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fo
ERNST
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THE SCIENCE OF THE LIVING VSi
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THIS
IS
BIOLOGY
Also by Ernst
One Long Argument:
Charles
Darwin and
Mayr
the Genesis of
Modern Evolutionary
Thought (1991)
Toward a
New
Philosophy of Biology: Observations of an Evolutionist (1988)
The Growth of Biological Thought:
Diversity, Evolution,
and Inheritance (1982)
The Evolutionary Synthesis (with William Provine, 1980, 1998) Evolution
and
the Diversity of Life: Selected Essays (1976)
Populations, Species,
and Evolution (1970)
Principles of Systematic Zoology (1969)
Charles Darwin,
On
the Origin of Species:
A
(Editor, 1964)
Animal
Species
and Evolution (1963)
Birds of the Southwest Pacific (1945)
Systematics
and
the Origin of Species (1942)
Facsimile of the First Edition
THIS
IS
BIOLOGY
The Science of the Living World
ERNST MAYR
THE BELKNAP PRESS OF HARVARD UNIVERSITY PRESS Cambridge, Massachusetts
London, England
SOUTH BOSTON
SB BR QH307.2 -M39 1997 Copyright
Kinu /ct9q
©
1997 by Ernst Mayr
All rights reserved
Printed in the United States of America Sixth printing, 1998
First
Harvard University Press paperback edition, 1998
Library of Congress Cataloging-in-Publication Data
Mayr, Ernst, 1904This
is
biology
:
the science of the living world p.
/
Ernst Mayr.
cm.
Includes bibliographical references and index.
ISBN 0-674-88468-X
(cloth)
ISBN 0-674-88469-8
(pbk.)
1.
Biology.
I.
QH307.2.M39
574— dc20 Designed by
L
Title.
1997
96-42192
Gwen
Frankfeldt
In
memory
of
my
mother,
Helene Pusinelli Mayr, to
whom so
I
owe
much
Contents
Guide
to Topics
Covered
viii
Preface
xiii
1
What
Is
the
2
What
Is
Science?
3
4
How How
5
Does Science Advance?
6
How
7
"What?" Questions: The Study of Biodiversity
8
"How?" Questions: The Making of
9
"Why?" Questions: The Evolution of Organisms
175
10
What Questions Does Ecology Ask?
207
11
Where Do Humans
227
12
Can Evolution Account Notes
Meaning of
1
24
Does Science Explain the Natural World?
45
Does Biology Explain the Living World?
64
Are the
Glossary
a
New
Fit into Evolution?
293
305
Acknowledgments 315
79
Life Sciences Structured?
273
Bibliography
Index
"Life"?
313
for Ethics?
107
Individual
124 151
248
Guide
1.
What
Is
The
the
to Topics
Meaning of
Covered
"Life"?
Physicalists
3
The flowering of physicalism The
Vitalists
The The
13
Organicists
16 19
The Distinguishing
What
Is
Characteristics of Life
Biology an
Autonomous
Science?
The Concerns of Science
How How How
How A
30
33
science differs
from theology
33
from philosophy
35
science differs
from the humanities
Scientific
Research
37
39
Science and the scientist
42
Does Science Explain the Natural World?
45
Brief History of the Philosophy of Science Verification
New
and
models of
falsification scientific
explanation
Discovery and Justification Internal
and external
factors in theory formation
Testing
The
Practicing Biologist Five stages of explanation
L
26
science differs
The Objectives of
3.
20
24
Science?
The Origins of Modern Science Is
5 8
decline of vitalism
Emergence
2.
1
48 49 50 51
52 53
55 56
Guide
Commonsense
to Topics
Covered
realism
The language of
56
science
57
Defining Facts, Theories, Laws, and Concepts
60
Facts versus theories
4.
How
61
Universal laws in the physical sciences
61
Concepts in the
62
life
sciences
Does Biology Explain the Living World?
Causation in Biology
66 67
Pluralism
67
Probabilism
69
Case studies in biological explanation
69 71
Closed and open programs
The Quest
74
for Certainty
76
Does Science Advance? Scientific
Advance
79
in Cell Biology
83
Understanding the nucleus
86
Understanding the chromosomes
89
Does Science Advance through Revolutions?
91
Progress in systematics
92
Progress in evolutionary biology
94
Gradualism
96
in biological advances
Does Science Advance through
Why
scientific
consensus
is
a
Darwinian Process?
so hard to achieve
The Limits of Science 6.
How
Are the
New
99 101
104
Life Sciences Structured?
107
The Comparative and Experimental Methods
in Biology
Attempts to Structure Biology
109 112
"What?" questions
113
"How?" and "Why?" questions
115
A new
119
Power
classification
Shifts within
based on "How?" and "Why?"
Biology
120 122
Biology, a Diversified Science 7.
64
Proximate and ultimate causations
Cognitive Evolutionary Epistemology
5.
ix
"What?" Questions: The Study of Biodiversity
124
Classification in Biology
125
Microtaxonomy: The Demarcation of Species
127
Competing
131
species concepts
Species concept, species category,
Macrotaxonomy: The
and
species taxa
Classification of Species
133
134
Guide
Downward Upward
to Topics
Covered
classification
135
classification
136
Evolutionary or Darwinian classification
136
Numerical phenetics
142
Cladification
143
Storing and Retrieving Information
146
Names
147
The System of Organisms
148
"How?" Questions: The Making of
a
New
Individual
151
The Beginnings of Developmental Biology
153
The Impact of the
155
Cell
Theory
Epigenesis or preformation?
156
Differentiation, the divergence of developing cells
158
Formation of the germ
161
layers
Induction
163
Recapitulation
164
Developmental Genetics
165
The impact of molecular biology Development and Evolutionary Biology
167 169
Recapitulation reconsidered
171
How
172
evolutionary advances occur
"Why?" Questions: The Evolution of Organisms
175
The Manifold Meanings of "Evolution"
176
Darwin's Theory of Evolution as Such
178
The
origin of
Darwin's Theory of
The
origin of
178
life
Common
Descent
humans
179 182
Darwin's Theory of the Multiplication of Species
182
Darwin's Theory of Gradualism
183
Mass extinctions
185
Darwin's Theory of Natural Selection
Competing
theories of evolutionary change
186 186
Natural selection
188
Chance or
189
necessity?
Sexual selection
The Evolutionary
Synthesis
191
and After
Explaining macroevolution
Does Evolution Progress?
Why
organisms are not perfect
Current Controversies
Group
selection
192
194 197 198
200 202
Guide
10.
to Topics
Covered
XI
Sociobiology
203
Molecular biology
205
Multiple causes, multiple solutions
205
What Questions Does Ecology Ask? A Brief History of Ecology
207 208
Ecology today
209
The Ecology of the Individual
210
The Ecology of
210
Species
Niche
211
Competition
212
Reproductive strategies and population density
214
and coevolution
Predators, prey,
215
The food chain and the pyramid of numbers Life histories
11.
216
and taxonomic research
217
The Ecology of Communities
218
Succession and climax
219
Ecosystem
221
Diversity
222
Paleoecology
223
Controversies in Ecology
224
Where Do Humans
Evolution?
Fit into
The Relationship of Humans The
rise
of
Homo
habilis,
to the
227
Apes
228
H. erectus, and H. sapiens
230
The Neanderthals and Cro-Magnon Man Classifying fossil
Becoming
232
hominid taxa
233
Human
236
Coevolution of language, brain, and mind
238
Cultural Evolution
The
Human
241
birth of civilization
Races and the Future of the
The meaning of human 12.
243
Human
244
Species
247
equality
Can Evolution Account for Ethics? The Origin of Human Ethics
248
Inclusive fitness altruism
251
250
Reciprocal altruism
253
The emergence of genuine altruism
254
How
Does
a Cultural
Group Acquire
Its
Particular Ethical
Reason or random survival?
How
257 259
Does the Individual Acquire Morality?
An open behavior program What Moral System Is Best Suited
Norms?
260 262
for
Humankind?
265
Preface
Some
ago the then-President of France, Valery Giscard
years
d'Estaing, declared the twentieth century "the century of biology."
perhaps not entirely accurate for the century as a whole,
If this is
has certainly been true for the second field
of inquiry.
We
Today, biology
is
it
a thriving
have witnessed unprecedented breakthroughs in
cellular biology,
genetics,
half.
and neuroscience,
as well
as spectacular
advances in evolutionary biology, physical anthropology, and ecology.
A whole
industry has grown out of research in molecular biology; the
results are readily
apparent in
animal breeding, and
The prospects Scientific
War
II,
human
fields as diverse as
nutrition, to
medicine, agriculture,
name only
for biology have not always
a few.
been so bright. From the
Revolution in the seventeenth century until well after World
science for
chemistry,
most people meant the "exact" sciences
mechanics, astronomy
—
all
—
physics,
of which relied heavily on
mathematics and emphasized the role of universal laws. During
this
time physics was considered the exemplar of science. By comparison, the study of the living world was considered an inferior endeavor.
Even today most people continue to hold profound misconceptions about the is
life
sciences.
For example, a failure to understand biology
frequently displayed in the media, whether the topic
is
the teaching
of evolution, the measurement of intelligence, the possibility of detecting extraterrestrial
smoking.
life,
the extinction of species, or the risks of
Preface
xiv
many biologists themselves have an obsolete notion of the life sciences. Modern biologists tend to be extreme specialists. They may know all about particular bird species, sex hormones, More
regrettable,
parental behavior, neuroanatomy, or the molecular structure of genes, yet they often are
uninformed about developments outside
their field
of expertise. Rarely do biologists have the time to stand back from
own
the advances in their
specialty
and look
at the life sciences as a
whole. Geneticists, embryologists, taxonomists, and ecologists sider themselves to
ciation of
be biologists, but most of them have
what these various
specialties
all
little
appre-
common and how
have in
some
they differ fundamentally from the physical sciences. To shed light I
on
these issues
is
a
major purpose of
have been a naturalist almost since
of plants and animals led
me
to
I
book.
this
learned to walk, and
approach the living world
Fortunately, the teaching of biology at the
say that the focus was
and inanimate environment.
on
life
history, behavior,
and chemistry, both of which something entirely
I
school
little
to
I
inter-
its
would now
ecology. Physics
also studied in high school,
and had
different,
and
We
my love
holistically.
German high
attended around 1920 centered on the whole organism and actions with the animate
con-
do with
were
living plants
and animals. During the years when about medicine, and questions as
"What
I
was a medical student,
far too busy, to
is
biology?"
I
was
far
too excited
pay any attention to such basic
and "What makes biology
Indeed, there was not any subject taught at that time
—
—
a science?"
at least
not in
German universities which was designated "biology." What we would now call biology was taught in departments of zoology and the
botany, both of which strongly emphasized the study of structural types
and
their phylogeny.
To be
sure, courses
physiology, genetics,
and other more or
but there was
integration of the subjects,
little
framework of the experimentalists was
less
were also given in
experimental disciplines,
and the conceptual
largely incompatible with that
of the zoologists and botanists, whose work was based in natural history.
After switching
my
studies
from medicine
birds) following the completion of
to zoology (particularly
my preclinical
examinations,
I
took
xv
Preface
my
courses in philosophy at the University of Berlin. But to
disap-
pointment, they built no bridges between the subject matter of the biological sciences
and
and 30s
that of philosophy. Yet in the 1920s
a
was developing that would eventually be designated "phi-
discipline
losophy of science." In the 1950s, teachings of this
field, I
philosophy of science;
and the physical
when
was again
I
became acquainted with the
bitterly disappointed. This
was a philosophy of
this
sciences.
It
logic,
had almost nothing
concerns of biologists. Around this time
I
sat
was no
mathematics,
do with the
to
down and made
a
list
of the major generalizations of evolutionary biology stated in books
and published papers myself
—and
—
a few of which,
found that not a
by
single
this time,
I
had contributed
one of them was adequately
covered in the philosophical literature; most of them were not even
mentioned. at this
Still,
I
me
to lay aside temporarily
different biology I
My
science.
was
My
effect.
various essays
my
on
these topics
and symposia, which
researches in evolutionary
in certain respects
from
I
had been interested
my
how
very
physics. For example, in
was invited by Daniel Lerner of the Massachusetts series dealing
Institute of
with cause and
in the
problem of biological causation
Serin finch paper in 1926
and another paper on the origin
of bird migration in 1930. Therefore sort
a contribution to the
only intention was to point out
Technology to participate in a lecture
since
make
to
result of invitations to conferences
theory and systematics.
1960
had no plans
and philosophy of
history
were the forced
point
through
my
I
welcomed
thoughts on this subject.
I
this
opportunity to
had long been aware of
a categorical difference between the inanimate
and the
living world.
Both worlds obey the universal laws discovered and analyzed by the physical sciences, but living organisms obey also a second set of causes,
the instructions from the genetic program. This second type of causation first
my
is
nonexistent in the inanimate world.
Of
course,
I
was not the
biologist to discover the duality of causation in organisms, but
1961 published paper from the lecture series was the
first
to provide
a detailed analysis of the subject.
In truth, sciences
L
my
various essays about the differences between the
and the physical sciences were directed not so much
life
at
Preface
xvi
philosophers and physicists as at wittingly adopted
many
my
fellow biologists,
who had un-
physicalist concepts in their writings. For
example, the claim that every attribute of complex living systems can
be explained through a study of the lowest components (molecules, genes, or whatever) struck
hierarchy of ever tissues,
me
as absurd. Living
more complex
organisms form a
from molecules,
systems,
through whole organisms, populations, and
cells,
species. In
and each
higher system, characteristics emerge that could not have been predicted
At
from a knowledge of the components.
first I
called,
was
thought that
this
phenomenon of emergence, and indeed,
restricted to the living world;
gave in the early 1950s in Copenhagen,
made
I
as
it is
now
in a lecture
I
the claim that emer-
gence was one of the diagnostic features of the organic world. The
whole concept of emergence was metaphysical.
When
at that
time considered to be rather
the physicist Niels Bohr,
who was
stood up to speak during the discussion period, for
an annihilating refutation. However,
much
I
to
in the audience,
was
my
fully
prepared
surprise,
he did
my
notion
object to the concept of emergence but only to
not at
all
that
provided a demarcation between the physical and the biological
it
sciences. Citing the case
of water, whose "aquosity" could not be
predicted from the characteristics of
and oxygen, Bohr
stated that
its
emergence
two components, hydrogen is
rampant
in the inanimate
world. In addition to reductionism, another particularly objectionable bete
noire for
me was
typological thinking, later baptized "essentialism"
the philosopher Karl Popper.
It
by
consisted of classifying the variation
of nature into fixed types (classes), invariant and sharply demarcated against other such types. This concept, going back to Plato
and Py-
thagorean geometry, was singularly unsuited to evolutionary and population biology, where one finds not classes but aggregates of
unique individuals, that
is,
populations. Explaining variable
ena in living nature in terms of populations
—seems
thinking
thinking.
to be difficult for those
phenom-
so-called population
accustomed to
physicalist
repeatedly argued this problem with the physicist Wolfgang
who was most anxious to understand what we biologists had mind. He finally came close to understanding it when I suggested
Pauli,
in
I
—
xvii
Preface
to
him
to think of a gas consisting of only 100 molecules, each differing
from the others
in direction
and speed of movement. He
called
an
it
"individual gas."
many of those attempting to construct a history of science. When Thomas Kuhn's Structure of Scientific Revolutions was published in 1962, I was puzzled as to why it should have caused such a commotion. To be sure, Kuhn had refuted Biology has also been misunderstood by
some of science
the most unrealistic theses of the traditional philosophy of
and had
called attention to the
importance of historical
But what he offered as a replacement seemed to
Where
me just
factors.
as unrealistic.
were the cataclysmic revolutions and
in the history of biology
where were the long periods of normal science postulated by Kuhn's theory?
From what
No doubt
Darwin's
I
knew of the
On
history of biology, they did not exist.
the Origin of Species, published in 1859,
revolutionary, but ideas about evolution
had been
Moreover, Darwin's theory of natural selection
century.
mechanism
in evolutionary adaptation
almost a century after
its
publication.
—was not
Throughout
fully this
minor revolutions but never any period of "normal"
to grasp
coming from
what had happened
a
background
—the
time there were
science. it
in physics
in the study of living
key
accepted until
or not Kuhn's thesis was valid for the physical sciences, biology. Historians
was
in the air for a
Whether
did not
fit
seemed not
organisms over
three centuries.
More and more
clearly
different kind of science
mentally in
its
phy. While
all
physics
began to see that biology was a quite
I
from the physical
subject matter,
its
sciences;
history, its
it
differed funda-
methods, and
its
philoso-
biological processes are compatible with the laws of
and chemistry,
living
organisms could not be reduced to these
physicochemical laws, and the physical sciences could not address
many
aspects of nature that were unique to the living world.
classical physical sciences,
on which
was based, were dominated by a
set
The
the classical philosophy of science
of ideas inappropriate to the study
of organisms: these included essentialism, determinism, universalism,
and reductionism. Biology, properly understood, comprises population and historical nar-
thinking, probability, chance, pluralism, emergence, ratives.
What was needed was
a
new philosophy of
science that could
xviii
Preface
incorporate the approaches of
all
and
sciences, including physics
bi-
ology.
When
planning
however.
I
this
wanted
volume,
had
I
in
mind
more modest task, would
a
to write a "life history" of biology that
introduce the reader to the importance and richness of biology as a
whole, while helping the individual biologist approach a problem that is
steadily
becoming more formidable: the information explosion.
workers annually join those already in the lanche of
new
publications. Practically
with have complained to
up with the disciplines.
domain
literature
And
that
is
field
world, in
and
yet often
sees
all its
a conceptual
that they
own
in their is
it
field
and add
biologists
come it
much
less
to keep
adjacent
feedback from outside one's narrow
into view
when one
as part of a larger
wonderful
to the ava-
have ever talked
I
no longer have time specialty,
decisive for a conceptual advance.
research frequently
own
me
all
New
diversity.
I
New
directions for
back from one's
steps
endeavor to explain the living
hope that
framework from which working
this
book
will provide
biologists can attain this
broader perspective on their specific research agenda.
Nowhere
is
the information explosion
lecular biology.
A
more apparent than
detailed discussion of this field
volume not because
I
think molecular biology
is
in
mo-
absent from this
is less
important than
other parts of biology but for exactly the opposite reason. Whether
we
deal with physiology, development, genetics, neurobiology, or be-
havior, molecular processes are ultimately responsible for whatever
happens, and every day researchers are making fresh discoveries in these domains. In Chapters 8
and 9
have highlighted some of the
I
major generalizations ("laws") discovered by molecular it
strikes
me
that while
all
biologists.
Still,
we have identified many trees, we have not may disagree; in any case, a comprehensive
yet seen the forest. Others
overview of molecular biology requires a competence
The same can be
the biology of mental processes. exploration,
and
I
I
do not have.
said for another exceedingly important discipline,
simply do not
We
are
command
still
in
a stage of local
the required knowledge
of neurobiology and psychology to attempt a broad analysis.
final
The
genetic
plays a decisive role in every aspect of an organism's
life: its
area not covered in great detail in this
program
A
volume
is
genetics.
xix
Preface
structure, development, functions,
and
activities.
Since the rise of
molecular biology, the emphasis in genetics has shifted to develop-
mental genetics, which has become virtually a branch of molecular biology,
and
However,
I
for this reason
hope
that
my
I
have not attempted to survey
treatment of biology as
helpful in an eventual "life history" of these
and other
this field.
a whole critical
will
be
branches
of biology that were not the direct focus of this volume.
and others
If biologists, physical scientists, philosophers, historians,
with a professional interest in the in the chapters that follow, this its
primary
goals.
sciences discover useful insights
life
book
will
have accomplished one of
But every educated person should have an under-
standing of basic biological concepts tition, extinction, adaptation,
—
evolution, biodiversity,
compe-
natural selection, reproduction, devel-
opment, and a host of others that are discussed
in
this
book.
Overpopulation, the destruction of the environment, and the malaise of the inner
cities
cannot be solved by technological advances, nor by
literature or history,
but ultimately only by measures that are based
on an understanding of
"know
thyself," as the
the biological roots of these problems.
ancient Greeks
commanded
us, entails first
To and
foremost knowing our biological origins. To help readers gain a better
understanding of our place in the living world, and of our responsibility to the rest
of nature,
is
the major objective of this book.
Cambridge, Massachusetts
September 1996
CHAPTER ONE What
the
Is
Primitive humans
Meaning of
lived close to nature. Every
"Life"?
day they were occu-
pied with animals and plants, as gatherers, hunters, or herdsmen.
And
death
—of
infants
and
women
elders,
was forever present. Surely our
earliest ancestors
with the eternal question, "What Perhaps, at living
first,
no
organism and a
men
in strife
must have wrestled
is life?"
clear distinction in
spirit
in childbirth,
was made between
life
a nonliving natural object.
primitive people believed that a spirit might reside in a
in a
Most
mountain or
a spring as well as in a tree, an animal, or a person. This animistic
view of nature eventually waned, but the belief that "something" in a living creature distinguished
from the body this
at the
it
moment
from inanimate matter and departed
of death held strong. In ancient Greece
something in humans was referred to
larly in the Christian religion,
By
the time of Descartes
it
was
as "breath." Later, particu-
called the soul.
and the
Scientific Revolution,
animals
(along with mountains, rivers, and trees) had lost their claim to a soul.
But a dualistic to
split
between body and soul
be almost universally accepted and
many
Why soul
is
in
human
beings continued
even today
still
believed by
people. Death was a particularly puzzling problem for a dualist.
should left
this soul
the body, did
suddenly either die or leave the body? it
go somewhere,
such as to
If
the
some nirvana or
heaven? Not until Charles Darwin developed his theory of evolution
through natural selection was a
scientific,
rational explanation for
THIS
BIOLOGY
IS
death possible. August Weismann, a follower of Darwin
was the
the nineteenth century,
at the
end of
author to explain that a rapid
first
new genotypes
sequence of generations provides the number of
re-
quired to cope permanently with a changing environment. His essay
on death and dying was the beginning of standing of the meaning of death.
When
biologists
and philosophers speak of
usually are not referring to
but rather to object.
life
new
a
life
(that
as contrasted
is,
"life,"
however, they
living) as contrasted
with the
To elucidate the nature of
era in our under-
this entity called "life"
has been one
of the major objectives of biology. The problem here
some
suggests
"thing"
—
a substance or force
—and
no
avail.
One can
noun
In reality, the
of the process of living.
It
"life" is
life
substance or
merely a reification
does not exist as an independent
cannot do with the abstraction
what
living
one can attempt
to
is;
"life."
One can
one can define what a
make
entity.
1
describe, even attempt
organism
living
a demarcation between living
Indeed, one can even attempt to explain
how
life is,
and how one should explain
a subject of heated controversy
the situation was
this:
and
and nonliving.
living.
can
2
living processes, has
been
since the sixteenth century. In brief,
There was always a camp claiming that living
organisms were not really different
at
all
from inanimate matter;
sometimes these people were called mechanists,
was always an opposing camp
that living organisms
is;
living, as a process,
be the product of molecules that themselves are not
there
that "life"
deal with the process of living scientifically, something one
to define,
What
is
for centuries phi-
losophers and biologists have tried to identify this vital force, to
with death
of an inanimate
lifelessness
—
had properties
later physicalists.
called vitalists
—claiming
that could not be
found
And
instead in inert
matter and that therefore biological theories and concepts could not
be reduced to the laws of physics and chemistry. In some periods and at certain intellectual centers the physicalists
and
other times and places the
in
the upper hand. In this century
it
vitalists
seemed seemed
to be victorious,
to have achieved
has become clear that both camps
were partly right and partly wrong.
The
had been right component and that
physicalists
physical
life
in insisting that there at the
molecular
is
no meta-
level life
can be
What
the
Is
Meaning of
"Life"?
explained according to the principles of physics and chemistry. At the
same
time, the vitalists
had been
right in asserting that, nevertheless,
organisms are not the same as inert matter but have numerous
living
autonomous
characteristics, particularly their historically acquired ge-
unknown
netic programs, that are
Organisms are
in inanimate matter.
many-level ordered systems, quite unlike anything found in the inani-
mate world. The philosophy that eventually incorporated the best principles excesses) is
from both physicalism and vitalism
became known
dominant
as organicism,
and
this
(after discarding the is
the paradigm that
today.
The
Physicalists
Early beginnings of a natural (as opposed to supernatural) explanation
of the world were
made
in the philosophies of various
including Plato, Aristotle, Epicurus, and
many
others.
Greek thinkers,
These promising
beginnings, however, were largely forgotten in later centuries.
Middle Ages were dominated by a
strict
God and
the Scriptures, which attributed everything in nature to laws.
But medieval thinking, particularly
terized
by a
belief in
all
The
adherence to the teachings of
in folklore,
was
His
also charac-
sorts of occult forces. Eventually this animistic,
magical thinking was reduced,
if
not eliminated, by a
new way
of
looking at the world that was aptly called "the mechanization of the
world picture" (Maier 1938). 3
The
influences leading
up
to the mechanization of the
world picture
were manifold. They included not only the Greek philosophers, transmitted to the Western world by the Arabs along with rediscovered original writings, but also technological developments in late medieval
and
early Renaissance times.
and other automata
There was great fascination with clocks
—and indeed with almost any kind of machine.
This eventually culminated in Descartes's claim that except
humans were nothing
all
organisms
but machines.
became the spokesman for the Scientific Revolution, which, with its craving for precision and objectivity, could not accept vague ideas, immersed in metaphysics and the supernatural, Descartes (1596-1650)
such as souls of animals and plants. By restricting the possession of
THIS
IS
BIOLOGY
humans and by declaring animals
a soul to
to be nothing but automata,
Descartes cut the Gordian knot, so to speak. With the mechanization
of the animal soul, Descartes completed the mechanization of the
world picture. 4 It
a
is
difficult to
little
why
understand
the machine concept of
organisms could have had such long-lasting popularity. After
machine has ever built been able to procure
own
its
ganism and a machine
replicated
itself,
energy.
programmed
itself,
The
all,
itself,
no or
between an or-
similarity
exceedingly superficial. Yet the concept did
is
not die out completely until well into this century.
The
success of Galileo, Kepler,
and Newton
to reinforce their explanations of the
cosmos
mathematics
in using
also contributed to the
mechanization of the world picture. Galileo (1623) succinctly captured
when he
the prestige of mathematics in the Renaissance
book of nature "cannot be understood comprehend the language and read the It is
unless one
which
letters in
written in the language of mathematics, and
triangles,
circles,
and other geometric
humanly impossible one wanders about
word of
Revolution
a
step
further,
and the
The
set
composed.
characters are
which
it
is
without these
shortly thereafter carried the
turning
mechanicism of the early period into a more based on a
it;
learns to
dark labyrinth."
The rapid development of physics Scientific
it is
its
figures without
to understand a single in a
said that the
first
the
more
general
specific physicalism,
of concrete laws about the workings of both the heavens
earth. 5
physicalist
movement had
the
enormous merit of refuting much
of the magical thinking that had generally characterized the preceding centuries. Its greatest achievement perhaps
explanation of physical reliance
phenomena and
on the supernatural
that
went too
energetic
far
in
some
new movement.
failure to explain
is
its
respects, this
Yet because of
its
of the
virtually
outgrowth into physiinevitable for
an
one-sidedness and
its
was
any of the phenomena and processes particular to
living organisms, physicalism
ment
much
was previously accepted by
everybody. If mechanicism, and particularly calism,
was providing a natural
eliminating
induced a rebellion. This countermove-
usually described under the umbrella term vitalism.
What
From
strictly
Meaning of "Life'
modern times
Galileo to
between
the
Is
mechanistic and more
Eventually, Cartesianism reached
de La Mettrie's
there has been a seesawing in biology
Vhomme
its
vitalistic
explanations of
life.
culmination in the publication of
machine (1749). Next followed a vigorous
flowering of vitalism, particularly in France and in Germany, but further triumphs of physics
and chemistry
in the mid-nineteenth It
was
so, since
no-
century inspired yet another physicalist resurgence in biology. largely confined to
where it
else
Germany, perhaps not surprisingly
did biology flourish in the nineteenth century to the extent
did in Germany.
THE FLOWERING OF PHYSICALISM
The nineteenth-century The
first
physicalist
one was a reaction
movement
by Johannes Muller (1801-1858), who
arrived in two waves.
moderate vitalism adopted
to the quite
in the 1830s switched
from
pure physiology to comparative anatomy, and of Justus von Liebig (1803-1873), well
known
for his incisive critiques
bring the reign of inductivism to an end.
former students
Reymond, Ernst
of Muller Briicke,
to
motion by four
Helmholtz,
Emil DuBois-
was
and Matthias Schleiden. The second wave,
which began around 1865, Julius Sachs,
—Hermann
which helped
set in
It
is
identified with the
names Carl Ludwig,
and Jacques Loeb. Undeniably, these
physicalists
made
important contributions to physiology. Helmholtz (along with Claude
Bernard in France) deprived "animal heat" of
and DuBois-Reymond
dispelled
ology by offering a physical
much
(electric)
its vitalistic
connotation,
of the mystery of nerve physiexplanation of nerve
activity.
Schleiden advanced the fields of botany and cytology through his insistence that plants consist entirely of cells
and
that
all
the highly
diverse structural elements of plants are cells or cell products. holtz,
DuBois-Reymond, and Ludwig were
Helm-
particularly outstanding in
the invention of ever-more sophisticated instruments to record the
measurements
precise
in
which they were
interested. This permitted
them,
among
force"
by showing that work could be translated into heat without
other achievements, to rule out the existence of a "vital
residue. Every history of physiology written since that time has docu-
mented
these
and other splendid accomplishments.
THIS Yet, the
IS
BIOLOGY
underlying philosophy of this physicalist school was quite
naive and could not help but provoke disdain a
background
among
biologists with
in natural history. In historical accounts of the
achievements of the physicalists, their naivete
when
came
it
many
to living
processes has frequently been ignored. But one cannot understand the vitalists'
one
is
passionate resistance to the claims of the physicalists unless
acquainted with the actual explanatory statements the physi-
calists offered. is
It
ironic that the physicalists attacked the vitalists for invoking
an unanalyzed
"vital force"
and
yet in their
own
explanations they
used such equally unanalyzed factors as "energy" and "movements."
The
definitions of
by the
lated
life
and the descriptions of
living processes
physicalists often consisted of utterly
formu-
vacuous statements.
For example, the physical chemist Wilhelm Ostwald defined a sea urchin as being, like any other piece of matter, "a spatially discrete cohesive
sum
of quantities of energy." For
acceptable vitalistic statement
replaced by the (1895),
many
equally undefined term
"energy."
whose work brought experimental embryology
stated that
development
is
an un-
physicalists,
became acceptable when
vital force
was
Wilhelm Roux into full flower,
"the production of diversity
owing
to the
unequal distribution of energy."
Even more fashionable than "energy" was the term "movement" to explain living processes, including developmental ones.
DuBois-Reymond (1872) wrote
"consists in explaining
movement of
all
atoms," that
and adaptational
that the understanding of nature
changes in the world as produced by the is,
"by reducing natural processes to the
mechanics of atoms ... By showing that the changes in bodies can be explained as a constant
sum
... of potential
all
natural
and
kinetic
energy, nothing in these changes remains to be further explained." His
contemporaries did not notice that these assertions were only empty words, without substantial evidence and with precious
little
explana-
tory value.
A
belief in the
importance of the movement of atoms was held not
only by the physicalists but even by some of their opponents. For
Rudolf Kolliker (1886)
—
a Swiss cytologist
who
recognized that the
chromosomes in the nucleus are involved in inheritance and that
What spermatozoa are
non
cells
Is
the
Meaning of "Life"?
—development was
a strictly physical
controlled by differences in growth processes: "It
postulate the occurrence in the nuclei of regular
is
and
phenome-
sufficient to
typical
move-
ments controlled by the structure of the idioplasm."
As exemplified
in statements
by the botanist Karl Wilhelm von
Nageli (1884), another favorite explanation of the mechanists was to
invoke "movements of the smallest parts" to explain "the mechanics
of organic
cytoplasm
life."
The
6
of a nucleus on the rest of the
effect
cell
—
—the
was seen by E. Strasburger, a leading botanist of the time,
propagation of molecular movements ... in a manner which
as "a
might be compared to the transmission of a nervous impulse." Thus it
did not involve the transport of material; this notion was, of course,
entirely
wrong. These physicalists never noticed that their statements
about energy and movement did not
Movements, unless
directed, are
really explain
random,
like
anything at
thing has to give direction to these movements, and this
what
their vitalist
all.
Brownian motion. Someis
exactly
opponents always emphasized.
The weakness of
a purely physicalist interpretation
obvious in explanations of
fertilization.
When
F.
was particularly
Miescher
(a student
of His and Ludwig) discovered nucleic acid in 1869, he thought that the function of the spermatozoon was the purely mechanical one of getting cell division going; as a consequence of his physicalist bias,
Miescher completely missed the significance of his
own
discovery.
Jacques Loeb claimed that the really crucial agents in fertilization were
not the nucleins of the spermatozoon but the ions.
One
is
almost
embarrassed when reading Loeb's statement that "Branchipus freshwater crustacean which,
if
The
knowledge. Even Sachs,
who
was not matched by
it is
their biological
studied so diligently the effects of various
on growth and
differentiation, never
given any thought to the question
why
seems to have
seedlings of different species
of plants raised under identical conditions of
would
that case
sophistication of the physicalists in chemistry,
particularly physical chemistry,
extrinsic factors
a
raised in concentrated salt solution,
becomes smaller and undergoes some other changes. In called Artemia."
is
light, water,
and food
give rise to entirely different species.
Perhaps the most uncompromising mechanistic school in modern
THIS
BIOLOGY
IS
biology was that of Entwicklungsmechanik, founded in the 1880s by
Wilhelm Roux. This school of embryology represented a
rebellious
reaction to the one-sidedness of the comparative embryologists,
who
were interested only in phylogenetic questions. Roux's associate, the embryologist Hans Driesch, was at
anything, even
first, if
more mecha-
but he eventually experienced a complete conversion from an
nistic,
extreme mechanist to an extreme separated a sea urchin
embryos of one
cell
embryo
vitalist.
This happened
at the two-cell stage into
when he
two separate
each and observed that these two embryos did
not develop into two half organisms, as his mechanistic theories
demanded, but were able
to
compensate appropriately and develop
somewhat smaller but otherwise
into
physicalistic explanations of life
who, however, were usually that organisms
and
perfect larvae.
and even absurdity of these purely
In due time, the vacuousness
became apparent
to
most
biologists,
adopt the agnostic position
satisfied to
living processes simply could not
be exhaustively
explained by reductionist physicalism.
The
The problem of explaining
"life"
Vitalists
was the concern of the
vitalists
the Scientific Revolution until well into the nineteenth century;
not really become the subject matter of rise
from it
did
scientific analysis until the
of biology after the 1820s. Descartes and his followers had been
unable to persuade most students of plants and animals that there
were no essential differences between
living
organisms and inanimate
matter. Yet after the rise of physicalism, these naturalists a
new look
at the
nature of
life
and attempted
(rather than metaphysical or theological)
had
to take
to advance scientific
arguments against Descartes's
machine theory of organisms. This requirement led
to the birth of
the vitalistic school of biology. 7
The
reactions of the vitalists to physicalist explanations were diver-
sified, since
what
it
the physicalist paradigm itself was composite, not only in
claimed (that living processes are mechanistic and can be
reduced to the laws of physics and chemistry) but also in what to take account of (the differences
between
living
it
failed
organisms and simple
What
Is the
Meaning of "Life'
matter, the existence of adaptive but
Kant's Zweckmassigkeit
—
in animals
planations). Each of these claims
much more complex and
Some
tures,
vital properties,
others
still
ment of
others
on the
properties
and evolutionary
and omissions was
or the other opponent of physicalism.
explained
plants,
criticized
vitalists
by one
focused on un-
holistic nature
on adaptedness or directedness
ex-
of living crea-
(as in the develop-
the fertilized egg).
All these
opposing arguments to the various aspects of physicalism
have traditionally been lumped together as vitalism. In some sense, this is
not altogether wrong, because
all
of the antiphysicalists defended
the life-specific properties of living organisms. Yet the label vitalist
conceals the heterogeneity of this group. 8 For instance, in
some
biologists (which Lenoir calls teleomechanists)
Germany
were willing to
explain physiological processes mechanically but insisted that this failed to
account for either adaptation or directed processes, such as
the development of the fertilized egg. These legitimate questions were raised again
and again by distinguished philosophers and until the
remarkably
little effect
as
biologists
end of the nineteenth century, but they had
from 1790
on the writings of the leading
physicalists
such
Ludwig, Sachs, or Loeb. Vitalism,
from
its
emergence in the seventeenth century, was decid-
edly an antimovement.
It
was a rebellion against the mechanistic
philosophy of the Scientific Revolution and against physicalism from Galileo to is
Newton.
It
passionately resisted the doctrine that the animal
nothing but a machine and that
all
manifestations of
life
exhaustively explained as matter in motion. But as decisive
vincing as the
vitalists
can be
and con-
were in their rejection of the Cartesian model,
they were equally indecisive and unconvincing in their
own
explana-
tory endeavors. There was great explanatory diversity but no cohesive theory. Life,
according to one group of
a special substance
vitalists,
was connected
either with
(which they called protoplasm) not found in
inanimate matter, or with a special state of matter (such as the colloidal state),
which,
it
was claimed, the physicochemical sciences were not
equipped to analyze. Another subset of
vitalists
held that there
is
a
special vital force (sometimes called Lebenskraft, Entelechie, or elan
THIS
10 vital) distinct
from the
IS
BIOLOGY
forces physicists deal with.
Some of
those
accepted the existence of such a force were also teleologists believed that
invoked
existed for
life
psychological
Lamarckism) to
had
physicalists
some
ultimate purpose. Other authors
mental
or
who who
forces
account for aspects
psycho-
(psychovitalism,
of living organisms that the
failed to explain.
Those who supported the existence of a views of the nature of this force.
had highly
vital force
From about
diverse
the middle of the
seventeenth century on, the vital agent was most frequently characterized as a fluid (not a liquid), in analogy to Newton's gravity
to caloric, phlogiston,
and other "imponderable
from a warm
invisible
and so was the heat
hence,
was not considered disturbing or unlikely that the
was
it
also invisible, even
J.
vital fluid,
though
German
much
as gravity was. 9
and subject
The concept of
a vital fluid
vital force.
