flaunted a supposed absence of desired information, and could not be conceptualized as a positive topic for test and study. By representing stasis as an active, interesting, and predictable feature of most lineages most of the time, punctuated equilibrium converted an unconceptualized negative to an intriguing, and highly-charged positive, thereby forging a field of study. Nonetheless, we cannot argue that a proven predominance of stasis within lineages can establish the theory of punctuated equilibrium by itself. Punctu¬ ated equilibrium implies and requires such stasis, but remains, primarily, a theory about characteristic tempos and modes of branching events, and the primary patterning of phyletic change by differential birth and death of species. Stasis has emerged from the closet of disappointment and consequent non¬ recording. At the very least, paleontologists now write, and editors of jour¬ nals now accept, papers dedicated to the rigorous documentation of stasis in particular cases—so skeptics, and scientists unfamiliar with the fossil record, need not accept on faith the assurances of experienced paleontologists about predominant stasis in fossil morphospecies (see pp. 752-755). Moreover, sta¬ sis has also become a subject of substantial theoretical interest (see pp. 874885), if only as a formerly unexpected result now documented at far too high a frequency for resolution as an anticipated outcome within random systems (Paul, 1985); stasis must therefore be actively maintained. In any case, pale¬ ontologists are now free to publish papers with such titles as: “Cosomys pri¬ mus: a case for stasis” (Lich, 1990), and “Apparent prolonged evolutionary stasis in the middle Eocene hoofed mammal Hyopsodus” (West, 1979). The study of McKinney and Jones (1983) may be taken as a standard and
Punctuated Equilibrium and the Validation of Macroevolutionary Theory symbol for hundreds of similar cases representing a characteristic mixture of satisfaction and frustration. These authors documented a sequence of three successional species of oligopygoid echinoids from the Upper Eocene Ocala Limestone of Florida. The two stratigraphic transitions are abrupt, and there¬ fore literally punctuational. But available evidence cannot distinguish among the mutually contradictory explanations for such passages: gradualism, with transitions representing stratigraphic gaps; rapid anagenesis for a variety of plausible reasons including population bottlenecks or substantial environ¬ mental change; punctuated equilibrium based on allopatric speciation else¬ where (or unresolvably in situ, given coarse stratigraphic preservation), and migration of new species to the ancestral range. Hence, frustration. (More¬ over, as this pattern represents the most frequent situation in most ordinary sequences of fossils, we can readily understand why the testing of punctua¬ tional claims within the theory of punctuated equilibrium requires selection of cases—fortunately numerous enough in toto, however modest in relative frequency—with unusual richness in both spatial and temporal resolution.) At the same time, however, we gain satisfaction in eminent testability for the set of claims representing the second key concept of stasis. Any species, if well represented throughout a considerable vertical span marking the hun¬ dreds of thousands to millions of years for an average duration, can be reli¬ ably assessed for stasis vs. anagenetic gradualism by criteria outlined pre¬ viously (pp. 765-774). McKinney and Jones (1983) compiled excellent evidence for stasis in each of their three species—the basis, after all, for using these taxa in establishing biozones for this section. (As argued on pp. 751752, biostratigraphers have always used criteria of stasis and overlapping range zones in their practical work on the relative dating of strata.) McKinney and Jones conclude (1983, p. 21): “These observations suggest there is little chance of species misidentihcation due to ontogenetic or phylo¬ genetic effects when using this lineage for biostratigraphic purposes.” Smith and Paul (1985) studied vertical variation of the irregular echinoid Discoides suhucula in a remarkably complete and well-resolved sequence of Upper Cretaceous sands. The species occurred throughout 8.6 m of section, apparently representing continuous sedimentation within one ammonite zone spanning less than 2 million years. The authors were able to sample meter by meter through a section with an interesting inferred environmental his¬ tory: “The sediment that was then being deposited changed from clean, wellwashed sand to a very muddy sand, and so one might expect to find evidence of phyletic gradualism in response to these changes” (1985, p. 36). Smith and Paul did measure a steady change in shape towards a more coni¬ cal form, a common response of irregular echinoids to muddy environments. But such an alteration can be ecophenotypically induced during ontogeny, and the authors see no reason to attribute this single modification to geneti¬ cally based evolution (while not, of course, disproving the possibility of such genuine gradualism). Otherwise, stasis prevails throughout the section: “In other, more important characters, D. suhucula remains morphologically static and shows no evidence of phyletic gradualism” (1985, p. 29).