Even such a reputable
vital force as
indispensable for
in the
and
1800-1840 period
others. In France,
erful,
it
is
vitalistic ideas,
The outstanding
histologist
F.
vitalism J.
was
Hunter,
J.
still
and
strong
C. Prichard,
where Cartesianism had been particularly pow-
Montpellier school (a group of
and the
and
in the writings of
not surprising that the
equally vigorous.
life.
the physiologists of the sixteenth, seventeenth,
eighteenth centuries had
this
real
be nevertheless very
Johannes Miiller accepted a
all
naturalist
extinction, creation, ca-
explaining the otherwise inexplicable manifestations of In England,
vital fluid
and spontaneous generation) considered
was eventually replaced by that of a scientist as
on
extensively
invisible, to
to scientific study,
eighteenth-century
late
Blumenbach (who wrote
tastrophes, mutability,
to a cold object;
though not necessarily something supernatural.
For instance, the influential F.
that flowed
and
Gravity was
fluids."
vitalists'
countermovement was
representatives in France were the
vitalistic
physicians
and
X. Bichat. Even Claude Bernard,
physiologists)
who
studied
such functional subjects as the nervous and digestive systems and considered himself an opponent of vitalism, actually supported a
number of
vitalistic
rather vitalistic in It
was
in
and reached
Germany its
most Larmarckians were
notions. Furthermore,
some of
their thinking.
that vitalism
greatest diversity.
had
its
most extensive flowering
Georg Ernst
Stahl, a late seventeenth-
What
the
Is
Meaning of "Life"?
century chemist and physician best of combustion, was the
first
great
known
1
for his phlogiston theory
opponent of the mechanists. Perhaps
he was more of an animist than a
vitalist,
but his ideas played a large
role in the teaching of the Montpellier school.
The next impetus
to the vitalistic
movement
in
Germany was
the
preformation versus epigenesis controversy, which dominated devel-
opmental biology
in the
second half of the eighteenth century. Pre-
formationists held that the parts of an adult exist in smaller
The
the very beginning of development.
form
at
epigenesists held that the
adult parts appear as products of development but are not present as parts in the beginning. In
had
some
to invoke
J.
He
it
by
epigenesis,
he
would convert the completely
egg into the adult of a particular
called this agent the vis essentialis.
Blumenbach
F.
fertilized
the embryologist Caspar
and replaced
causal agent that
unformed mass of the species.
when
1759,
Friedrich Wolff refuted preformation
rejected the vague vis essentialis
and proposed
instead that a specific formative force, nisus formativus, plays a decisive role not only in the
regeneration, irritability
development of the embryo but
and reproduction. He accepted
and
sensibility, as
Blumenbach was
also in growth,
other forces, such as
contributing to the maintenance of
quite pragmatic about these forces, considering
essentially as labels for
the causes.
still
observed processes of which he did not
They were black boxes
life.
them
know
for him, rather than metaphysical
principles.
The branch of German philosophy called Naturphilosophie, advanced by F. W. J. Schelling and his followers early in the nineteenth century,
was a decidedly metaphysical
vitalism, but the practical phi-
losophies of working biologists such as Wolff, Blumenbach, and eventually Miiller
were antiphysicalist rather than metaphysical. Mtiller has
been maligned unfair.
A
as
an unscientific metaphysician, but the accusation
collector of butterflies
had acquired the
and plants from
naturalist's habit
his
is
boyhood on, he
of looking at organisms
holistically.
This perception was lacking in his students, whose leanings were
more
toward mathematics and the physical sciences. Miiller realized that the slogan
"life is
a
movement of
particles"
was meaningless and without
explanatory value, and his alternative concept of Lebenskraft
(vital
THIS
12
though a
force),
was
failure,
IS
BIOLOGY
closer to the concept of a genetic
program
than the shallow physicalist explanations of his rebellious students. 10
Many
of the arguments put forth by the
were intended to
vitalists
explain specific characteristics of organisms which today are explained
by the genetic program. They advanced a number of perfectly valid refutations of the
machine theory but, owing
backward
to the
up with the
state
come
of biological explanation available at that time, were unable to
correct explanation of vital processes that were eventually
found during the twentieth century. Consequently, most of the argumentation of the
vitalists
was
From
negative.
the 1890s
argued, for example, that physicalism could not explain in
embryonic
phenomena,
and reproduction, and psychic
structures, regeneration
like
memory and
on Driesch
self- regulation
intelligence. Yet
it
is
remarkable
ever his
These
word
"Entelechie"
vitalists
is
replaced by the phrase "genetic program."
knew
not only
how
when-
often perfectly sensible sentences emerge in Driesch's writings
that there
was something missing
in the
mechanistic explanations but they also described in detail the nature
of the plain.
phenomena and
processes the mechanists were unable to ex-
11
many weaknesses and even contradictions in vitalist may seem surprising how widely vitalism was adopted and how long it prevailed. One reason, as we have seen, is that at that Given the
explanations,
it
time there was simply no other alternative to the reductionist machine theory of tion.
life,
which, to
Another reason
is
many
biologists,
that vitalism
was
clearly out of the ques-
was strongly supported by
several
other then-dominant ideologies, including the belief in a cosmic pur-
pose (teleology or finalism). In Germany, Immanuel Kant had a strong influence
on
an influence finalism
is
vitalism, particularly still
on the school of teleomechanism,
evident in Driesch's writings.
evident in the writings of most
A close
vitalists.
connection with
12
In part because of their teleological leanings, the vitalists strongly
opposed Darwin's selectionism. Darwin's theory of evolution denied the existence of any cosmic teleology
"mechanism"
for evolutionary
change
and substituted
—natural
in
selection:
its
place a
"We
see in
Darwin's discovery of natural selection in the struggle for existence the
most
decisive proof for the exclusive validity of mechanically
What
the
Is
Meaning of "Life"?
13
operating causations in the whole realm of biology, and the definitive demise of
teleological
all
and
organisms" (Haeckel 1866). Selectionism
we
see in this
vitalistic interpretations
made
of
vitalism superfluous in
the realm of adaptation.
Driesch was a rabid anti-Darwinian, as were other
vitalists,
but his
arguments against selection were consistently ridiculous and showed clearly that
he did not in the
by supplying denying any of a
mechanism
a
or
finalistic
new paradigm
this theory.
Darwinism,
for evolution while at the
same time
understand
least
view of
vitalistic
to explain
life,
became the foundation
"life."
THE DECLINE OF VITALISM
When
was
vitalism
first
Furthermore,
time
at that
it seemed to "What is life?"
proposed and widely adopted,
provide a reasonable answer to the nagging question,
was
it
a legitimate theoretical alternative
not just to the crude mechanicism of the Scientific Revolution but also to nineteenth-century physicalism. Vitalism seemingly explained
the manifestations of
machine theory of Yet considering
long a period it
collapsed.
its
The
last
its
First, vitalism
it is
vitalism
was
in biology
surprising
how
rapidly
A
considerable
number of
for
how
different factors
downfall.
was more and more often viewed
rather than a scientific concept.
the vitalists had
and
and completely
support of vitalism as a viable concept in biology
disappeared about 1930. contributed to
successfully than the simplistic
opponents.
how dominant
prevailed,
it
more
far
life
no method
It
was considered
to test
it.
would
unscientific because
By dogmatically
existence of a vital force, the vitalists often constitutive reductionism that
as a metaphysical
asserting the
impeded the pursuit of
a
elucidate the basic functions of
living organisms.
Second, the belief that organisms were constructed of a special substance quite different from inanimate matter gradually lost support.
That substance,
it
was believed through most of the nineteenth century,
was protoplasm, the was
13 Later cellular material outside the nucleus.
it
called cytoplasm (a term introduced by Kolliker). Because protoplasm seemed to have what was called "colloidal" properties, a flour-
THIS
14
IS
BIOLOGY Biochem-
ishing branch of chemistry developed: colloidal chemistry.
however, together with electron microscopy, eventually estab-
istry,
lished the true composition of cytoplasm
of
its
and elucidated the nature
various components: cellular organelles, membranes, and mac-
romolecules.
It
was found that there was no
special substance "pro-
and the word and concept disappeared from the
toplasm,"
The nature of the
literature.
biochemically,
and
biological
was likewise explained
colloidal state
colloidal chemistry ceased to exist.
Thus
for a separate category of living substance disappeared,
all
and
evidence
it
became
possible to explain the seemingly unique properties of living matter in terms of
macromolecules and their organization. The macromole-
cules, in turn, are
as
composed of the same atoms and small molecules
inanimate matter. Wohler's synthesis in the laboratory of the organic
substance urea in 1828 was the of inorganic Third,
all
compounds of the
first
proof of the
artificial
conversion
into an organic molecule.
vitalists'
attempts to demonstrate the existence of
ended
a nonmaterial vital force
in failure.
Once
physiological
and
developmental processes began to be explained in terms of physicochemical processes at the cellular and molecular tions left
no unexplained residue
pretation. Vitalism simply
Fourth,
new
that
level,
these explana-
would require a
vitalistic inter-
became superfluous.
biological concepts to explain the
phenomena that used Two advances in
to be cited as proof of vitalism were developed.
One was
the rise of genetics,
to the concept of the genetic
program. This made
particular were crucial for this change.
which ultimately led it
possible to explain
all
goal-directed living
phenomena,
principle, as teleonomic processes controlled
Another seemingly
teleological
phenomenon
at least in
by genetic programs. newly interpreted
to be
was Kant's Zweckmassigkeit. This reinterpretation was achieved by the second advance, Darwinism. Natural selection made adaptedness possible
by making use of the abundant
variability of living nature. Thus,
two major ideological underpinnings of vitalism tiselectionism
—were
destroyed. Genetics
providing valid interpretations of the vitalists
force.
—
teleology
and an-
and Darwinism succeeded
phenomena claimed by
not to be explicable except by invoking a
vital
in
the
substance or
What If
one were
Is
Meaning of "Life"?
the
15
to believe the writings of the physicalists, vitalism
was
nothing but an impediment to the growth of biology. Vitalism took
phenomena of
the
so
life,
and transferred them
it
was claimed, out of the realm of science
to the realm of metaphysics. This criticism
indeed justified for the writings of some of the more mystical
but
it
not
is
vitalists,
when raised against reputable scientists such as even more so, Muller, who specifically articulated
fair
Blumenbach and, all
is
the aspects of
that
life
were
left
unexplained by the physicalists.
That the explanation Muller adopted was a
failure
does not diminish
the merit of his having outlined the problems that
had
still
to
be
solved.
There are
many
similar situations in the history of science
where
unsuitable explanatory schemes were adopted for a clearly visualized
problem because the groundwork been
laid.
for the real explanation
Kant's explanation of evolution
probably
example.
It
necessary
movement
is
to
justifiable to
by teleology
had not is
a
yet
famous
conclude that vitalism was a
demonstrate the vacuity of a shallow physi-
calism in the explanation of rightly stated, the vitalists
Indeed, as Francois Jacob (1973) has
life.
were largely responsible for the recognition
of biology as an autonomous scientific discipline. Before turning to the organicist paradigm which replaced both vitalism
and physicalism, we might note
twentieth-century
among
phenomenon
physicists. Niels
special laws not
He thought
found
—the development of
Bohr was apparently the
in inanimate nature
A
to suggest that
might operate in organisms.
Erwin Schrodinger and other
vitalistic
and Eugene Wigner.
It
is
ideas of the physicists Walter
curious that a form of vitalism
some reputable minds of reputable
survived in the minds of
become
vitalistic beliefs
supported similar ideas. Francis Crick (1966) devoted a
whole book to refuting the Elsasser
first
of these laws as analogous to the laws of physics except
for their being restricted to organisms. physicists
in passing a rather peculiar
extinct in the
further irony, however,
is
that
many
physicists long after
it
had
biologists.
biologists in the post- 1925
period believed that the newly discovered principles of physics, such as the relativity theory, Bohr's
complementarity principle, quantum
mechanics, and Heisenberg's indeterminacy principle, would offer
new
THIS
16
BIOLOGY
IS
insight into biological processes. In fact, so far as
can judge, none
I
of these principles of physics applies to biology. In spite of Bohr's searching in biology for evidence of complementarity, and perate analogies to establish this, there really as that principle.
different
is
some
no such thing
The indeterminacy of Heisenberg
des-
in biology
something quite
is
from any kind of indeterminacy encountered
in biology.
Vitalism survived even longer in the writings of philosophers than
But so
in the writings of physicists.
among
after 1965. still
far as
I
know, there are no
the group of philosophers of biology
Nor do
I
know
who
mann)
who had
are
vitalistic
no longer
who
of a single reputable living biologist
supports straightforward vitalism. The few
biologists
vitalists
started publishing
late twentieth- century
leanings (A. Hardy,
Wright, A. Port-
S.
alive.
The Organicists
By about 1920
vitalism
seemed
Haldane (1931) stated quite
to be discredited.
The
physiologist
rightly that "biologists have almost
imously abandoned vitalism as an acknowledged
belief."
J.
S.
unan-
At the same
time, he also said that a purely mechanistic interpretation cannot
account for the coordination that particularly puzzled
is
so characteristic of
development. After showing the invalidity of both the the mechanistic approaches, Haldane stated that different theoretical basis of biology, based
the
The demise of
is
normal
vitalism,
mechanicism, resulted in a
digm accepted exhaustively
for
and
"we must find that
a all
so coordinated that
an adult organism."
rather than leading to the victory of
new
explanatory system. This
new
para-
that processes at the molecular level could be explained
by physicochemical mechanisms but that such mecha-
nisms played an increasingly smaller, levels
What
vitalistic
on the observation
phenomena concerned tend towards being
they express what
life.
Haldane was the orderly sequence of events during
if
not negligible, role
at
higher
of integration. There they are supplemented or replaced by
emerging characteristics of the organized systems. The unique characteristics
of living organisms are not due to their composition but
rather to their organization. This
mode
of thinking
is
now
usually
What referred to as organicism.
Meaning of "Life"?
Is the
1
stresses particularly the characteristics of
It
highly complex ordered systems and the historical nature of the
evolved genetic programs in organisms.
According to W.
who
E. Ritter,
"Wholes are so related to
coined the term organicism in 1919, 14
their parts that not only does the existence
of the whole depend on the orderly cooperation and interdependence of
its
over his
parts,
but the whole exercises a measure of determinative control
and Bailey 1928).
parts" (Ritter
its
own
holistic
view of organisms
the view here presented parts. Natural wholes,
posite, consisting of
is
J.
C.
Smuts (1926) explained
as follows:
"A whole according to
not simple, but composite and consists of
such as organisms, are
many
.
.
complex or com-
.
and interaction of
parts in active relation
one kind or another, and the parts may be themselves such as
cells in
an organism." His statements were
lesser wholes,
condensed by
later
other biologists into the concise statement that "a whole the
sum
of
its
parts."
interchangeably. Perhaps, at
as Niels
is
term, since
still
useful today. But holism
many inanimate
Bohr has pointed out
restricted
holism was more frequently used,
first,
adjective "holistic"
strictly biological
term "organicism"
systems are also
important feature of the
The objection of the
now
is
used more frequently.
It
not a
holistic,
more
encom-
program
is
an
new paradigm.
organicists
aspects of physicalism as to
its
was not so much
reductionism.
The
them
far
more was
reductionist explanations. For reductionists, the in principle resolved as
soon
to the mechanistic
physicalists referred
to their explanations as mechanistic explanations,
were, but what characterized
is
is
correctly. Therefore, in biology the
passes the recognition that the existence of a genetic
tion
more than
and organicism have been used
Since the 1920s, the terms holism
and the
is
15
which indeed they that they were also
problem of explana-
as the reduction to the smallest
components has been accomplished. They claim
that as soon as
one
has completed the inventory of these components and has determined the function of each of them,
it
should be an easy task to explain also
everything observed at the higher levels of organization.
The
organicists
demonstrated that
because explanatory reductionism
is
this claim
is
simply not true,
quite unable to explain charac-
THIS
18
IS
BIOLOGY
of organisms that emerge at higher levels of organization.
teristics
Curiously, even
most mechanists admitted the
instance,
(1961), for
conceded "that there are large sectors of biological study in
which physico-chemical explanations play no a
insufficiency of a purely
The philosopher Ernest Nagel
reductionist explanation.
number of outstanding
and
role at present,
biological theories have
that
been successfully
exploited which are not physico-chemical in character." Nagel tried to
by inserting the words
save reductionism
but
"at present,"
it
was
already rather evident that such purely biological concepts as territory, display, predator thwarting,
and so on could never be reduced
to the
terms of chemistry and physics without entirely losing their biological
meaning. 16
The pioneers of holism
(for
example, E.
Russell
S.
and
J.
S.
Haldane)
argued effectively against the reductionist approach and described
how
convincingly
well a holistic approach
fits
phenomena of
the
behavior and development. But they failed to explain the actual nature
when
of the holistic phenomena. They were unsuccessful
trying to
explain the nature of "the whole" or the integration of parts into the
whole.
Smuts, and other early proponents of holism were
Ritter,
somewhat metaphysical) in their explanations. some of Smuts's wordings had a rather teleological flavor. 17
equally vague (and
Indeed,
Alex Novikoff (1947), however, spelled out in considerable detail
why an
explanation of living organisms has to be holistic. level
become
parts
wholes are material
entities,
and integration
wholes on one
on
a higher one
.
results
.
.
"What
are
both parts and
from the
interac-
tion of parts as a consequence of their properties." Holism, since rejects reduction,
"does not regard living organisms as machines
it
made
of a multitude of discrete parts (physico-chemical units), removable like pistons
of an engine and capable of description without regard
to the system
from which they are removed." Owing
of the parts, a description of the isolated parts properties of the system as a whole.
It
is
to the interaction
fails
to convey the
the organization of these
parts that controls the entire system.
There
is
an integration of the parts
tissues, organs,
at
each
level,
from the
cell
to
organ systems, and whole organisms. This integration
What found
is
the
Is
biochemical
at the
whole organisms
Meaning of
"Life"?
1
the developmental level,
level, at
at the behavioral level.
18
They
organization.
is
in
no
All holists agree that
system can be exhaustively explained by the properties of
components. The basis of organicism
and
its
isolated
the fact that living beings have
are not just piles of characters or molecules, be-
cause their function depends entirely on their organization, their
mutual
interrelations, interactions,
and interdependencies.
EMERGENCE It is
of
now
clear that
two major
pillars in the
modern biology were missing
in
all
explanatory framework
the early presentations of
holism. One, the concept of the genetic program, was absent because it
had not
yet
of emergence
been developed. The other missing
—
that in a structured system,
new
pillar
was the concept
properties emerge at
higher levels of integration which could not have been predicted from a
knowledge of the lower-level components. This concept was absent
because either
had not been thought of or
it
it
had been dismissed
cepts of the genetic
antireductionist
and
program and of emergence, organicism became yet
remained mechanistic.
Jacob (1973) describes emergence this way: "At each relatively well-defined size
form
and almost
integration of sub-units is
may be
level, units
identical structure associate to
given the general
name
formed by assembling integrons of the
'integron'.
level
below
takes part in the construction of the integron of the level above."
integron has level
new
characteristics
and
capacities not present at
An
it;
it
Each
any lower
of integration; these can be said to have emerged. 19
The concept of emergence gan's
of
Each of these units formed by the
a unit of the level above.
integron
as
and metaphysical. By eventually incorporating the con-
unscientific
first
received prominence in Lloyd
book on emergent evolution
(1923). Darwinians
who
emergent evolution nevertheless had some misgivings about they were afraid that gentists
were also
delism; that
is,
it
was
antigradualistic. Indeed,
saltationists, particularly
some
it
Mor-
adopted because
early emer-
during the period of Men-
they believed that evolution proceeded in large, dis-
continuous leaps, or
saltations.
These misgivings have
now been
THIS
20 overcome, because
it is
now
IS
BIOLOGY
understood that the population (or spe-
rather than the gene or the individual,
cies),
one can have
different
the unit of evolution;
is
forms (phenetic discontinuities) within popu-
—
—
lations by recombination of existing DNA while a population as a whole must by necessity evolve gradually. A modern evolutionist would
more complex
say that the formation of a
emergence of a new higher
and level
level, is strictly a
system, representing the
matter of genetic variation
through natural
selection. Integrons evolve
selection,
and
every
at
they are adapted systems, because they contribute to the fitness
of an individual. This in no
way
conflicts
with the principles of
Darwinism.
To sum up, organicism
is
best characterized
by the dual
the importance of considering the organism as a whole,
same time the firm conviction
that this wholeness
is
and
level
at the
not to be con-
sidered something mysteriously closed to analysis but that
be studied and analyzed by choosing the right
belief in
it
should
of analysis. The
organicist does not reject analysis but insists that analysis should be
continued downward only to the lowest
level at
which
yields relevant
new information and new
insights.
integron, loses
some of
when
its
characteristics
this
approach
Every system, every
taken apart, and
many
of the important interactions of components of an organism do not
occur
And and
at the
physicochemical level but at a higher level of integration.
finally, it is
activities
the genetic
program which controls the development
of the organic integrons that emerge at each successively
higher level of integration.
The Distinguishing
Characteristics of Life
Today, whether one consults working biologists or philosophers of science, there
seems to be a consensus on the nature of
living organ-
—and
most
isms. At the molecular level, their functions
residue that
all
at the cellular level,
—of
obey the laws of physics and chemistry. There
would require autonomous
vitalist principles. Yet,
is
no
organ-
isms are fundamentally different from inert matter. They are hierarchically ordered systems with in
many emergent
properties never found
inanimate matter; and, most importantly, their
activities are
gov-
What
Is
the
Meaning of
21
"Life"?
erned by genetic programs containing historically acquired informa-
something absent
tion, again
As a This
is
result, living
in inanimate nature.
organisms represent a remarkable form of dualism.
not a dualism of body and soul, or body and mind, that
is,
a
dualism partly physical and partly metaphysical. The dualism of modern biology
is
consistently physicochemical,
that organisms possess both a genotype
and
and
type, consisting of nucleic acids, requires for
tionary explanations.
it
arises
from the
a phenotype. its
fact
The geno-
understanding evolu-
The phenotype, constructed on the
basis of the
information provided by the genotype, and consisting of proteins, lipids,
and other macromolecules, requires functional (proximate)
planations for
understanding. Such duality
its
is
unknown
ex-
in the
inanimate world. Explanations of the genotype and of the phenotype require different kinds of theories.
We may
tabulate
some of
the
phenomena
that are specific to living
beings:
Evolved programs. Organisms are the product of 3.8 billion years of evolution. All their characteristics reflect this history. Development,
behavior, trolled
and
all
other activities of living organisms are in part con-
by genetic (and somatic) programs that are the
result of the
genetic information accumulated throughout the history of
His-
life.
been an unbroken stream from the origin of
torically there has
and the simplest prokaryotes up
life
to gigantic trees, elephants, whales,
and humans. Chemical properties. Although ultimately living organisms consist of the
same atoms
for the
as
inanimate matter, the kinds of molecules responsible
development and function of
peptides, enzymes,
living
organisms
—
nucleic acids,
hormones, the components of membranes
—
are
macromolecules not found in inanimate nature. Organic chemistry
and biochemistry have shown organisms can be broken
down
that
all
substances found in living
into simpler inorganic molecules
and
can, at least in principle, be synthesized in the laboratory.
Regulatory mechanisms. Living systems are characterized by
all
sorts
of control and regulatory mechanisms, including multiple feedback
mechanisms, that maintain the steady
state
of a sort never found in inanimate nature.
of the system, mechanisms
THIS
22
IS
BIOLOGY
Organization. Living organisms are complex, ordered systems. This explains their capacity for regulation
and
for control of the interaction
of the genotype, as well as their developmental and evolutionary constraints.
Teleonomic systems. Living organisms are adapted systems, the result
of countless previous generations having been subjected to natural selection.
These systems are programmed for teleonomic (goal-di-
rected) activities
and behavioral
from embryonic development
to the physiological
of the adults.
activities
Limited order of magnitude. The size of living organisms occupies a limited range in the middle world, largest cells
whales and
and
cellular
trees.
Life cycle.
and evolutionary
Organisms,
life
from the smallest viruses
flexibility.
at least sexually
reproducing ones, go through
cycle beginning with a zygote (fertilized egg)
and passing
through various embryonic or larval stages until adulthood
The complexities of the in
some Open
life
cycle vary
from
species an alternation of sexual systems.
materials
to the
basic units of biological organization,
components, are very small, which gives organisms
great developmental
a definite
The
is
reached.
species to species, including
and asexual generations.
Living organisms continuously obtain energy and
from the external environment and eliminate the end-
products of metabolism. Being open systems, they are not subject to the limitations of the second law of thermodynamics.
These properties of living organisms give them a number of capacities
A A A A A
not present in inanimate systems: capacity for evolution
capacity for
self- replication
capacity for growth
and
differentiation via a genetic
capacity for metabolism (the binding
and
program
releasing of energy)
capacity for self-regulation, to keep the complex system in steady
state (homeostasis, feedback)
A
capacity (through perception and sense organs) for response to
stimuli
A
from the environment
capacity for change at two levels, that of the phenotype and that
of the genotype.
What
Is
the
Meaning of
23
"Life"?
All these characteristics of living organisms distinguish gorically
them
from inanimate systems. The gradual recognition of
catethis
uniqueness and separateness of the living world has resulted in the
branch of science called biology, and has led to a recognition of the
autonomy of
this science, as
we
will see in
Chapter
2.
CHAPTER TWO What
Biology
encompasses
living organisms.
the
life
sciences
—
physical sciences,
all
Science?
Is
of the disciplines devoted to the study of
Sometimes these
disciplines are referred to as
a useful term that distinguishes biology
whose focus
is
sciences, political science, military science,
specialties,
we
and such putative
Why
ence and creationist science. themselves "science"?
that distinguish
and
in addition to these
frequently encounter Marxist science, Western
science, feminist science,
call
social
and many others comprise
yet other systematized bodies of knowledge,
academic
from the
on the inanimate world. The
What
do
all
sciences as Christian sci-
of these various disciplines
are the characteristics of a true science
from other systems of thought? Does biology have
it
these features? It
should be easy to answer these basic questions, one would think.
Doesn't everybody evident
when one
know what
science
is?
That
this is
not the case
but also the enormous professional literature dealing with tion.
1
T
is
studies not just the offerings of the popular press this
ques-
H. Huxley, a friend of Charles Darwin and a popularizer of
Darwin's theories, defined science as "nothing but trained and organized
common
sense." Alas, this
is
not true.
corrected by science. For instance, earth
is
flat
and
Common
common
sense
sense
tells
is
frequently
us that the
that the sun circles the earth. In every branch of
science there have been
commonsense opinions
that have subsequently
What
been proven wrong. One might go so activity consists
A number
25
Science?
Is
far as to say that scientific
of either confirming or refuting
common One
encountered in agreeing upon a definition of science. that science
sense.
of factors account for the difficulties philosophers have
is
both an
activity (that
which
of them
is
do) and a body
scientists
of knowledge (that which scientists know). Most philosophers today, in their definition of science,
emphasize the ongoing
entists: exploration, explanation,
and
testing.
activity
of
sci-
But other philosophers
tend to define science as a growing body of knowledge, "the organization
and
principles."
classification of
knowledge on the basis of explanatory
2
Emphasis on the collection of data and the accumulation of knowledge
is
a residue of the early days of the Scientific Revolution,
when
induction was the preferred method of science. There was a widespread
misconception
among
inductionists that a pile of facts
would not only
permit generalizations but almost automatically produce as if
by spontaneous combustion.
ally agree that facts
Actually, philosophers today gener-
all.
concern. As far back as 1861, Charles Darwin wrote,
anyone should not see that
To be
if it is
sure,
to be of
any
all
all
not a
new it is
is
observation must be for or against
service."
most authors who use the word "knowledge" mean
to include not just facts but also less
"Are not
"How odd
observations theory-laden?" they have asked. Even this
some view
theories,
alone do not explain, and they even argue a great
deal over the question whether pure facts exist at
that
new
an interpretation of the
confusing, however, to use the
word "understanding"
meaning. Hence the definition, "The aim of science
is
to
it
facts; it is
for this
advance our
Some philosophers would add "by solving Some have gone further and have said, "The
understanding of nature." scientific
problems." 3
aims of science are to understand, predict, and control." Yet there are
many branches of science in which prediction plays a very subordinate role, and in many of the nonapplied sciences the question of control never comes up.
Another reason
on
for the difficulties philosophers have
a definition of science
is
that the endeavors
had
which we
in agreeing call
science
THIS
26
IS
BIOLOGY
have changed continually over the centuries. For example, natural
—the study of nature the purpose of understanding God's legitimate branch of science intentions —was considered about
theology
for
a
150 years ago. As a
him
until
1859 some of Darwin's
result, in
critics
chided
for including in his account of the origin of species such
"unscientific" factor as chance, while ignoring as the
hand of God
in the design of
in the twentieth century scientists'
all
we have witnessed
notion of
how
clearly
and
saw
small. Yet
a complete reversal in
view of random phenomena: in both the
the physical sciences there has been a change nistic
what they
creatures great
an
life
from a
sciences
strictly
and
determi-
the natural world works to a conception that
is
largely probabilistic.
To take another example of how science
is
gradually changing, the
strong empiricism of the Scientific Revolution led to a heavy emphasis
on the discovery of new to the
facts,
while curiously
little
important role that the development of
reference was
new
made
concepts plays in
the advancement of science. Today, concepts such as competition,
common as laws
descent, territory,
and
and altruism
are as significant in biology
discoveries are in the physical sciences,
importance was strangely ignored until quite reflected, for
Even
if
and
yet their
recently. This neglect
is
example, in the provisions established for Nobel Prizes.
there were a Nobel Prize in biology (which there
Darwin could not have been awarded the concept of natural selection
is
not),
a prize for the development of
— —because
surely the greatest scientific achieve-
ment of
the nineteenth century
attitude
which favors discoveries over concepts continues into the
it
was not
a discovery. This
present day, but to a lesser extent than in Darwin's time.
No one knows what future
may
bring.
The
other changes in our image of science the
best
one can do under the circumstances
is
to
try to present an outline of the kind of science that prevails in our
time, at the
end of the twentieth century.
The Origins of Modern Science
Modern
science began with the Scientific Revolution, that remarkable
achievement of the
human
intellect characterized
by the names Coper-
What nicus, Galileo, Kepler,
many
27
Science?
Newton, Descartes, and Leibniz. At that time
of the basic principles of the scientific method were developed
which ence
Is
still
is,
What one considers scisome respects Aristotle's
largely characterize science today.
of course, a matter of opinion. In
biology was also science, but
lacked the methodological rigor and
it
comprehensiveness of the science of biology as
it
developed from 1830
to the 1860s.
The
scientific disciplines that
gave
the prevailing concept of
rise to
science during the Scientific Revolution were mathematics, mechanics,
and astronomy. original
large a contribution scholastic logic
framework of
determined;
The
How
it
major
ideals of this
role in Descartes's thinking.
new, rational science were objectivity, empiricism,
inductivism, and an endeavor to eliminate ics
—
that
is,
to the
has not yet been fully
this physicalist science
certainly played a
made
all
remnants of metaphys-
phenomena
magical or superstitious explanations of
that
were not grounded in the physical world. Virtually
all
architects of the Scientific Revolution
remained devout
Christians, however; and, not surprisingly, the kind of science they
created was very
much
a branch of the Christian faith. In this view,
the world was created by
God and
thus
it
could not be chaotic.
It
was
governed by His laws, which, because they were God's laws, were universal. to
An
be sound
explanation of a if it
phenomenon
or process was considered
was consistent with one of these
laws.
With the
workings of the cosmos thus ultimately clear-cut and absolute,
it
should be possible eventually to prove and predict everything. The task of God's science, then,
was
to find these universal laws, to find
the ultimate truth of everything as test their
As
truth by
far as
way of
embodied
predictions
in these laws,
and
to
and experiments.
mechanics was concerned, matters conformed rather well
to this ideal. Planets orbited the
sun and
planes in a predictable manner. Perhaps
balls rolled it
history that mechanics, being the simplest of to develop a set of coherent laws
down
inclined
was not an accident of all
sciences,
and methods. But
was the
first
as the other
branches of physics developed, exceptions to the universality and
determinacy of mechanics were found again and again, requiring various modifications. Indeed, in everyday
life
the laws of mechanics
THIS
28
are often so completely thwarted
determinacy appears to be
BIOLOGY
IS
by random
much
For instance, so
totally absent.
bulence usually accompanies the
(stochastic) processes that
movement of
air
tur-
masses and water
masses that the laws of mechanics do not permit long-term predictions in either
meteorology or oceanography.
The mechanists'
worked even
recipe for the natural world
There was no room in the
for the biological sciences.
less well
scientific
method
of the mechanists for the reconstruction of historical sequences, as
occurred in the evolution of causations that impossible. tificness"
make
When
evolutionary biology was examined for
This was particularly true
it
in
it
so valuable
were the only inferior
taste to call one's colleagues
came
to
bad
be called
term was for centuries pejoratively attached
our basic knowledge in
The younger
a science
is,
the
all
sciences
more
is
based upon descrip-
descriptive
it
has to be to lay
foundation. Even today, most publications in molecular
biology are essentially descriptive. is
test.
to the favorite investigative
to be treated as if
good
"scien-
sciences.
life
factual
came
its
flunked the
The experiment was
these other nonexperimental sciences
Actually,
a
it
was not
descriptive sciences. This
tion.
came
it
it
method. Any other method was considered
But since
scientists,
when
the experiment.
in this field that eventually
valid scientific
to the
nor for the pluralism of answers and
according to the criteria of mechanics,
method of mechanics:
science.
life,
prediction of the future in the biological sciences
"observational," for
all
What
description
is
is
meant by
really
"descriptive"
based on observation, whether
by the naked eye or other sense organs, by simple microscopes or telescopes, or
by means of highly sophisticated instrumentation. Even
during the Scientific Revolution, observation (rather than experimentation) played a decisive role in the
advance of science. The cosmo-
logical generalizations of Copernicus, Kepler,
Newton were based on observation
rather than
ments. Today, the underlying theories in astrophysics, cosmology, planetary science,
quently as a result of
new
and
for the
most part
on laboratory
fields
experi-
such as astronomy,
and geology change
observations that have
little if
fre-
anything to
do with experimentation.
One might put
it
another way and say that the findings described
What by Galileo and
his followers
29
Science?
Is
came from
the experiments of nature they
were able to observe. The eclipses and occlusions of planets and are natural experiments, as are earthquakes, volcanic eruptions,
magnetic
craters,
and erosion
shifts,
meteor
events. In evolutionary biology,
and South America
the joining of North
stars
in the Pliocene
through the
Isthmus of Panama, which resulted in a massive faunal interchange of the two continents, volcanic islands
one such experiment; the colonization of
is
and archipelagos such
as Krakatau, the Galapagos,
much
recolonization of
of the northern hemisphere owing to the
Pleistocene glaciations, are other natural experiments. in the observational sciences
is
discovered, critically evaluated,
ments if
in fields
where
and
and subsequent
the Hawaiian Islands, not to mention the defaunation
due
to the genius of
Much progress those who have
and compared such natural
a laboratory
experiment
experi-
highly impractical,
is
not impossible.
A
revolution in thought though the Scientific Revolution was
—by
abandoning superstition, magic, and the dogmas of medieval theologians
—
it
nevertheless did not include a revolt against allegiance to
the Christian religion,
and
this ideological bias
had adverse conse-
quences for biology. The answer to the most basic problems in the study of living organisms depends on whether or not one invokes the
hand of God. This
is
particularly true for
subject matter of interest to creationists)
all
questions of origin (the
and design
(the subject matter
of interest to natural theologians). The acceptance of a universe containing nothing but God,
human
fine for the physical sciences
souls, matter,
of the day, but
it
and motion worked worked
against the
advance of biology. 4
As a
result,
biology was basically dormant until the nineteenth and
twentieth centuries. Although a considerable
amount of factual knowl-
edge in natural history, anatomy, and physiology was accumulated
during the seventeenth and eighteenth centuries, the world of that time
true for
was considered
life at
to belong to the realm of medicine; this
anatomy and physiology, indeed, even
for botany,
which
was
largely
consisted of the identification of medicinally important plants. To be sure, there as a
was
hobby or
also it
some
natural history, but either
was pursued
it
was practiced
in the service of natural theology. In
THIS
30 retrospect,
good
evident that
it is
BIOLOGY
IS
some of this
early natural history
was very
science; but, not being recognized as such at that time,
it
did
not contribute to the philosophy of science. acceptance of mechanics as the exemplar of science led
Finally, the
to the belief that organisms are in
From was
this
no way
different
from
inert matter.
followed logically the conclusion that the goal of science
to reduce
of biology to the laws of chemistry and physics. In
all
due time developments
in biology
made
this position
untenable (see
The eventual overthrow of mechanicism and
Chapter
1).
vitalism,
and the acceptance
its
in the twentieth century of the
nemesis,
paradigm
of organicism have had a profound impact on the position of biology
among
the sciences
—an
impact not yet
fully appreciated
by many
philosophers of science.
7s
Biology an
Autonomous
Science?
After the middle of the twentieth century, one could discern three
very different views on the position of biology in the sciences. According to one extreme, biology
because
it
to be excluded
is
from science altogether
lacks the universality, the law-structuredness,
quantitative nature of a "true science"
(meaning
to the other extreme, biology not only has
all
and
strictly
physics). According
the necessary attributes
of a genuine science but differs from physics in important respects so that
it is
to
be ranked as an autonomous science, equivalent to physics.
Between these two extremes
is
the view that biology should be ac-
corded the status of a "provincial" science, because
and because
its
The question "Is
the
two
lacks universality
findings can ultimately be reduced to the laws of
and chemistry.
physics
in
it
"Is
biology an autonomous science?" can be rephrased
parts: "Is biology, like physics
biology a science exactly first
question,
we might
and chemistry,
like physics
a science?"
and
and chemistry?" To answer
consult John Moore's eight criteria for
determining whether a certain activity qualifies as science. According to
Moore
(1993): (1)
A
science
must be based on data
collected in
the field or laboratory by observation or experiment, without invoking
supernatural factors. (2) Data must be collected to answer questions,
What and observations must be made
31
Science?
Is
to strengthen or refute conjectures.
methods must be employed
(3) Objective
possible bias. (4) Hypotheses
minimize any
in order to
must be consistent with the observations
and compatible with the general conceptual framework. potheses must be tested, and,
if
possible,
(5) All hy-
competing hypotheses must
be developed, and their degree of validity (problem-solving capacity)
must be compared.
(6)
must be
Generalizations
universally valid
within the domain of the particular science. Unique events must be explicable without invoking supernatural factors. (7) In order to elimi-
nate the possibility of error, a fact or discovery
only
if
(repeatedly) confirmed
characterized by the steady
by other
must be
fully
accepted
investigators. (8) Science
improvement of
is
by the
scientific theories,
replacement of faulty or incomplete theories, and by the solution of previously puzzling problems.
Judging by these
criteria,
should be considered,
But
is
most people would conclude
like physics
and chemistry,
that biology
a legitimate science.
biology a provincial science, and therefore not on a par with
the physical sciences?
introduced,
it
When
was used
as
the term "provincial science" was
first
an antonym to "universal," meaning that
biology dealt with specific and localized objects about which one could
not propose universal laws. The laws of physics,
as
on
know
earth. Biology,
by
contrast,
is
was
it
limitations of time or space; they are as valid in the
have no
said,
Andromeda
provincial because
galaxy
that
all life
we
of has existed only on the earth, and only for 3.8 billion of the
more
10 billion or
years since the Big Bang.
This argument was convincingly refuted by Ronald
who showed
that
none of the fundamental
Munson
(1975),
laws, theories, or principles
of biology are either implicitly or explicitly restricted in their scope or range of application to a certain region of space or time. There a great deal of uniqueness in the sorts of generalizations is
also unique,
currents.
As
world of
"What
all
but we can establish laws and theories about ocean
for the
is
but one can make
about unique phenomena. Each ocean current
argument that the
earth deprives biological principles of ask,
life,
is
restriction of all
known
life
to the
we must known to exist
universality, here
universal'?" Since inanimate matter
is
outside the earth, any science dealing with inanimate matter
must be
THIS
32
BIOLOGY
IS
applicable extraterrestrially in order to be universal. Life, so
been demonstrated for the earth only; yet
far,
laws and principles
its
has (like
those of inanimate matter) are universal because they are valid on the
known domain
earth, the
of
existence.
its
can see no reason for
I
withholding the designation "universal" from a principle that
domain
for the entire
More
when
often,
meant
for
which
biology
is
it is
true
is
applicable.
described as a "provincial" science,
and chemistry, and
that
ultimately the findings of biology can be reduced to chemical
and
what
is
is
that
physical theories.
By
it is
a subset of physics
an advocate of the autonomy of biology
contrast,
might argue in the following way:
many aspects
and, moreover,
life
of physics). In this sense physics
it
is
no reason
was the
make
it
first
which
an exemplar merely because
well-organized science. That historical fact does not its
younger
it is
It
would be
A
sibling, biology.
unity
accepted that science contains
separate provinces, one of which
biology.
is
as provincial a science as biology.
is
any more universal than
number of
any other science outside
(or to
to consider physics as
of science cannot be achieved until a
attributes of living organisms
of the physical world studied by physicists
are not relevant to the study of
There
Many
cannot be reduced to physicochemical laws,
that interest biologists
futile to try to
is
physics, another of
"reduce" biology, one
provincial science, to physics, another provincial science, or vice versa. 5
Many,
ment
if
not most, of the promoters of the unity of science move-
in the late nineteenth
and
were phi-
early twentieth centuries
losophers rather than scientists and had
little
awareness of the het-
—which
erogeneity of the sciences. This applies to the physical sciences
include elementary particle physics, solid state physics,
quantum me-
chanics, classical mechanics, relativity theory, electromagnetism, not to
mention geophysics, astrophysics, oceanography, geology, and oth-
ers
—and
sciences.
increases exponentially
The
when we
impossibility of reducing
common denominator
all
many
think of the
these
domains
life
to a single
has been demonstrated again and again during
the past 70 years.
So to
reiterate: Yes,
But biology
autonomous
is
biology
not a science
science
on
is,
like physics
like physics
and chemistry, a
and chemistry;
a par with the equally
it is
science.
rather an
autonomous
physical
What
certain
not
if
sciences, in spite of their
all
amount of autonomy, did not
the tasks of the philosopher of biology features are
share is
also in principles
would define
in
unique features and a
common
to establish
which biology shares with the other
methodology but features
33
Science?
one would not be able to speak of science
sciences. Nevertheless,
the singular
Is
One
features.
what the
of
common
sciences, not only in
and concepts. And these
common
a unified science.
The Concerns of Science It
has been said that the scientist searches for truth, but
who
The world and
are not scientists claim the same.
many
all
that
people is
in
it
are the sphere of interest not only of scientists but also of theologians,
philosophers, poets, and politicians.
between
HOW
their concerns
How can
and those of the
one make a demarcation
scientist?
SCIENCE DIFFERS FROM THEOLOGY
The demarcation between
science
and theology
is
perhaps
because scientists do not invoke the supernatural to explain
and they do not
natural world works,
understand natural
it.
When
phenomena,
early
humans
rely
on divine
tried to
easiest,
how
revelation to
give explanations for
particularly for disasters, invariably they invoked
supernatural beings and forces, and even today divine revelation legitimate a source of truth for
Virtually
scientists
all
the
known
to
many
me
pious Christians as
is
is
as
science.
personally have religion in the
do not invoke supernatural
best sense of this word, but scientists
causations or divine revelation.
Another feature of science that distinguishes
it
from theology
is its
openness. Religions are characterized by their relative inviolability; in revealed religions, a difference in the interpretation of even a single
word
new
in the revealed
may
founding document
religion. This contrasts dramatically
active field of science,
any theory. refuted,
Indeed,
New
and it
is
at
where one finds
conjectures are all
made
lead to the origin of a
with the situation in any
different versions of almost
continuously, earlier ones are
times considerable intellectual diversity
by a Darwinian process of variation and
exists.
selection in
THIS
34
and
the formation
Chapter
BIOLOGY
IS
testing of hypotheses that science advances (see
5).
Despite the openness of science to
be said that virtually a set of
what we might
of the natural world. is
all scientists
facts
—somewhat
One
and hypotheses, like
theologians
of these axiomatic assumptions
human
called the principle of objectivity
(as
it
must
—bring
with them to the study
call "first principles"
a real world, independent of
commonsense
new
that there
is
perceptions. This might be
opposed
to
or
subjectivity)
realism (see Chapter 3). This principle does not
mean
that individual scientists are always "objective" or even that objectivity
among human beings is possible in any absolute sense. What it does mean is that an objective world exists outside of the influence of subjective human perception. Most scientists though not all believe
—
—
in this axiom.
Second, scientists assume that this world
some way, and
structured in
not chaotic but
is
that most, if not
all,
structure will yield to the tools of scientific investigation. tool used in
new
all scientific
activity
Every
testing.
is
is
aspects of this
new
A
primary
and every
fact
explanation must be tested again and again, preferably by different
investigators using different
methods
(see
Chapters 3 and
4).
Every
confirmation strengthens the probability of the "truth" of a fact or
and every
explanation, ability that
teristic features
to
abandon
proposed
is
falsification or refutation strengthens the
an opposing theory of science
is
this
is
One
correct.
prob-
of the most charac-
openness to challenge. The willingness
a currently accepted belief
when
a new, better
an important demarcation between science and
one
is
religious
dogma.
The method used on whether one
is
to test for "truth" in science will vary
testing a fact or
a continent of Atlantis
when no such
depending
an explanation. The existence of
between Europe and America became doubtful
continent was discovered during the
first
few Atlantic
crossings in the period of discoveries during the late fifteenth
and
early sixteenth centuries. After complete oceanographic surveys of the
Atlantic
Ocean were made and, even more
graphs from
satellites
were taken in
conclusively proved that
convincingly, after photo-
this century, the
no such continent
exists.
new
evidence
Often, in science,
What
Is
35
Science?
the absolute truth of a fact can be established.
much
The absolute
and usually
truth of
much
an explanation or theory
is
longer, to gain acceptance.
The "theory" of evolution through natural
selection
was not
and even
years;
do not
believe
harder,
by
fully accepted as valid
some
today, in
among
100
who
religious sects, there are people
is
and causal
historical
phenomena in the material universe, and they domain of legitimate scientific study everything
all
include within the
known
scientists for over
it.
Third, most scientists assume that there
continuity
takes
happen
to exist or to
But they do not go
in this universe.
beyond the material world. Theologians may
also
be interested in the
physical world, but in addition they usually believe in a metaphysical
or supernatural realm inhabited by souls, this
of
heaven or nirvana
is
believers after death.
all
spirits, angels,
or gods, and
often believed to be the future resting place
Such supernatural constructions are beyond
the scope of science.
HOW
SCIENCE DIFFERS FROM PHILOSOPHY
The demarcation between
science
and philosophy
is
more
determine than that between science and theology, and
difficult to
this led to
tension between scientists and philosophers throughout most of the
nineteenth century. Philosophy and science were a single endeavor at the time of the Greeks.
The beginning of
a separation of the
place in the Scientific Revolution; but right
William Whewell, and William Herschel, to the
Ernst
up
to
two took
Immanuel Kant,
many people who
contributed
advance of science were also philosophers. Later authors,
Mach
like
or Hans Driesch, started out as scientists and then switched
to philosophy. Is
there, perhaps,
no demarcation
at all
between science and phi-
losophy? The search for and discovery of facts
of science; but elsewhere there
is
is
surely the business
a considerable area of overlap.
Theorizing, generalizing, and establishing a conceptual framework for their field
indeed,
is
it is
considered by most scientists to be part of their job; this that
of science have
felt
makes the
real scientist. Yet
that theorizing
domain of philosophy. For
many
philosophers
and concept formation
are the
better or for worse, in recent decades
most
THIS
36 of this endeavor has
IS
now been
BIOLOGY
taken over by
scientists,
and some
basic
concepts developed by biologists have subsequently been taken up by philosophers and are
To
now
also concepts of philosophy.
replace their former chief concern, philosophers of science have
specialized in elucidating the principles
are formed.
which
They search
scientists
whereby theories or concepts
for the rules that specify the operations
by
answer the "What?" "How?" and "Why?" questions
they encounter. The major domain of philosophy relating to science
now
is
the testing of "the logic of justification"
of explanation (see Chapter
At
3).
and the methodology
worst, this type of philosophy
its
tends to degenerate into logic-chopping and semantic quibbling. At its
best,
it
has forced scientists into responsibility and precision.
Although philosophers of science often logical rules are
seem
state that their
merely descriptive and not prescriptive,
to consider
it
their task to
no
doing. Scientists usually pay
determine what
many
Perhaps the greatest a
result, the so-called
it
be
attention to this normative advice but
may
differ
from case
most quickly
to case.
of the philosophy of science, until only
failing
few years ago, was that
As a
of them
scientists should
rather choose that approach which (they hope) will lead to results; these approaches
methodo-
took physics as the exemplar of science.
philosophy of science was nothing but a
philosophy of the physical sciences. This has changed under the influence of the younger philosophers, the philosophy of biology.
many
of
whom
The intimate connection
between philosophy and the
life
sciences
is
specialize in
that exists today
evident from the
many
articles
published in the journal Biology and Philosophy. Through the
efforts
of these young philosophers, the concepts and methods used
in the biological sciences
have
now become
important components of
the philosophy of science.
This biology.
most desirable development
is
a
It
should be the aim of every
his views of nature so that they
make
for
both philosophy and
scientist to eventually generalize
a contribution to the philosophy
of science. As long as the philosophy of science was restricted to the laws and methods of physics,
it
was not possible
such a contribution. Fortunately,
this
is
for biologists to
no longer the
make
case.
The incorporation of biology has modified many of
the tenets of
What
As we
the philosophy of science. rejection of strict
Is
37
Science?
will see in
Chapters 3 and
4,
the
determinism and of reliance on universal laws, the
acceptance of merely probabilistic prediction and of historical narratives,
the
acknowledgment of the important
role of concepts in theory
formation, the recognition of the population concept and of the role
many
of unique individuals, and
other aspects of biological thought
have affected the philosophy of science fundamentally. abilism
now dominant,
With prob-
aspects of logical analysis that are based
all
become highly vulnerable. The complete certainty which, following Descartes, had been the ideal of the philosophers of science seems less and less important as a goal. on
typological assumptions have
HOW As
SCIENCE DIFFERS FROM THE HUMANITIES the demarcation between science
far as
and the humanities
is
concerned, the tendency of writers in the past to ignore the heterogeneity of both fields has led to difference
misconceptions. There
more
is
—both of which
are branches of science
—than between evolutionary biology (one of
and history (one of the humanities). Literary
the sciences)
has virtually nothing in
common
of the humanities and even
When
many
between physics and evolutionary biology
C.
P.
Snow wrote
less
his
criticism
with most of the other disciplines
with science.
Two Cultures
in 1959,
what he
actually
described was the gap between physics and the humanities. Like others
of that era, he naively assumed that physics could stand for science as a whole.
The gap between
pointed out,
is
physics and the humanities, as he rightly
indeed virtually unbridgeable. There
pathway from physics
to ethics, culture,
mind,
is
simply no
free will,
and other
humanistic concerns. The absence in physics of these important topics contributed to the alienation of scientists and humanists that decried. Yet, life
all
these concerns have substantial relationships with the
sciences.
Similarly,
when
E.
M. Carr
with "the sciences," he found tory,
(2)
Snow
he
said, deals exclusively
(1961), a humanist, contrasted history
five respects in
which they
differ:
is
His-
with the unique, science with the general.
History teaches no lessons. (3) History, unlike science,
to predict. (4) History
( 1 )
necessarily subjective, while science
is is
unable objec-
THIS
38 tive.
And
(5)
BIOLOGY
IS
upon
history, unlike science, touches
and morality. What Carr
was that these differences are
failed to see
valid only for the physical sciences
issues of religion
and
for
much
of functional biology.
and 5 apply as well to evolutionary biology
However, statements
1, 3,
as to history, and, as
Carr admits, some of these claims (statement
2,
for instance) are not strictly true even for history. In other words, the
sharp break between the "sciences" and the "nonsciences" does not exist,
once biology
admitted into the realm of science. 6
is
Quite often the estrangement between science and the humanities is
assigned to the failure of scientists to appreciate the
ment"
as they
go about their research. Yet not
be shouldered by
scientists.
A
all
"human
ele-
of the blame should
rudimentary knowledge of certain
findings of science, particularly of evolutionary biology, behavioral science,
human
development, and physical anthropology,
most work
sable for
in the humanities. Yet,
too
all
is
indispen-
many humanists
have failed to acquire such a knowledge and display an embarrassing ignorance of these subjects in their writings.
Many
understanding of science with the statement, mathematics." Actually, there
little
is
"I
excuse their poor
have no ability in
mathematics in those parts of
biology with which the humanists should most familiarize themselves.
For instance, there
is
not a single mathematical formula in Darwin's
Origin of Species or in
understanding of arable
human
component of
how
Growth of
Biological
Thought (1982).
An
biology should be a necessary and insep-
studies in the humanities. Psychology, formerly
with the humanities,
classified Yet,
my
is
now
considered a biological science.
can one write anything in the humanities, whether in history
or literature, without having a considerable understanding of
human
behavior?
Snow correctly emphasized this point. There is a deplorable ignoamong most people of even the simplest facts of science. For
rance
example, writer after writer eye is
is
still
states that
the result of a series of accidents.
that the writer has
selection,
which
is
he cannot believe that the
What
this
statement reveals
no understanding of the workings of natural
an antichance
—
rather than an accidental
—
process.
Evolutionary change occurs because certain characteristics of individuals are better
suited to the current environmental circumstances of a
What species than are others,
39
Science?
Is
and these more adaptive
features
become
concentrated in later generations through differential rates of survival
and reproduction
—
in other words,
selection
—
the primary
through
selection.
Darwin knew very
plays a part in evolution, as
Chance
well,
certainly
but natural
mechanism of evolutionary change
—
is
not an
accidental process.
An
ignorance of the findings of biology
whenever humanists are forced
is
particularly
to confront such political
damaging
problems
as
global overpopulation, the spread of infectious diseases, the depletion
of nonrenewable resources, deleterious climatic changes, increased agricultural requirements worldwide, the destruction of natural habi-
the proliferation of criminal behavior, or the failures of our
tats,
educational system.
None
of these problems can be satisfactorily ad-
dressed without taking into account the findings of science, particularly biology,
and
yet too often politicians proceed in ignorance.
The Objectives of It is
Research
what
good
for?
rather different answers to this question have been given.
The
often asked
Two
why do we do
Scientific
insatiable
curiosity of
science? Or,
human
beings,
understanding of the world they interest in science
by most
and the
live in, is
desire
for a better
the primary reason for an
based on the conviction that
scientists. It is
none of the philosophical or purely
science
is
ideological theories of the world
can compete in the long run with the understanding of the world
produced by
science.
To make a contribution is
to this better understanding of the
a source of great satisfaction to a scientist; indeed
for exhilaration.
The emphasis
is
times plays a role, but the joy
often is
on
discovery,
it is
world
an occasion
where luck some-
perhaps even greater when one
succeeds in the difficult intellectual achievement of developing a
new
concept, a concept that can integrate a mass of previously disparate facts,
or one that
is
more
successful as the basis of scientific theories. is
the incessant need for dull
data-collecting, the disappointment (if not
embarrassment) of invalid
Offsetting the joy of research, of course,
THIS
40
BIOLOGY
IS
theories, the recalcitrance of certain research subjects,
and
a multitude
of other frustrations. 7
An
entirely different objective
the world,
its
particularly health,
and
and
forces
by applied
is
to use science as a
scientists (including those in
it is
ills
to control
is
when
that
is
held
medicine, public
and the average
agriculture), engineers, politicians,
But what some politicians and voters forget the
means
resources. This second objective
it
citizen.
comes
to
of pollution, urbanization, famine, or the population explosion,
not sufficient to fight the symptoms.
One
does not cure malaria
with aspirin, and one cannot fight social and economic
Our way of
going into the causes.
ills
without
dealing with racial discrimination,
crime, drug addiction, homelessness, and similar problems, and the success
extent
we
will
have in eliminating them, will depend to a considerable
on our understanding of
their biological roots.
These two objectives of science
improvements
world
in the
—
—
are not entirely different domains, be-
cause even applied science, particularly policy
is
based, relies
largely motivated
puzzling
in
all
basic science. In
by the simple
phenomena
In both basic
on
and making
satisfying curiosity
science
most
on which public
cases scientists are
desire for a better understanding of
our world.
and applied
science,
any discussion of the objectives
of scientific research always entails questions of values. To what extent
can our society afford certain big science projects,
like the
supercon-
ducting supercollider or the space station, considering the narrowness of the results
we can
expect to obtain? To what extent should one
consider certain experiments, particularly with keys, apes) as unethical? Is
embryonic materials might lead to unethical
ments
in
human
mammals
there a danger that
psychology or
clinical
(dogs,
mon-
work with human
practices?
What
experi-
medicine might be harmful to
the experimental subjects?
As long
as the physical sciences
were dominant, science was usually
considered to be value-free. During the student rebellion of the 1960s,
some groups who resented
this
arrogance promoted the slogan
with value-free science." Since the genetics
and evolutionary biology,
rise it
"Down
of biology, and particularly of
has
become
clear that scientific
What
Is
and theories have an impact on
findings
41
Science?
values,
though
what extent
to
Some of
unclear (see Chapter 12).
science can generate values
is
Darwin's opponents, such as
Adam
Sedgwick, accused Darwinism of
destroying moral values. Even today, creationists fight evolutionary
biology because they are convinced that
why
human
in this century clearly
genetics.
And
the reason
sociobiology was attacked so viciously in the 1970s was that
seemed its
values from the science of
its
undermines the values of
The eugenics movement
Christian theology.
derived
it
promote
to
it
certain political values incompatible with those of
opponents. Almost
major religious and
all
political ideologies
up-
hold values that are claimed to derive from science, and almost
all
uphold other values that are incompatible with certain
ideologies
findings of science.
Paul Feyerabend (1970) has ventured to suggest (as have other
contemporary writers) that a world without science "would be more pleasant than the world true.
There would be
we
live in today."
pollution
less
I
am
not sure that
and pollution-caused
this
is
cancer, less
crowding, and fewer of the adverse by-products of mass society. But it
would
35-40
also
years,
be a world with high infant mortality, a
life
against severe winter cold.
It is all
too easy to forget the vast benefits
of science (including agricultural and medical science)
complaining about evils
span of only
no way of escaping summer heat and protecting oneself
deleterious side effects.
its
Most of
when one
these so-called
of science and technology could be eliminated; scientists
what should be done, but
and
legislation
its
the politicians and
My own is
ment of
knowledge must be translated into
much
of the voting public.
view of the contributions of science
who had
the greatest,
the
know
enforcement, and this has so far been resisted by
that of Karl Popper,
science
their
is
human
is
more
in line with
"Next to music and
this to say:
art,
most beautiful and most enlightening achieve-
spirit. I
abhor the
at
present so noisy intellectual
fashion that tries to denigrate science, and
I
admire beyond anything
the marvelous results achieved in our time by the
and biochemists and made over our beautiful earth."
available
work of
through medicine
biologists
to sufferers
all
THIS
42
BIOLOGY
IS
SCIENCE AND THE SCIENTIST
One
frequently hears that science can
that,
but of course
A
thing.
it is
scientists
who
her best
scientist at his or
do
this,
or science cannot do
either can or cannot is
do some-
dedicated, highly motivated,
scrupulously honest, generous, and cooperative. Scientists are only
human, however, and do not always
live
up
to these professional ideals.
or financial considerations that arise from outside
Political, theological,
of science should not, but often do, affect scientific judgment. Scientists
learn
from
own
have their
specific traditions
and
values,
which they
a mentor, older colleague, or other role model. This includes
not only the avoidance of dishonesty or fraud but also giving appropriate credit to competitors if they have priority in
making
A
priority claims, but
good
same time he
at the
and
will
more
is
a discovery.
usually anxious to please the leaders in his field
sometimes follow
their authority even
cheating or manufacturing of data the
is
end of a
when he should be
is
whose
is
discovered sooner or later
career; for that reason alone, fraud
option in science. Inconsistency there
own
defend his
critical.
Any and
scientist will tenaciously
probably no
scientist
is
is
not a viable
perhaps a more widespread
who
entirely escapes
it.
failing;
Charles
Lyell,
Principles of Geology influenced Darwin's thinking, preached
uniformitarianism, but
it
struck even
nonuniformitarian was his
Darwin himself was lation thinking
when
own
some of his contemporaries how new species.
theory of the origin of
also capable of inconsistency;
he applied popu-
explaining adaptation by natural selection, but
he employed typological language in some of his discussions of speciation.
Lamarck proclaimed loudly
that he
was a
strict
mechanist,
endeavoring to explain everything in terms of mechanical causes and forces,
and
yet his discussion of inevitable perfection through evolu-
tionary change strikes the
modern
reader as a subconscious adherence
None of Darwin's adhermore forcefully than A. R. Wallace, but to man, Wallace "chickened out."
to a (nonmechanistic) perfecting principle.
ents stressed natural selection
when it came Some flaws
to applying
it
and hypotheses of
in the findings
induced by wishful thinking.
chromosomes
in the
human
When
scientists are clearly
an early investigator found 48
species, this discovery
was subsequently
What
Is
43
Science?
confirmed by numerous other investigators because that
number
they expected to find. The correct until three different
new
is
the
number
(46) was not established
techniques had been introduced.
Recognizing that error and inconsistency are widespread in science, Karl Popper in 1981 proposed a set of professional ethics for the scientist.
The
principle
first
Second,
all
scientists at all times
One should
to be unavoidable.
found, and learn from them;
others,
who
them when others
call
own
he has
an unforgivable
from one's
when
a scientist
depends on such factors
made and what most
it
seem them when
errors; they
sin to conceal errors.
must be supplemented one must acknowledge
errors,
And
finally,
one always must
calling attention to those of others.
is
his prestige
among
as
how many
important discoveries
his peers. This
his contribution to the conceptual structure
of his discipline has been. so important to
important,
is
attention to them. errors
The major reward of prestige
commit
can help discover and correct ones errors.
In order to be able to learn
be aware of one's
authority; scientific
search for errors, analyze
it is
Third, while such self-criticism
by criticism by
no
is
beyond what any one person can master, including
inferences go well specialists.
that there
is
Why
scientists?
are priority
and recognition by peers
Why do a few scientists try to deniHow is a scientist rewarded for
grate their peers (or competitors)?
achievements?
and the
What
is
the relationship of scientists to one another,
relation of scientists to the rest of society? All such questions
have been asked by researchers in the sociology of science, most importantly by Robert Merton,
who
virtually
founded the
As Merton has shown, much modern science
is
discipline.
done by research
groups, and alliances are often formed under the flag of certain
dogmas. 8 But despite a certain degree of dissension
in science,
impresses outsiders most
among
the remarkable consensus
is
what
scientists
in the last half of the twentieth century.
This consensus
is
of science. English
and
particularly well reflected in the internationality
is
rapidly
becoming the lingua franca of science, Germany, and France,
in certain countries, such as Scandinavia,
prominent
scientific journals
primarily English-language country, even an American
have adopted English names and publish articles.
A
scientist traveling to
another
visiting Russia or Japan, feels quite at
home
THIS
44
when
company of
in the
articles are
BIOLOGY
IS
colleagues
from those countries. Numerous
published these days in scientific journals in which the
coauthors are from different countries.
One hundred
years ago scien-
papers and books very often had a distinctly national
tific
becoming
this is
All scientists
rarer
who
all
flavor,
but
the time.
reach worthwhile goals tend to be ambitious and
hard-working. There
is
no such thing
Many work
as a 9-to-5 scientist.
15 to 17 hours a day, at least during certain periods of their career. Yet
most of them have broad
number of
raphies; quite a
interests, as
scientists are
evident from their biog-
is
amateur musicians, for
stance. In other respects scientists are as variable a lot as any
group.
Some
prolific,
Some
are extroverts, others shy introverts.
in-
human
are exceedingly
while others concentrate on the production of a few major
books or papers.
I
do not think
that there
is
a definite
temperament
or personality that one could identify as the typical scientist. Traditionally
cation or
one became a
by growing up
more common
as a
biologist either
young
for a youngster to
through a medical edu-
naturalist.
become
At present
museum
on
There are also thousands of young bird watchers, some of professional biologists (as
dient
a fascination with the
is
life
television,
(often the dinosaur hall), or an inspiring teacher.
become stays
much
excited about the
sciences through the media, particularly nature films visits to a
it is
I
did).
whom
The most important
wonders of
with most biologists for their entire
living creatures. life.
They never
will
ingre-
And
this
lose the
excitement of scientific discovery, whether empirical or theoretical, nor
new organisms. And so much in biology has a direct bearing on one's own circumstances and personal values. Being a biologist does not mean having a job; it means choosing a way of life. 9
the love of chasing after
new
ideas,
new
insights,
CHAPTER THREE How
Does Science Explain the Natural World?
The
earliest
attempts to explain the natural world invoked the
supernatural.
From
the most primitive
animism
to the great
monotheistic religions, anything that was puzzling and seemingly inexplicable
was attributed
The ancient
to the activities of spirits or gods.
Greeks initiated a different approach. They attempted to explain the
phenomena of
the world through natural forces. Philosophy, which
developed in the sixth century bc, became occupied increasingly with the task of explaining the world
and attempting
the ideal of "knowing" should be.
The Greeks based
on observation and considerable role. science that
The
we
to determine
what
their explanations
thinking, though metaphysics always played a
From
these early beginnings, the philosophy of
recognize today gradually developed.
third kind of explanatory endeavor
was
which arose
science,
during the Scientific Revolution. Supernatural explanations, philosophy,
and science
are perhaps best considered not three consecutive
stages but rather three
complementary approaches
knowing. The history of
human thought shows
to the
problem of
that these differing
endeavors evolved from one another without sharp breaks. For example,
many
of the great philosophers, even Kant, included
explanatory schemes. Prior to Darwin,
God was
explanatory factor by most biologists. After the
losophy continued to
exist
God
in their
also accepted as rise
an
of science, phi-
and prosper; what changed was
its
objective.
As science gradually became emancipated from philosophy, philoso-
THIS
46
BIOLOGY
IS
phers began to stand back reflectively from the work of scientists and to focus
on
analysis of scientists' activities.
The ultimate aim of world
The
—on
that point
science
both
is
to
advance our understanding of the
and philosophers of science
scientists
about that which
scientist raises questions
understood and attempts to answer them. The
is
not
first
known
answer
is
agree.
or not called a
conjecture or hypothesis and serves as a tentative explanation. But
what
really
When
an explanation?
is
a puzzling
phenomenon
countered in the everyday world, most frequently terms of what
known
is
moon must
of the
or what
be due to the
rational.
is
is
en-
"explained" in
For example, an eclipse
shadow
earth's
it is
falling
on the moon,
or the fauna and flora of the Galapagos Islands must have gotten there
by overwater
dispersal,
because these volcanic islands obviously never
had any connection with the South American continent. But merely having a rational explanation that the answer
is
is
not enough.
One must
also
make
sure
true, or at least as close to the truth as available
knowledge permits. This goal of the
scientist
is
precisely also the
objective of the philosopher of science.
What
has been controversial
the Greeks to
modern times
is
among how an
world should be constructed and
philosophers from the age of explanation about the natural
tested. Scores
of philosophers have
endeavored to formulate principles by which our understanding of the world could be advanced (or, as
be found).
Hume,
Among
was often
said,
how
truth could
those usually listed are Descartes, Leibniz, Locke,
Kant, Herschel, Whewell, Mill, Jevons, Mach, Russell, and
Popper. Curiously, the list,
it
name
of Darwin
is
rarely included in such a
even though he was clearly one of the greatest philosophers of
times.
1
In
fact, to
all
modern philosophy of biology
a large extent the
was founded by Darwin.
Were
these philosophers of science simply attempting to describe
faithfully the
methods of the
scientist, as
philosopher, or was their endeavor to
seen through the eyes of a
tell scientists
how
they should
construct their explanations and tests so that their findings constitute truly "good" science? 2 If the latter
had
little effect. I
is
do not know of a
the case,
I
fear that so far
single biologist
it
has
whose theorizing
How Does
47
Science Explain the Natural World?
was much affected by the norms proposed by philosophers of Scientists usually
science.
much
go about their research without paying
The one exception
tion to the fine points of methodology.
atten-
is
Karl
Popper's insistence on falsification (see below), which was widely
accepted by biologists in principle, though
worked out
rarely
it
in
practice.
Why way
are philosophers of science
that scientists construct
science has
still,
Of
today, so worried about the
explanations? After
test their
had an almost unbroken
Scientific Revolution.
series
of successes ever since the
it
is
is
soon refuted in the contest among
theories. Cases of a refutation of a
major
are remarkably rare. Overall, the reliability of the
science
all,
course, occasionally an erroneous theory
temporarily adopted, but
competing
and
scientific
theory
major claims of
unquestionable. Giere (1988) suggests that the heritage of
is
Cartesian skepticism during the Scientific Revolution
is
responsible
for the continuing doubts of the philosophers.
The media, with
new
their daily sensational
and challenges
discoveries
announcements of major
to existing theories, tend to mislead
no
the nonscientist into believing that science can yield "truth" about anything.
many of them
as
much
certainty or
To the contrary, the basic theories of
as
science,
50 or even 150 years old, are being confirmed
again and again. Even in a field as controversial as evolutionary biology,
framework established by Darwin
the basic conceptual
in 1859 has
turned out to be remarkably robust. All attempts in the years to invalidate
last
130
Darwinism (and there have been hundreds) have
been unsuccessful, and the same
is
true for
most other
areas of
biology.
Nevertheless fallible
it
must be acknowledged
and our reasoning even more
task of philosophy to scrutinize the
knowledge
—indeed,
formulate and
today. 3
It
is
the
so. It is therefore a legitimate
methods by which
to advise scientists as to the
test theories.
the problem of what
mology.
that our sense organs are
scientists obtain
most
The branch of philosophy
we know and how we know
reliable
way
to
that deals with
it is
called episte-
main preoccupation of the philosophy of
science
THIS
48
A Not
IS
BIOLOGY
Brief History of the Philosophy of Science
surprisingly, the rise of interest in epistemology coincided with,
or was caused by, the Scientific Revolution. With astronomy and
mechanics the most active sciences
and
that time, observation
at
mathematics were highly regarded, and
Sir Francis
Bacon (through
induction) and Descartes (through geometry) became their apostles.
Through Bacon, induction became the for
two
any prior hypotheses or preconceived expecta-
tions without having
When
induction was fashionable in England in the early nine-
teenth century,
Darwin proclaimed
was a true follower of
that he
Bacon, while in reality what he adopted was a more or 4
deductive approach (see below). Later Darwin saying that
if
one did believe
in this
into a gravel pit, count the pebbles,
Liebig (1863)
was one of the
first
less
made fun of induction,
and describe the prominent
incisive critique
itself
as well
go
colors."
scientists to repudiate
Baconian induction, arguing convincingly that no
ganum. Induction by
hypothetico-
method, "one might
scientist
or could ever, follow the methods described in Bacon's
had
ever,
Novum
Or-
cannot generate new theories. Liebig's
helped to bring the reign of inductionism to an end, 5
and from then on
it
was considered derogatory
inductionist (or "stamp-collector"). cal
method
by simply recording, measuring, and describing observa-
his theories
tions.
established scientific
centuries. According to this philosophy, the scientist develops
Many
of the
to call
critics
someone an
of this empiri-
approach, however, overlooked the fact that the data underlying
any
scientific
endeavor remained as indispensable as ever; what was
to be criticized facts
were used
was not the
collecting of facts per se but
in theory formation. In
biology) which rely essential scientific
some
on the construction of
method today
is
how
these
sciences (particularly in historical narratives, the
basically inductive.
Later in the nineteenth century, particularly under the influence of
work of Frege (1884) and of other logicians and mathematicians, logic became a dominant influence in the philosophy of mathematics the
and
physics. This
was particularly illuminating where mathematically
formulated, universal laws played an important role, as in the physical sciences.
.
It
was
less
appropriate for biology, where pluralism, prob-
How Does and purely
abilism,
49
Science Explain the Natural World?
qualitative as well as historical
phenomena abound,
while strictly universal laws are virtually absent. As a consequence, a
philosophy of science developed that was tailored for the situation in the physical sciences but was to a large extent inappropriate for biology.
VERIFICATION AND FALSIFICATION In this century, the philosophy that long dominated Anglo-American science
was
logical empiricism,
which grew out of the Vienna Circle
of logical positivists (Reichenbach, Schlick, Carnap, Feigl) in the 1920s
and 1930s. Logical empiricism was
work of
built
on
three foundations: (1) the
number of twentieth -century mathematicians and
a
empiricism of David
(2) the classical
Mill to Russell
and Mach; and
Hume
logicians;
as transmitted
through
(3) the physical sciences, particularly
the classical physical sciences as they were understood prior to relativity
and quantum mechanics.
The approach positivists
and
to scientific confirmation
was the
verification
endorsed by the logical
traditional hypothetico- deductive
(H-D) method,
through repeated testing was considered to be the best
criterion for the goodness of a theory. If testing confirms a theory,
they would say that the theory had been verified. Verification greatly strengthens theories and sometimes leads to constructive modification.
One must not assume,
however, that verification "proves" unambigu-
ously that a given theory to a verification of
is
Popper agreed with the 4
true.
These methods have sometimes led
what ultimately turned out
to be a
wrong
theory. 6
logical positivists that a theory will
be
regarded the more satisfactory the greater the severity of the inde-
pendent
tests
it
has survived," but he insisted that falsification was the
only way to finally eliminate an invalid theory. test, it It is
has been
not
like
falsified. Falsification is
proving that 2
+
2
is
not
If
the theory
fails
a
not a simple matter, however. 5. It is
particularly ill-suited
for the testing of probabilistic theories,
which include most theories
The occurrence of exceptions
to a probabilistic theory does
in biology.
not necessarily constitute
falsification.
And
in fields such as evolution-
ary biology, in which historical narratives must be constructed to explain certain observations, to decisively falsify
it is
often very difficult,
if
not impossible,
an invalid theory. The categorical statement that a
THIS
50
IS
abandonment of
single falsification requires the
true for theories based
but
is
on
BIOLOGY a theory
might be
the universal laws of the physical sciences,
often not true for theories in evolutionary biology. 7
NEW MODELS
OF SCIENTIFIC EXPLANATION
The modern philosophy of
science began in 1948 in a paper written
by Carl Hempel and Paul Oppenheim and elaborated by Hempel 1965. In these essays
Hempel proposed
new model of
a
in
scientific
explanation, which he called the deductive-nomological (D-N) model.
This schema had
its
heyday
in the 1950s
and 60s and was
also
known
as the "received view."
The tific
idea behind deductive-nomological explanation
explanation
is
a deductive
argument
scribing the to-be-explained event
is
in
which
is this:
A scien-
a statement de-
deduced from one or more true
universal laws in conjunction with statements of particular facts (cor-
respondence
According to
rules).
this view, a scientific
theory
an
is
"axiomatic deductive system," whose premises are based on a law.
The
original
D-N model was
very typological and deterministic, and
was soon modified to cope with probabilistic or year
new papers
Each
or books were published suggesting ways and means
to correct actual or
were proposed
statistical laws.
seeming flaws in the received view. Some of these
as genuinely
new
theories, although ultimately they
were derived from the Hempel model.
One
of these modifications became
known
as the
semantic concep-
tion of theory structure. 8 For Beatty (1981, 1987), a proponent of this
new model,
a theory
is
the definition of a system, and applications of
a theory are instantiations of theory.
Such applications may or may
not be spatio-temporally restricted. Theories are neither general nor
permanent, and are therefore compatible with plural solutions and with evolutionary change. This fact that there are so
generalizations.
The
last
point
is
important in view of the
few spatio-temporally unrestricted biological
ability
of the semantic view to faithfully represent
evolutionary theorizing has induced Beatty,
Thompson,
Lloyd,
and
other philosophers to adopt the semantic view. 9
Although ceived view,
this it
theory escapes several of the weaknesses of the
faces
two
difficulties as far as the
re-
working biologist
is
How Does concerned. The
first
Science Explain the Natural World?
is
that
when one
51
asks for a definition of this
approach, one gets exceedingly different versions from different semanticists.
The second stumbling block
is this:
How
can the semantic
view be applied by the working biologist? What the philosopher offers is
a description of theories that have
But such a description
how
new
to develop
is
its
considered
appreciated logical rules
least so
question perhaps
this
is
more or
less obsolete).
What is
When
does
a semantic theory?
much
why
do
I
acceptance in
view (which
has been increasingly
not a matter of simple
that rationality has to be construed in broader terms
than either deductive or inductive logic
offer.
Each of the various explanatory schemes of its
scientist.
the biologist
the reason
is
distinct advantages over the received
that the assessment of a theory
and
tell
seems to me.
it
the semantic approach has achieved
biology, in spite of
today
At
measure up to the specifications of
a theory fail to
feel that
not sufficiently normative to
theories.