825
826
THE STRUCTURE OF EVOLUTIONARY THEORY This case becomes particularly interesting, and merits consideration here, as a demonstration of how far reliable inference can extend, even when the tempo and mode of origin for a descendant species cannot be directly resolved (the usual situation in paleontology). The potential descendant, D. favrina, enters the section near the top and overlaps in range with D. subucula, thus implying cladogenetic origin rather than anagenesis. The descendant’s larger size and hypermorphic morphology suggest a simple het¬ erochronic mechanism for the production of all major differences, hence in¬ creasing our confidence in (although clearly not proving) a hypothesis of di¬ rect evolutionary filiation. Finally, the fact that no morphological differentia of the species undergo any phyletic transformation within the lifetime of the putative ancestor further underscores the punctuational character of the tran¬ sition, whatever the mode followed. The one character that does change dur¬ ing the tenure of D. subucula (perhaps only ecophenotypically, as discussed above) does not move towards the morphology of descendant D. favrina. The authors conclude (1985, pp. 36-37): Clearly the sedimentary record is complete enough and represents a suf¬ ficiently long period of time to be able to detect phyletic gradualism. Yet throughout this period D. subucula remains otherwise morphologically static. Characters that have been modified in closely related species show no evidence of undergoing gradual transformation within the duration of the species . . . The overlapping ranges of the two species and the total absence of phyletic gradualism in the characters that serve to distinguish the species suggests that punctuated equilibrium is a better model for speciation in this particular case. In a later section (pp. 854-874), I shall discuss the generality of stasis within taxa or times under the more appropriate heading of empirical work on relative frequencies. But I shall also note this broader argument here, and in passing, if only to underscore the strong psychological bias that still per¬ vades the field, thereby conveying a widespread impression that gradualism maintains a roughly equal relative frequency with punctuated equilibrium, whereas I would argue that, in most faunas, only a small minority of cases (surely a good deal less than 10 percent in my judgment) show evidence of gradualism. Under this largely unconscious bias, most researchers still single out rare cases of apparent gradualism for explicit study, while bypassing ap¬ parently static lineages as less interesting. Johnson (1985), for example, studied 34 European Jurassic scallop species, and concluded (p. 91): “One case . . . was discovered where . . . the sudden appearance of a descendant form could fairly be ascribed to rapid evolution (within no more than one million years). Inconclusive evidence of gradual change over some 25 million years was discovered in one of the other lineages studied . . . but in the remaining 32 lineages morphology appears often to have been static/’ Yet Johnson virtually confines his biometrical study to the two cases of putative change, presenting only a single figure for just one of the
Punctuated Equilibrium and the Validation of Macroevolutionary Theory 32 species in stasis. Johnson’s title for his excellent article also records this bias in degrees of relative interest—for he sets the unmentioned but over¬ whelmingly predominant theme of stasis in opposition to his label for the en¬ tire work: “The rate of evolutionary change in European Jurassic scallops.” The most brilliantly persuasive, and most meticulously documented, exam¬ ple ever presented for predominant (in this case, exclusive) punctuated equi¬ librium in a full lineage—Cheetham’s work on the bryozoan Metrarabdotos, more fully treated on pp. 843-845 and 868-870—began as an attempt to il¬ lustrate apparent gradualism. Cheetham wrote (in litt. to Ken McKinney, and quoted with permission from both colleagues): “The chronocline I thought was represented ... is perhaps the most conspicuous casualty of the restudy, which shows that the supposed cline members largely overlap each other in time. Eldredge and Gould were certainly right about the danger of stringing a series of chronologically isolated populations together with a gradualist’s ex¬ pectations.” Cheetham’s biometry led him to the opposite conclusion of ex¬ clusive stasis: “In nine comparisons of ancestor-descendant species pairs, all show within-species rates of morphological change that do not vary sig¬ nificantly from zero, hence accounting for none of the across-species differ¬ ence” (Cheetham, 1986, p. 190). The establishment of stasis as an operational and quantifiable subject be¬ hooves us to develop methods and standards of depiction and characteriza¬ tion. Several studies have simply presented mean values for single characters in a vertical succession, but such minimalism scarcely seems adequate. At the very least, variances should be calculated (and included in published dia¬ grams in the form of error bars, histograms, etc.)—if only to permit statistical assessment of significances for mean differences between levels, and for corre¬ lations of mean values with time. Smith and Paul (1985), for example, presented both ontogenetic regres¬ sions and histograms for samples from each meter of sediment to illustrate stasis in relative size of the peristome in Discoides subucula (Fig. 9-14). Cronin (1985) also used both central tendency and variation to illustrate sta¬ sis throughout 200,000 years of intense climatic fluctuation (during Pleisto¬ cene ice cycles) for the ostracode Puriana mesacostalis. Cronin (Fig. 9-15) en¬ circled all specimens of the species at three expanding levels of time in a multivariate plot of the first two canonical axes (encompassing 92 percent of total information): (1) variation in a single sample spanning 100 to 1000 years; (2) in one formation encompassing 20,000 to 50,000 years; and (3) across two formations, representing 100,000 to 200,000 years. Two features of this pattern provide insight into the anatomy of stasis: first, relatively small increase in the full range of variation over such marked extensions in lengths of time; second, the concentric nature of the enlarging ellipses, indicating no preferred direction in added variation, but merely the regular expansion an¬ ticipated in any random system with increasing sample size. As Cronin notes, this lack of directionality seems all the more surprising when we recognize that this lineage persisted in stasis through several ice-age cycles. Stasis must
827
828
THE STRUCTURE OF EVOLUTIONARY THEORY be construed as a genuine phenomenon, actively maintained—and not as an absence of anything.' Cronin writes (1985, pp. 60-61): “Total within-sample variability representing 102 to 103 years is only slightly less than variability over 105 to 2 X 105 years. Puriana mesacqstalis shows no secular trends in its morphology over this time interval that might be evident from a lack of concentricity of the ovals—stasis is directionless. Yet high-amplitude environ¬ mental fluctuations occurred during this time that could have catalyzed speciation or caused extinction.” Once we construe stasis as an interesting evolutionary phenomenon, ac¬ tively promoted within species, we then become eager to know more about its fine-scale anatomy and potential causes. A remarkable series of studies by Michael A. Bell on the Miocene stickleback fish Gasterosteus doryssus (Bell and Haglund, 1982; Bell, Baumgartner and Olson, 1985; Bell and Legendre, 1987) provide evidence at a maximal level of paleontological resolution, for these fossils occur in abundance in varved sediments with yearly bands— surely a summum bonum for attainable temporal precision! Bell and col-
0 to 100cm
number of specimens 10-3
test diameter
peristome diameter test diameter
x 1 00
9-14. An impressive demonstration of stasis in the peristome size of the echinoid Discoides subucula. From Smith and Paul (1985). All specimens are shown for each narrow collecting interval, spaced one meter apart.
Punctuated Equilibrium and the Validation of Macroevolutionary Theory MORPHOLOGIC STASIS IN P. MESACOSTALIS AND P. FLORIDAN A
levels of SPECIES
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9-21. Note that both species of Ischyromys live sympatrically and remain in sta¬ sis in some parts of their range, particularly in Nebraska and Wyoming.
in relative abundance) and gradualism (minor size increase within the larger species at the end of its range) in a total pattern, but he concluded (correctly, I think, in my own biased way) that punctuated equilibrium had shown greater utility in challenging previous as¬ sumptions that had stymied proper conclusions. He closed his paper by writing (1993, p. 307): “So, Ischyromys displays features of both ‘punctuated equilibria’ and ‘phyletic gradualism’ as defined by Eldredge and Gould (1972). But the primary revelation of this study is that what was thought to be a single gradually evolving lineage must now be seen as the replacement of one stable species by another.” (In the fairness of full disclosure, Heaton did his graduate work under my direction. But I really do encourage independence and contrariness, and some of my students have documented gradualism, even in their Ph.D. dissertations, when truly (and for the only time) beholden to my “official” ap¬ proval—e.g. Arnold, 1982.)