Lack of an answer to not
is
been developed by the
this
century has had
vogue for ten years or more and then has been replaced by an
amended
version or an entirely
new scheme.
10
The 1980s have been
particularly active in the philosophy of science, but this activity has
on how
best to
construct and test a scientific explanation. In his recent survey,
Salmon
not led to any consensus
(1988) writes: "It seems to
among
me
schools of thought at present the mechanists
—and
the philosophers
that there are at least three powerful
—the pragmatists, the
deductivists,
and
that they are not likely to reach substantial
agreement in the near future."
Discovery and Justification
Most
scientists
that science
of
new
nature,
is
and philosophers of science seem a two-step process.
facts, irregularities,
The
first
step involves the discovery
exceptions, or seeming contradictions in
and the formation of conjectures, hypotheses, or theories
explain them.
The second
step deals with justification
by which such theories are
tested
and
to
—the procedures
validated.
For most philosophers, the pathway to a
making
basically to agree
new theory
begins with
a conjecture or hypothesis to solve a puzzle; this hypothesis
THIS
52 is
IS
BIOLOGY
then subjected to rigorous testing. But the working scientist
even
earlier.
starts
During the discovery phase he engages
in a great deal of
When
he encounters an
simple observation and description of
facts.
among the facts available to him, induces him to ask a question, and that
unexplained irregularity or anomaly the discovery of this puzzle
question leads eventually to a conjecture or hypothesis.
Every scientist occasionally has "hunches" about the meaning or explanation of this or that observation. But
hunches that moves
testing of these
—how one goes about hy—has become preoccupation of philosophers
of "truth." Justification potheses, or theories
only the successful
it is
scientific discovery to the stage
testing conjectures,
a
of science, in large part because justification
amenable to
is
logical
Discovery only rarely follows "logically" from the preceding
analysis.
situation,
and therefore most philosophers have
traditionally not con-
sidered aspects of discovery to be their business. Rather, they usually ascribe discovery to chance, to psychological factors, to the Zeitgeist,
worse, to prevailing socioeconomic conditions.
or,
Popper (1968), for instance, idea occurs to a
man
knowledge. The
latter is
...
is
asserted,
pothesis
scientist, the is
it
happens that a new
not concerned with questions of
only with questions of justification or
working
"How
irrelevant to the logical analysis of scientific
method one
fact
validity." Yet in the eyes
new theory
.
.
but
of the
uses to refute an erroneous hy-
usually of trivial interest, while the discovery of a
or the formulation of a
.
is
new
fact
frequently of the most funda-
mental significance. 11
INTERNAL AND EXTERNAL FACTORS IN THEORY FORMATION
No
scientist lives in a
economic, and
vacuum. He
lives in
social, as well as scientific,
an
intellectual, spiritual,
environment.
What impact
do these influences have on the nature of the theories he develops? Intellectual historians tend to
ments within science
—
hold internal factors
—
as primarily responsible for
that
new
is,
develop-
theories
and
concepts. Social historians, by contrast, search for external factors that
is,
components of the socioeconomic
sociologists have fact that
milieu.
been remarkably unsuccessful
On
the whole the
in their endeavor. 12
Charles Darwin and Alfred Russel Wallace,
The
who came from
How Does such
socioeconomic
different
totally
pendently
backgrounds,
irrelevance of external factors. Indeed,
know
I
illustrates the
of no evidence what-
on the development
of a specific biological theory 13 The reverse, however, true: scientific or pseudoscientific theories political activists to
Among economic latter it
promote
and the
is
sometimes
have frequently been used
their particular agenda. 14
the external factors, one factors
inde-
arrived
same theory of evolution
at virtually the
soever of the influence of a socioeconomic factor
by
53
Science Explain the Natural World?
must
distinguish between socio-
Zeitgeist, or intellectual milieu.
While the
seems to play only a small role in the proposal of new theories,
seems to play a very large
role in the resistance to intellectual shifts
that are in conflict with established beliefs. This
was the reason why
Darwin's theory of natural selection encountered such massive
world of Cuvier or Agassiz,
tance; in the conceptual to
accommodate
it
resis-
was impossible
a theory of evolution. 15
TESTING
How does is
valid?
new hypothesis The philosopher who wants
a scientist go about determining whether his
By
subjecting
to determine the
undertaken by
it
scientists
taken by philosophers,
than working
to certain tests.
goodness of a theory does the same, but the testing is
sometimes quite different from that under-
who
scientists do.
16
much more
rigidly
of rules will be applied
differs,
tend to apply rules
Which
set
however, depending on the school to which a given philosopher belongs.
For example, philosophers of science since the days of the logical positivists
have placed great
The
predictions. it
stress
better a theory
on is,
permits. Prediction in this context
that such
and such
the capacity of theories to
the
more
means
make
correct the predictions
logical prediction:
a constellation of factors exists,
provided
one can expect
such and such an outcome to occur. This use of prediction in logic is
different
from the everyday use of the word
means being
"prediction,"
able to foretell the future. Foretelling the future
logical prediction.
Many
is
which
chrono-
authors (including myself in the past) have
confused the two kinds of prediction. Science, often even the physical sciences,
is
only rarely able to
make
chronological predictions. For
THIS
54 instance, nothing
as unpredictable as the future course of evolution.
is
The dinosaurs were the most at the
BIOLOGY
IS
successful
group of
terrestrial vertebrates
beginning of the Cretaceous; that they would be extinct by the
end of that
owing
era,
an asteroid with the earth,
to the collision of
was unpredictable.
The
biologist, like the physicist, also applies the test of prediction
and searches failure
for exceptions,
of a prediction to
regularities rarely
but he
come
is less
disturbed by the occasional
knows
true because he
that biological
have the universality of physical laws. The usefulness
of prediction in testing biological theories
is
Some
highly variable.
theories, particularly in functional biology, have high predictive value,
whereas others are controlled by so complex a consistent prediction
owing
probabilistic,
nomena and owing tiplicity
set
of factors that
unachievable. Predictions in biology are at best
is
most
to the great variability of
biological phe-
to the occurrence of contingencies
and the mul-
of interacting factors that affect the course of events. For the
biologist,
it
prediction;
not so important that his theory survive the
is it
is
more important
that his theory
is
of
test
useful in solving
problems. 17 In the functional sciences, theories are best tested with the help of
experiments. But in the sciences in which experiments are not possible
and prediction and
this
is
is
of limited value in testing a particular hypothesis
usually the case in historical sciences
tions have to be
—
additional observa-
made. For instance, the theory of
claims that the animals and plants of are descendants of those
more
common
from older geological periods.
and
descent
if
one found
fossil
It
would
elephants
giraffes in the early Cretaceous. Likewise, dinosaurs originated in
the Mesozoic,
descent
and therefore
if fossil
facts.
it
would contradict the theory of common
dinosaurs were found in the Paleozoic.
Another way of of
common
and
Giraffes
elephants, for instance, are descendants of early Tertiary taxa. discredit the theory of
descent
recent geological periods
testing a theory
For instance,
if
on the
is
to use
an entirely different
basis of morphological evidence
I
constructed a phylogenetic tree of a certain group of organisms,
set
have I
can
use one of several types of molecular (biochemical) evidence to construct
an independent phylogeny, and then
test
the degree of congru-
How ence of the two
two
Does Science Explain the Natural World?
trees.
Whenever
there
55
between the
a disagreement
is
independent evidence must be used as a further
trees, additional
check. In biogeography, theories about former land connections or
about dispersal capacities of different taxa can be tested in various ways, and biogeographic theories can thus be refuted or strengthened. In order to prove that the dinosaurs truly at the
end of the Cretaceous, additional
became completely
extinct
early Tertiary deposits in
remote areas of the world have to be examined. The nature of the observations and tests required are different from one problem to the next, tests
though
agreement about what
specialists are usually largely in
or observations should be considered valid in a given
The Practicing
None of
many
the
field.
Biologist
philosophies of science proposed in this century
based as they were on laws and logic
—has been
well suited for theory
development in evolutionary biology. This realization led Popper in 1974 to conclude, not that the
but "that Darwinism
is
scientific
not a testable
method prescribed scientific
is
flawed,
theory but a meta-
physical research program." Other philosophers, also with a back-
ground
in physics or mathematics,
recanted a few years after
later,
made
similar statements.
and the philosophy of
Popper
logical empiricism,
having been dominant for some 40 years, was abandoned owing
to the critiques of
Kuhn, Lakatos,
Beatty,
Laudan, Feyerabend, and
other philosophers. In the long run, what logical empiricism
managed
among many
biologists
to accomplish in the
life
sciences
was
to foster
a distrust in the philosophy of science.
Nevertheless, the average biologist,
worried over the given time.
state
When
every biologist
I
of
seems to me,
affairs in the
in the 1950s
knew
it
is
philosophy of science
and 60s Popper was the
insisted that he
was
a Popperian,
whatever he wanted to do. Labels are sometimes
tell
apart.
typical refined
convenient
of the story
could never
Yale. After four
So he sent one to Harvard, the other to
boy had become a
any
and then did
mean nothing. (The situation reminds me who had two identical twin sons whom he
years the Harvard
at
great rage,
politically
but they often of the father
not particularly
Boston Brahmin,
THIS
56
become a them apart.)
while the Yale boy had still
could not
tell
The working
BIOLOGY
IS
typical Yale bulldog
—and
the father
whether he should follow the
biologist does not ask
prescriptions of this or that school of philosophy.
When one
studies
the history of various theories in science, one sympathizes with Fey-
erabend (1975),
who
claimed, "Anything goes." Indeed this attitude
seems to be what guides the biologist in most of his theorizing. does what Francois Jacob (1977) has called "tinkering."
moment most
will get
him
at the
EXPLANATION
—where
chance, pluralism, history, and uniqueness play
such important roles (see Chapter 4)
make
method
uses whatever
—
a flexible system of theory
more appropriate than
construction and testing would seem principles.
He
respect to natural selection
conveniently to the solution of his problem.
FIVE STAGES OF In biology
He
—with
Such a system might be captured
observations
on undisturbed
rected experiments,
some of which
rigid
in five words. (1) Scientists
nature, or during specifically di-
are unexplained
by current theories
or are in conflict with generally held views. (2) These observations lead the scientist to formulate questions of
"How?" and "Why?"
(3)
To
answer these questions, the investigator constructs a tentative conjecture or working hypothesis. (4) In order to determine correct,
he subjects
to rigorous testing,
it
the probability that
it
is
which
weaken
valid, or
it;
if this
conjecture
is
will either strengthen
tests consist
of making
additional observations, preferably using different strategies or path-
ways
as well as carefully designed experiments. (5)
The explanation
ultimately adopted will be the conjecture that has been
most
successful
during the testing procedure.
COMMONSENSE REALISM Philosophers have endlessly speculated about whether there
world outside of organs,
us, as indicated
and whether
this
organs and by science.
world
exactly as
One extreme
from accept
us. it
Biologists
known
to
as a fact that a "real
a real
by the stimuli received by our sense
is
me
we
are told
by our sense
represented by Bishop Berkeley's
is
suggestion that the outside world 18
is
is
simply an outward projection
are
commonsense
world" outside of us
realists.
exists.
They
We now
How Does have so tion,
many ways
57
Science Explain the Natural World?
of testing our sense impressions by instrumenta-
and the predictions based on such observations come true so
would seem
invariably, that there
little
benefit in challenging the
pragmatic or commonsense realism on the basis of which biologists
normally conduct their researches.
Common much
sense
is
prefer to rely
not a fashionable tool
on
among
contrast,
syllogisms appear to be virtually identical equations.
comfortable with
common
nature of causation, a
who
philosophers,
To a nonlogician, by
logic.
He
most
more
is
sense. Also in the determination of the
commonsense approach
often the
is
most
comfortable and productive one. The rigorous approach of the logician
might have been suitable for a deterministic, erned by universal laws, but
seems
it
less
essentialistic
world gov-
appropriate in a probabilistic
world ruled by contingencies and chance, a world in which one forever asked to explain unique
phenomena. White,
pied,
ravens as well as black and black-necked swans (they
make
a
good case
is
and brown
all exist!)
do not
for a superiority of logic.
THE LANGUAGE OF SCIENCE Each branch of science has
and concepts of
its field.
own
its
When
a
terminology for the
term
an object or individ-
refers to
—mitochondria, chromosomes, nucleus, gray dawn redwood— poses no problem. But
wolf, Japanese beetle,
ual
it
refer to
usually
more heterogeneous phenomena or
facts, processes,
a large class of terms
processes; competition,
evolution, species, adaptation, niche, hybridization, variety are that are encountered in biology.
exactly the necessary.
19
same way by However,
all
When
workers, they are helpful and indeed
as the history of science has
not the case, and the result
is
some
these terms are understood
shown, that
is
often
misunderstanding and controversy.
Three kinds of problems with language are encountered by the
working
scientist. First, the
meaning of
a term
knowledge of the subject grows. Such changes surprising, since scientific terms are usually
language and have usage.
Terms
all
may change in
meaning
as
our
are not
borrowed from
daily
the vagueness and imperfections of this prior
like force, field, heat,
and so on used
in
modern
physics
have distinctly different meanings from earlier periods. The complex gene of the modern molecular biologist, with
its
flanking sequences,
THIS
58
BIOLOGY
IS
exons and introns, and other elaborations,
on
early "beads
concept of H.
J.
by Johannsen almost
1909,
is
from the
utterly different
"gene,"
which was
used to describe
still
first
introduced
this entity.
Because
terms undergo a certain amount of change,
scientific
all
word
Muller; yet the
in
is
and even from the more sophisticated
a string" notion
would be most confusing
new term with
to introduce a
it
every minor
change of meaning; new terms should be reserved for truly drastic changes. Indeed, technical terms must have a good deal of "openness" to permit the incorporation of further findings.
The second problem
working
for the
is that some terms phenomenon or proc-
scientist
have been unwittingly transferred from a given ess to
an entirely different one. This
application of
De
Vries's
the genetic material; for
is
well illustrated by T. H. Morgan's
term "mutation" to any sudden change
De
Vries, a
mutation was an evolutionary
change that would instantaneously make a new evolutionary
some 30 the
same
more than
in
a genetic concept.
It
species.
It
was an
took the nongeneticists
to 40 years to understand that Morgan's mutations were not as
De Vries's
mutations. 20
of science that a term which
is
It is
in
a basic principle of the language
more or
less universal
use as the
designation of a particular entity should not be transferred to a different entity. Violation of this principle invariably leads to confusion.
Perhaps most frequent and most confusing
term for several different phenomena. In literature, a great deal
much
of logical sophistication
analysis of certain terms, but surprisingly
term's possible basic heterogeneity. 21
used for
is
little
at least four entirely different processes;
selection),
which again
"evolution,"
of the philosophical is
employed
attention
Examples are
is
in the
paid to a
"teleological," a
"group"
refers to four different kinds of
which has been applied
same
the use of the
(as in
term
group
phenomena;
to three very different processes
or concepts; and "Darwinism," a term which has continuously changed its
meaning. 22 Terminological ambiguity has had dire consequences from time to
time in the history of biology. Darwin's failure to realize that the term "variety"
was used
differently
by zoologists and botanists got him
completely confused about the nature of species and of speciation. 23
A similar fate befell Gregor Mendel. He was uncertain about the nature
How Does
59
Science Explain the Natural World?
of the kinds of peas he crossed and, heterozygotes "hybrids."
When
like
most plant breeders, he
found by using "other" hybrids that were actually
real species hybrids,
The use of the same term "hybrid" for two entirely biological phenomena thwarted his later research efforts. 24 he
failed.
By
far the
most
called
he tried to confirm the laws he had
practical solution for such
different
homonymy is the adopAnd whenever the
tion of different terms for the different items.
possibility of confusing equivocation exists, precise definitions for each
term in question should be proposed.
phenomenon changes appropriately.
The
its
the designated concept or
If
meaning, the definition should be revised
definitions of
most terms used
in science are con-
tinuously modified as our knowledge increases. Just about every basic
term
and
in the physical sciences, for example, has
been redefined again
again. 25
Most philosophers seem and perhaps
this
to be quite reluctant to provide definitions,
accounts for the
sophical literature.
The reason
many
equivocations in the philo-
for this reluctance
is
"definition" in the classical philosophical literature
meaning
that
was a holdover from the
based on the principles of essentialism. 26
that the
had
a
scholastic tradition It
term
specific
and was
seems that many philoso-
phers use the term "explication" for that which a working scientist calls a definition.
To
me
the need for clear definitions
so obvious that
is
I
have never
been able to understand why so many philosophers have been opposed to giving definitions. Popper,
one of the most adamant opponents of
definitions, revealed in his autobiography,
he held
this view.
He
Unended Quest (1974), why
said he learned early in his youth that
one should
"never argue about words and their meanings, because such arguments are specious
and
insignificant."
He was
later readings "that the belief in the
astonished to discover in his
importance of the meanings of
words, especially definitions, was almost universal." This, he says, was
outcome of the power of essentialism. When Popper read Spinoza he found his writings "full of definitions which seem to me arbitrary, pointless, and question-begging." Popper here reveals what evidently the
he was opposed
to. It is
the
game of
logicians to lay
down
of words and then to operate in syllogisms with these. 27
definitions
THIS
60
What Popper overlooked definition, scientist
he
is
is
BIOLOGY
IS that
when
a scientist
demands
a clear-cut
talking about something entirely different.
demands
is
an elimination of equivocation.
What
the
If further scientific
advance shows that the definition of a concept or process
is
incomplete
or erroneous, the definition must be and will be changed. Without clear-cut definitions at
tion of concepts scientist that
and should
all
times, however,
and theories
is
possible.
no progress
It is
in the clarifica-
my feeling as
a practicing
philosophers should give up their antipathy to definitions
test
by
precise definitions whether or not the terms they
use refer only to a single subject or to a heterogeneous mixture. This
would put an end
to a considerable
philosophical literature.
number of
controversies in the
28
Defining Facts, Theories, Laws, and Concepts Quite a large philosophical discussion has developed around the meaning of terms such as hypothesis, conjecture, theory,
example, philosophers pothesis
insist
and a theory, but
I
on making
am
and
fact,
a distinction
law. For
between a hy-
unaware of a definition of theory that
always permits such a sharp demarcation, especially in the
life
sciences.
In any case, the scientist in the field or at the laboratory bench
is
usually not as precise in his use of these terms as the philosopher at his
may when what he is
desk might wish. Whenever a scientist has a brainwave, he
say, "I just
discovered (or invented) a
actually describing
new
theory,"
might be considered by a philosopher a conjecture
or hypothesis.
Another term that has become extremely popular is
"model." To the best of
my
in the entire scientific literature
the last 20 years or so.
hypothesis? Does a
terms
—
on evolution or
systematics prior to
How exactly does a model differ from a working to be mathematical? How does it differ
model have
from an algorithm? cate the
in recent times
knowledge the term was not used once
I
deliberately ask such
"dumb" questions
need for more explanation from philosophers.
to indi-
All of these
—
conjecture, hypothesis, model, algorithm, theory
are
some-
times used interchangeably by practicing scientists in formulating their
How Does
Science Explain the Natural World?
explanations. (The reader
is
warned
that
I,
61
word
too, often use the
"theory" in this looser sense.)
FACTS VERSUS THEORIES
A
theory, to be sound, has to have a factual basis, but
draw the
line
between a theory and a
fact?
When
where does one
does a universally
supported and repeatedly verified theory come to be considered a
fact?
For instance, a modern evolutionist might say that the theory of evolution
is
now
converted into a planets
a fact. Strictly speaking, of course, a theory
fact; rather,
theory
is
Uranus and Neptune showed
replaced by
fact.
When
is
never
the outer
irregularities in their orbits, the
theory was advanced that there was a ninth planet, and in due time Pluto was indeed discovered. At that
was no longer a theory of
DNA
—
it
was now a
was discovered and
established, theories
its
moment,
the existence of Pluto
fact. Similarly, after
the structure
control over protein synthesis was
were proposed about a code that controls the
correct translation of the information in the
DNA.
Rather quickly one
of these theories proved to be the correct one, and the now-accepted genetic code
was no longer considered a theory but simply a
1859 Darwin's ideas about the inconstancy of species and descent were considered to be theories. favor of these "theories"
fact.
In
common
The amount of evidence
and the absence of any counterevidence
in
has,
since then, led biologists to accept these theories as facts. Facts, then,
may be
defined as empirical propositions (theories) that
have been repeatedly confirmed and never refuted. Theories that have
not yet been converted into or replaced by
facts are nevertheless useful
heuristic devices, particularly in areas of science
are insufficient, such as in the microscopic in sciences (such as
where the sense organs
and biochemical realm, or
cosmology and evolutionary biology) that con-
struct historical narratives to explain past events.
UNIVERSAL LAWS IN THE PHYSICAL SCIENCES
What
is
the relationship of theories
refer to processes
and
facts to universal laws?
with a predictable outcome, but
many
Laws
of the laws
of physics, such as the law of gravity or the laws of thermodynamics,
THIS
62
BIOLOGY
IS
could just as well simply be called
although universally true,
is
simply a
Those who have a high regard
That birds have
facts. fact,
feathers,
not a law.
for natural laws are mostly thinking
Our human schedules are based on nature's summer that it will be followed by winter, and that each year trees will add a new ring of growth. Lyell's uniformitarianism was based on such observations. What happened in the of the regularity of nature. regularities.
We know in
past can be expected to cists
the
happen today and
in the future.
When
physi-
wished to defend the certainty with which they held their theories,
they would point out that theories in physics are based on universal laws that are without exceptions and are spatio-temporally unrestricted.
Regularities are regularities are abilistic
abundant
but most of these
in the living world, too,
not universal and without exception; they are prob-
and very much
restricted in space
and time. Smart (1963),
Beatty (1995), and other philosophers have maintained that there are
few
if
many
any universal laws in biology. Of course
at the
molecular
level
of the laws of chemistry and physics are equally valid for
biological systems,
and these
any, regularities that have
are widespread in biology. But few, if
been observed in complex systems
the rigorous definition of laws adopted by physicists
Most of the
time, biologists
who
a logical general statement that
is
use the
word "law" simply mean
directly or indirectly
servational confirmation or falsification,
and
satisfy
and philosophers.
that can be
open
to ob-
employed
in
explanations and predictions. Such "laws" are the basic constituents
of any scientific analysis or explanation. But
concept "law" to such an extent that or generalization in biology, then
becomes rather questionable.
its
it is
CONCEPTS
word IN
one modifies the
usefulness in theory construction
Probabilistic theories, based
so-called laws, rarely give the kind of certainty
using the
if
applicable to any regularity
one
is
on such
aiming for when
"law."
THE LIFE SCIENCES
In biology, concepts play a far greater role in theory formation than
do
laws.
The two major contributors
are the discovery of
new
concepts.
When
new
to a
new theory in
facts (observations)
one goes
the
life
sciences
and the development of
to a dictionary for the
meaning of the
How Does term "concept," one
Science Explain the Natural World?
think of
which
it
is
a concept,
this definition, the
and so
a concept. But
is
of ideas speaks of concepts, he applies a
and
yet there does not
this
narrower sense.
seem
Still,
good
to be a
a biologist
what the important concepts of
concept
number
3
may be when I
every other figure; every object of
is
can form a mental image
I
A
broad definition.
gets a very
any mental image. According to
63
is
when
much narrower
a student definition,
definition for "concept" in
virtually never in
doubt
as to
his field are. In evolutionary biology,
for example, they include selection, female choice, territory, competition, altruism, biopopulation,
and many
others.
Concepts, of course, are not restricted to biology; they also occur in the physical sciences.
What Gerald Holton
(1973)
apparently what biologists refer to as concepts.
however, that the physical sciences
number of
and
basic concepts
facts is
leaders in these fields have
made
all
rather limited in the
very important. Indeed, some
statements indicating that they
progress in their science
In most biological sciences,
facts.
large role.
is
in such fields of functional biology as physiology,
where the discovery of new
assume
is
themata
calls
have the impression,
I
is
due
to the discovery of
new
on the other hand, concepts play
Not every new concept has
as revolutionary
an impact
a as
did natural selection in evolutionary biology, but most recent advances
more complex
in the
biological sciences (ecology, behavioral biology,
evolutionary biology) are due to the proposal of
The
classical
philosophy of science has
made
new
concepts.
curiously
little
refer-
ence to the important role of concepts in theory formation. The longer I
study theory formation, however, the more
I
am
impressed by the
fact that theories in the physical sciences are usually
those in biology contrast
on concepts. One can
try to soften the seeming
by saying that concepts can be formulated
can be stated as concepts. But
when
Here
focus
on
is
a
is
and laws
and "concept"
apt to run into difficul-
problem area which the philosophy of
science, in
its
physics, has rather neglected.
In the next chapter biologists
as laws,
the terms "law"
are rigorously defined, such a transformation ties.
based on laws,
must take
we
will
look more closely
at
into account as they formulate
explanations of the living world.
unique factors
and
test their
CHAPTER FOUR How
Does Biology Explain
the Living World?
When
a biologist tries to answer a question about a unique
"Why
occurrence such as
Old World?" or "Where did the
no hummingbirds
are there
species
Homo
cannot rely on universal laws. The biologist has to study facts relating to the particular
problem, infer
from the reconstructed constellations of construct a scenario that
all
in the
sapiens originate?" he
sorts of
factors,
all
the
known
consequences
and then attempt
would explain the observed
facts
to
of this
particular case. In other words, he constructs a historical narrative.
Because
this
approach
law explanations, the logic,
is
so fundamentally different
classical
from the causal-
—coming from —considered
philosophers of science
mathematics, or the physical sciences
it
quite in-
admissible. However, recent authors have vigorously refuted the nar-
rowness of the
classical
historical-narrative
view and have shown not only that the
approach
is
valid but also that
it
is
perhaps the
only scientifically and philosophically valid approach in the explanation of unique occurrences. It is,
1
of course, never possible to prove categorically that a historical
narrative
is
"true."
science works, the
The more complex
more
a system
is
with which a given
interactions there are within the system,
and
these interactions very often cannot be determined
by observation but
can only be inferred. The nature of such inference
is
on the background and the previous experience of the
likely to
depend
interpreter;
and
How Does
65
Biology Explain the Living World?
therefore, not surprisingly, controversies over the "best" explanation
frequently occur. Yet every narrative
is
open
to falsification
and can
be tested again and again. For instance, the demise of the dinosaurs was once attributed to the occurrence of a devastating disease to which they were particularly
vulnerable, or to a drastic change of climate caused events. Neither ever,
by geological
assumption was supported by credible evidence, how-
and both ran
into other difficulties. Yet,
when
1980 the asteroid
in
theory was proposed by Walter Alvarez and, particularly, after the
presumed impact ries
crater
was discovered
were abandoned, since the new
Among role are
in Yucatan,
facts
fit
all
previous theo-
the scenario so well.
the sciences in which historical narratives play an important
cosmogony
(the study of the origin of the universe), geology,
paleontology, phylogeny, biogeography,
and other parts of evolutionary
biology. All these fields are characterized living species
is
But uniqueness
unique and so is
is,
by unique phenomena. Every
genetically speaking, every individual.
not limited to the world of
planets of the solar system
is
unique.
On
life.
Each of the nine
earth, every river system
and
every mountain range has unique characteristics.
Unique phenomena have long frustrated the philosopher.
Hume
noted that "science cannot say anything satisfactory about the cause of any genuinely singular phenomenon."
mind
He was
correct
that unique events cannot be fully explained
if
he had in
by causal
laws.
we enlarge the methodology of science to include historical narratives, we can often explain unique events rather satisfactorily, and sometimes even make testable predictions. The reason why historical narratives have explanatory value is that earlier events in a historical sequence usually make a causal contribuHowever,
if
2
tion to later events. For instance, the extinction of the dinosaurs at
the
end of the Cretaceous vacated a
and thus
large
number of
set the stage for the spectacular radiation
ecological niches
of the
mammals
during the Paleocene and Eocene, owing to their invasion of these vacant niches. The most important objective of a historical narrative is
to discover causal factors that contributed to the occurrence of later
events in a historical sequence.
The establishment of
historical narra-
THIS
66 tives is
does not in the
mean
least
BIOLOGY
IS
the
abandonment of
causality,
a particularistic causality arrived at strictly empirically.
relate to
any law but,
rather, explains a simple,
unique
but
it
does not
It
case. 3
Causation in Biology
A
explanation
scientific
is
very often considered to be true
if it is
based on the discovery of the cause for an observed phenomenon, particularly of an unexpected
teractions
often highly predictive. In such cases
is
certain chemical reactions certainty.
phenomenon. 4 Causality
—
for instance, in
a definite cause can be designated with
Most of the standard treatments of
on problems
sophical literature are based
—
in simple in-
causality in the philo-
where the
in physics,
of laws such as those of gravity and thermodynamics
unambiguous answer
to the question
However, such a simple solution at the cellular-molecular level.
whenever the
effect
is
is
"What
the cause of
is
The problem
is
misleading. in
It
approach
may be
is
makes us search
It is
at the
usually not successful; in fact,
difficult, if
last step in a
An
long chain reaction. Here
final effect
we may have
interaction between series
perhaps
beginning effect. it
is
But
often
being the
to
adopt a different
two individuals, prior
is
not
strictly
to
its
conclusion, goes
of stages, during most of which each of the
acting individuals has several options available.
determined
at the
Which of these he
can usually be construed only of the chain of actions process (even
when
somewhat
its
is
when
looked
will
beginning of the stage but
depends on a number of factors and contingencies.
causal
.
thinking.
through a whole
choose
.
.
not impossible, to pinpoint the cause
an interaction of complex systems, with the
way of
an
?"
particularly perplexing
of the process for the cause producing the predictable end in biology this
effect
give
rarely available in biology, except
the end of a whole chain of events.
a residue of teleological thinking that
may
Strict causality
the chosen option at each step
at retrospectively. In fact, the
whole
random components) can be considered to have been considered. One could therefore say,
retrospectively
paradoxically, that causation in
complex
situations
is
an
How Does
a posteriori reconstruction,
of
a series
67
Biology Explain the Living World?
or, to
put
it
differently, causation consists
of steps which, taken together, can be called the cause.
PROXIMATE AND ULTIMATE CAUSATIONS There
a further complication as far as causation in biology
is
concerned. Every
phenomenon
or process in living organisms
two separate causations, usually referred
result of
(functional) causations
from
a
is
the
to as proximate
and ultimate (evolutionary) causations.
activities or processes involving instructions
is
program
All the
are proxi-
mate causations. This means particularly the causation of physiological,
developmental, and behavioral processes that are controlled by
and somatic programs. They are answers
genetic
to
"How?"
questions.
Ultimate or evolutionary causations are those that lead to the origin of
new
genetic programs or to the modification of existing ones
other words,
all
process of evolution. the genotype.
They
in
changed
are the past events or processes that
They cannot be
or physics but
—
causes leading to the changes that occur during the
investigated
must be reconstructed by
testing of historical narratives.
They
by the methods of chemistry historical inferences
—by the
are usually the answer to
"Why?"
questions. It is
nearly always possible to give both a proximate and an ultimate
causation as the explanation for a given biological instance, for the existence of sexual
a
phenomenon. For
dimorphism one can
proximate physiological explanation
give either
(hormones, sex-controlling
genes) or an evolutionary explanation (sexual selection, aspects of
predator thwarting).
Many famous
controversies in the history of
biology came about because one party considered only proximate causations and the other party considered only evolutionary ones.
One
of the special properties of the living world
two
is
that
it
has these
sets
of causations. In the inanimate world, by contrast, there
one
set
of causations
—
that provided
is
by the natural laws (often
only
com-
bined with random processes).
PLURALISM
When
one looks carefully
discover
more than one
at a biological
problem, one can usually
causal explanation. Darwin, for instance (as
THIS
68
we
Chapter
will see in
9),
BIOLOGY
IS
believed in both allopatric and sympatric
speciation as explanations for the diversity of
life,
in natural selection
and in inheritance of acquired characters as explanations for evolutionary change, in particulate inheritance (reversions) and in blending
Such pluralism of
inheritance.
beliefs presents a
problem
for
both
verification
and
would not
necessarily falsify the inheritance of acquired characters,
and
falsification.
Producing evidence for natural selection
falsifying the inheritance
of acquired characters would not neconly other possible cause of
essarily leave natural selection as the
evolutionary change. Curiously, pluralism in biological explanation was preciated by the old-time naturalists than by
geographers from
Zimmermann on
much
modern
better ap-
specialists. Bio-
(in the eighteenth century) fully
understood that discontinuities could be primary (dispersal jumps) or secondary (vicariance), but the present-day vicarianists not only act as if vicariance
had been the
were the only possible solution but
first
to think of
Some
it!
it
is
earlier
recent punctuated equilibrium
was the only theory of evolutionary
enthusiasts write as though this
change possible, while
also act as if they
authors adopted plural solutions. Indeed,
quite possible that in biology the majority of
processes
must be explained by
of science that cannot cope with pluralism In biology a plurality of causal factors, in the chain of events, often
makes
it
to determine the cause of a given
organisms found on a given island connected to the mainland arrived
by overwater
tional discontinuity
at
phenomena and
a plurality of theories.
an
is
not suitable for biology.
very
difficult, if
not impossible,
have colonized
instance, the it
when
it
was
may have Any distribu-
earlier period, or they
dispersal at a later period, or both.
may
philosophy
combined with probabilism
phenomenon. For
may
A
be due to a secondary break of an originally
continuous range (vicariance) or to dispersal across unsuitable terrain.
A
species
species,
may
have become extinct owing to competition with another
persecution by humans, a change of climate, an asteroid
impact, or a combination of these. In many, perhaps most, instances it
is
not possible to determine with certainty which particular cause
or combination of causes was responsible for a particular case of extinction in the geological past.
In almost
all
the classical controversies in biology, the opponents
How Does
69
Biology Explain the Living World?
neglected to consider a third alternative to the two controversial viewpoints. For example, the reductionist explanations of the physi-
could not explain biological
calists
phenomena
that have
no equivalent
in the limited inorganic realm, while the vitalistic counterproposals
were equally
deficient; organicism, a third
viewpoint which combined
the best of both, eventually prevailed (see Chapter
between chance and
necessity, natural selection
And
solution that ended the debate.
in the old
1).
In the argument
emerged
as the third
preformation versus
epigenesis argument, the solution to the controversy turned out to be
the genetic program. Almost every protracted controversy in biology
was terminated by the rejection of both previous explanations and the adoption of a
new
one.
PROBABILISM In the days of strict physicalism,
when
everything was believed to be
determined by an identifiable cause, to permit an outcome of a process to be also affected
by chance or accident was considered
unscientific.
Therefore, Darwin's process of natural selection (which, though
it
did
not proceed by chance, nevertheless assumed a good deal of randomness)
was referred
by the
to
physicist Herschel as the "law of the
higgledy-piggledy." Actually, already in Laplace's day the role of stochastic
(random) processes was appreciated by some
The reason why the
outcome
is
so
many
biological theories are probabilistic
simultaneously influenced by several factors,
them random, and particular mutation
is
random,
that locus could be anything
it
for the
outcome.
does not
mean
is
If
we
of
say that a
that a mutation at
under the sun but merely that
unrelated to any current needs of the organism or
that
many
any one factor
this multiple causation prevents
from being 100 percent responsible
way
scientists.
is
it
is
not in any other
predictable.
CASE STUDIES IN BIOLOGICAL EXPLANATION
When
philosophers of science discuss the formulation of scientific
theories, almost all of the case studies cited deal with the physical sciences. Yet as
we have
seen, explanation in biology,
ticularly in evolutionary biology,
may be
and more par-
rather different
from
that in
THIS
70
the physical sciences. Thus,
Let
me
might be helpful to examine a few cases
it
that illustrate this difference
BIOLOGY
IS
more
fully.
5
begin with the following simple situation.
Members of
the
camel family are found in the living fauna only in Asia (and north Africa)
and
in
South America.
How
can one explain such a discon-
tinuous pattern of distribution? Louis Agassiz applied his theory of creation real
and simply postulated
that
God had
created camelids twice,
camels in the old world and llamas in South America.
suggestion
became unacceptable
When
after 1859, the hypothesis
this
was pro-
posed that camels must have existed in former times also in North
America but then became confirmed
this conjecture
extinct in that area. Paleontology has since
through finding a rich
fossil
camel fauna in
North America.
A somewhat more
difficult
problem, of which Darwin was already
One
aware, was the discontinuity of the fossil record.
of the more
important components of Darwin's evolutionary paradigm was continuity.
Evolution proceeds by gradual change. Yet,
at living nature, all
conspicuous in the entirely
new
fossil record.
New
species,
types of organisms, turned
record, with
when one looked
one saw was discontinuity. This was particularly
up
and more importantly,
quite suddenly in the fossil
no intermediates being found between them and
their
presumptive ancestors. To be sure, occasionally a "missing link" was found, such as Archaeopteryx between birds and fossil
was separated by
the true birds.
large gaps
from
its
Darwin stubbornly (and
rightly) insisted that there
that the fossil record
was
reptiles,
as
we now
must have been complete far
but even
reptilian ancestors
this
and from
believe, quite
continuity, but
too spotty to demonstrate
this.
His
conclusion was not widely accepted for almost 100 years after the publication in 1859 of the Origin.
A
contribution to a solution was provided in
speciational evolution.
I
my
population could undertake a considerable ecological restructuring netic lineage.
and become the It is
shift
ideal starting point for a
and genetic
new
phyloge-
highly unlikely that such a small population
be preserved in the speciation
1954 paper on
proposed that a peripherally isolated founder
fossil record,
would
however. This theory of geographic
was adopted and elaborated by Eldredge and Gould (1972)
How Does in their theory of
punctuated equilibrium. 6
conspicuous conceptual theory. Indeed,
Biology Explain the Living World?
my
it is
from an
shift
71
What we have
impression that
all
more
here
is
a
populational
essentialistic to a
drastic theory shifts
in biology are the result of a conceptual shift.
In
many
new causation may be postulated, while new theory remains remarkably similar to the old
instances, a totally
the bulk of the
theory. For example,
roads" of Glen
Roy
Darwin
1839 explained the so-called "parallel
in
in Scotland as old shorelines, ascribing their origin
Having found marine
to a drastic elevation of the land. altitudes in the
Andes, having observed the dramatic
coast after an earthquake,
rise
shells at
high
of the Chilean
and making use of many other observations,
Darwin did not consider such
major
a
rise
of that area in Scotland
improbable, particularly since there was no other reasonable theory available.