849
850
THE STRUCTURE OE EVOLUTIONARY THEORY also strongly implicating punctuated equilibrium as the major generator of larger trends, if only because the fruits of anagenesis get plucked so quickly unless they can be “locked up” in cladogenetic iterations. *
The “dissection” of punctuations to infer both existence and modality Once a literal punctuation has been noted, and a cladogenetic origin inferred by such criteria as the documentation of ancestral survival, further testing of punctuated equilibrium as the mode of origin for the new species may be achieved by several standards that might be characterized (somewhat meta-
Slim Buttes, South Dakota 85 Whitneyan
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9-29. This cartoon of changing form and range in the histogram of complexity through life’s phylogeny illustrates how we fall into error when we treat extreme values as surrogates or epitomes of entire systems. A view that emphasizes speciation and diversity might recognize the constancy of the bacterial mode as the outstanding feature of life’s history. From Gould, 1996a.
897
898
THE STRUCTURE OF EVOLUTIONARY THEORY an icon for an argument, not a quantification. The full vernacular under¬ standing of complexity cannot be represented as a linear scale, although meaningful and operational surrogates for certain isolated aspects of the ver¬ nacular concept have been successfully designated for particular cases-—see McShea, 1994. I do not see how anyone could mistake the extreme value of a small tail in an increasingly skewed distribution through time for the evident essence, or even the most important feature, of the entire system. The error of construing this conventional trend of extremes as the essential feature of life’s history be¬ comes more apparent when we switch to the more adequate iconography of entire ranges of diversity through time. Consider just three implications of the full view, but rendered invisible when the sequential featuring of extremes falsely fronts for the history of the whole. 1. The salience of the bacterial mode. Although any designation of most sa¬ lient features must reflect the interests of the observer, I challenge anyone with professional training in evolutionary theory to defend the extending tip of the right tail as more definitive or more portentous than the persistence in place, and constant growth in height, of the bacterial mode. The recorded history of life began with bacteria 3.5 billion years ago, continued as a tale of prokary¬ otic unicells alone for probably more than a billion years, and has never expe¬ rienced a shift in the modal position of complexity. We do not live in what older books called “the age of man” (1 species), or “the age of mammals” (4000 species among more than a million for the animal kingdom alone), or even in “the age of arthropods” (a proper designation if we restrict our focus to the Metazoa, but surely not appropriate if we include all life on earth). We live, if we must designate an exemplar at all, in a persisting “age of bacte¬ ria”—the organisms that were in the beginning, are now, and probably ever shall be (until the sun runs out of fuel) the dominant creatures on earth by any standard evolutionary criterion of biochemical diversity, range of habi¬ tats, resistance to extinction, and perhaps, if the “deep hot biosphere” (Gold, 1999) of bacteria within subsurface rocks matches the upper estimates for spread and abundance, even in biomass (see Gould, 1996a, for a full develop¬ ment of this argument). I will only remind colleagues of Woese’s “three do¬ main” model for life’s full genealogy (see Fig. 9-30), a previously surprising but now fully accepted, and genetically documented, scheme displaying the phylogenetic triviality of all multicellular existence (a different issue, I fully admit, from ecological importance). Life’s tree is, effectively, a bacterial bush. Two of the three domains belong to prokaryotes alone, while the three king¬ doms of multicellular eukaryotes (plants, animals, and fungi) appear as three twigs at the terminus of the third domain. 2. The cause of the bacterial mode. “Bacteria,” as a general term for the grade of prokaryotic unicells lacking a complex internal architecture of or¬ ganelles, represent an almost ineluctable starting point for a recognizable fos¬ sil record of preservable anatomy. As a consequence of the basic physics of self-organizing systems and the chemistry of living matter—and under any
Punctuated Equilibrium and the Validation of Macroevolutionary Theory
popular model for life’s origins, from the old primordial soup of Haldane and Oparin, to Cairns-Smith’s clay templates (1971), to preferences for deep-sea vents as a primary locale—life can hardly begin in any other morphological status than just adjacent to what I have called (see Fig. 9-29) the “left wall” of minimal conceivable preservable complexity, that is, effectively, as bacteria (at least in terms of entities that might be preserved as fossils). I can hardly imagine a scenario that could begin with the precipitation of a hippopotamus from the primordial soup. Once life originates, by physico-chemical necessity, in a location adjacent to this left wall (see Kauffman, 1993), the subsequent history of right-skewed expansion arises predictably as a fundamental geometric constraint of this initial condition combined with the principles of Darwinian evolution—that is, so long as the most genuine trend of life’s history then prevails: “success” measured variationally, in true Darwinian fashion, as expansion in diversity and range through time. If life continues to add taxa and habitats, then structural constraints of the system virtually guarantee that a right tail of complexity will develop and in¬ crease in skew through time as a geometric inevitability, and not necessarily for any overall advantage conferred by complexity. As noted above, life must begin, for physico-chemical reasons, next to the left wall of minimal com¬ plexity. Little or no “space” therefore exists between the initial bacterial mode and this natural lower limit; variation can expand only into the “open” domain of greater complexity. The vaunted trend to life’s increasing complex¬ ity must be reconceived, therefore, as a drift of a small percentage of species from the constant mode of life’s central tendency towards the only open di-
BACTERIA
ARCHAEA
EUCARYA Animals
9-30. In life’s full genealogy, all three multicellular kingdoms grow as twigs at the terminus of just one branch among the three great domains of life’s history. The other two domains are entirely prokaryotic. From work of Woese and col¬ leagues, as presented in Gould, 1996a.
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THE STRUCTURE OF EVOEUTIONARY THEORY rection for expansion. To be able to formulate this alternative view at all, we must reconceive the history of life as expansion and contraction of a full range of taxa under constraints of systems and environments, rather than as a flux of central tendencies, valued extremes^ or salient features. 3. The right tail as predictable, but passively generated. A critic might re¬ spond that he accepts the reformulation but still wishes to assert a vector of progress as life’s central feature in the following, admittedly downgraded, way: yes, the vector of progress must be construed as the expanding right tail of a distribution with a constant mode, not as a general thrust of the whole. But this expanding tail still arises as a predictable feature of the system, even if we must interpret its origin and intensification as the drift of a minority away from a constraining wall, rather than the active trending of a totality. The right tail had to expand so long as life grew in variety. This tail therefore originated and extended for a reason; and humans now reside at its present terminus. Such a formulation may not capture the full glory of Psalm 8 (“Thou hast made him a little lower than the angels”), but a dedicated anthropocentrist could still live with this version of human excellence and dom¬ ination. But the variational reformulation of life’s system suggests a further implica¬ tion that may not sit well with this expression of human vanity. Yes, the right tail arises predictably, but random systems generate predictable consequences for passive reasons—so the necessity of the right tail does not imply active construction based on overt Darwinian virtues of complexity. Of course the right tail might be driven by adaptive evolution, but the same configuration will also arise in a fully random system with a constraining boundary. The issue of proper explanations must be resolved empirically. By “random” in this context, I only mean to assert the hypothesis of no overall preference for increasing complexity among items added to the distri¬ bution—that is, a system in which each speciation event has an equal proba¬ bility of leading either to greater or to lesser complexity from the ancestral design. I do not deny, of course, that individual lineages in such systems may develop increasing complexity for conventional adaptive reasons, from the benefits of sharp claws to the virtues of human cognition. I only hold that the entire system (all of life, that is) need not display any overall bias—for just as many individual lineages may become less complex for equally adaptive rea¬ sons. In a world where so many parasitic species usually exhibit less complex¬ ity than their freeliving ancestors, and where no obvious argument exists for a contrary trend in any equally large guild, why should we target increasing complexity as a favored hypothesis for a general pattern in the history of life? The location of an initial mode next to a constraining wall guarantees a temporal drift away from the wall in random systems of this kind. This sit¬ uation corresponds to the standard paradigm of the “drunkard’s walk” (Fig. 9-31), used by generations of statistics teachers to illustrate the canonical ran¬ dom process of coin tossing. A drunkard exits from a bar and staggers, en¬ tirely at random, along a line extending from the bar wall to the gutter (where he passes out and ends the “experiment”). He winds up in the gutter on every