However, only a few years
advanced
his ice-age theory,
and
it
after
Darwin's publication, Agassiz
became
quite clear that the parallel
roads were the shorelines of a glacial lake. Although Darwin himself later called his interpretation "a great failure,"
The
close to the correct solution.
was actually quite
it
essential insight
was that the
parallel
roads were shorelines. Prior to the proclamation of the ice-age theory, the only
way
to explain such shorelines
was
to consider
in the geological literature, particularly
through the writings of Dar-
win's teacher, Charles Lyell. Explaining these to glacial activities
A
was not
was possible
same
shorelines as
and
in
many
on design written by the natural
to take over almost
due
major change.
similar situation pertains to that vast,
magnificent, literature It
really a
them ocean
were well established
shores; furthermore, massive elevations of land
all
respects very
theologians.
of this literature into Darwinism
simply by replacing the explanatory causal factor:
it
was not God who
perfected the design but the action of natural selection. Scores of similar cases could surely be
a theory
was
left
found
in
which the
essential structure of
untouched; only the basic causal factor was replaced.
Cognitive Evolutionary Epistemology All of epistemology
and how we know
is it.
concerned with the problem of what we know In the last 25 years a
movement
called evolu-
THIS
72 tionary epistemology
new way
BIOLOGY
IS
has arisen which promotes a supposedly
(e.e.)
of looking at the acquisition of knowledge.
representatives has referred to
it
Copernican revolution ," while
its
in
opponents consider
misleading and the contributions of
The term
shall call
I
e.e.
with
new
e.e.
claim to be
this
to be rather trivial.
and cognitive evolutionary epistemology.
claims that certain "structures" in the brain, which
evolved through a Darwinian selection process, permit the reality
major
its
Darwinian evolutionary epistemology (ana-
lyzed in detail in Chapter 5)
Cognitive
of
has actually been applied to two entirely different
e.e.
processes which
One
such extravagant terms as "a
of the outside world, and that
with their world
if
humans
to deal
humans could not
deal
they did not have these brain structures. All
individuals that were inferior in this capacity were sooner or later
eliminated without leaving descendants.
Modern "real
scientists fully
many perceptions our human senses provide
understand that
world" are possible, and that
of the only a
very limited sampling of the characteristics of this world. The students of protozoans (beginning with Jennings) have revealed to us what the
world
is
like to a one-celled creature.
how
described
now
different the
realize that
human
Von Uexkull has
world of a dog
is
from our world.
spectrum of electromagnetic waves.
We
flowers have ultraviolet coloration which us.
is
among
a vast
do know of infrared
manifested in warmth and of ultraviolet
but not by
We
beings see only the small slot of wavelengths
represented by the colors from red to violet, from
insects
graphically
rays.
We know
that
rays
some
perceived by bees and other
Other animals can perceive and act upon
magnetic information or hear above and below the range of sounds accessible to
much
of
not to
it
humans.
accessible to other
that there
is
mammals and
a vast olfactory world, certainly to insects, but
us.
What determined total
We know
the selection of those particular aspects of the
world that can be perceived by a human? The most plausible
theory
is
that the ancestors of
all
organisms were able to survive and
reproduce because they had the capacity to sense those aspects of their
environment that are most important for course,
is
equally true for the
human
their survival,
and
this,
of
species. This thinking suggests
How Does that there are
many
Biology Explain the Living World?
"worlds ," of which only one
That part of the world that ceptions
as the
the world of intermediate dimensions.
Milky
Way
beyond
A
Below
galaxy.
is
it
accessible to us.
important for humans and their per-
is
sometimes referred to
is
is
73
It
middle world (mesokosmos), ranges from molecules to the
and
the world of elementary particles,
it is
the transgalactic world of spacetime.
remind
solid table, the physicists
us,
"in reality" not at
is
all
solid
but consists of atomic nuclei and electrons that are far distant from
one another. Most
biologists
know
I
accept the reality of this expla-
nation and others (ranging from genes and quarks, to quasars, black holes,
and dark matter,
to the peculiar relations
between the world of
subatomic particles and the world of the ultragalactic cosmos). These
phenomena cannot be people
realist, as
perceived by
who
hold
human
sense organs.
view are sometimes
this
The
scientific
called, believes
that the success of a theory warrants a belief in the existence of a
postulated theoretical entity, and that such theoretical entities are as real as the
observed ones. This
scientists
know.
I
scientific realism
But, frankly, in their everyday lives a table in this way,
and
shared by
all
the
most people do not understand
most
this includes
advance in our understanding of
is
physicists.
Furthermore, no
this smallest or largest
world makes
any contributions whatsoever to our understanding of the middle world, the "real world" as tion provided
by
humans
physicists
perceive
it.
Although instrumenta-
and engineers has opened up the
ing subatomic world as well as the transgalactic one,
other worlds
is
part of our normal sensory world,
contributes to our
commonsense
realism.
And
fascinat-
none of
these
and none of them
understanding them
is
not essential to our survival.
But how, then,
is it
possible that
universal properties as time directly?
we can have ideas on such basic we cannot perceive them
space, if
Here the philosophy of Kant had
the thinking of rectly,
and
some
epistemologists. Kant,
believed that the brain
is
a considerable if I
understand him cor-
so structured that
information about these properties of the universe. ber that Kant was an essentialist in
much
impact on
one
is
born with
One must remem-
of his thinking and was
convinced that the variable world of phenomena was represented in
THIS
74
IS
our thinking by one eidos for each he called the Ding an
sich.
BIOLOGY class
of variable
phenomena which
existed a priori, that
It
before any
is,
experience, hence prior to birth.
When Konrad
Lorenz occupied Kant's chair in Konigsberg in 1941,
he developed a theory of evolutionary epistemology based on Kant's notion "that the perception and the thinking of structures
has functional
cognitive structures in his brain in just the fins for
to
be
newborn must have various same way that the newborn
able to cope with the world, Lorenz said, a
whale has
man
which precede any individual experience." In order
swimming. As our hominid ancestors
shifted
from
one adaptive zone to another, appropriate mental structures were selected,
were
by exactly the same process whereby structural adaptations
selected.
These innate structures of our perception and thinking,
says Lorenz, are the exact equivalent of morphological or
kind of adaptations.
same
the
It
seems to
me
that Lorenz's suggestion
as the fact that eyes are laid
down
any other is
basically
embryo long before
in the
they can be used for seeing. 7 Even the most primitive protists have an
apparatus for sensing and responding to the dangers and opportunities they encounter in their habitat.
More than
selection have elaborated the genetic
from
a billion years of natural
program of the human
that of a simple protozoan into that of
species
mankind. Thus the new
biological understanding of the nature of genetic
programs has
finally
explained what for such a long time had been a great mystery for the philosophers.
one must accept the idea that during the evolution
believe that
I
of
humans from
problems considerably beyond the capacity even of a chimpanzee.
solve
But
primates, a brain rapidly evolved that was able to
this
still
leaves
structuring of the
unanswered the question:
modern human
"How
specific
is
the
brain?"
CLOSED AND OPEN PROGRAMS There
is
much
present capacity nearly 100,000 years ago, at
were culturally
human brain reached its a time when our ancestors
to indicate that physically the
still
at a
brain of 100,000 years ago
very primitive level (see Chapter 11). The is
the
same brain
computers. The highly specialized mental
that
is
activities
now we
able to design
see in
humans
How
Does Biology Explain the Living World?
75
today seem not to require an ad hoc selected brain structure. All the
human
achievements of the
intellect
specifically selected for these tasks
To be
sure, different
human
were reached with brains not
by the Darwinian process.
capacities are controlled
by
different
areas in the brain. But in view of our present great ignorance about
the workings of the
human
brain,
it
would be misleading
to
become
too specific at this time in our speculations about the brain structures
human cognition and on what we know at this moment,
recognition of the world. Yet based
that permit
it
appears that one might recognize
three kinds of areas in the brain.
the brain seems to contain areas that
First,
programmed.
are rigidly
from the very beginning
Instincts in the lower animals,
and most locomotory patterns
in
and
reflexes
both lower and higher animals, are
examples of these "closed programs." But whether more complex behaviors of the category
is
human
species (and if so which) belong to this
unknown. Research
temperament
behaviors than
The brain
in the area of infant behavior
indicates that there
we used
also
may be more
rigidly
and
programmed
to think. 8
seems to contain areas that are suitable for "open
programs." This information
is
not rigidly programmed in the way
that instincts are, but specific areas in the brain are set aside to accept
such information organism.
if it is
available in the
Many components
environment of the young
of our cognitive equipment, such as the
capacity to learn languages or to adopt ethical norms, are apparently best acquired at certain early ages
and
are not easily displaced or
forgotten once acquired. These categories of learning
much
in
young
on the
common of
its
an early
sensitive period,
is
is
inserted
an area evidently ready for the acceptance of
information. Similarly, every
human
becomes "imprinted"
mother. This "object-to-be-followed"
in the gosling's brain in this
to have
with the simple "imprinting" of the ethologists. The
gosling, during gestalt
seem
new
experience of a developing
recorded in the appropriate brain space and reinforces
had previously been recorded by the brain. 9 The components of our knowledge of the world with which we are associated experiences that
born, as described by Kant as well as Lorenz and other evolutionary epistemologists, are perhaps best understood as
open programs.
THIS
76 Finally, the
brain seems to contain generalized areas which permit
(memory) of
the storage
the course of
BIOLOGY
IS
all
sorts of information acquired
At the present time we
life.
know
throughout
virtually nothing
concerning a possible subdivision of the brain for different categories
memory may
of such general information. Short-term and long-term
be examples of these subdivisions. Cognitive evolutionary epistemology the second class in this
list.
It
is
particularly concerned with
deals with brain areas that evolved
through selection to supply the newborn with suitable open programs in
which
is
nothing metaphysical or
to store important
are simply a product of
unknown
specific cognitive information.
essentialistic
about such brain
Darwinian evolution. What
the degree of specificity of these areas.
is
probable that indicated
and
much
by the
of the specificity
relative ease
is
is
It
There
areas; they still
largely
would seem
acquired after birth. This
with which, in a young person,
is
many
functions of large destroyed parts of the brain can be taken over by
other areas.
How e.e.? I
does
all
this
add up with respect
to
an evaluation of cognitive
conclude that highly specific brain structures are not needed
for the perception
would seem
and understanding of our world.
that the evolutionary
On
improvement of the
the whole,
it
central nervous
system does not necessarily lead to highly specific neural structures
but rather to a continuously improved general structure of the brain.
As a
result,
not only
faced primitive
is it
able to cope with the actual challenges that
humans but
it
also has capabilities, such as those
required for playing chess, that were not being called
when
these
appears to
improvements of the brain were
me
that cognitive
e.e. is
upon
selected.
at the
time
As a whole,
it
nothing revolutionary but a natural
outgrowth of applying Darwinian evolutionary thought to neurology
and epistemology.
The Quest for Certainty
The aim of is
science
is
often described as the search for truth, but what
truth? Darwin's Christian opponents never questioned the truth of
every word in the Bible, leading
them
to the conclusion that everything
How Does in this
77
Biology Explain the Living World?
world had been created by God. What
former eras were
in
daring unorthodoxies, such as that the earth moves around the sun,
now
are
flat (as
considered absolute truths. That the earth is
no longer denied by any reasonable
historian of science
knows how many "unquestioned
was previously believed)
The
person.
round and not
is
truths" of former periods have subsequently been
Prior to Kepler, astronomers took
it
shown
to be errors.
for granted that the orbits of
heavenly bodies are perfect circles. Prior to Darwin,
all
most philosophers
were sure that species are constant. Until the 1880s,
was universally
it
accepted that characteristics acquired during one's lifetime could be
None of
passed on to one's offspring. tions our generation
makes
us
knows what
silent
that will ultimately be refuted
assump-
by further
scientific advances.
That the sequence of fossils
now
is
documents evolution
in the earth's strata
accepted by scientists as an irrefutable truth. But
findings of science are certainty,
still
tentative.
They may have
many
other
a high degree of
but we would not be greatly disturbed by their eventual
replacement by an either slightly or drastically revised alternate theory. Scientists
no longer
insist
on "absolute
particular theory has withstood
explains everything that
it is
all
truth."
They
are satisfied
attempts at falsification and
supposed to explain. For centuries
it
if
a
if it
was
believed that Newton's equations were the ultimate truth. Eventually,
however,
it
was shown by
under
Einstein's relativity theories that
certain conditions these equations are not correct,
adequate they are in the normal
no matter how
terrestrial situation.
The commonsense consensus would seem
to be that
most conclu-
sions of science are so well established that they can be considered to
be
certainties, while others are
degrees of certainty. If there it
is
only provisional truths with varying
competition between two theories and
cannot be clearly established which of the two
(1977) suggests adopting that theory which solving problems, or which has solved the
The
truth of explanations, however,
is
is
"more
is
more
is
almost surely true, but
distant past,
it
like
Laudan
successful in
most problems.
often vulnerable. That birds
acquired their feathers assisted by natural selection that
true,"
most things
that
is
a proposition
happened
in the
—
can probably never be established unequivocally
that
THIS
78
is, it
Why the
cannot be proven.
advantage
is
even more
cold in these
IS
BIOLOGY
acquisition of feathers
difficult to prove:
warm-blooded
Was
it
was of
selective
for protection against
vertebrates, or for protection against
excessive solar radiation? 10
There are observations in every branch of science that are unexplained.
Why has
still
totally
the phenotype of certain invertebrates (particu-
larly so-called living fossils)
remained
virtually
unchanged
for
more
than a hundred million years, while their associates in the same faunas either have
become
extinct or evolved drastically?
Why do
two kinds
of birds seem equally successful, one in which the male actively takes part in the raising of the
not? (The answer
young and the other where the male does
may be what
The number of such puzzles was in the
meantime
a
the
young
far greater
are fed, insects or fruit.)
50 or 100 years ago, and
remarkably high percentage of such cases has been
explained satisfactorily
—
for instance,
why members
of the
sterile cast
of social insects participate with such devotion in the raising of the offspring of the queen. 11 Biochemistry has been able to elucidate the
nature of almost
all
physiological puzzles.
The most important
re-
maining puzzles concern the explanation of the most complex processes in organic
adult stage
life,
the development of the fertilized egg
and the functioning of
up
central nervous systems.
to the
Most
individual processes in these two important fields are already reason-
ably well understood, but the explanation of the integration of the individual processes
and
their
control
is
still
a
little
beyond our
some
nonscientists
comprehension. In the light of these remaining uncertainties,
have gone to the extreme of claiming that nothing found by science has any degree of certainty. tioned whether
we can
And
even some philosophers have ques-
ever find the ultimate truth about anything.
This uncertainty has led to the question which
Chapter
5:
"Does science advance?"
we
will consider in
CHAPTER FIVE Does Science Advance?
Practically all working scientists, and indeed most lay people with an interest in science, are convinced that we are making steady advances in our understanding of nature, as successive generations of scientists
fill
in
more and more
world works. According to
we it
will
never be able to answer
constructed as
of questions can
it
is?"),
still
parts of the "true" story of
this view, there
("Why
is
may be some
how
the
questions that
there our world?"
"Why
is
but in every branch of science a vast number
be identified that would seem to be accessible
to further research.
This conviction that science has advanced, and will continue to advance, last
by no means shared by everybody, however. During the
is
50 years the
minism and
shift in the
philosophy of science from
a belief in absolute truth to a position in
approach to truth (or presumed truth) preted by
some commentators
as
advance. This has led the antiscience is
a wasteful activity because
it
is
strict deter-
which only an
recognized has been inter-
evidence that science does not
movement
to argue that science
does not lead to any
final truth
about
the world around us.
When
one reads the current biological
understand
how
literature,
one can perhaps
such a negative view could have arisen. To outside
observers, the seemingly unresolved controversies surrounding punc-
tuated equilibria, the role of competition in ecosystems and of dispersal in biogeography, the control of biological diversity, the adaptationist
THIS
80
BIOLOGY
IS
program, and the definition of species
(to
mention only a few of the might
issues discussed in the chapters that follow)
conclusion that progress.
no consensus
Even a handful of
scientists
easily lead to the
hence no hope for any
in sight,
is
real
we may
themselves believe that
be reaching the limit of the questions that can be answered by science.
Throughout the philosophy of
science,
1
one finds widespread objec-
which Kitcher (1993) has
tion to the notion of scientific progress,
referred to as "the Legend." According to the Legend, science has been
very successful in attaining "the goals of science erations of scientists have plete true story of the
confession that
I
these critics consider
know
to
is,
world
.
.
.
.
.
Successive gen-
.
more and more parts of Champions of Legend .
the .
.
toward ... a better and better approximation to
overall trend
My
filled in
com-
saw an truth."
adhere to the Legend will undoubtedly make
me
old-fashioned. But what
what science do these
developments in the sciences
I
refer?
critics
know
best
fit
I
I
would
like to
must admit
that
the Legend remarkably
well.
For example, the history of geology from Werner and Lyell up to
modern
plate tectonics, taken together with the history of organic
evolution from Lamarck to the evolutionary synthesis of the 1940s,
must surely be considered
as progress over the previous belief in
an unchanging world. The progression from Ptolemy to Copernicus, Kepler,
Newton, and modern astrophysics
improvement tific
in
is
a story of continuous
our understanding of the cosmos. Changes in scien-
thinking from Aristotle to Galileo, Einstein, and
chanics
is
quantum me-
another saga of steady advance.
Similar series of progressive stages can be cited for morphology,
and ecology. The development of molecular biology since the 1940s has been an uninterrupted run of achievements. Where there was virtually nothing prior to the
physiology, systematics, behavioral biology,
1940s,
we now have
a well-established megascience. All the
advances in medicine sciences.
I
show how
rest
on advances
major
in biology or other basic
could take up one problem in biology after the other and successive theories have
in explaining the
known
become
increasingly
facts.
But what exactly do we mean by the terms "scientific progress"?
more powerful
We mean by them
"scientific
advance" or
the establishment of scientific
Does Science Advance?
more and
theories that explain
81
better than earlier ones
and
are less
vulnerable to refutation. In most sciences, better theories permit better predictions,
and they
Which of two
or
are less likely to be replaced
more
theories
point of a scientific controversy. that in are
by other conjectures.
the "better" one
The
is
often exactly the
history of science shows, however,
due time the controversies concerning a particular problem
somehow
and eventually one theory
resolved,
edged to be better than historical controversies
theories
is
and
their
competitors.
its
was achieved by the
generally acknowl-
is
The
many
resolution of
rejection of both opposing
replacement by a third one.
Quite often, a theory becomes so successful that in the end
it
no
longer has any competitors. Yet the fact that a particular theory at a certain time
is
phenomenon does word. The large number of
the only one explaining a process or
not necessarily
mean
that
it
is
the final
theories that were once universally accepted but were subsequently so
thoroughly refuted that they are is
now unanimously
A
further evidence of scientific progress.
among
literally
considered invalid
few of the best known
hundreds of such theories might be mentioned:
Schwann's theory of the origin of
new
cells
from the nucleus, blending
inheritance, the quinarian relationship of taxa, the inheritance of
acquired characters, and untold theories in physiology. These nowrefuted theories,
when
they were
first
proposed, were usually the best
possible explanation at that time, based
tion
and conceptual framework of the
satisfied it
on the field.
then-existing informa-
But
scientists are rarely
with any theory; they always attempt to improve
it
or replace
by a better or more comprehensive one. The theories that have taken
their place have withstood
numerous attempts
consistent with the available evidence
Some
authors,
among whom
up
at refutation
and
are
to the present time.
Charles Darwin
is
perhaps foremost,
have a remarkably high batting average for the success of their theories.
But even Darwin proposed theories that have since been refuted.
Among them good
and sympatric speciation owing
are pangenesis
principle of divergence.
The
to the
history of genetics provides particularly
illustrations for the conclusion that
much advance
in science
consists of the refutation of erroneous theories.
To be
sure, not every theory
of progress. Indeed,
when
change in science
is
necessarily evidence
in the late 1890s the theory that "nuclein"
THIS
82
BIOLOGY
IS
was the genetic material was abandoned discovered to have been a backward
later
it
was
The same can be
said
in favor of proteins, step.
Mendelians
for the typological-saltational evolutionary theories of the
who
(Bateson, DeVries),
rejected the prevailing
Darwinian concept of
gradual populational evolution. The historiography of biology abounds
What
examples of such temporary retrograde developments.
in
cases have taught us
that
is
refuted theory until
it
it is
an error to
totally
abandon
these
a seemingly
has been exhaustively tested and found to be
unquestionably erroneous.
The pathway Indeed
it
often
new
to
insights
by no means necessarily
is
a "zeroing-in," a zigzag approach
is
rectilinear.
making use of the
principle of reciprocal illumination. Every solution of a scientific
question, large or small, leads to
new
questions; there
unexplained residue, the so-called black boxes
assumptions that are
in
still
need of
usually an
is
—somewhat
arbitrary
and explanation.
fuller analysis
In that sense, there will never be an end to science.
Not
all activities
and attention
that engage scientists' time
lead to scientific advance. In every field there are clerical
enjoy preparing
lists
and other compilations, who
banks and become occupied by other
—
perhaps for good reasons
problems of their
fields.
—
minds who
like to establish
activities that will
other workers but do not noticeably advance the ers
necessarily
field.
data
be helpful to
Most work-
are afraid to look at the great unsolved
Instead they essentially duplicate what has
already been done by others. For instance, they will study in Drosophila virilis
what has already been established
Others produce a rich body of generalizations
from these
Some workers and
fail
restrict
new
in Drosophila melanogaster.
facts
but
fail
to develop
any
facts.
themselves to a highly specialized problem
to establish intellectual
with workers in neighboring
and
particularly conceptual contact
explanations often make number of adjacent fields, may often have repercussions
fields. Scientific
use of information and concepts from a
and
a theoretical advance in
in several related fields.
one
field
Sometimes progress
in science
is
expressed
not by simply refuting another theory but by broadening the explanatory basis which unites or synthesizes several scientific disciplines.
Most of those who have attacked
the notion of scientific progress
Does Science Advance? have been philosophers or other nonscientists
83
who
simply do not have
the expertise to be able to evaluate whether or not there has been any real progress in
me
leads
our understanding. Everything
know about
I
to disagree with the claims of these critics.
Most
and theories of current science have remained unshaken 100, is
some more than 200
now
years.
Our
science
principles
for 30, 50,
basic understanding of the world
remarkably robust.
There are a few major exceptions, such as our understanding of the brain and of the cohesion of the genotype, but
must be emphasized
it
these are exceptions. Yet skepticism about scientific advance sufficiently
widespread outside of science to
justify further
still
is
documen-
tation for steady progress in various fields of science, particularly biology. In order to substantiate the claim for real progress,
shall
I
analyze in detail a concrete case study.
Scientific
Cytology
—the
2
purpose. This scope.
The
Hooke under
study of
scientific field
first
the
in Cell Biology
cells
—
is
particularly suitable for this
was made possible by the invention of the micro-
work
for the first time.
Advance
title
in cytology
was published
Micrographia, in which the
in 1667
word
by Robert
"cell"
was used
Although many microscopic objects were described
in the ensuing 150 years
by three outstanding microscopists, Grew,
Malpighi, and Leeuwenhoek, the study of microscopic objects was
more an amusing entertainment than
new was
serious science. Little that
described from 1740 to 1820. Although
referred to, the references
seemed
gitudinal structures rather than
to
emphasize
cells
was
were occasionally
fibers
and other lon-
cells.
The major advances between 1820 and about 1880 or 1890 were made possible by technical improvements in the lenses (the most important of which were made by Abbe) and by the discovery of
oil
immersion. The lighting of objects was also constantly improved as well as the finally,
methods of
by the use of
all
fixing tissues
sorts of dyes to
wall, cytoplasm, nucleus,
and
and any
living material, and,
produce contrast between
cellular organelles.
Some of
the
cell
most
important early discoveries by investigators such as Brown, Schleiden,
THIS
84
BIOLOGY
IS
and Schwann were made with remarkably primitive self-made microscopes. In the early nineteenth century, however, a
number of
firms began to produce ever-improved microscopes, facilitated the
study of
and helped
cells
and
optical
this greatly
The
to popularize cytology.
inadequacy of the early instruments often led to erroneous observations,
and was one of the reasons
for
some of
the early controversies
in cytology.
One
from most
gains the impression
study of
cells
histories of biology that the
began with Schleiden and Schwann. However,
Meyen (1804-1840) published even earlier a remarkably well-informed monograph on plant cells. He described 3
cation of cells
plant
cells,
by
F.
the multipli-
used iodine to stain starch inclusions in
division,
and gave an exact description of
But Meyen was not alone; there were
period about half a dozen other investigators contributions to the accurate description of In
body
who made
all cells
speculating
on
which he
its
called the nucleus.
significance. This
his discovery of a
was done by M.
cell
of
he
said,
it
Schleiden in a cells
originate
the cytoblast.
was formed de novo from the liquid of the
content. Quite evidently this
cells, fitting
J.
new
by the growth of the nucleus. He therefore renamed itself,
at that
substantial
But he refrained from
paper published in 1838 in which he claimed that
The nucleus
name
cells.
November 1831 Robert Brown announced in
had
chloroplasts. If he
not died so young, no doubt his would have become an honored in the history of biology.
F.
J.
accurate and
was an epigenetic theory of the origin
into an intellectual environment in
which any kind of
preformation was frowned upon. Nevertheless Meyen immediately published a rejoinder to Schleiden in which he reiterated his observation of the formation of
new
cells
by the division of old
cells,
process that for Schleiden presumably smacked of preformation.
It
a
did
not help Meyen's thesis that he held a number of other ideas about the
cell
nucleus that were rather erroneous.
Schleiden, a botanist, had
plant cells that
done
with their well-formed
Meyen had
his cytological investigations
cell walls.
He confirmed
essentially already reached,
on
a conclusion
which was that a plant
consists of nothing but cells, even
though some of them are greatly
modified. But what about animals?
Do
they also consist of
cells?
This
Does Science Advance?
was demonstrated
show
for
in
one animal
these tissues,
1839 by Theodor Schwann,
different they cells.
from
originate
nuclei.
Few
He
able to
components of
seemed from one another,
Schwann, however, also confirmed,
in a very detailed investigation, Schleiden
nuclei could originate
who was
tissue after the other that the
no matter how
were nothing but modified
cells
85
s
erroneous theory that
only added another process that
from unformed
new new
intercellular material.
publications in biology have ever caused such a sensation as
Schwann's magnificent monograph. plants consist of the
It
demonstrated that animals and
same building blocks
—
cells
—and
that a unity
therefore exists throughout the entire organic world. Furthermore, the cellular
composition of both animals and plants demonstrates that
cells are
the elementary
components of organisms.
It
was a vigorous
endorsement of reductionist thinking. Schleiden later published a detailed presentation of his theory of
on induction and
science with strong emphasis
a severe critique of
the then-fashionable theories of science of Schelling and Hegel. Yet,
was not nearly
is
quite clear that Schleiden
as
he thought he was, and his ultimate conclusions were
He
clearly
as inductive
all
teleological.
based his theory of science on Kant by way of
similar teleological worldview
was true
for
it
and empirical
Fries.
A
Schwann, who was a devout
Catholic.
The Schleiden -Schwann theory of the origin of new nuclei from the cytoplasm or other unformed organic substances fit well not only into the epigenetic thinking of the embryologists but also into the
theory of spontaneous generation, which was that time.
It is
the acceptability of theories.
formation of
new
nuclei
The theory of the
and
cells in
thoroughly refuted by Robert
cell.
cell
widely accepted
at
Remak
possibility of a free
unformed organic material was in 1852.
developing frog embryo, beginning with the every
still
another illustration of the influence of ideologies on
He showed first
that in a
cleavage division,
of every tissue was the result of the division of a preexisting
In 1855 he followed this
up with
monomore thor-
a larger, well-illustrated
graph, in which the Schleiden-Schwann theory was even
oughly refuted. In the same year Virchow adopted Remak's conclusions
and coined the famous motto omnis
cellula e cellula ("all cells
from
THIS
86
Not
cells").
surprisingly,
BIOLOGY
IS
Virchow was
also a
determined opponent of
the theory of spontaneous generation. It is
not altogether easy to determine what really caused the change
in the theory of the origin of cells.
Presumably the improvement of
microscopes and microscopic techniques was involved, as well as the choice by
Remak
On
frog embryo.
of a particularly suitable material, the developing the other hand, the
new theory was
in
seeming
opposition to epigenesis and to the theory of spontaneous generation,
both
still
prevalent at that time.
It
seems, at least in this case, that the
empirical findings simply swept away any misgivings over the seeming violation of broadly held ideas.
UNDERSTANDING THE NUCLEUS The new
cell
theory originally had no use for the nucleus, even though
Remak had shown division of the
clearly that a division of the nucleus
cell; this
preceded the
observation was categorically denied by others,
including the otherwise so pioneering Hofmeister. As a result
another 30 years before the slogan omnis nucleus e nucleo
("all
it
took
nuclei
from nuclei") could be coined by Flemming. It
was
really the process
most important (for the egg)
of fertilization that ultimately provided the
clues. It started
and by Gegenbaur
reproductive elements are
with the proof supplied by Kolliker
(for the
cells.
and development was, however,
What
spermatozoon) that these two
role they played in fertilization
in the
beginning very controversial.
For the physicalists fertilization was nothing but a physical phenome-
non, consisting of a transmission of excitation produced by the contact of the spermatozoon with the egg
cell.
simply the signal that initiated the cleavage of the egg
opponents
it
them was
Fertilization for cell.
For their
was the "message" which the spermatozoon brought
the egg that was
to
the truly significant aspect of fertilization.
Before the latter view could eventually achieve victory, a
number of
erroneous ideas about development had to be eliminated. Most important
among
these
was preformation, the
belief that a miniature
organism was encapsulated in either the egg or the spermatozoon. Beginning with Blumenbach,
this idea
was ridiculed so unmercifully
87
Does Science Advance?
eventually was replaced by the theory of epigenesis, the belief
that
it
that
development started from an
given form by
some extraneous
The second
idea that
had
entirely
unformed mass which was
force.
to be accepted
was
that of the equal
contribution of egg and spermatozoon to the characteristics of the
developing embryo, in other words, a consideration of the genetic aspects of fertilization. This proof
who
proved
in the 1760s
this
idea that the
made
first
provided by Koelreuter,
point conclusively in his hybridization
experiments. Although Koelreuter's similar to his were
was
work was widely
ignored, findings
by many others, and the much more important role than
in subsequent years
spermatozoon played
a
merely initiating the cleavage of the
fertilized
egg was eventually
accepted. Amazingly, as late as the 1870s, Miescher, the discoverer of
nucleic acid,
still
The entry of
adhered to the physicalist interpretation. 4
and sometimes even
the spermatozoon into the egg,
the fusion of the male nucleus with the egg nucleus,
had been observed
repeatedly between the 1850s and 1876, but these observations were
misinterpreted owing to the erroneous conceptual framework of the investigator.
It
was Oskar Hertwig (1876) who
fertilization consisted
egg, that the
clearly
showed
that
of the penetration of a spermatozoon into the
spermatozoon provided a male nucleus which fused with
the egg nucleus,
and
that the
development of the embryo was
initiated
by the division of the newly formed nucleus of the zygote that had been formed by the fusion of the male and egg nucleus. These observations were fully confirmed
The cleus
—widespread
idea
is
and expanded by H. Fol
in the preceding decades
—
dissolved prior to every division of a cell
refuted, at least for the process of fertilization,
by the mitosis of the
What was not
fully
cell
understood
it
was now
clearly
cell
division
was
nucleus. at the
the spermatozoon played a dual role.
of the father into the egg but
that the cell nu-
and improved micro-
scopic techniques soon demonstrated that every initiated
in 1879.
It
time was that
fertilization
by
imported the genetic material
also gave the signal for the beginning
of the development of the zygote. That these are two entirely different roles
was not understood by the
physicalists.
When Loeb was
able
by
THIS
88
chemical means to
made
IS
development of unfertilized
initiate the
claims about such
BIOLOGY
artificial
the 1870s
it
he
parthenogenesis which showed that
he was entirely unaware of the genetic role of
By
eggs,
had become quite
fertilization.
clear to the foremost
workers in
and the egg
the field that the fusion of the spermatozoon nucleus
nucleus had a genetic significance. Just what this significance was and
how the two was
still
nuclei could transmit the genetic properties of the parents
What was
totally obscure.
next required was the discovery
and correct description of the reduction division during the meiosis of the maturing germ
component of plished by
The
and the appreciation
cells
that the essential
the nucleus were the chromosomes; this was accom-
Weismann, van Beneden, and
who
empiricists, those
Boveri.
did the superb microscopic work, often
missed the correct interpretation of their findings simply because they did not have an appropriate theoretical framework. Often they did not ask the question
exemplary.
He
why something was
asked very perceptively:
mitosis necessary? This process
unnecessarily complex.
Why
one half to one daughter
Roux
is
happening. Here Roux was
Why
the complex process of
so wasteful in time
and so seemingly
not simply halve the nucleus and give
and the other
cell
is
to the other daughter cell?
quite correctly concluded that the elaborateness of the process
of mitosis
is
justifiable
only
if
each daughter nucleus receives different
What
components of the is
the nuclear material
method must be used
highly heterogeneous and a
its
to
qualitatively
is
make
sure that
share of each of the qualitatively
original nucleus.
equally interesting about this period
is
that
many
correct
observations and theories were subsequently ignored, only to be rediscovered
later.
discovered only
own
Perhaps later.
I
should
say,
For instance, Roux more or
valid theory of mitosis because
some of
"with the true significance" being
the observations
it
seemed
on developing
abandoned
his
to be contradicted
by
eggs.
less
And van
Beneden's
completely correct observation that the chromosomes of the sperm nucleus did not fuse with those of the egg nucleus, providing the observational basis for Mendel's findings, was largely ignored until after 1900.
None of the
speculations
on theory formation found
in the literature
Does Science Advance? of the philosophy of science
ments of
this period,
89
valid for the highly involved develop-
is
including
wrong observations and
false guesses.
Advances were sometimes made by new discoveries, sometimes by new theorizing.
was sometimes material from
It
permitted advances,
and sometimes new technologies,
frog embryo,
so successfully used by the later cytologists. perfectly evident
is
observations and
new
Even
theories
would turn out
may be
it
that
the aniline stains
like
The one thing
that
is
what was needed was an abundance of new
that
could operate. Sooner or tation
new organism
a
Oskar Hertwig's sea urchin egg and Remak's
like
on which
a
Darwinian selection process
one particular observation or interpre-
later,
and be accepted
to be unassailable
as "true."
ultimately refuted, just as the assumption that proteins
was eventually
are the material of inheritance
refuted,
though
it
had
more or less the truth. The protein hypothesis was so firmly established that when it was finally replaced by the DNA hypothesis, some leading investigators, such as Goldbeen accepted for 30 or 40 years
schmidt,
still
refused to believe
During the 40 years
as
it.
after 1880,
improvements in microscopy per-
mitted ever more accurate descriptions of nuclei and their changes in the mitotic
and meiotic
these changes.
The
cycles
and an explanation of the meanings of
acquisition of this understanding
plex story, with contributions
made both by superb
is
a highly
technicians,
com-
who
supplied excellent descriptions of the various aspects of the processes
of maturation and fertilization, and by brilliant theoreticians. 5
UNDERSTANDING THE CHROMOSOMES The
starting point of the ensuing speculations
was the observation
that well-formed chromatin bodies, later called
observed during
by
a granular
cell
chromosomes, are
division (mitosis) but were seemingly replaced
mass or a network of thin threads
in the resting stage
of the nucleus. The problem was to find a meaning for what happens
when
this
irregular chromatic material
defined chromosomes, particularly after species difficult
had a at
fixed
first
number of
is it
converted into the well-
had been shown that each It was rather
mitotic chromosomes.
to develop a theory as long as
one had no idea
whatsoever as to what the biological role of the chromatin was. Al-
THIS
90
though
it
had been
stated early
nuclein, this conclusion
no one
really
At
was located
it
that chromatin
more
precise
either.
was Weismann who
in the
was nothing but
the function of nuclein was, this
was of no help
this stage
on
was by no means generally accepted, and since
knew what
identification
BIOLOGY
IS
insisted that the genetic material
chromosomes, and although the
of inheritance were quite wrong,
it
details
of his theory
steered attention in the right
The person who contributed the most to an understanding of the chromosomes was Boveri. He started with the simple observation that there was a fixed number of chromosomes during mitosis, direction.
and
he could demonstrate the individuality of
in favorable material
these
chromosomes, that
chromosomes had
he could recognize that each of the
is,
certain diagnostic characteristics. After these chro-
mosomes had been
"dissolved" into the nuclear matter of the resting
nucleus, Boveri could demonstrate that during the next mitotic cycle the very cycle,
same number of chromosomes re-formed
and furthermore
teristics as in
that they
as in the last mitotic
had the same individual charac-
the previous cycle. This led
him
to the continuity theory,
according to which the chromosomes never lose their identity during the resting phase of the nucleus, but continue throughout the the
cell.
Although
this
life
of
theory was heavily attacked by other leading
it eventually became the basis of the chromosome theory of inheritance. Boveri's theory was based on inference. The continuity of the chromosomes could not be observed directly. Was there some underlying
cytologists, including Hertwig,
Sutton-Boveri
deeper concept or ideology that gave Boveri the strength of conviction that he
was
right?
Did
opponents have some other underlying
his
concept or ideology which induced them to be sure that Boveri was quite
wrong? Unfortunately
existing literature, to
come
suspect, however, that there tual
I
have been unable, on the basis of the
to a conclusion
on
this question.
must have been something
background of Boveri and Hertwig that led
difference of opinion. Needless to say, neither of
laws to back
up
I
rather
in the concep-
to
their
drastic
them invoked any were based on
their opinions. Their conclusions
observations and what each of
from these observations.
Up
them thought was
to
now
this
a logical inference
disagreement has not yet
Does Science Advance?
been explained in any terms that would shed controversies
chromosomes through
the resting phase of the
remnant of the preformation-epigenesis controversy,
a
still
on the philosophers'
light
on theory formation. Was perhaps the argument about
the continuity of the
nucleus
91
with Hertwig the epigenesist and Boveri the preformationist?
down
There was no slowing of the
cell after
and
genetics
cell
1900. At
first
physiology, subsequently followed
of the fine structure of the
and
finally the
in the advances of the
study of
all
understanding
the major contributions were
cell
made by
by the exploration
with the help of electron microscopy,
components of the cytoplasm by molecular
biology.