Punctuated Equilibrium and the Validation of Macroevolutionary Theory
iteration of this sequence (and with a predictable distribution of arrival times) simply because he cannot penetrate the bar wall and must eventually “reflect off” whenever he hits this boundary. (Of course, he will also end up in the gutter even if he moves preferentially towards the bar wall; in this case, the average time of arrival will be longer, but the result just as inevitable.) The issue of active drive (a small bias in relative frequency fueled by the general Darwinian advantages of complexity) vs. passive drift (predictable movement in a random system based on the model of the drunkard’s walk) for the expansion of the right tail must be resolved empirically. But the macroevolutionary reformulation of life’s history in variational terms estab¬ lishes a conceptual framework that permits this question to be asked, or even conceived at all, for the first time. Initial studies on mammalian vertebrae and teeth, foraminiferal sizes, and ammonite sutures have been summarized in Gould, 1996a, based on pioneering studies of McShea, Boyajian, Arnold, and Gingerich. This initial research has found no departure from the random model, and no overall preference for increase in complexity in studies that tabulate all events of speciation. General rules.
The older literature of paleobiology focused on the
recognition and explanation of supposedly general “rules” or “laws” regulat¬ ing the overt phenomenology of life’s macroevolutionary pattern. As the modern synthesis developed its core of Darwinian explanation, several lead¬ ing theorists (see especially Haldane, 1932, and Rensch, 1947, 1960) tried to render these laws as large-scale expressions of evolution’s control by adaptive anagenesis in populations under Darwinian natural selection. This subject fell out of favor for several reasons, but in large part because non-adaptationist explanations, deemed less interesting (and certainly less coordinating) than accounts based on natural selection, provided an ade¬ quate compass for most of these “laws.” Thus, for example, Dollo’s law of ir-
9-31. The standard statistical model of the “drunkard’s walk” shows that even the expanding right tail of life’s right-skewed histogram of complexity may arise within a random system with equal probabilities for the movement of any de¬ scendant towards either greater or lesser complexity. From Gould, 1966a.
901
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THE STRUCTURE OF EVOEUTIONARY THEORY reversibility (see Gould, 1970b) only restates the general principles of mathe¬ matical probability for the specific case of temporal changes based on large numbers of relatively independent components. And Williston’s law of reduc¬ tion and specialization in modular segments may only record a structural constraint in random systems, thus following the same principles as my previ¬ ous argument about the expanding right tail of complexity for life’s total¬ ity. Suppose that, in overall frequency within the arthropod clade, modu¬ lar species (with large numbers of similar segments) and tagmatized species (with fewer fused and specialized groupings of former segments) always en¬ joy equal status in the sense that 50 percent of habitats favor one design, and 50 percent the other. (I am, of course, only presenting an abstract “thought experiment,” not an operational possibility for research. Niches don’t exist independent of species.) But suppose also that, for structural reasons, modu¬ lar designs can evolve toward tagmatization, but tagmatized species cannot revert to their original modularity—an entirely reasonable assumption under Dollo’s law (founded upon the basic statements of probability theory) and generalities of biological development. Then, even though tagmatization en¬ joys no general selective advantage over modularity, a powerful trend to tagmatization must pervade the clade’s history, ultimately running to comple¬ tion when the last modular species dies or transforms. However, one of these older general rules has retained its hold upon evolu¬ tionary theory, probably for its putative resolvability in more conventional Darwinian terms of general organismic advantage: Cope’s Law, or the claim that a substantial majority of lineages undergo phyletic size increase, thus im¬ parting a strong bias of relative frequency to the genealogy of most clades—a vector of directionality that might establish an arrow of time for the history of life. A century of literature on this subject had been dominated by proposed ex¬ planations in the conventional mode of organismic adaptation fueled by nat¬ ural selection. Why, commentators asked almost exclusively, should larger size enjoy enough general advantage to prevail in a majority of lineages? Pro¬ posed explanations cited, for example, the putative benefits enjoyed by larger organisms in predatory ability, mating success, or capacity to resist extreme environmental fluctuations (Hallam, 1990; Brown and Maurer, 1986). The speciational reformulation of macroevolution has impacted this sub¬ ject perhaps more than any other, not because the theme exudes any spe¬ cial propensity for such rethinking—for I suspect that almost any conven¬ tional “truth” of macroevolution holds promise for substantial revision in this light—but because its salience as a “flagship,” but annoyingly unre¬ solved, issue inspired overt attention. Moreover, the conventional explana¬ tions in terms of organismal advantage had never seemed fully satisfactory to most paleontologists. The rethinking has proceeded in two interesting stages. First, Stanley (1973), in a landmark paper, proposed that Cope’s Law emerges as a passive consequence of Cope’s other famous, and previously unrelated, “Law of the Unspecialized”—the claim that most lineages spring from founding species with generalized anatomies, under the additional, and quite reasonable, as-
9-32. Cope’s Law shown, under a speciational perspective, as a differential movement of speciation events towards larger size from a constraining boundary imparted by a small founding member of the lineage. Adapted from Stanley, 1973.
sumption that the majority of generalized species also tend to be relatively small in body size within their clades. These statements still suggest nothing new so long as we continue to frame Cope’s Rule as anagenetic flux in an average value through time—that is, as a conventional “trend” under lingering Platonic approaches to macroevolu¬ tion. But when we reformulate the problem in speciational terms—with the history of a Cope’s Law clade depicted as the distribution of all its species at all times, and with novelty introduced by punctuational events of specia¬ tion rather than anagenetic flux—then a strikingly different hypothesis leaps forth, for we now can recognize a situation precisely analogous (at one fractal level down) to the previous construction of life’s entire history: an evolving population of species (treated as stable individuals), in a system with a left wall of minimal size (for the given Bauplan), and a tendency for founding members to originate near this left wall (Fig. 9-32). Therefore, just as for all of life in my previous example, if the clade pros¬ pers with an increasing number of species, and even if new species show no directional tendency for increasing size (with as many species arising smaller than, as larger than, their ancestors), then the mean size among species in the
904
THE STRUCTURE OF EVOLUTIONARY THEORY clade must drift to the right, even though the mode may not move from initial smallness, just because the space of possible change includes substantial room in the domain of larger size, and little or no space between the founding lin¬ eage and the left wall. Thus, as Stanley (1973) stated so incisively, Cope’s Law receives a reversed interpretation as the structurally constrained and passive evolution (of an abstracted central tendency, I might add) from small size, rather than as active evolution towards large size based on the organismic ad¬ vantages of greater bodily bulk under natural selection. But we must then carry the revision one step further and ask an even more iconoclastic question: does Cope’s Law hold at all? Could our impressions about its validity arise as a psychological artifact of our preferential focus upon lineages that grow larger, while we ignore those that remain in stasis or get smaller—just as we focus on fishes, then dinosaurs, then mammoths, then humans, all the while ignoring the bacteria that have always dominated the diversity of life from the pinnacle of their unchanging mode throughout geo¬ logical time? Again, we cannot even ask this question until we reformulate the entire is¬ sue in speciational terms. If we view a temporal vector of a single number as adequate support for Cope’s Law, we will not be tempted to study all species in a monophyletic clade that includes signature lineages showing the docu¬ mented increase in size. But when we know, via Stanley’s argument, that Cope’s Law can be generated as a summary statement about passively drifting central tendencies in random systems with constraining boundaries, then we must formulate our tests in terms of the fates of all species in monophyletic groups. Jablonski (1997) has published such a study for late Cretaceous mollusks of the Gulf and Atlantic coasts (a rich and well-studied fauna of 1086 species in Jablonski’s tabulation) and has, indeed, determined that, for this prominent group at least, prior