Even though observations were almost invariably the starting
point of
new developments,
theory formation clearly was not the result
of simple induction. Rather the observations raised puzzling questions,
which led to conjectures that were
new
mately resulting in
The
theories
either falsified or confirmed, ulti-
and explanations.
history of cytology illustrates in the
most graphic manner the
gradual progress of science, the failure of erroneous theories, the
among competing
struggle
theories,
and the ultimate victory of the
interpretation that at present has the greatest explanatory value. it is
indisputable that the interpretation of the
which
is
now adopted
is
cell
infinitely superior to the
and
its
And
components
concept of the
cell
prevalent 150 years ago.
Does Science Advance through Revolutions? If
we can conclude from
this
and other case
studies that science
makes
steady advances in our understanding of nature, the question must
next be asked
how these
advances occur. This highly controversial issue
occupies a large portion of the literature of the contemporary phi-
losophy of science.
Kuhn's theory of
S.
(2)
distinguish
two major schools:
scientific revolutions versus
normal
( 1 )
Thomas
science,
and
Darwinian evolutionary epistemology.
Few a
One can
publications in the philosophy of science have created as great
stir as
did Kuhn's Structure of Scientific Revolutions in 1962. According
to Kuhn's original thesis in the first edition, science advances
through
occasional scientific revolutions, separated by long periods of "normal
THIS
92
During
science."
BIOLOGY
IS
a scientific revolution, a discipline adopts
new "paradigm which then dominates ,"
an entirely
the ensuing period of normal
science.
Revolutions (paradigm
and periods of normal science
shirts)
only two aspects of Kuhn's theory. Another one mensurability between the old and the critics
has claimed that
ways in the
different
Kuhn
of these concepts, trix." is,
A
Kuhn used
first
later
according to Kuhn, a
There
is
new paradigm. One of Kuhn's
the term paradigm in at least twenty
most important
introduced the term "disciplinary ma-
more than a new theory; it system of beliefs, values, and symbolic genis
a considerable similarity between Kuhn's disci-
and terms such
plinary matrix
are
a supposed incom-
edition of his book. For the
disciplinary matrix (paradigm)
eralizations.
is
used by other
as "research tradition"
philosophers. 6
Many authors were able to many more were unable to do
confirm Kuhn's conclusions; perhaps so.
The numerous
cannot be discussed profitably without looking
aspects of his thesis
at concrete cases
and
asking whether theory change did or did not follow Kuhn's generalizations.
I
have therefore analyzed a number of major theory changes
mind.
in biology with this question in
PROGRESS IN SYSTEMATICS In the science of animal
Chapter
7),
we can
and plant
classification
distinguish an early period,
(systematics; see
from the
the sixteenth century to Linnaeus, during which
most
herbalists in
classifications
were constructed by logical division, and the nature of the changes
made from one
classification to
classified species
and on the weighting of
This type of methodology In due time a
it
was
is
number of
different kinds of characters.
downward classification. downward classification was really
referred to as
realized that
method of identification, and
method, upward
another depended on the
classification,
it
was supplemented by
a very different
consisting of the arrangement in a
hierarchical fashion of ever-larger groups of related species into classes.
However, the method of downward side
by
side,
being used in keys in
graphs and in
all
classification
continued to
exist
taxonomic revisions and mono-
field identification guides.
Upward
classification
was
Does Science Advance?
used by some herbalists,
first
(1763), but
this
later
93
by Magnol (1689) and Adanson
method did not begin
to be generally
the last quarter of the eighteenth century. There was
adopted until
no revolutionary
replacement of one paradigm by another one, since both continued
now with different objectives. One would have expected that the adoption of
to exist, although
common
descent in 1859 would have produced a major revolution in
taxonomy, but
was not the
this
case. In
are recognized on the basis of the greatest
Not
Darwin's theory of
upward
classification,
number of shared
groups
characters.
surprisingly, the taxa thus delimited consisted usually of descen-
dants of the nearest
common
plied the justification for the
ancestor. Hence, Darwin's theory sup-
method of upward
classification,
but
it
did not result in a scientific revolution in systematics.
One hundred years later, after 1950, two new schools of macro taxonomy were established, numerical phenetics and cladistics. Did these amount
to revolutions? Phenetics
sifications so that
supplied a if
it
produced rather unsatisfactory
new methodology but not
one looks
at the
clas-
did not have a major impact. Furthermore,
new concept. By
really a
volume of the current
literature,
it
contrast,
one might be
inclined to say that cladistics indeed produced a major revolution. Actually, the
approach of recognizing taxa by jointly derived characters
was already previously widely practiced, (1950). Yet,
it is
as
Hennig himself points out
evident that the vigorous and consistent application
of cladistic analysis unquestionably had a major impact.
Even it
if
one were to designate
this a scientific revolution,
however,
did not proceed as described by Kuhn. There was no sudden
replacement of one paradigm by a different one, because two systems
were existing side by
side:
Hennig's ordering system (cladification) and
the traditional Darwinian methodology (evolutionary classification).
They
differed not only in
cladistic
methodology but
also in objective.
The
system was interested only in discovering and representing
phylogeny, while the endeavor of the evolutionary system was to construct taxa of the most similar and most closely related species, an
approach particularly useful in ecology and
two approaches can continue entirely in their objectives.
to exist side
life
by
history studies.
side, since
The
they differ
THIS
94
BIOLOGY
IS
PROGRESS IN EVOLUTIONARY BIOLOGY Evolutionary biology provides another testing ground for the theory of scientific revolution. The simple picture of the biblical creation story began to lose credence
by the end of the seventeenth century.
when
In the eighteenth century,
the long duration of geological
when
astronomical time was beginning to be appreciated,
and
the bio-
geographic differences of the different parts of the world were established,
and when an abundance of
were described, various new
fossils
scenarios were proposed, including repeated creations,
new
however, operating with
by
origins.
side with the biblical story of creation,
by the vast majority. The
many
Buffon,
first
to seriously
which was
world picture of
his time. Indeed,
When
first
was from
it
Blumenbach,
ideas that the evolutionary thinking of Diderot,
Herder, Lamarck, and others was derived.
proposed the
supported
still
undermine these views was
of whose ideas were in complete opposition to the
essentialistic-creationist
his
of them,
all
These new theories existed side
1800 Lamarck
in
made few
theory of genuine gradual evolution, he
converts; he did not start a scientific revolution. Furthermore, those
who
followed him, like Geoffroy and Chambers, differed widely from
Lamarck and from each other had not
By
in
many
effected the replacement of
contrast,
produced
no one can deny
conform
all
Lamarck
Indeed
The
of a whole package of theories,
of Darwin's
The
first
first
9).
7
Matters become
and second
five
much
at all
Darwinian revo-
paradigm actu-
of which are most
clearer if
one speaks
scientific revolutions.
common
one consisted of the acceptance of evolution by
descent. This theory
one.
does not
it
analysis of the
lution encounters considerable difficulties because his
important (see Chapter
certainly
new
often called the
it is
scientific revolutions. Yet,
to Kuhn's specifications.
ally consisted
a
that Darwin's Origin of Species (1859)
a genuine scientific revolution.
most important of
respects.
one paradigm by
was revolutionary
in
two
respects. First,
it
replaced
the concept of special creation, a supernatural explanation, with that
of gradual evolution, a natural, material explanation. replaced the lutionists,
of
life.
model of
straight-line evolution,
And
adopted by
second,
it
earlier evo-
with that of branching descent, requiring only a single origin
This was at
last
a persuasive solution for
what numerous
Does Science Advance?
on (and
authors, from Linnaeus
"natural" system.
It
rejected
more deprived man of animal
series.
formed the most
had attempted
to find, a
supernatural explanations.
all
It
further-
unique position and placed him in the
his
Common
earlier),
95
descent was remarkably rapidly adopted and
and perhaps the most successful research
active
program of the immediate post-Darwinian period. The reason why
of morphology and systematics
fitted so well into the research interests is
that
it
it
supplied a theoretical explanation of previously discovered
empirical evidence, such as the Linnaean hierarchy and the archetypes
of
Owen and von
Baer.
paradigm. Furthermore,
It
did not involve any drastic shift of a
one were
if
to accept the period
from Buffon
(1749) to the Origin (1859) as a period of normal science, one would
have to deprive a
number of
smaller revolutions, which took place
also within this period, of their revolutionary status. This includes the
discovery of the great age of the earth, of extinction, of the replacement
of the scala naturae by morphological types, of biogeographic regions, of the concreteness of species, and so on. All of these were necessary prerequisites for Darwin's theory
of the
first
and could be included
Darwinian revolution,
as
components
shifting the beginning of the first
Darwinian revolution back to 1749. 8
The second Darwinian revolution was caused by the theory of natural selection. Although proposed
and
fully explained in 1859,
encountered such solid opposition owing to
its
it
conflict with five
prevailing ideologies (creationism, essentialism, teleology, physicalism,
and reductionism) that
it
was not generally accepted
tionary synthesis of the 1930s and 40s.
some other
countries there
is
And
until the evolu-
in France,
considerable resistance to
Germany, and it
even
at the
present time.
When
did this second Darwinian revolution take place
was proposed
Can one
in 1859 or
when
it
was broadly adopted
—when
it
in the 1940s?
consider the period from 1859 to the 1940s a period of
normal science? Actually a considerable number of smaller revolutions in biology
took place in
this period,
inheritance of acquired characters
such as the refutation of an
(Weismann
blending inheritance (Mendel 1866, and
many
1883), the rejection of
subsequent papers), the
development of the biological species concept (Poulton, Jordan, Mayr),
THIS
96
BIOLOGY
IS
the discovery of the source of genetic variation (mutation, genetic
recombination, diploidy), the appreciation of the importance of stochastic processes in evolution (Gulick, Wright), the founder principle,
the proposal of
numerous
quence, and so on.
Many
genetic processes of evolutionary conse-
of these had indeed a rather revolutionary
impact on the thinking of evolutionists, but without any of the Kuhnian attributes of a scientific revolution. After the general adoption of the synthetic theory,
1950 on, modifications of almost synthesis were proposed
be
little
all
aspects of the
let
us say from
paradigm of the
and some adopted. Nevertheless, there can
doubt that throughout the period from 1800 to the present
there were periods of relative quiet in evolutionary biology,
and other
periods of rather vigorous change and controversy. In other words,
Kuhnian image of well-defined short revolutions and
neither the
intervening long periods of normal science steady, It
is
correct,
nor that of slow,
even progress.
would be
but has not yet been done, to look
interesting,
breakthroughs in various other
fields
at
of biology and see to what extent
they qualify as revolutions, whether they led to the replacement of
one paradigm by another, and
how much
time
it
took before the
replacement was completed. For instance, was the origin of ethology (put forth by Lorenz and Tinbergen) or the Schleiden)
cell
development of biology
in
theory (Schwann,
most revolutionary
a scientific revolution? Perhaps the
the twentieth century was the rise of
new scientists, new problems, new experimental methods, new journals, new textbooks, and new culture heroes, but conceptually the new field was nothing molecular biology.
It
resulted in a
new
field,
with
but a smooth continuation of the developments in genetics preceding 1953; there was rejected. 9
no revolution during which the previous
science
There were no incommensurable paradigms. Rather
the replacement of coarse-grained by fine-grained analysis
development of entirely new methods. The
was revolutionary, but
GRADUALISM Virtually
all
IN
it
was not
a
rise
it
was
was
and the
of molecular biology
Kuhnian revolution.
BIOLOGICAL ADVANCES
authors
who
have attempted to apply Kuhn's thesis to
theory change in biology have found that
it
is
not applicable in this
Does Science Advance?
Even in the cases
field.
change,
it
there
all,
science."
in
97
which there was a rather revolutionary
did not take place in the form described by Kuhn. First of
was no clear-cut difference between revolutions and "normal
What one
finds
major theory changes.
is
of minor revolutions take place even
any of the periods that Kuhn might designate
in
Up
to a point this
did not induce
and normal
now
is
him
science.
to
abandon
his distinction
a
revolutionary theory
many
may
as three or four
science.
this realization
exist side
paradigms
by
As a
results always in result,
may
evolution,
coexist at the
saltationism,
marckism competed with selectionism
the
new
side with the old one. In fact,
same
Darwin and Wallace had proposed natural
mechanism of
the
normal
between revolutions
new paradigm by no means
the immediate replacement of the old one.
instance, after
as
by Kuhn, but
also admitted
10
The introduction of
as
between minor and
a complete gradation
A number
time. For
selection as
orthogenesis,
and La-
for the next 80 years. 11
It
was
not until the evolutionary synthesis of the 1940s that these competing
paradigms
lost their credibility.
Kuhn makes no discoveries
distinction
and those
between theory changes caused by new
that are the result of the
development of
entirely
much
new
concepts. Changes caused by
less
of an impact on a paradigm than conceptual upheavals. For
new
discoveries usually have
instance, the ushering in of molecular biology through the discovery
of the structure of the double helix had only minor conceptual consequences, and therefore there was virtually
no paradigmatic change
during the transition from genetics to molecular biology.
The same new theory may be
far
more revolutionary in some good illustration.
sciences than in others. Plate tectonics supplies a
That
this
effect
theory had a revolutionary, one might almost say cataclysmic,
on geology
is
obvious. But what about biogeography? As far as
avian distributions are concerned, the historical narrative inferred prior to plate tectonics
had
to be
Atlantic connection in early Tertiary
changed hardly is
at all
(a
North
the only exception) as a result
of the adoption of plate tectonics. 12 To be sure, avian distribution in all
with plate tectonic reconstructions,
work showed
that the geological reconstructions
Australonesia did not agree at
but
later geological
were
faulty,
while the revised construction fitted the biological postu-
THIS
98 lates quite well.
13
BIOLOGY
IS
That there must have been a Pangaea in the Per-
mian -Triassic had been postulated by
paleontologists long before the
proposal of plate tectonics. In other words, the interpretation of the history of
on earth was not nearly
life
much
as
affected
by the
acceptance of plate tectonics as was that of geology.
The major impact of
the introduction of a
new paradigm may be
a massive acceleration of research in the area. This illustrated
is
particularly well
by the explosion of phylogenetic researches
posal by
Darwin of the theory of common
anatomy
as well as in paleontology,
was directed
much
after the pro-
descent. In comparative
of the research after 1860
to the search for the phylogenetic position of specific
taxa, particularly primitive
and aberrant ones. There
are
many
other
had relatively little impact on The unexpected discovery by Meyen originate by the division of old cells and
instances where remarkable discoveries
the theory structure of the
and Remak that new
cells
field.
not by the conversion of a nucleus into a little
impact. As far as genetic theory
covery that the genetic material
is
did not lead to a major paradigm
The
situation
is
somewhat
is
new
cell
had remarkably
concerned, likewise, the dis-
nucleic acids rather than proteins shift.
different with the
development of new
When Darwin's theorizing forced the inclusion of man in of common descent, it caused indeed an ideological revolution.
concepts. the tree
By
contrast, as
was correctly emphasized by Popper (1975), Mendel's
new paradigm of
inheritance did not.
that changes in concepts have far
For instance, the replacement of
had a revolutionary impact biology,
effect
on the
cannot be overemphasized
more impact than new
essentialistic
discoveries.
by population thinking
in the fields of systematics, evolutionary
and even outside of science
profound
It
(in politics). This shift
interpretation of gradualism, speciation, macro-
evolution, natural selection,
and racism. The
rejection of cosmic tele-
ology and of the authority of the Bible has had equally drastic
on the
had a
interpretation of evolution
effects
and adaptation.
Finding virtually no confirmation of Kuhn's thesis in a study of theory changes in biology inevitably forces us to ask what induced
Kuhn
to propose his thesis? Since
much
of explanation in physics deals
with the effects of universal laws, such as
we do not have
in biology,
Does Science Advance? it
is
99
indeed possible that explanations involving universal laws are
Kuhnian revolutions. But we must
subject to
was a
physicist
and
also
remember
that
Kuhn
that his thesis, at least as presented in his early
writings, reflects the essentialistic-saltationistic thinking so widespread
among
physicists.
Each paradigm was
at that time, for
Kuhn, of the
nature of a Platonic eidos or essence and could change only through its
replacement by a
new
eidos.
Gradual evolution would be unthink-
able in this conceptual framework. Variations of an eidos are only "accidents," as
it
was
called
by the scholastic philosophers, and there-
fore variation in the period irrelevant,
between paradigm
shifts
is
essentially
merely representing normal science.
Does Science Advance through a Darwinian Process?
The
Kuhn
picture of theory change that
painted in 1962 was congenial
to the essentialistic thinking of physicalists, but
with the thinking of a Darwinian.
It is
it
was incompatible
therefore not surprising that
the Darwinians favored an entirely different conceptualization for
theory change in biology, usually referred to as Darwinian evolutionary epistemology. As Feyerabend (1970) points out, this
is
actually a very
old philosophical concept: "The idea that knowledge can be advanced
by
a struggle of alternative views
was
first
Mill (especially in
On
it
it
depends on proliferation
(this
has been emphasized by
that
put forth by the Presocratics
Popper himself) and
is
and
was developed into
Liberty).
decisive for science, too,
The
a general philosophy
by
idea that a struggle of alternatives
was introduced by Mach (Erkenntnis und
Irrtum) and Boltzmann (Popularwissenschaftliche Vorlesungem), mainly
under the impact of Darwinism."
The
principal thesis of Darwinian evolutionary epistemology
science advances very
much
as
does the organic world
Darwinian process. Epistemological progress thus variation
and
selection.
More
ability, etc.,
the
characterized by
"More robust ideas or ideas explanatory power or greater
survive better from one generation to the
next in the struggle for acceptance"
document
that
precisely,
with greater verisimilitude or greater
problem solving
is
is
—through
(Thompson
this process, for instance, in
Darwin's
1988:235).
own
One can
theorizing. In
THIS
100 his
IS
BIOLOGY
younger years he proposed one evolutionary theory
always eventually rejecting
them
by descent through natural
until
he
after the other,
finally arrived at his evolution
selection. 14
Or
the post-Darwinian
in
period one could describe the great variation of evolutionary theories that
competed with natural
thogenesis
—Lamarckism,
saltationism, or-
selection as the successful survivor.
indeed great similarity with natural selection in the compe-
There
is
tition
among
logical
selection
—leaving only natural conjectures
and hypotheses dealing with an epistemo-
problem, with one or the other eventually being victorious,
least temporarily.
historical
At a superficial
advance of
level there
scientific theories
is
no doubt
at
that the
has a strong resemblance to
the Darwinian process of evolutionary change.
When
more closely, however, epistemological change actually occurs in a manner that in many respects is different from genuine evolutionary change. 15 The variation, for instance, among the analyzed
various theories
is
not caused by chance, as
by the reasoning of the promoters of these is
is
genetic variation, but
theories.
While
not a weighty argument because the source of variation
true, this is
of
little
consequence for the Darwinian process. Darwin, for instance, accepted
some
since-refuted so-called Lamarckian processes, such as "use
disuse"
and
variation.
a direct effect of the
Even
environment
as sources of
in the synthetic theory of the 1940s,
many
horizontal transfer, hybridization, and others. Hence, is
epistemology
is
Also, the
more
it
is
irrelevant
produced by chance or not.
The transmission from generation cultural transmission,
genetic transmission, to
sources of
mutation, recombination, biased variation,
variation are accepted:
whether the variation
and
new
to generation in evolutionary
something very different from
mention only another of a
series
of differences.
major theory advances ("Kuhnian revolutions") are perhaps
drastic than the genetic changes compatible with the nature of
biological populations.
Even though
it
is
thus quite obvious that epistemological changes
are not isomorphic with theless true that they
Darwinian evolutionary changes,
do occur according
to the basic
it is
never-
Darwinian model
of variation and selection. In a group of competing theories, the one that will ultimately prevail
is
the one which runs into the fewest
Does Science Advance?
and
difficulties
factorily
—
in
is
able to explain the greatest
other words, the one that
Darwinian process. In epistemology, is
new
a continuous production of
tures.
by
Some of them
more
are
still
successful
fit
number of the
is
This
"fittest."
variation, that
is,
of
new
is
a
—
conjec-
that
they
is,
be accepted until modified or replaced
will
great variation in the size of the
is
— many of them being very minor, others being
drastic to deserve to
facts satis-
as in biological populations, there
the situation better than others
and
better explanations. There
changes
101
sufficiently
be called revolutions. Branching descent, natural
selection, nucleic acid instead of proteins as carriers of genetic infor-
mation are among the advances that have had the most revolutionary impact.
From (1)
draw the following conclusions:
these observations one can
There are indeed major and minor revolutions in the history of
biology. Yet even the
major revolutions do not necessarily represent
sudden, drastic paradigm
paradigm may coexist commensurable.
shifts.
(2)
is
always a series of minor revo-
found only
inappropriate to
call
in inactive branches of biology,
far better
than Kuhn's description of
fit
and some of them
are
more
new conjectures (Darwinian One
eliminated so that in the end the only theory
And
be more strongly affected by a
left is
(5) a prevailing
better ones, or
and theories that
which
paradigm
new concept than by
IS
a
is
new
is
are
most
likely to
discovery.
SO
nation or theory has been proposed, cases
still
HARD TO ACHIEVE assume that once a new scientific
SCIENTIFIC CONSENSUS
Nonscientists often naively
Darwin-
successful than others.
inferior or invalid conjectures
successful in explanation.
(4)
theory change in biology
can say that these are "selected," until replaced by
one can say that
would seem
it
scientific revolutions. Active areas
of biology experience a steady proposal of variation)
but
such quiet periods "normal science."
ian evolutionary epistemology seems to
ally,
a subsequent
are not necessarily in-
between the major revolutions. Periods without such revolu-
tions are
WHY
and
earlier
They
branches of biology seem to experience no
(3) Active
periods of "normal science." There lutions
An
for long periods.
it
expla-
will quickly be adopted. Actu-
where a sudden new insight led
to a revolutionary instan-
THIS
102
taneous illumination of a
BIOLOGY
IS
have occurred only very
field
Most
rarely.
major tenets of modern science had to overcome years of resistance both from within and from outside of science. As we have seen, Darwin's and Wallace's theory of natural selection was not adopted by the majority of scientists from 1859 until about 1940. Continental drift
was
first
advanced by Wegener in 1912, although there had already
been a number of forerunners. The geophysicists opposed
known
almost unanimously, arguing simply that no force was could
move
entire continents
all
the geology of the ocean floor.
map nor
over the
Some of
theory
this
could
it
that
explain
the biogeographic cases cited
in support of drift (Pleistocene distribution patterns)
sen and easily refuted. However, eventually
were badly cho-
more and more evidence through the researches
for continental drift accumulated, particularly
when in the early 1960s sea floor spreading magnetic phenomena were discovered, continental drift
of paleontologists, so that
and correlated
was accepted within a few
years. 16
Another theory that was proposed long before geographic speciation (multiplication of species).
Galapagos evidence, Darwin geographic speciation. But
at first (in the
it
was accepted was
On
1840s) supported strict
later (in the 1850s)
he accepted also sym-
patic speciation, and indeed eventually thought that frequent and
more important
1889) that speciation
is
and
and
view (1864,
mapping of
the distribution of
closely related species of birds,
mam-
almost universal conviction that
snails led to the is
was the more
usually geographic was a minority view until
subspecies, incipient species,
geographic speciation
it
process. 17 Moritz Wagner's
1942. 18 In the 80 years after 1859, the
mals, butterflies,
the basis of the
mode of time, so many
the major, perhaps almost exclusive
speciation in sexually reproducing organisms. Since that
new arguments have been advanced
in favor of sympatric
and other
forms of nongeographical speciation that the question as to whether these other
modes of
speciation occur
and
if so,
to
what
extent,
is still
controversial. Conceptual positions are clearly involved in this argu-
ment, some authors approaching the problem from the point of populational geography, while others derive their arguments from local ecology.
The reasons why some
theories have to struggle for the better part
Does Science Advance?
103
of a century before they are accepted, while a few
new
almost instantaneously, are manifold;
of them. 19
One
I
will list six
ideas succeed
reason that consensus takes a long time to achieve
is
that
different sets of evidence lead to different conclusions. For instance,
the student of geographic speciation
is
consistently impressed
by the
gradualness of the speciation process and considers this powerful
evidence for gradual evolution. By contrast,
many
been equally impressed by the universality in the
between species this equally
as well as
paleontologists have fossil
record of gaps
between higher taxa and have considered
convincing evidence for saltational evolution. The resulting
challenge, then,
is
to
show how
the discontinuous fossil record can be
reconciled with the gradual process of speciation. This was attempted
by Mayr, Eldredge and Gould, and
A second reason why consensus
is
Stanley. 20
hard to achieve
is
that disagreeing
adhere to different underlying ideologies, making certain
scientists
one group which are impossible
theories acceptable to
for another
group. For instance, the theory of natural selection was unacceptable in
(and ensuing years)
1859
teleologists,
and deterministic
gies ("deep
paradigms") meets
ment of erroneous
theories.
to
creationists,
physicalists.
more
far
who
held
who
as vitalism, essentialism,
them and were not
Opposing concepts therefore spread only
ideolo-
resistance than the replace-
Such viewpoints
and natural theology were an
creationism, teleology,
the worldview of those
ents
natural theologians,
The replacement of
slowly,
essential part of easily given up.
by recruiting adher-
did not yet have a firm worldview.
A third
reason
is
to account for the
that at a given time several explanations
same phenomena equally
well.
An
may seem
instance
is
long-distance orientation in birds, which has been attributed to sun orientation, magnetism, olfaction,
In
some
instance, either
cases there
is
factors.
completed speciation
may be
achieved by the acquisition of
premating or postmating isolating mechanisms; or
rapid geographic speciation relict
and other
actually a pluralism of possible answers. For
may
relatively
take place either in founder or in
populations; or species status
may be
attained by
chromosomal
reorganization.
Sometimes a consensus cannot be reached because one
biologist
is
THIS
104
IS
BIOLOGY
concerned with proximate, the other with evolutionary, causations. For T.
H. Morgan, sexual dimorphism was explained by the sex chromo-
somes and hormones (proximate causations), while of evolution
it
is
for the students
explained by selection for reproductive success (an
evolutionary causation).
Some
factors that
strictly scientific.
work
against acceptance of
new
ideas are not
Perhaps one author was disliked or had even offended
the current establishment, while another
had unexpected success with
a subsequently refuted theory because he belonged to a powerful clique.
When
the scientists involved belong to different schools or
countries in which different explanatory schemes have been traditional,
consensus
may be
of the other
five
harder to achieve. Presumably in these cases one
reasons listed above had been primary, but once a
tradition
was established
face of
opposing evidence.
all
by many French authors
was tenaciously maintained even
it
An example
for a
is
in the
the long-lasting preference
Lamarckian interpretation of evolution,
while in most other countries selectionism had already been victorious.
The
scientific
accept the least
establishment of a country
work of an author of
their
own
is
usually
more ready
nation, or one
who
to
has at
published in their language, than the writings of foreign authors.
Important work published in Russian, Japanese, or even non-English western European languages ignored altogether. Even
if
is
likely to
cation
is
if
not
the ideas contained in such neglected
publications are eventually adopted,
rediscovered
be widely neglected,
them subsequently, and
it is
often because
someone
else
the priority of the earlier publi-
forgotten.
The Limits of Science In his
famous essay Ignoramus, ignorabimus ("We do not know, we
number of scien-
will
never know") DuBois-Reymond in 1872
tific
problems which he was sure science would never be able
Yet
listed a
by 1887 he had to admit that some had already been
some of were on
his critics claimed that all
the
way
had been solved
to solve.
solved. Indeed, in principle or
to solution.
Occasionally one reads the overenthusiastic statement that science
Does Science Advance? can find the solution to that this
is
not true. 21
our problems. Every good
all
Some
ciple.
scientist
knows
of the limitations of science are practical,
while others are a matter of principle. There certain experiments with
105
human
general agreement that
is
subjects are out-of-bounds
on
prin-
They violate our moral standards, perhaps even our moral sense.
Certain experiments in "big physics," on the other hand, are simply
too expensive to justify support. Here again there
though
A
in this case the limitation
is
one of
serious practical limit to science
is
a definite limit,
is
practicality.
the difficulty of exhaustively
explaining the workings of a highly complex system.
due time we
in
will
I
am
sure that
understand, in principle, the workings of devel-
opment, of the brain, and of an ecosystem. But considering, for
more than
instance, the
1
billion
neurons in the brain, the complete
analysis of a particular thought process
may
forever be too
complex
for a detailed analysis.
The same
made about
practical point can be
the regulatory mecha-
nisms of the genome, which are highly complex and which are far
from being understood. What
amounts and
different types of
is
still
the function
(if
any) of the vast
DNA?
In
some organisms
noncoding
DNA than the total of the coding genes. To assume that all of this DNA is merely an unwanted byproduct ("junk")
this
adds up to more
of various molecular processes
is
not a palatable solution for a Dar-
winian. There have been non-Darwinian proposals, but they are not convincing. Here clearly that
some of
selected)
the
DNA
is is
an area of unfinished
science.
guess
is
indeed an unselected (or not yet counter-
byproduct of molecular process, but that other components
are part of the
complex regulatory machinery of the genome.
Most problems
relating to
"What?" and "How?" questions
least in principle, accessible to scientific elucidation. It
"Why?" questions. Many of the
latter,
the color of gold?
Why
is
Why
certain wave-
Why
are rho-
the capacity to translate light into
Why do bodies respond composed of elementary particles?
nerve impulses?
does gold have
do electromagnetic waves of a
length produce in our eyes the sensation of redness?
dopsins the only molecules to have
are, at
different with
particularly those relating to the
basic properties of molecules, are unanswerable.
nuclei
My
to gravity?
Why
are atomic
THIS
106
Some chanics,
IS
BIOLOGY
of these are probably solvable by chemistry,
and molecular
quantum me-
biology. But there are other "ultimate ques-
tions," particularly pertaining to values, that
can never be answered.
many unanswerable questions often asked by non"Why do I exist?" "What is the purpose of the world?" and
This includes the scientists.
"What was there before the beginning of the universe?" All such and there is an endless number of them, deal with problems
questions,
outside the
domain of
The question
is
science.
sometimes raised
as to the future of science.
Con-
sidering man's unquenchable thirst for knowledge, the incompleteness
of our present understanding, and the high success of science-based technology, there to flourish
Bush has
is little
and advance
doubt as
it
in
my mind
has for the
said so rightly, science indeed
that science will continue
last
is
250
years.
an endless
As Vannevar
frontier.
CHAPTER How Are
SIX
the Life Sciences
Structured?
Biology,
as
it
exists today,
Part of the reason isms, ranging It
and genes
interactions
is
that
it
an extraordinarily diversified science. deals with exceedingly varied organ-
from viruses and bacteria
also deals with
cules
is
many
to fungi, plants,
hierarchical levels,
to cells, tissues, organs,
and animals.
from organic macromole-
and whole organisms, and the
and organization of whole organisms
into families,
com-
munities, societies, populations, species, and biota. Each level of activity
and organization
cytology,
anatomy,
mention only a applications
is
an area of specialization with
genetics,
systematics,
ethology,
few. Furthermore, biology has a
and has given
rise to,
or
is
its
own name
or
ecology,
to
wide range of practical
at least involved in,
numerous
applied fields such as medicine, public health, agriculture, forestry, plant raphy,
and animal breeding, pest
control, fisheries, biological oceanog-
and so on.
Even though biology
as a
modern
science originated as recently as
the middle of the nineteenth century,
back to the ancient Greeks. 2,000 years ago are
still
Two
its
roots, as
we have
seen, go
distinct traditions that arose over
recognizable today: the medical tradition,
represented by Hippocrates and his predecessors and followers, and the natural history tradition. in the ancient
The medical
tradition, reaching a climax
world in the work of Galen
(c.
130-200), led to the
development of anatomy and physiology, while the natural history tradition, culminating in Aristotle's History of Animals and his other
THIS
108
IS
BIOLOGY
biological works, eventually gave rise to systematics, comparative biology, ecology,
and evolutionary
biology.
The separation of medicine from natural history continued through and the Renaissance. The two traditions were, how-
the Middle Ages
by botany because
ever, linked
history, focused
Indeed,
on the
this field,
although a branch of natural
plants believed to have medicinal properties.
the leading botanists from the sixteenth to the
all
eighteenth centuries
—
that
is,
from Cesalpino
end of the
—were phy-
to Linnaeus
with the single exception of John Ray. The more
sicians,
biological
components of medicine,
in
strictly
due time, became anatomy and
and those of natural history became botany and zoology,
physiology,
while paleontology was associated with geology. This classification of the
life
sciences prevailed
the twentieth century.
The
Scientific
had the most
from
late in the
eighteenth until well into
1
Revolution had only a minor impact on biology.
decisive effect
was the discovery
What
in the seventeenth
and
eighteenth centuries of the almost unimaginable diversity of the faunas
and
floras in different parts
back by
official
of the world. The rich booty brought
voyages and individual explorers (such as the plant-
collecting students of Linnaeus) led to the
and museums and favored an emphasis on systematics
collections
Chapter
founding of natural history
7).
entirely of systematics, except for the study of at
(see
Indeed, biology in the age of Linnaeus consisted almost
anatomy and physiology
medical schools.
Almost tive. It
all
work
would be
in the life sciences during that period
was descrip-
a mistake, however, to consider this period of biology
as conceptually sterile.
Through the natural
history of Buffon, the
physiology of Bichat and Magendie, the idealistic morphology of
Goethe, the work of Blumenbach and his followers Cuvier, Oken, and
Owen, and laid for
the speculations of Naturphilosophie, the foundations were
most of the subsequent conceptual breakthroughs.
Still,
in
view of the enormous diversity and uniqueness in the living world, a
much broader sciences. This
factual basis
was
laid
was needed
in biology than in the physical
not only through systematics but also through
comparative anatomy, paleontology, biogeography, and related ences.
sci-
How Are
The term biology was introduced into the by Lamarck, Treviranus, and Burdach. But 2
no
1800
literature as early as at first there
was
actually
of research deserving this name. The term indicated, however,
field
a trend or a goal tive,
109
the Life Sciences Structured?
and
away from a
signified a turning
strictly descrip-
taxonomic preoccupation and a move toward a greater
interest
in living organisms. Treviranus (1802:4) offers this description: "The
subject matter of our investigations will be the various forms
manifestations of
life,
the conditions
and the causes by which
ence,
occupies
itself
is
effected.
The
and
their exist-
which
science,
with these subjects, we shall designate by the
biology or science of
The
this
and laws controlling
name
life."
origins of the science of biology as
between 1828 and 1866 and
is
we know
it
names von Baer
associated with the
(embryology), Schwann and Schleiden
today took place
theory), Muller, Liebig,
(cell
Helmholtz, DuBois-Reymond, Bernard (physiology), Wallace and Dar-
win (phylogeny, biogeography, evolutionary theory), and Mendel netics).
On
of
led to
From
The excitement of
this
the Origin of Species in 1859.
Developments
in these 38 years
we
find today.
The Comparative and Experimental Methods
in Biology
most of the subdisciplines of biology
the Greek kosmos to
(ge-
period was capped by the publication
modern
that
times, philosophers
and
scientists
have used two major approaches in their search for some underlying order in nature. The
first
regularities they observed.
By
this
was
at first
was the search
for laws to account for the
The other was the search
meant not phylogenetic
"having items in common."
And
this
for "relationship."
relationship but simply
could be established only by
comparison.
work when they developed comparative mor-
The comparative method achieved of Cuvier and his associates phology. At
first this
its
greatest
in the
was a purely empirical endeavor, but
common
proposal by Darwin in 1859 of the theory of
and more became a rigorous
method turned out
triumph
scientific
after the
descent
it
more
method. The comparative
to be so successful that
it
was applied
to other
biological disciplines, leading to comparative physiology, comparative
THIS
110
IS
BIOLOGY Modern macrotax-
embryology, comparative psychology, and so on.
onomy
A
is
almost exclusively comparative.
major impetus to the new science of biology was the invention
and development of new instrumentation. Instruments invented by Johannes Mtiller and his students and by Claude Bernard were decisive in the pioneering
No
developments of physiology.
however, had a greater impact on the
improved microscope. This resulted biological disciplines,
rise
other instrument,
of biology than the steadily
in the
development of two new
embryology and cytology. 3
After 1870 a split developed in biology, the reasons for which were
not understood (with
at the time.
The biology of evolutionary causations
almost exclusive emphasis on phylogeny) was based on
its
comparison and on inferences from observations
by
their opponents).
The biology of proximate
(called speculation
causations,
on the other
hand (primarily physiology and experimental embryology),
stressed
experimental approaches. Representatives of these two schools of bi-
ology argued vehemently over which of the two was the right one. Today, of course,
it
is
clear that
both
sets
of questions must be
answered.
When the
it
same
mode
was discovered that the structure and function of
in animals
and
plants,
and
that this
was
was
cells
also true for the
of inheritance of individual characteristics, the old division into
botany and zoology no longer made very
much
sense.
This was
especially true after the great similarity, indeed virtually identity, of
molecular processes in the two kingdoms was discovered, and
all
the distinctness of the fungi
or plant
kingdom had been
and prokaryotes from
established.
It
became
that in a classification of biological concepts for
new
increasingly obvious
one would have
to look
ordering principles, not based on the type of organisms.
After the development of cellular
people argued that there was
botany
after
either the animal
at
all.
now no
However, in certain
and molecular
biology,
some
longer any need for zoology and
areas,
such as taxonomy and mor-
phology, there remained a need to deal with animals and plants separately.
Development and physiology
rather different in plants
are, likewise,
on the whole
and animals, and behavior concerns only
How Are animals.
may
No
how
matter
Ill
the Life Sciences Structured?
brilliant the
advances in molecular biology
need for a biology of whole
be, there continues to exist a vital
organisms, even though such a biology might have to be organized very differently from the traditional one.
But aside from these exceptions, plants
all
biological problems concern
What is particularly interesting about the new biological disciplines is that equivalent were made by students both of plants and of animals. Brown discovered the cell nucleus, and the botanist
and animals
equally.
origin of the various
contributions
The
botanist
Schleiden with the zoologist
problem of
made by and
Schwann proposed
who came from
developed by Virchow,
fertilization likewise
and
botanists
the
was solved by
zoologists,
and
cell
theory, further
zoology and medicine. The
this
is
a series of discoveries
equally true for cytology
later for genetics.
Numerous attempts have been made tion of
to develop a rational classifica-
enormous range of
biological disciplines, to deal with the
all
phenomena brought them has been
together under the heading biology, but
entirely successful so
far.