assertions of Cope’s Law only represent an ar¬ tifact of biased attention (see commentary of Gould, 1997b). Jablonski found that 27-30 percent of genera do increase in mean size through the sequence of strata. But the same percentage of genera (26-27 percent) also decrease in mean size—although no one, heretofore, had sought them out for equal ex¬ amination and tabulation Moreover, and more notably for its capacity to lead us astray when we op¬ erate within a conceptual box defined by anagenetic flux rather than varia¬ tion in numbers of taxa, an additional 25-28 percent of genera fall into a third category of generally and symmetrically increasing variation through the sequence—that is, the final range for all species within the genus includes species both smaller and larger than the extremes of the ancestral spread. I strongly suspect that a previous inclusion of these genera as affirmations of Cope’s Law engendered the false result of dominant relative frequency for phyletic size increase. Older treatments of the topic usually considered ex¬ treme values only, and affirmed Cope’s Law if any later species exceeded the common ancestor in size—thus repeating, in miniature, the same error gener¬ ally committed for life’s totality by ignoring the continuing domination of bacteria, and using the motley sequence of trilobite to dinosaur to human as evidence for a central and defining thrust. Obviously, from a variational and
Punctuated Equilibrium and the Validation of Macroevolutionary Theory speciational perspective, successful genera with substantially increasing num¬ bers of species through time will probably expand their range at both ex¬ tremes of size, thus undergoing a speciational trend in variation, not an anagenetic march to larger sizes! Particular cases. This lowest macroevolutionary level of individ¬
ual monophyletic clades has defined the soul of paleontological discourse through the centuries. Only the histories of particular groups can capture the details that all vivid story-telling requires; the “why” of horses and humans certainly elicits more passion than the explanation (or denial) of Cope’s Law, or the pattern of increasing mean species longevity in marine invertebrates through time. Yet, even here on such familiar ground, our explanations re¬ main so near and yet so far—for these “closer” stories of particular histories must also be reexamined in a speciational light. Consider just two “classics” and their potential revisions. 1. HORSES AS THE EXEMPLAR OF “LIFE’S LITTLE JOKE.” As noted above (Fig. 7-3, and 580-581), the line of horses, proceeding via three major trends of size, toes and teeth from dog-sized, many-toed, “eohippus” with lowcrowned molars to one-toed Equus with high-crowned molars (see Fig. 9-33 for W. D. Matthew’s classic icon, linearly ordered by stratigraphy) still marches through our textbooks and museums as the standard-bearer for adaptive trending towards bigger and better. I do not deny that, even in a refomulated speciational context, several as¬ pects of the traditional story continue to hold. MacFadden (1986), for exam¬ ple, has documented a clear cladal bias towards the punctuational origin of new species at larger sizes than their immediate ancestors, so both the iconic transition from ladders to bushes, and the recognition that several specia¬ tional events lead to smaller sizes, even to dwarfed species, throughout the range of the lineage, does not threaten (but rather reinterprets in interesting ways) the conventional conclusion that horses have generally increased in size through the Cenozoic. Moreover, I do not doubt the usual adaptational sce¬ nario that a transition from browsing in soft-turfed woodlands to grazing on newly-evolved grassy plains (grasses did not evolve until mid-Tertiary times) largely explains the adaptive context for both the general reduction in num¬ bers of toes from splayed feet on soft ground to hoofs on harder substrates, and the increasing height of cheek teeth to prevent premature wear from eat¬ ing grasses of high silica content. Nonetheless, a speciational reformulation in terms of changing diversity as well as anatomical trending tells a strikingly different, and mostly opposite, story for the clade as a whole. Modern perissodactyls represent but a shade of their former glory. This clade once dominated the guild of large-bodied mammalian herbivores, with speciose and successful groups, especially the titanotheres, that soon became extinct, and with diversity in existing groups far exceeding modern levels. (The rhinoceros clade once included agile run¬ ning forms, the hyrachiids, wallowing hippo-like species, and the indricotheres, the largest land mammals that ever lived.) Modern perissodactyls exist as three small clades of threatened species: horses, rhinos, and tapirs. Horses have declined precipitously from their maximal mid-Tertiary abun-
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