Among
none of
the classifications
all
of biology that have been proposed over time, none has been
more
misleading than the one that recognized three branches of biology:
and experimental. Not only were
descriptive, functional,
of biology
(like
much
this classification,
but
entire fields
of evolutionary biology) virtually excluded by it
in all parts of biology,
ignored the fact that description
and that the experiment
is
is
a necessity
a major tool of
analysis almost exclusively in functional biology. Furthermore, the
experiment
is
most important not so much
as
a
means of data
collecting but rather for the testing of conjectures.
Driesch revealed
how
when he remarked how chairs
were
now
little
he understood the structure of biology
fortunate
it
was that
at
German
universities
given only in experimental biology and none in
taxonomy. Here he lumped evolutionary biology, ethology, and ecology with taxonomy, and considered
all
parts of organismic biology purely
descriptive sciences because they were not experimental. Gillispie's
comment
that
taxonomy does not
interest the historian
example of a misconception of different biological
is
another
disciplines.
THIS
112
New
BIOLOGY
IS
Attempts
to Structure
Biology
symposium devoted
In 1955 the Biology Council organized a special to the analysis of the concepts of biology
the structure of biology. 4
The
criteria
and how best
to represent
by which various authors pro-
posed to divide biology into disciplines were exceedingly varied. Widely favored was Mainx's division into morphology, physiology, embryology,
and a few other standard cytology, histology,
subjects, often hierarchically subdivided into
whole organ physiology, and so on, on the
of morphological considerations. Another widely accepted tion,
proposed by
P.
Weiss, chose a
more or
less hierarchical
basis
classifica-
approach:
molecular biology, cellular biology, genetic biology, developmental biology, regulatory biology,
group and environmental biology. 5
Many
of the review panels of the National Science Foundation were labeled according to this classification. the experimentalist Weiss
no
interesting (and
It is
lumped
all
surprise) that
aspects of organismic biology
(systematic, evolutionary, environmental,
and behavioral biology) un-
der one category, "group and environmental biology," while reserving categories of equal weight for hierarchical levels
five
below whole
organisms. Generally, the criteria of classification that any given author suggests are greatly influenced
by
his educational
background.
If
he comes from
the physical sciences or was strongly influenced by them, he to stress experiment, reduction,
centrate
on functional
and unitary components and
processes. 6
were raised as naturalists tend to tions, systems, inferences
In 1970 the
Committee on the
Life Sciences of the National last three
lution (10)
nutrition,
While
this
(11)
biology, (8)
aspects.
Academy
of which are applied
fields:
(2) genetics, (3) cell biology,
developmental biology,
and systematic
who
uniqueness, popula-
stress diversity,
molecular biology and biochemistry,
(4) physiology, (5)
to con-
contrast, those biologists
from observation, and evolutionary
recognized twelve categories, the (1)
By
likely
is
(6)
morphology,
(7) evo-
ecology, (9) behavioral biology,
disease mechanisms,
and
(12)
improved on some of the other systems,
it
pharmacology. 7
too had problems,
such as considering systematics and evolutionary biology to be a single discipline.
How Are Eventually,
it
was
scientific research
113
the Life Sciences Structured?
one asks
realized that the types of questions
might help
in leading to a
more
in
logical classification
of the biological disciplines. The three big questions
are:
"What?"
"How?" and "Why?" "what?" questions
One cannot do factual basis
—
science,
that
is,
any science, without
first
establishing a solid
recording the observations and findings on which
theories are based. Description thus
is
a very important aspect of any
scientific discipline.
Curiously, attaching the
always has had a
word
somewhat
tended to
call
speaking,
most of the
"descriptive" to a scientific discipline
pejorative implication.
the morphologists'
work
physiologists'
that of the morphologists.
The
descriptive, even though, strictly
own work was
Some molecular
as descriptive as
biologists have confessed
embarrassment that so much of the work published nothing but a recording of
physiologists
facts (descriptive).
There
in their field is
no need
such embarrassment, because molecular biology, being a needs, like
all
new
is
for
field,
other branches of science, to go through this descriptive
phase. It
would be misleading
biology. Description
is
to recognize a separate discipline, descriptive
the
first
step in any
omy, the recognition of species and higher than
much
genome
branch of biology. Taxon-
taxa,
of molecular or cellular biology,
project. Description
is
no more
or, for that
descriptive
matter, the
should never be maligned, because
the indispensable foundation of
all
it is
explanatory and interpretive re-
search in biology. 8
What
is
rather surprising
to Rensch, Mayr, Simpson,
worth of
their
own
cies in Biological
is
that the taxonomists themselves, prior
and Hennig, had
that has
no
appreciation of the
Tenden-
Theory" the distinguished ant taxonomist
Wheeler (1929:192) said that taxonomy
wrong
little
discipline. In a discussion called "Present
"is
theory, being merely diagnostics
this idea was,
was made
W
M.
the one biological science
and
clear, for instance,
classification."
How
by the publications
of Hennig, Simpson, Ghiselin, Mayr, Bock, Ashlock, and Hull. 9 All sciences deal
both with phenomena and with processes, but in
THIS
114
some
IS
BIOLOGY
phenomena
sciences the study of
prevails, in others the study
of processes. The physiologists, concerned with the explanation of the
machinery of
deal almost exclusively with processes.
life,
The
evolu-
tionary biologists, however, deal also with processes, those that lead to evolutionary changes, particularly to
new
new
adaptations and
taxa.
But one of the principal concerns of the naturalists has always been the study of the diversity of special concern of
and ecology.
It
many
life.
The study of organic
diversity
biological disciplines, particularly
involves an interaction of
is
the
taxonomy
complex systems, and requires
from the
a rather different strategy, for example,
analysis of simple
physiological processes, as studied in the laboratory.
The study of
diversity invariably
This
sive description as a first step.
(including paleontology
and
and
demands is
precise
and comprehen-
particularly true for
taxonomy
parasitology), biogeography, autecology,
branches of comparative biology (including comparative bio-
all
chemistry). This descriptive basis permits the comparisons that lead to the generalizations characterizing the various subdisciplines of evo-
lutionary biology. Criticism
is
justified
only
beyond description. The most important
and theories
generalizations
when
scientists
never go
results of science are the
that are derived
from the raw
factual
material.
In any
field,
the data-collecting phase
is
rarely ever completed.
Not
only does science as a whole have an endless frontier, but so does each of
its
many
become
subdivisions.
available,
this are the
Whenever new methods
for data collecting
whole new horizons open up to view. Examples of
advent of electron microscopy in cytology, scuba gear for
shallow water research, or
new methods
for collecting the fauna in
the canopy of tropical forests. Invertebrate zoology
vances the
when
bottom
fauna,
made major
ad-
technologies were developed to collect the meiofauna of
layer of the ocean, the pelagic as well as benthic
deep sea
and the organisms associated with volcanic hot vents
in the
ocean deeps.
Looking back rassed at
how
at the history
of biology, a biologist
neglected were
all
is
almost embar-
organisms that were not higher
animals or higher plants. For instance, everything that was not clearly
an animal was traditionally considered to belong to the domain of
How Are Only very
botany. are
recently have biologists realized
from plants (indeed, they are more
even more recently
and
relatives) are
plants,
how
115
the Life Sciences Structured?
how
different fungi
and
closely related to animals)
strikingly different the prokaryotes (bacteria
from the eukaryotes (including the
protists, fungi,
and animals). The Prokaryota are now recognized
as a separate
super kingdom, and provide a remarkable example of the endless frontier that exists in biology even at the descriptive level.
"how?" and "why?" questions Answers
to the
"What?" questions alone
solution to the problem of
Hence, we must
now
"How?" tion?
failed to
produce a satisfactory
to classify the subdivisions of biology.
turn to the "How?" and "Why?" questions. 10 In
functional biology, as in level to the
how
all
aspects of physiology
from the molecular
function of whole organs, research deals primarily with
questions.
How
does a particular molecule perform
By what pathway does
a
whole organ function? Such questions,
which deal with the here and now, have been referred to of proximate causations. This
func-
its
field,
from the molecular
as the study
up
level
to
whole organisms, deals primarily with the analysis of processes.
"How?" it
is
the
most frequent question
in the physical sciences,
led to the discovery of the great natural laws.
It
and
was the dominant
question also in biology until the early 1800s because the then-leading biological disciplines, physiology physicalist thinking.
and embryology, were dominated by
These two disciplines were almost exclusively
concerned with the study of proximate causations. To be sure, "Why?" questions were also asked, but with Christianity being at that time the
dominant ideology of the Western world, such questions
bly yielded the facile answer: the
Law-Maker
(physicalism),
God the and God
inevita-
Creator (creationism),
God
the Designer (natural theol-
ogy)-
"Why?" questions deal with the that account for
all
existed in the past.
World?
Why are
historical
and evolutionary
aspects of living organisms that exist
Why
are
hummingbirds
now
factors
or have
restricted to the
desert animals usually colored like the substrate?
do insect-eating temperate zone birds migrate
New Why
in the fall to subtropical
or tropical areas? Such questions, usually relating to adaptations or to
THIS
116
BIOLOGY
IS
organic diversity, have traditionally been referred to as the search for ultimate causations.
"Why?" questions did not become and more
tions until after the proposal of evolution after
when Darwin proposed
1859
particularly until
mechanism
a concrete
ques-
scientific
for change:
natural selection.
Very few people
realize that
it
was Darwin who was responsible
And by
for
making "Why?" questions
scientifically legitimate.
questions he brought
of natural history into science. Physicalists
all
asking these
Herschel and Rutherford had excluded natural history from science
like
because
it
did not conform to the methodological principles of physics.
The nature of inanimate
objects, not having a historically acquired
genetic program, cannot be elucidated by "Why?" questions. What
Darwin did was equipment of
to
add a most important new methodology
The terminology of proximate and ultimate causations has history,
a long
perhaps going back to the days of natural theology, when
"ultimate" referred to the
hand of God.
It
has been said that Herbert
Spencer spoke of ultimate and proximate causes, but the reference
I
have been able to find
(1897:98) wrote to
ments ...
Darwin
of force.
is
in 1880:
No doubt
"To
an obscure disease this
is
which G.
in a letter offer
as a full explanation of heredity
that the cause, say, of
requires
to the
science.
.
.
.
J.
earliest
Romanes
molecular move-
seems to
me
like
saying
like diabetes, is the persistence
the ultimate cause, but the pathologist
some more proximate cause
if
his science
is
to be of
any
value."
Considering the vagueness of it
this statement,
it is
not surprising that
took another 40 years until a better defined usage was introduced
into the literature
by John Baker (1938:162).
in full his use of these terms:
It is
of interest to quote
"Animals have evolved the capacity to
respond to certain stimuli by breeding. In cold and temperate climates it
is
up
usually clear that the season adopted allows the
in favorable climatic conditions,
and one may say
young
to
grow
that in a sense
these conditions are the ultimate cause of the breeding season being at that particular time.
There
is,
of course, no reason to suppose that
the particular environmental conditions favorable to the
young
are
How Are necessarily the
one or ones which constitute the proximate cause and
stimulate the parents to reproduce. the
117
the Life Sciences Structured?
young might be the
Thus abundance of
insect food for
and length of day the proximate
ultimate,
cause of a breeding season."
David Lack I
(in 1954)
adopted
(in 1961)
took
this
terminology over from Baker, and
from both of these authors (even though
it
after
Darwin ultimate causation simply meant evolutionary causation). The concept was quickly further developed by Orians (1962) and some
Even before 1961, perceptive biologists understood well
ethologists.
two
that there are these
"All biological
stated:
mechanisms
as well as
on the
to stay
until
I
products of evolution
side of the repeatable
leave the singular others."
sides to biology. Weiss (1947:524), for instance,
systems have a dual aspect. They are causal .
.
.
Physiology
may want
and controllable phenomena and
and non-repetitive cause of
But neither Weiss nor anyone
else
historic evolution to
enlarged
upon
these hints
formalized the distinction in 1961.
Proximate causes as well as
its
relate to the function
of an organism and
its
parts
development, as investigated from functional morphology
They
to biochemistry.
deal with the decoding of genetic
and somatic
programs. Evolutionary (historical or ultimate) causes, on the other hand, attempt to explain
why an organism
is
the
way
it is,
as a
product
of evolution. They explain the origin and the history of genetic pro-
grams. Proximate causes are usually the answer to the question "How?" while evolutionary causes are usually the answer to the question
"Why?"
much of the history of biology of the last were made to explain biological phenomena
Unfortunately, through
130 years endeavors
exclusively in terms of either
one or the other of these two causations.
The experimentalists would say
that
development was
entirely
due to
physiological processes in the developing embryo, while evolutionary biologists
would
into a fish,
and
were considered. such
egg of a fish would always develop
that of a frog into a frog, also that such
as recapitulation
past,
stress that the
phenomena
would not make sense unless the evolutionary
Many
aspects
of the great controversies in biology of the
as the controversy
between the nature and nurture schools
THIS
118
and behavior, or the
inheritance
in
BIOLOGY
IS
of the
rebellion
Entwick-
lungsmechaniker against the Haeckelian comparative embryologists, 11 result of this one-sidedness.
were the
The continuing confounding of questions dealing with proximate and ultimate causations
rationale
is
particularly apparent in the writings of the
is
and the
so-called structuralists
and rather
antiselectionist
morphologists. Their basic
idealistic
teleological; they see logic, order,
rationality in the biological realm. 12
and
principle,
is
frowned upon and
is
Chance, as an explanatory
always considered an alternative to
directional processes rather than
selective
a simultaneous process.
Consideration of the "historical" (evolutionary) component of bio-
phenomena
logical
is
to be avoided if at
possible. 13
all
That both
causations have to be considered in most biological explanations,
purely physicochemical ones excepted,
The recognition these
two very
and
has led to methodological
it
(what method to use when) and to a clearer demarcation
between various biological
disciplines.
It
has also called attention to
the historical aspect of ultimate causations
mechanisms involved that
most
broken down into
different questions has helped to resolve various con-
ceptual controversies in biology, clarification
not seen by the structuralists.
is
that biological inquiry can be
and
in proximate causations,
biologists are
to the physiological
and
on the whole students
it
of proximate causations, owing to their choice of
work.
Yet, as
I
has demonstrated
either of ultimate or
have always insisted, no biological
field in
which
phenomenon
is
to
fully
explained until both proximate and ultimate causations are illuminated. Even
though most biological
one or the other
set
disciplines concentrate
on
either
of questions, each of these disciplines, to a lesser
or greater extent, has to consider also the other type of causations. Let
me
illustrate this for
molecular biology.
a functional role in an organism. interacts with other molecules, cell,
and so on
the
it
played in the history of
life,
But when we ask
evolution,
it
given molecule has
performs
this role,
how
it
role in the energy balance of the
these questions result in a study of proximate cau-
why
sations.
role
—
How
its
A
how and why
it
differs
contains this molecule, what
cell
how
it
may
have changed during
from homologous molecules
organisms, and similar questions, then
we
in other
are dealing with ultimate
How Are causations.
119
the Life Sciences Structured?
The study of both kinds of causations
is
equally legitimate
and indispensable.
The study of animal behavior a particular type of
does
is
another area that demonstrates the
Why
organism displays the behavior components
it
But to explain the neurophysiology of a
a result of evolution.
particular behavior
is
between the two types of causation.
particularly close connection
requires
a
study of the proximate causations
through neurophysiological studies. Proximate causations impinge on the phenotype, that
is,
on mor-
phology and behavior; ultimate causations help explain the genotype
and
its
Proximate causations are largely mechanical; ultimate
history.
causations are probabilistic. Proximate causations occur here and now,
moment,
at a particular
at a particular stage in the life cycle
individual, during the lifetime of
of an
an individual; ultimate causations
have been active over long periods, more specifically in the evolutionary past of a species. Proximate causations involve the decoding of an
program; ultimate causations are respon-
existing genetic or somatic sible for the origin
of
new
genetic programs
determination of proximate causations
is
and
their changes.
The
usually facilitated by experi-
mentation, of ultimate causations by inference from historical narratives.
A
NEW CLASSIFICATION BASED ON "HOW?" AND "WHY?"
What
classification
to arrange strictly
them
on the
of the
life
sciences might
one adopt
if
one were
either with proximate or with evolutionary causations,
basis of their
major concern?
All of physiology (organ
physiology, cellular physiology, sensory physiology, neurophysiology,
endocrinology, and so on), most of molecular biology, functional
morphology, developmental biology, and physiological genetics
fit
best
with proximate causations. Evolutionary biology, transmission genetics, ethology, systematics, comparative morphology,
and ecology
fit
best
with evolutionary causations. This tentative division immediately results in certain
difficulties,
such as the necessity of splitting genetics into transmission (and population) genetics and physiological genetics, or of splitting mor-
phology into functional and comparative morphology. However, these
THIS
120
BIOLOGY
IS
had already been conceptually separated
disciplines
even though covered by a single instance,
is
for a long time,
Functional morphology, for
label.
often studied by descriptive morphologists,
make
of phylogeny
hard to place;
extensive use of molecular methods. Ecology
is
complex systems, and therefore
deals largely with
it
and students
most ecological problems involve both proximate and ultimate causations.
When
in the nineteenth century the cell theory
by Schwann, Schleiden, and Virchow phology and so
modern
it
was
still
cellular biology
is
Power
it
was
clearly a
heyday of electron microscopy, but
in the
largely molecular biology.
Shifts within Biology
The ongoing restructuring of biology could not good
and
deal of tension, controversy,
subdiscipline
and would
became
successful,
it
much
try to take as
away from the established
was developed
branch of mor-
would
fight for
attention
disciplines.
take place without a
dislocation.
Whenever
its
a
new
place in the sun
and resources
Sometimes a new
as possible field
would
my Ph.D. in Berlin in 1926, several knowledgeable zoologists advised me to switch to Entwicklungsmechanik, if I were to choose academic zoology as my career. When
establish a virtual
monopoly.
"Spemann
the vacant chairs," they told me.
fills all
I
got
DuBois-Reymond
never concealed his contempt for the "descriptive zoology" of his teacher Johannes Miiller, even though in retrospect the achievements
of his
own
ever field
competing time
this
research are by comparison not
was dominant fields
at
all
and capture
as
many
The biochemist George Wald loudly proclaimed he
said.
it
is
The
positions as possible.
happened was when molecular biology had
biology and that
Which-
that impressive.
any time would try to squeeze out the
molecular biology;
all
that there
of biology
is
is
last
flowering.
its first
only one
molecular,
At several universities in the United States most or
all
organ-
ismic biologists were at that time replaced by molecular biologists.
With the physical
sciences traditionally favored
elections to the National
Academy,
by Nobel
in advisory roles of the
and by industry, those parts of biology
Prizes, in
government,
closest to the material
and the
thinking of the physical sciences were always favored by the govern-
How Are
the Life Sciences Structured?
121
ment, while other aspects of biology, such as the study of biodiversity,
were consistently neglected. The origin of
this diversity,
one of the
two principal problems of evolutionary biology, was almost
totally
ignored by evolutionary genetics prior to the evolutionary synthesis.
Medicine- related biology has, for obvious reasons, always been a vorite
among
fa-
granting agencies. Equivalent projects usually get far
when supported by the National Institutes of Health when supported by the National Science Foundation.
higher financing
than
Botany particularly suffered from these developments. In the days of Linnaeus, botany was the scientia amabilis, and right up to the early
many
twentieth century there were ologists.
botanists
among
the leading bi-
This was particularly true for cytology and ecology. All three
of Mendel's so-called rediscoverers (DeVries, Correns, and Tschermak)
were botanists. But then began a
series
of setbacks. The study of fungi
(mycology) was removed from botany and became an independent field;
even more importantly, so did the study of the prokaryotes. Most
zoologists,
around 1910, had become
after
specialists
and so on, and
genetics, neurophysiology, behavior,
felt
in cytology,
that they were
phenomena and wanted to be called biologists rather than zoologists a word which, rightly or wrongly, always seemed to remind them of morphology or taxonomy. The word "biological" increasingly often was used comprehensively for the comdealing with basic biological
—
bination of botany and zoology. For example, in 1931 at Harvard the Biological Laboratories were established in a
In this
new department
there
still
Department of
were professors
who
Biology.
taught
strictly
botanical subjects, like plant morphology, plant physiology, plant tax-
onomy, and plant reproductive biology, but now they were rubbing
who
specialized in equivalent zoological
Institute
of Biological Science (AIBS) was
elbows with other biologists subjects.
When founded
the
American
in 1947,
disciplines.
The
it
included botany, zoology, and
botanists, however,
all
other biological
were apprehensive (with consider-
able justification) that the unique characteristics of plants
forgotten
if
the consolidation into biology went too
the National
Academy reorganized
its
far.
would be
When
in 1975
subdivisions, the section of
zoology was abolished and replaced by a section on population biology,
THIS
122
and
evolution,
ecology.
The
BIOLOGY
IS
botanists were invited to
preferred to preserve their section. a section of plant biology
would lead
A number
properties of plants.
They maintained
do likewise but abandoning
that
unique
to a neglect of the
of botanists, however,
left
the section
of plant biology and joined such general biological sections as the section of genetics or the section of population biology. 14
But botany has by no means been obliterated. For instance,
assumed leadership
make important
botanical journals continue to
and botany departments
ogy,
universities. Indeed, in the
ment, botany
is
it
in the study of tropical biology. Herbaria
now
again
are
still
has
and
contributions to biol-
many
active at
colleges
and
wake of the modern conservation move-
more productive than
it
had been
in the
preceding period.
Almost invariably the representatives of a new
new
ders of a
discipline, think that this
tradition, the foun-
makes one of the
classical
subdivisions of biology obsolete. Actually, even the most traditional
—
branches of biology
—
ogy
them
are endless unfinished frontiers
to
still
and physiol-
systematics, anatomy, embryology,
are
needed, not just as data banks but also because
round out our view of the
have a golden period, and
and
all
living world.
many
of them are
Each
still
discipline
all
of
needed
seems to
of them have several. But even after
the law of diminishing returns has taken over, there for abolishing a discipline that has
become
is
"classical."
no
justification
15
Biology, a Diversified Science
Chapters
1
and
2 emphasized the distinguishing features
and concepts
of biology as compared with the physical sciences, theology, philosophy,
and the humanities. Almost equally important
differences within biology.
bank,
its
own
set
larities
among
and
but even they
theories
its
own
its
own
conceptual framework,
scientific societies.
To be
its
data
own
sure, there are simi-
the biological disciplines that deal with proximate
causations, as well as tions,
Each branch of biology has
of theories,
textbooks, journals,
are the conceptual
among
differ
those that specialize in ultimate causa-
remarkably in the nature of the prevailing
and fundamental concepts.
To do such an
analysis for
all
special areas in biology
would have
How Are required far
more space than
my
have far exceeded
that follow, however,
is
available in this
competence. is
What
I
volume, and
—
developmental biology, evolution, and ecology of the struggle
among opposing
But before embarking on that a point
made
disciplines in
in the Preface
my
analysis.
task,
do.
genetic
It
program
informative
way
its
is
functions,
and
down
now
More
become
certainly true for genetics.
the structure of an organism, Didactically, the
I
I
would be
a
most
to use
my
have tried to do in
dealt only with transmission
to the rise of molecular biology, the
shifted to developmental genetics,
virtually
should elaborate on
to deal with the concepts of genetics
Growth of Biological Thought. But there
Owing
I
for not including certain
its activities.
the history of genetics as the vehicle. This
genetics.
systematics,
the underlying factor of everything organisms
plays a decisive role in laying
development,
its
is
—
convey the nature
biological disciplines relate to every-
thing that concerns living organisms. This
The
fields
to
fields.
perhaps
—my reasons
Some
would
concepts and the relative maturity of
framework of these
the current conceptual
it
attempt in the chapters
will
sample analysis of four
a
123
the Life Sciences Structured?
and
this
emphasis has
kind of genetics has
branch of molecular biology.
formidable, and perhaps quite insurmountable, are the prob-
lems posed by molecular biology. Whether we deal with physiology, development, genetics, neurobiology, or behavior, molecular processes are ultimately responsible for
what happens. Some unifying phenom-
ena are already apparent, such as the homeoboxes; others can be dimly perceived. But every time
I
have attempted to present a bird's-eye view
of molecular biology as a whole,
mass of detail. For
some of
though
in Chapters 8
discipline
The reason not that
that
I
I
less
—quite the contrary—but
that
I
is
I
devoted
do not have. The same
is
this
important than other parts of its
treatment requires a com-
true for neurobiology and psy-
chology, which are also exceedingly important. However,
my
is
have highlighted
have not devoted more space to
consider
it
petence
and 9
the major generalizations ("laws") discovered by molecular
biologists.
biology
have been overwhelmed by the
reason no special section of this book
this
to molecular biology,
I
I
hope
that
treatment of biology as a whole will shed some light on those
branches of biology that are not covered in detail in
this
volume.
CHAPTER SEVEN "What?" Questions: The Study of Biodiversity
The most impressive aspect of the
living
world
is its
diversity.
No
two individuals in sexually reproducing populations are the same, nor are any two populations,
Wherever one
species, or higher taxa.
looks in nature, one finds uniqueness.
Our knowledge of tially
during the
last
the diversity of
300 years.
and the work of individual
It
life
has been growing exponen-
began with the voyages of exploration
explorers,
whose recorded observations
and
collections revealed differences in the faunas
new
continent and island explored. Next came the study of freshwater
and ocean organisms, including those
in the
deep
and
sea,
floras
of every
which revealed
another dimension of biodiversity. The investigation of microscopic plants
and animals,
parasites,
and
fossil
remains has acquainted us
further with uniqueness in the earth's biota. Finally
and
scientific
fossil.
to describe
The
and
particular field of research
classify this vast diversity
still
the discovery
study of the prokaryotes (bacteria and their
both living and
became
came
relatives),
whose
of nature
is
task
it
called
taxonomy. After an initial burst of interest in classification by Aristotle
and
Theophrastus around 330 bc, taxonomy experienced a long decline until the Renaissance.
The
field
had
a
second great flowering through
work of Linnaeus (1707-1778), followed by another decline that was halted only when Darwin published his Origin of Species in 1859. the
1
This work was essentially the result of taxonomic research, and tax-
"What?" Questions: The Study of Biodiversity
onomy
125
has continued to play an important role in the development
of evolutionary theory, providing the basis for the biological species
concept and for major theories of speciation and of macroevolution (see below).
Realizing that the task of studying biodiversity
description and inventory-taking,
"taxonomy" should be ifying,
is
mere
greater than
Simpson suggested
term
that the
restricted to the traditional aspects of class-
while the term "systematics" should be applied to "the scientific
study of the kinds and diversity of organisms, and of any and
among
relationships
science of diversity,
adopted by
them." Systematics thus was conceived as the
and
biologists.
all
this
new broadened concept
has been widely
2
Systematics includes not only identification and classification of
organisms but also the comparative study of species as well as
an interpretation of the
economy of nature and
taxa in the
all
characteristics of
and higher
role of lower
in evolutionary history.
branches of biology depend entirely on systematics;
Many
includes
this
biogeography, cytogenetics, biological oceanography, stratigraphy, and certain areas in molecular biology. 3
a synthesis of
It is
knowledge, theory, and method applied to
The ultimate
task of the systematist
diversity of the living
standing.
is
all
many
kinds of
aspects of classification.
not merely to describe the
world but also to contribute to
its
under-
4
Classification in Biology
In daily
life,
one can deal with a
large
number of very
different items
only by classifying them. Classifications are used for the ordering of tools, drugs,
ideas.
and
When we
art objects, as well as for theories, concepts, classify,
their shared attributes.
are similar
and
A
related to
we group
and
objects into classes according to
class, then, is
an assemblage of
entities that
one another.
Every classification system has two major functions: to
facilitate
information retrieval and to serve as the basis of comparative research. Classification field.
is
the key to the system of information storage in any
In biology, this information storage system consists of
museum
THIS
126
and the
collections
by
ability to
its
homogeneous
and
vast scientific literature in books, journals,
The
other publications.
BIOLOGY
IS
quality of any classification
facilitate
is
judged
the storing of information in relatively
and
divisions
scheme
to permit rapid discovery
and
retrieval of
this information. Classifications are heuristic systems.
human
Considering that classifying has been a with our most primitive
human
uncertainty and disagreement tion.
And
areas of science,
all
science after
who
person rules
how
considering
it is
still
exists
how much
about the nature of
classifica-
this subject.
attempts to classify organisms can derive activities
store: (1)
However, the
some elementary
such as classifying books in a
Items that are to be classified should
be assembled into classes that are as homogeneous as possible. individual item the greatest for
is
included in the class with whose
number of
any item that
is
in
is
curious the extent to which philosophers of
Whewell (1840) have neglected
goods in a
beginning
activity
surprising
it is
important the process of classifying
from everyday human
library or
ancestors,
attributes. (3)
A
members
separate class
is
(2)
it
An
shares
established
too different to be included in one of the previously
established classes. (4)
The degree of
expressed by arranging
them
difference
among
the classes
is
a hierarchy of nested sets. Each
in
categorical level in the hierarchy represents a certain level of distinctness.
These rules apply also to the
though for the Considering
not
all,
living
how
indispensable taxonomic research
branches of biology, one
prestige in recent years.
plines
classification of organisms,
is
surprised by
The major method
in
comparison, and yet no comparison
is
conclusions that
is
even
world some additional rules are required.
its
many
is
many,
if
and low
biological disci-
will lead to
not based on sound taxonomy. In
to
neglect
fact,
meaningful there
is
no
—from comparative anatomy and comnot ultimately comparative psychology—
branch of comparative biology parative physiology to entirely
is
based on the findings of taxonomy.
The multiple follows:
that
(
1 ) It is
roles of
taxonomy
in biology
can be summarized as
the only science that provides a picture of the existing
organic diversity on earth. (2)
It
provides most of the information
needed for a reconstruction of the phylogeny of
numerous
interesting evolutionary
life.
(3) It reveals
phenomena and makes them
avail-
"What?" Questions: The Study of Biodiversity able for causal study
by other branches of biology.
127
(4)
supplies
It
almost exclusively the information needed for entire branches of biology (such as biogeography and stratigraphy). (5)
It
supplies ordering
systems or classifications that are of great heuristic and explanatory value in most branches of biology, such as evolutionary biochemistry,
immunology,
Through
(6)
ecology,
ethology,
genetics,
and
foremost exponents, systematics has
its
geology.
historical
made important
conceptual contributions, such as population thinking (see Chapter
would not otherwise be
that
8),
biologists.
easily accessible
experimental
to
These conceptual contributions have significantly broadened
biology and have led to a better balance within biological science as a whole.
The taxonomist brings order in
two
steps.
The
first is
into the bewildering diversity of nature
the discrimination of the species, an endeavor
referred to as microtaxonomy. species into related groups,
The second
an
is
the classification of these
activity referred to as
macrotaxonomy
Consequently taxonomy, the combination of the two, was defined by
Simpson (1961)
and
as "the theory
practice of delimiting kinds of
organisms and of classifying them."
Microtaxonomy: The Demarcation of Species
The
quite different
is
an
activity
from other concerns of the taxonomist.
It
is
an area
replete with semantic as the "species
and delimitation of
species
recognition, description,
and conceptual
difficulties,
usually referred to
problem." The term "species" simply means "kind of
organism," but because variation
is
so
rampant
in the living world,
one must define precisely what one means by "kind."
A
male and a
female are also different kinds of organisms, as are infants and adults.
As long
as
its
was believed that each species had been separately
was believed
to consist of the descendants of the
created,
first
pair
naturalist dealing with higher organisms such as birds
and
a species
of
it
kind created by God.
The
mammals
rarely
had any doubt
as to
what
species were.
A
species for
him was simply a group of organisms different from other such groups, where "different" meant differing in visible morphological features.
THIS
128
IS
BIOLOGY
This species concept was quite widely, indeed almost universally,
adopted until the differed
last third
somewhat
less
of the nineteenth century. Organisms that
than
full species
naeus and even by Darwin. This
were called
varieties
by Lin-
was referred
species concept
to as
the typological or essentialistic species concept (and incorrectly as the
morphological species concept).
The typological teristics:
( 1 )
species concept postulated four species charac-
species consist of similar individuals sharing in the
"essence"; (2) each species
is
discontinuity; (3) each species
and
(4) the possible variation
separated from is
same
others by a sharp
all
constant through space and time;
within any one species
is
severely limited.
Philosophers referred to such essentialistically conceived species as "natural kinds."
In the course of the nineteenth century the weaknesses of this typological or essentialistic species concept
became more and more
apparent.
Darwin conclusively refuted the notion
constant.
The
studies of geographic variation
analysis of local population samples
and
that species are particularly the
confirmed that species are com-
posed of populations which vary from location to location and whose individuals vary within a given population. Types or essences
do not
exist in living nature.
In addition to these conceptual objections to the typological species concept, there was the purely practical one that
it
was often of no
help in the delimitation of species taxa. Morphological variation within
breeding populations, and from one population to another within the
same
"kind,"
was often greater than the differences between morpho-
logically similar populations that did
morphological criterion was not a tation. cies
not interbreed. Hence a purely
reliable criterion for species delimi-
Making matters even worse was
—natural
the discovery of sibling spe-
populations that were reproductively isolated (that
is,
not capable of interbreeding because of physiological or behavioral barriers) but
which could not be distinguished from one another
morphologically. Such populations have
higher taxa of animals and occur also
now been found in almost all among plants. It became nec-
essary to search for a different criterion for delimiting species, this
was discovered
in the reproductive isolation of populations.
and
"What?" Questions: The Study of Biodiversity
From
of noninterbreeding came the so-called biologi-
this criterion
cal species concept.
A
species, according to this concept,
interbreeding natural populations that isolated barriers.
129
is
group of
a
reproductively (genetically)
is
from other such groups because of physiological or behavioral
The only way
to fully
biological species concept are there species?
continuum of
Why
is
to ask
Why
Darwinian "Why?" questions:
do we not find
similar or
principle able to
understand the appropriateness of the
in nature simply
more widely diverging
an unbroken
individuals,
in
all
mate with one another? The study of hybrids provides
the answer. If the parents are not in the
same
species (as in the case
of horses and asses, for example), their offspring ("mules") will consist of hybrids that are usually more or viability, at least in
less
sterile
and have reduced
the second generation. Therefore, there
is
a selective
advantage to any mechanism that will favor the mating of individuals that are closely related (called conspecifics)
more
distantly related individuals. This
isolating
mechanisms of
species.
A
is
biological species thus
tution for the protection of well-balanced,
The
biological species concept
is
and prevent mating among
achieved by the reproductive is
an
insti-
harmonious genotypes.
called "biological" because
vides a biological reason for the existence of species
among
it
pro-
organisms,
among incompatible individumay also have other properties,
namely, the prevention of interbreeding als. It is
only incidental that a species
such as the occupation of a separate ecological niche and certain species-specific morphological or behavioral characteristics, that dis-
tinguish
A
it
from other
species. 5
major reason for the almost universal acceptance of the biological
species concept
is
its
usefulness in
most
areas of biological research.
Ecologists, students of behavior, students of local biota,
physiologists
and molecular
and even
biologists are interested in the kinds of
populations that can coexist without interbreeding. In
many
cases, the
students of living organisms recognize species not by the morphological criteria
of the typologist but by aspects of their behavior, their
life
history, or their molecules.
The
biological species definition can be applied without difficulty
whenever populations cality. It
runs into
in breeding condition coexist at the
difficulties,
same
lo-
however, under two kinds of circum-
THIS
130
The
stances.
BIOLOGY
IS
the case of uniparentally reproducing (asexual)
first is
organisms, which have no populations and do not interbreed. The biological species concept obviously cannot be applied to such organ-
isms. Exactly
among
what the best
asexual organisms
between clones
difference
are for discriminating species
criteria
not yet
is
Degrees of morphological
clear.
as well as differences in niche utilization
have been suggested but have not been adequately tested. Such agamic species are placed in the species category in the Linnaean hierarchy.
The second problem with applying the to the delimitation of species rarely confined to
is
one limited geographic
extend over a lesser or greater range. different
biological species concept
that populations within a species are locality. Rather,
When
they usually
such populations are visibly
from one another, they are usually recognized
as subspecies.
Subspecies are often part of a continuous series of populations and as
such freely interbreed and exchange genes. But
geographically isolated and have
and
no opportunity
as a result they diverge morphologically.
may new
eventually attain set
is
for gene exchange,
Over time such subspecies
A
species consisting of a
number of
called a polytypic species. Species that are not subdivided
into subspecies are called
When some
monotypic
species.
of the more distant populations are geographically
completely isolated from question
subspecies are
species status, because they have acquired a
full
of isolating mechanisms.
subspecies
many
arises:
parental species?
all
other populations within a species, the
Are these isolated populations
What
criteria
populations to recognize as into a polytypic species?
full
The
still
members of
the
can one use to decide which of these
and which others
species
to
combine
species status of geographically isolated
populations can be determined only by inference, particularly by degree of morphological difference. 6
The
biological species concept
stood the
gist
of
it,
7
that species status "is simply
He
was long
and Darwin, in
his
in
coming. Buffon under-
Transmutation notebooks, said
an instinctive impulse to keep separate."
referred to the "mutual repugnance" of species to intercrossing
and pointed out
that
good
species
might
"differ scarcely in
character," in other words, that species status
do with degree of morphological
had
little if
any external anything to
difference. Curiously, in his later
"What?" Questions: The Study of Biodiversity
Darwin gave up
writings,
131
concept and reverted to a
this biological
largely typological one.
and the
In the second half of the nineteenth century
the twentieth,
more and more
third of
first
naturalists referred to species in terms
of their biological characteristics. Even though they did not propose a formal definition, authors like Poulton, K. Jordan,
and Stresemann
evidently subscribed to a biological species concept.
It
not generally adopted until
proposed a formal definition
I
and provided massive support
than anything
in 1940
for the biological species concept in
1942 book, Systematics and the Origin of
What helped
was, however,
Species.
the acceptance of the biological species concept else
my
more
was the vulnerability of the competing concepts.
These included the nominalist species concept, the evolutionary species concept, the phylogenetic species concept, and the recognition species
None
concept.
of them
is
as practical as the biological species
in delimiting species, though each
still
concept
number of adherents
has a
today.
COMPETING SPECIES CONCEPTS According to the nominalist species concept, only individuals nature,
makes
and
species are a
species
trariness
is
human
artifact; that
sees, for instance,
species of titmice in a British
woodland or the
common
species of is
wood
naturalist
warblers in a
New
England
forest
knows
nothing arbitrary about species borders, but that these
species are products of nature.
same
A
who
common
in the
arbi-
unsubstantiated by the situation encountered in any actual
the four
me more
a person (not nature)
is,
by grouping individuals under a name. But such
exploration of the natural world.
that there
exist in
Nothing brought
forcefully than the fact that the Stone
mountains of
New
this
point
home
Age primitive
to
natives
Guinea discriminate and name exactly the
species that are distinguished
by the
naturalists of the West.
requires a vast ignorance of both living organisms
It
and human behavior
to adopt the nominalist species concept.
The evolutionary
species concept has
by paleontologists who follow
been promoted particularly
species through the time dimension.
According to Simpson's (1961:153) definition, "An evolutionary species
THIS
132
is
IS
BIOLOGY
a lineage (an ancestral-descendant lineage of populations) evolving
separately
from others and with
its
own
The main problem with
tendencies."
unitary evolutionary role and
this definition
is
that
population. Furthermore, "unitary role"
is
another. Finally,
it
and why
what
a
do not interbreed with one
phyletic lines its
not a
is
side-steps the crucial question of
actually fails in
it
applies
it
equally to almost any isolated population. Also, a lineage
objective, the delimitation of
species taxa in the time dimension, because in a single gradually
evolving phyletic lineage the evolutionary species concept does not
permit one to determine it
at
what point
a
new
species begins
ends and which part of such a lineage has a "unitary
and where role."
The
evolutionary species definition ignores the core of the species problem: the causation
and maintenance of
rary living species. species,
but
it fails
It is
discontinuities
among contempo-
rather an endeavor to demarcate taxa of fossil
even in that endeavor.
The evolutionary
species definition ignores the fact that there are
two processes by which new species may
originate: (1) the gradual
change of a phyletic lineage into a different species without changing
number of
the
species,
and
(2) the multiplication
of species through
geographic isolation (such as Darwin saw on the Galapagos Islands).
The
difficulties a
by the
taxonomist encounters are almost invariably caused
—the
latter
multiplication of species in the horizontal (space)
dimension, rather than the change of species in the vertical (time) dimension. The biological species definition specifically addresses the
problem of the multiplication of species, while the evolutionary species definition ignores
it,
when we speak of
dealing only with phyletic evolution. Ordinarily
speciation,
we mean
the multiplication of species.
many when a new "apomorphy" apomorphy may be as small as a
According to the phylogenetic species concept, adopted by cladists (see below), a
new
species originates
originates in any population. This
single gene mutation. Rosen, finding that the species of fishes in almost
any tributary of the Central American proposed that
all
rivers
endemic genes,
one gene.
One
of
remarked that with the high frequency of
neutral gene mutations, every individual at least
locally
these populations be raised to species rank. 8
his critics quite rightly
by
had
How
is
apt to differ from
would one then decide when
its
parents
a population
"What?" Questions: The Study of Biodiversity
was
enough
different
to
be considered a separate species? This obser-
showed the absurdity of trying
vation clearly
133
to apply the cladistic
concepts of macrotaxonomy to the species problem (for cladistics, see
more on
below).
The recognition
species concept, proposed
by H. Paterson,
is
nothing
but a different version of the biological species concept, misunderstood
by Paterson. 9
SPECIES CONCEPT, SPECIES CATEGORY,
The word nomena: cies taxa.
failure
"species"
is
(1) the species concept, (2) the species category,
and
(3) spe-
Endless confusion in the literature has resulted from the
of some authors to discriminate
meanings of the word
ferent
AND SPECIES TAXA
applied to three very different objects or phe-
among
these three very dif-
"species."
The species concept is the biological meaning or definition of the word "species." The species category is a particular rank in the Linnaean hierarchy placed.
so on)
—the
Each rank is
traditional hierarchy in
referred to as a category.
To determine whether
belongs in the species category, one tion. Species taxa are particular
that
comply with the
viduals")
which organisms are
in this hierarchy (such as species, genus, order,
tests
it
and
a population
against the species defini-
populations or groups of populations
species definition; they are particulars ("indi-
and thus cannot be defined, only described and demarcated
against each other.
At the time of Linnaeus, the identification of species was of concern primarily to the taxonomist, but this biologists
now know
Each species
is
no longer the
is
a biological experiment,
as far as
an incipient species
enters
a dead
is
is
that the species
is
case.
Evolutionary
the crucial entity of evolution.
and there
is
no way
concerned, whether the
end or the entrance into a
large
new
to predict,
new
niche
Even though evolutionists may speak of broad phenomena such trends, adaptations, specializations,
rable
from the progression of the
the species.
Owing
and
it
adaptive zone. as
regressions, they are not sepa-
entities that display these trends,
to their reproductive isolation, whatever evolution-
ary processes take place in a species are restricted to this species and
THIS
134
its
descendants. This
why
is
BIOLOGY
IS
the species
the coin of evolutionary
is
change.
The
species
is
also, to a large extent, the basic unit
ecosystem can be fully understood until
component
and
species,
composed,
is
No
of ecology.
has been dissected into
diversified interactions
its
of these
A species, regardless of the individuals of which
species are understood. it
until the
it
interacts as a unit with other species with
which
it
shares the environment.
In the case of animals, species are also important units in the
behavioral sciences.
Members of
behavior patterns, particularly
a species share
all
many
species-specific
those that have to do with social
behavior. Individuals that belong to the
same
same
species share the
signaling systems in their courtship behavior,
and communication
systems are largely species-specific. In olfactory species this includes the possession of species-specific pheromones.
The
an important
species represents
logical systems.
level in the hierarchy
an immensely useful ordering device for
It is
phenomena. Even though there
significant biological
name
the "science of species" (comparable to the science of it is
cells),
there
one of the most
is
no doubt
The branch of taxonomy Fortunately,
most
higher groups, such as
But what
is
modern
many
no name
for
"cytology" for the
and
that
biology.
Classification of Species
that deals with the classification (or group-
above the species species
is
that such a science exists,
active areas of
Macro taxonomy: The
ing) of organisms
of bio-
seem
to
fall
mammals and
level
is
called
macrotaxonomy.
into natural, easily recognized
birds or butterflies
and
beetles.
one to do with species that seem intermediate between
groups or do not seem to belong to any group? In the course of the history of taxonomy, there have been
proposals of methods and principles classifications resulting
different objectives,
and
The
from these principles sometimes had rather this
is,
perhaps, the reason
is no consensus among taxonomists method of classifying.
there
many
for classifying organisms.
as to
why even
which
is
today
the "best"
"What?" Questions: The Study of Biodiversity
DOWNWARD
135
CLASSIFICATION
Downward classification was the prevailing method of when medicinal botany flourished during and after the
classification
Renaissance.
primary purpose was the identification of different types of plants
Its
and animals. At zoology was
still
this
time knowledge of species in both botany and
very primitive, and yet
identify correctly the plant
Downward
classification
important to
this
logical division.
produces two
are either hairy or have feathers,
(mammals and
the resulting classes
vitally
method of
Aristotle's
Animals are either warm-blooded or not;
Warm-blooded animals
was
proceeds by dividing large classes into
through the use of
subsets,
it
which had the known healing properties.
classes.
and each of
birds) can again be subdivided
by
the process of dichotomy until finally one has arrived at the particular
which the specimen belongs that one was attempting
species to
to
identify.
The to the
principles of
classifications
in field guides
method is,
downward
classification
end of the eighteenth century and
which
is
dominated taxonomy up reflected in the keys
proposed by Linnaeus. The method
and
in the keys of
no longer
is
is
taxonomic
is
still
and
used today
revisions, except that the
referred to as classification but as
what
it
really
identification.
schemes had a number of serious weaknesses that
Identification
prevented them from being useful as true classification systems. They relied
entirely
on
single
characters
distinguishing feature or attribute, call
a characteristic),
Any
impossible,
what we would
and the sequence of characters
by the taxonomist controlled the divisions.
"character" in biology
(a
classes
is
a
in everyday usage arbitrarily
chosen
produced by the dichotomous
gradual improvement of such a classification was nearly
and the choice of
certain characters
sometimes resulted
in
highly heterogeneous ("unnatural") groups.
People had, of course, long recognized natural groupings such as fish
and
reptiles,
or ferns, mosses, and conifers. Toward the end of
the eighteenth century efforts were
made
to replace the largely artificial
scheme of Linnaeus with a more natural system based on commonly observed similarities and relationships. But there was great uncertainty as to
how
to determine these criteria.
THIS
136
BIOLOGY
IS
UPWARD CLASSIFICATION From about 1770 on, even Linnaeus Adanson promoted upward
as
Upward
approach.
as well as other taxonomists
classification as a
classification consists
such
more appropriate
of assembling species by in-
spection into groups (taxa) consisting of similar or related species.
The most
similar of such
newly formed taxa are then combined into
a higher taxon of the next higher
taxa has been formed. This
rank until a complete hierarchy of
method was simply
the application of
everyday classification methods to the grouping of species of organisms.
But proponents of upward methodology. There was
still
classification failed to
develop a rigorous
a strong tendency to give special weight
to conspicuous single characters,
and there was no theory
for the existence of reasonably well defined
to account
groups nor for the existence
of the hierarchy of taxa. Every taxonomist more or
less
developed his
own methodology. The years from about 1770 to 1859 was a transition period. The downward method was clearly abandoned, but upward classification was without a well-articulated methodology and often was employed arbitrarily.
this
A
subcategory of upward classification developed during
time which consisted of the so-called special-purpose
tions.
These
classifications
were not based on the
totality
but, for the sake of a special purpose, were based restricted
number of
classified for culinary
characters. For instance,
classifica-
of characters
on only one or
a
mushrooms might be
purposes into edible and inedible (or poisonous)
ones. Special-purpose classifications go back at least as far as Theo-
phrastus,
who
distinguished plants according to their growth form
into trees, shrubs, herbs, are
still
and
grasses. Special-purpose classifications
useful in ecology. For example, a limnologist
may
divide
plankton organisms into autotrophs, herbivores, predators, and detritus feeders. All such systems have a lower information content than a
Darwinian
classification system.
EVOLUTIONARY OR DARWINIAN CLASSIFICATION In the brilliant thirteenth chapter of
put
all
On
the Origin of Species
these taxonomic uncertainties to rest
by showing that
Darwin a
sound
"What?" Questions: The Study of Biodiversity
organisms must be based on two
classification of
(common change).
descent)
A
and degree of
similarity
criteria:
genealogy
(amount of evolutionary
based on both of these
classification
137
criteria
is
called
an
evolutionary or Darwinian classification system.
Philosophers and practical
classifiers
had long appreciated
that
if
explanatory (causal) theories exist for the grouping of objects, then these explanations
must be taken
into consideration in the delimitation
of such groups. Accordingly, the eighteenth-century classifications of
human ries
diseases
were replaced in the nineteenth and twentieth centu-
with systems based on the etiology of these diseases. Diseases were
classified into those
caused by infectious agents, by defective genes, by
aging,
by malignancy, by
so on.
Any
classification
toxic substances or
harmful radiation, and
which takes causation into account
to severe constraints that prevent
it
from becoming
is
subject
a purely artificial
system.
As soon
as
Darwin developed
his
theory of
common
consists of the descendants of the nearest
taxon
is
called monophyletic.
If a classification
descent, he
group of organisms)
realized that each natural "taxon" (or distinct
10
common
ancestor; such a
system
and exclusively on the monophyly of the included
is
taxa,
based it is
strictly
a genea-
logical ordering system.
But Darwin saw very clearly that genealogy "by classification." Classifying
alogy
is,
itself
does not give
organisms exclusively on the basis of gene-
in a way, merely a special-purpose classification.
The
criterion
of descent was, for Darwin, not a replacement of the criterion of similarity
but rather a constraint on the kind of similarity that could
be accepted as evidence for relationship. The reason
cannot be neglected
is
why
similarity
that the diverging branches of the phylogenetic
"undergo different degrees of modification," and
this "is expressed
by the forms being ranked under different genera,
families, sections
tree
or orders" (Darwin 1859:420). In other words, the degree of difference that arises during phylogenetic divergence
the delimitation classification.
on
A
and ranking of
sound Darwinian
must be duly considered
taxa, in
order to produce a true
classification, thus,
a balanced consideration of genealogy
difference).
in
and
must be based
similarity (degree of
THIS
138
To understand the
IS
BIOLOGY Darwinian
role of similarity in a
classification,
one must understand the concept of homology. Relationship among species
and higher taxa
characters.
A
indicated
is
by the existence of homologous
two or more taxa
feature in
is
homologous when
it is
derived phylogenetically from the same (or a corresponding) feature
of their nearest
common
ancestor.
Many
kinds of evidence can be
used to infer homology. These include the position of the structure in relation to neighboring ones; the connection of
by an intermediate stage
two dissimilar
stages
in a related form; similarity in ontogeny; the
existence of intermediate conditions in fossil ancestors;
and the com-
parative study of related monophyletic taxa. 11
But not
similarities
all
between organisms
result
from homology.
Three kinds of character changes during evolution can mimic homology; they are usually grouped together under the term homoplasy.
They
are convergence, parallelism,
and
reversal.
Convergence
is
the
independent acquisition of the same feature by unrelated evolutionary such as the acquisition of wings by both birds and
lineages,
Parallelism lineages
though
A
is
owing it
to a genetic predisposition for this character, even
was not phenotypically expressed
well-known example
in a
is
group of acalypterate
same advanced character logical
analysis
among
a given
common
in the
ancestor.
the independent acquisition of stalked eyes flies.
Reversal
is
the independent loss of the
in several lineages of a phylogeny.
would permit the untangling of these
A
genea-
similarities
group of organisms and the removal from a taxon of
those species (or higher taxa) whose similarities are not due to
mon
bats.
the independent realization of a character in two related
com-
descent.
The reason why Darwin includes degree of similarity among the classifying criteria is that branching and divergence are not absolutely correlated.
There are branching patterns
diverge at about the
same
rate.
the language families, they
("trees") in
which
all
branches
Although not exactly true for
do tend
in that direction.
trees of
The reason
is
that factors responsible for the evolution of languages are not adaptive
but stochastic.
When
the Anglo-Saxons crossed the North Sea
colonized England, their language did not have to
and
become adapted to when a branch
the British climate or to political changes. However,
"What?" Questions: The Study of Biodiversity of reptiles (dinosaurs) conquered the
adapted to the tion of
new way of life, and
and
ecological factors
Darwinian
their
to
become
changed
at
who remained
This consideration of
all.
impact on the phenotype characterizes a
classification.
Until 1965 Darwinian classification universally,
and
it
and grouping of the process,
testing of the
genealogical arrangement
difficulty
through similarity
related species
and the
ganisms can be
was the system
in use almost
continues to be popular today. 12 The delimitation
by which both of Darwin's
A
had
it
this resulted in a drastic modifica-
phenotype. Related branches of dinosaurs
its
in the ancestral niche hardly
their
niche,
air
139
satisfied.
is
the
is
monophyly of
the second step. This
sound
criteria for a
first
step of
these groups is
the only
and
way
classification of or-
13
encountered by the taxonomist
is
the discordant evo-
lution of different sets of characters. Entirely different classifications, for instance,
of the a
may
life cycle,
from the use of characters of
result
different stages
such as larval versus adult characters. In the study of
group of bees, Michener (1977) obtained four different
when he
sorted these species into similarity classes
characters
and
(4)
use of a
of: (1) larvae, (2)
male
new
genitalia. set
pupae,
Almost
of characters,
(3) external
invariably, it
leads to
on
classifications
the basis of the
morphology of
when a new
a taxonomist
adults,
makes
delimitation of taxa
or a change in rank. Even the characteristics of a single stage of the life
may change at very unequal rates during instance, when one compares humans with
evolution.
cycle
For tives,
the chimpanzees, one finds that
Homo
is
their nearest rela-
more
similar in certain
molecular characters to Pan than are some congeneric species of Drosophila to one another. Yet, as
even this closest relative
among
we
all
know, humans
differ
from
the anthropoid apes very drastically
in certain traditional characters (central
nervous system and
its
ca-
pacities) and in the occupation of a highly distinct adaptive zone.
Almost any organ system and group of molecules will
in a phyletic lineage
have a somewhat different rate of change from
rates are
not constant but
of evolution. Certain
may
DNA
speed up or slow
changes are
five
all
down
times as
of rodents as in the primates, for example. The
others.
These
in the course
fast in a
group
different rates of
THIS
140
BIOLOGY
IS
evolution of different components of the phenotype require great
caution in choosing the characters on which a classification
The use of
based.
may
different sets of characters
is
to
be
lead to rather
different classifications.
Each rank (such
as species, genus, order,
Linnaean hierarchy
is
and so on)
referred to as a category.
14
in the traditional
The lower the rank
of a given taxon (group) of organisms, the more similar the included species usually are
and the more recent
their
common
ancestor
is.
There are no operationally defined definitions for any of the higher
Many
categories.
higher taxa are extremely well delimited and can be
described unambiguously and with high accuracy (for instance, birds or penguins), but the category in which they are placed subjective
of genera call
and involves an element of judgment.
may be
called a tribe
A
is
often
particular group
by some authors, whereas others would
a subfamily or family.
it
Most current
classifications
were developed during the heyday of
comparative anatomy in the immediate post-Darwinian period. At that time,
when an
ancestor was looked
for, it
was conceived
as representing
not a single ancestral stem species but a whole taxon. Hence, the nearest
common
mammals (or
ancestor of the same or lower categorical rank of the
the therapsid reptiles
is
and
that of the birds
some other group of reptiles). Owing
of monophyly,
all
taxa
in
formed) were monophyletic. Also, under
no group
is
and
definition
taxonomy (when
correctly
to this concept
traditional
this
concept of monophyly,
paraphyletic. For a cladist, a group
is
paraphyletic
contains the stem clade (branch) of a derived taxon.
paraphyly makes no sense in a Darwinian
was monophyletic
then, a taxon
the nearest
and
rank,
common
if all
of
its
ancestral taxon of the
this definition
is still
the dinosaurs
is
if it
The concept of
classification.
For Darwin,
members descended from same or lower
categorical
maintained by Darwinian taxonomists
today.
A
typical
Linnaean hierarchy
discontinuity.
tween
reptiles
Among
living
is
characterized by a
organisms there
is
good deal of
no intermediacy be-
and mammals, nor between tubinares and penguins,
nor between turbellarians and trematodes. This observation has long
"What?" Questions: The Study of Biodiversity
141
been puzzling and has inspired a number of non-Darwinian saltational theories. Evolutionary researches, however, have helped in providing
an understanding of the pattern of
diversity.
Most new types of organisms do not transformation of a phyletic lineage, that a founder species enters a
new
originate
by the gradual
of an existing type. Rather,
is,
adaptive zone and succeeds in the
new
environment by making rapid adaptational adjustments for optimal fitness.
of
Once
stasis
it
has achieved
this,
which there may be
in
the
new
lineage
The 2,000+
The 5,000+
illustrate this situation.
are also only variations
on
enter a period
good deal of speciation but no
a
reconstruction of the structural type (bauplan).
Drosophila
may
species of
species of songbirds
a single theme.
The two evolutionary processes
that
produce species
— phenotypic — only
change over time and increase in diversity (speciation)
are
loosely correlated. In the traditional Linnaean hierarchy the gaps be-
tween taxa and the great variation in explained by this lack of correlation. a highly suitable adaptive zone,
may
it
size
When
of the higher taxa are
a founder species reaches
experience copious speciation
without experiencing any selection pressure for a change of the basic
The Darwinian
structural type. 15
classification
well suited to cope with taxa of highly
uneven
system
is
particularly
and
to reflect the
when
"horizontal"
size
gaps between ancestral and derived taxa.
But problems for the Darwinian classification
of living taxa
The recent biota evolutionary
is
classifier arise
expanded
to include the extinct biota.
consist of the endpoints of countless branches of the
tree.
Higher taxa are separated from one another by gaps
caused by divergent evolution and by extinction. Yet, a complete classification
are related
of organisms must include extinct groups,
by descent
classification
of
fossil
to
one another and
taxa raises
consensus has yet been reached. are intermediate
originate
The
between two
fossil
record
is
of which
to the living biota.
The
numerous problems about which no
How
should one treat
living ones?
by "budding," with the
all
New taxa,
fossil
taxa that
almost invariably,
ancestral taxon continuing to flourish.
generally far too incomplete to provide evidence
for the "stem species" of a derived
new
taxon.
THIS
142
The and
BIOLOGY
IS
Darwinian
two-criteria approach of
—was
similarity
essentially unchallenged
dle of the twentieth century.
To be
fully practice the conscientious
classification
from 1859
many
sure,
—genealogy
until the
mid-
taxonomists did not
adherence to monophyly testing and
new methods, however, were new methodologies makes
a careful weighting of similarity. Entirely
not proposed until the 1960s. Each of the use of only one of Darwin's two
on
numerical phenetics
criteria:
(Hennigian ordering)
similarity, while cladification
is
is
based
based on
genealogy.
NUMERICAL PHENETICS The and
objectives of the numerical pheneticists are to avoid
by sorting
arbitrariness
all
subjectivity
with numerical methods, into
species,
groups agreeing in a large number of joint characteristics. Pheneticists
common
believe that the descendants of a
multiplicity of characters
that
ancestor will share such a
they will automatically form well-
defined taxa.
Important objections to numerical phenetics are that
bersome method requiring the
analysis of very large
characters (more than 50, preferably
more than
give different weight to characters of different
it
is
a
cum-
numbers of
100); that
it
fails
to
taxonomic importance;
does not have a methodology for the ranking of taxa; that
that
it
fails
to allow for different evolutionary rates in different character
complexes; that
its
methods produce
different character sets are used;
and
different classifications
that
it
it
when
cannot be improved
gradually.
As long
as only
morphological characters were available, numerical
phenetics was unsatisfactory because there were simply not enough characters to count.
became tion
is
When
large
available, the situation
actually a phenetic
numbers of molecular
characters
DNA
hybridiza-
changed considerably.
method, but
it
avoids most of the standard
shortcomings of phenetic analysis owing to the very large number of characters that are taken into account.
Some of the
"distance"
methods
of computer taxonomy are also essentially phenetic methods. There still
is
considerable disagreement as to the value of these methods,
compared with other approaches (such
as parsimony).
"What?" Questions: The Study of Biodiversity
143
CLADIFICATION
The other
recent alternative to Darwinian classification
is
an ordering
system relying entirely on genealogy. In 1950 Willi Hennig published
German
method which, he claimed, would permit the establishment of an unambiguous genealogical classification. His most basic criteria were these: only groups based exclusively on the possession of in
a
unquestioned "apomorphies," that
is,
shared derived characters, should
be recognized, while ancestral ("plesiomorphic") characters should be ignored. Furthermore, each taxon should consist of a branch of the
phylogenetic tree containing the stem species of this branch and
of
descendants,
its
including
all
"ex-groups,"
modified descendants, such as birds and
that
drastically
is,
mammals from
all
the reptiles.
Hennig's reference system, thus, consists simply of branches (clades) of the phylogenetic (that
is,
tree,
without giving any consideration to similarity
amount of evolutionary
change).
many
In the Darwinian evaluation of similarity, as
characters as
possible are used, not only apomorphies. Hence, ancestral (plesiomor-
phic) characters are given appropriate consideration because they often
contribute strongly to the aspect, and hence the classificatory status,
of a taxon. The same
is
true for a consideration of autapomorphies
in the ranking of sister taxa.
gives a
Darwinian
The use of
classification
as
many
an additional
object to a particular classification [should]
tell
characters as possible virtue: "Assigning
us as
much
an
as possible
about that object. For an extreme perception of order, the ideal would
be that correct
classification
should potentially
tell
us everything about
an object" (Dupre 1993:18).
The Darwinian strict
ing
classification shares
with
cladistics, in contrast to
numerical phenetics, the conviction that the cause of the group-
must be given due consideration. Consequently, these two schools
of macrotaxonomy insist that the taxa recognized by them must be
monophyletic. According to the traditional definition, a taxon nophyletic
if all
ancestral taxon,
of
its
and
members descended from this
is
the definition
still
the nearest
of
all
is
mo-
maintained by the
Darwinian taxonomists. Hennig, however, proposed an ent principle. For him, a group
is
common
"monophyletic" when
entirely differit is
composed
the descendants of the stem species. Since this definition leads
THIS
144 to
IS
BIOLOGY
an entirely different delimitation of
the term "holophyletic" for Hennig's
monophyletic
is
taxa,
new
Ashlock (1971) proposed
The
concept.
term
traditional
a qualifying adjective for a taxon, while Hennig's
concept of holophyly refers to a method for delimiting taxa. Even
though the taxa delimited by the traditional method may the cladons delimited
from
differ
by Hennig's method, both hierarchies of taxa
are strictly genealogical.
A
clade of the Hennigian system does not correspond to a taxon
of the Darwinian classification and should therefore be given a ferent technical
name, "cladon."
16
includes) the "stem species," that first
apomorphic character of
this
Each cladon
classifications
The methodology of
traced back to (and
the species which displays the
is,
branch
(clade). Since clades rather
than classes form the basis of Hennig's system,
from genuine
is
dif-
by the term
it
may be
distinguished
"cladification."
partitioning characters into those that are
uniquely derived and those that are ancestral, the so-called cladistic analysis,
way
is
an excellent method of phylogenetic
to test taxa for
analysis.
monophyly. Anyone interested
It is
method
aspects of characters will find cladification an excellent
the ordering of species
However,
and taxa with reference
as valuable as a
cladogram
is
a suitable
in the phylogenetic
for phylogenetic studies,
violates almost all the principles of a traditional classification. its
for
to their phylogeny. it
Among
deficiencies are the following: (1)
Most
species
clades (cladons) are highly heterogeneous, with the stem
and other stem groups being
groups of
sister clades
far
more
than to the crown groups of their
In other words, dissimilar groups of species are
cladon,
and
similar to the stem
own
combined
clade.
into
one
similar groups of species (sister stem groups) are separated
into different cladons. (2) Either the
traditionally reptiles,
stem species or the entire stem group very often has
been included
the ancestors of
saurs, the
in
an ancestral taxon,
mammals, among
presumed ancestors of the
like the
the Reptilia,
birds, also
and the dino-
among
the Reptilia.
Taking these stem groups out of the taxon with which until
had always been associated makes
this
therapsid
now
they
taxon "paraphyletic," and,
according to cladistic principles, invalid as a taxon. The result
is
a
"What?" Questions: The Study of Biodiversity destruction of a high fraction of
and
taxa,
now
the currently recognized higher
all
recognized
fossil
taxa that have given
derived taxa.
rise to
(3)
this includes all
The requirement
taxonomic rank
that sister groups should be assigned the
unrealistic because sister
is
usually, differ in the
groups frequently,
number of autapomorphic
that has evolved very
gone a
little
since
its
origin
same if
not
characters, that
derived characters, restricted to this particular branch.
had
145
A
is,
group
sister
and one which has under-
drastic evolutionary transformation (for instance, the birds)
to be given the
same
categorical rank in the original
Hennigian
arrangement. (4)
His
There
own
is
no
valid theory of ranking in Hennig's methodology.
followers have
geological time
and
abandoned Hennig's only two ranking
criteria,
categorical equality of sister groups. Instead, they
have adopted the one criterion which Hennig himself specifically rejected, degree of difference
but have only subjective
criteria for their
evaluation. (5)
According to Hennig, every new synapomorphy (derived char-
acter) in a
stem species requires the assignment of a new categorical
rank. Although ignored
by the majority of the
have applied this principle as to differs
demand
at the species level
cladists,
some of them
and have gone so
far
that every population be raised to species level if
it
even by a single character (the phylogenetic species concept).
Such a pulverization of the system would, of course, lead to taxonomic chaos and make any information retrieval virtually impossible. (6) All
oldest
nonapomorphic characters
and most often confirmed
are neglected.
characters one utilized in a classification, the
on the whole, such
a classification will be.
It
taxonomy
rules of
more
is
one of the
that the
useful
and
more
reliable,
Even though quite
rightly
only derived characters can be used for a cladistic analysis, such a restriction
makes no sense when
classification. Indeed,
many
it
comes
taxa are characterized by the prevalence
of ancestral characters. Furthermore,
it
ary asymmetry in rates of evolution
if
ignored.
It
has
to delimiting taxa in a
become evident
completely conceals evolution-
autapomorphic characters are
that a Hennigian cladification actually
has the characteristics of an identification scheme rather than those
THIS
146
BIOLOGY
IS
of a traditional classification. Indeed, leading cladists have again and again stressed that their methodology
is
a search for characters with
diagnostic value.
Cladons, as delimited by a
(7)
because
sister
cladist, reflect a
one-sided relationship
groups, even though genetically
more
closely related
than far-distant descendants, are excluded from the cladons. According
modern descendants of Charlemagne are him than he was to his brothers and sisters.
to cladistic principles, the
more
closely related to
In principle, a cladistic classification tion.
The
clade, or "cladon,"
of the stem species. strictly
taxa.
17
Any
is
a single-character classifica-
characterized by the
is
first
single-character classification,
complying with phylogeny,
apomorphy even when
heterogeneous
results in artificial,
Leading taxonomists for more than a hundred years have rejected
single-character classifications.
A
good
classification,
they have said,
is
based on the greatest number of possible characters.
These shortcomings of Hennig's phylogenetic cladification show why it
cannot take the place of a traditional Darwinian
However,
one
if
is
classification.
interested only in phylogenetic information, then
one should use Hennig's system. In other words, both Hennig's cladification
and the
traditional
Darwinian
classification are legitimate
but have very different applications and objectives. 18
Storing In view of
all
and Retrieving Information
these difficulties,
it
occurs not infrequently that different
authors defend different classifications.
The answer that
is
is,
Which one should one choose?
one should choose the most
most apt
practical
one and the one
to maintain stability in information storage
retrieval. Stability is
and
one of the basic prerequisites of any communi-
cation system; the usefulness of a classification stands in direct relation to to
its stability.
The
traditional
be very stable and
Darwinian system of
point of view.
with traditional
by
classifications,
and the study of new
of homoplasies,
cladifications,
classification tends
contrast, are frequently in conflict
therefore ideal
Cladifications,
resolution
from
this
is
hence in
may
instability.
characters, as well as a
result
in
considerably
new
modified
"What?" Questions: The Study of Biodiversity
The sequence of taxa
147
in a collection or printed classification has to
be linear (one-dimensional) by necessity, but
common
three-dimensional branching phenomenon.
It
is
how one
branches and twigs and
cuts a phylogenetic tree into
its
arranges these twigs into a linear sequence. This
when
the phylogenetic tree
(dendrogram). this
problem:
A number
( 1 )
is
descent
somewhat
a
particularly true
is
bush (thamnogram) rather than a
a
is
arbitrary
tree
of conventions have been adopted to solve
Place obviously derived taxa after those
from which
they were derived, hence trematodes and cestodes after the turbellari-
more
ans. (2) List specialized taxa after the
more
generalized, seemingly
"primitive" taxa. (3) Avoid changing any widely adopted se-
quence without cogent reasons, because such a traditional sequence is
important for information storage and
adopted in the taxonomic
literature
and
having been
retrieval,
in collections. 19
NAMES The names
for the higher taxa serve as convenient labels for the
purpose of information Papilionidae to have
must mean the same thing
maximum
usefulness. 20
to the millions of organisms,
there
and terms such
retrieval,
was not an
efficient
and
to zoologists
would be
It
as
Coleoptera and
all
over the world
quite impossible to refer
to store information
about them,
if
and universally adopted system of name-
giving. For these practical reasons, taxonomists have
adopted a number
of rules dealing with the provision of names.
These rules are botanical,
laid
down
international codes of zoological,
in
and microbial nomenclature. The major
communication system of taxonomists
objectives of the
Preamble
are well stated in the
of the Code of Zoological Nomenclature (1985): "The object of the code is
to
promote
stability
and
animals, and to ensure that each
name
is
unique and
provisions are subservient to these ends." plant or animal
is
names of
universality in the scientific
composed of
The
a generic
distinct. All its
scientific
and
a
name
specific
of a
epithet
(Linnaean binomial nomenclature). For instance, the orange hawk-
weed
is
Hieracium (generic) aurantiacum
guage chosen for the franca
among
scientific
scientists in the
(specific epithet).
names of organisms
is
The
lan-
Latin, a lingua
period after the Middle Ages.
THIS
148
Original descriptions of
poorly
larly in
known
BIOLOGY
IS
new
species are often insufficient, particu-
groups, and
the actual species the describer
may
not provide certainty as to
had before him. For
this reason,
every
species has a
unique "type" which can always be examined to deter-
mine
species
what
to
it
belongs,
making use of
all
the additional
new
information acquired since the original description. The word "type" for this exemplar, based
naean period,
is
on the
essentialistic
philosophy of the Lin-
quite misleading because such a "type"
ticularly typical for the species,
and the modern
not par-
is
species description
is
not based exclusively on the type. Indeed, since every species and every population
is
must include the
variable, the description of the species
careful evaluation of this variability; in other words,
on
it
must be based
a large series of specimens.
The type of
a species
The
a specimen; the type of a genus
and the type of
(the type species);
a family
is
locality at
is,
is
a genus.
a species
The name of
must be formed from the stem of the name of the type genus. which the type specimen of a species was collected
the type locality. This information that
a family
is
is
important in
all
is
polytypic species,
in species that consist of several geographic subspecies.
If several
names
are available for a taxon, the oldest
the valid name. However, stages of
it
one
is
ordinarily
has happened, particularly in the early
taxonomy, that an older name was overlooked or was rejected
for various reasons,
and
a junior
name became
the universally adopted
name of the taxon. Information retrieval is severely handicapped when, at a much later period, the previously neglected older name is reinstated merely for the sake of priority. There are provisions in modern codes that state under what conditions such a prior name can be suppressed for the sake of the stability of nomenclature. The principle of priority
is
applied in zoological nomenclature only to the
names
of species, genera, and families, not to those of the higher taxa. 21
The System of Organisms
Up
to about the middle of the nineteenth century, organisms were
classified into
animals and plants. Anything not clearly an animal was
"What?" Questions: The Study of Biodiversity
149
placed with the plants. However, the closer study of fungi and micro-
organisms made
clear that they
it
had nothing
in particular to
plants but should be recognized as independent higher taxa. drastic revision of the classification of
insight achieved in the 1930s that the
of the bacteria and their
from
all
From
relatives,
organisms resulted from the
Monera
(prokaryotes), consisting
were something entirely different
other organisms (eukaryotes) with their nucleated the origin of
life
cells.
(about 3.8 billion years ago) until about
ago, only prokaryotes existed.
billion years
do with
The most
They
are
now
1.8
usually
divided into two kingdoms, the Archaebacteria and the Eubacteria,
mainly differing in their adaptations and in the structure of their ribosomes. 22
Around
1.8 billion years
ago the
first
one-cellular eu-
karyotes originated, characterized by a membrane-enclosed nucleus
with discrete chromosomes and by the possession of various cellular organelles.
The
latter evidently
biotic prokaryotes.
and
in particular
troversial.
The
as recently as
The
how
evolved through the inclusion of sym-
exact details of the origin of this symbiosis,
the nucleus
first fossil
came
into existence, are
still
con-
records of multicellular organisms appeared
about 670 million years ago.
There are a number of possible ways of classifying the eukaryotes. Until recently, for the sake of convenience, the unicellular eukaryotes
were usually combined into one taxon, the protists it
was
fully
(Protista).
Although
understood that some of the protists (Protozoa) were
were
closest to the animals, that others
closest to plants,
and
that
still
others were closest to the fungi, the traditional diagnostic criteria of plants
down
and animals (possession of chlorophyll, mobility) often broke at this level,
and there was simply too much uncertainty about
relationships to retain the convenient label "protists." particularly
New
researches,
by Cavalier- Smith, which make use of previously ne-
glected characters (for instance, the presence of certain
and of molecular
characteristics,
membranes)
have brought considerable
clarifica-
tion.
Although
it
may
still
be convenient to speak of unicellular eu-
no longer
defensible.
karyotes as protists, a formal taxon Protista
is
Whether
for these protists
to recognize 3 or 5 or 7
kingdoms
is still
150
THIS
IS
BIOLOGY
being argued between lumpers and is
splitters. 23
For the nonspecialist
it
probably convenient to recognize a smaller number. Thus the system
of organisms might be divided into two empires and their respective
kingdoms:
Empire Prokaryota (Monera)
Kingdom Kingdom
Eubacteria
Archaebacteria
Empire Eukaryota
Kingdom Kingdom Kingdom Kingdom Kingdom Kingdom
Archezoa Protozoa
Chromista
Metaphyta
(plants)
Fungi
Metazoa (animals)
CHAPTER EIGHT "How?" Questions: The Making of a New Individual
species consists of thousands, millions, or even billions of
Every
individuals.
new
ones. Although
mechanism a
Many
new
of them perish every day and are replaced by
we
usually think of sexual reproduction as the
for generating
individual
is
for
new
an existing one to
normal way of reproduction fungi,
and even
some
in
individuals, the simplest split into
in prokaryotes, in
way
two. This
many
make
to
is
protists
the
and
invertebrate phyla.
In addition to splitting, there are several other ways to reproduce
without
sex.
A
frequent pattern in
some
plants
and
invertebrates
is
to
produce a new individual by budding. Somewhere on the body wall a
bud
originates
which eventually breaks off and becomes
a
new
individual. Vegetative reproduction, particularly through subterranean
runners,
is
also frequent in plants. In
individuals develop
process
is
from eggs alone
—
some
asexual organisms,
fertilization
is
not required. This
called parthenogenesis. Aphids, planktonic crustaceans,
some other animals may
alternate
new and
between parthenogenetic and sexual
generations.
Most new sively
individuals in higher organisms
come
into being exclu-
through sexual reproduction, which involves
many complex
events in the production of eggs and sperm, mating of the two sexes,
and the care of the developing embryo. Not
surprisingly, this has
produced one of the most protracted controversies
in evolutionary
biology: to explain the selective advantage of this reproductive strategy.
THIS
152
A
BIOLOGY
IS
female producing offspring through parthenogenesis has seemingly
double the
fertility
who
of a female
wastes, so to speak, about half
her descendants on males that are not capable of reproducing them-
The ultimate explanation
selves. is
that
for the success of sexual reproduction
greatly increases the genetic variability of the offspring,
it
and
increased variability has multiple advantages in the struggle for survival
—the
reduction of vulnerability to diseases being only one of
them. Except for the workings of the brain, no other living
of a
world
new
miraculous and awe-inspiring
as
is
adult from a fertilized egg.
The
phenomenon
in the
development
as the
history of our understanding
of this process can be divided roughly into three periods. The period, ranging
from antiquity
to about 1830, focused
first
on describing
the developing embryo. This period was particularly concerned with the relative contributions to the
mother.
A
embryo made by
second period began with the
single
cell.
cells,
and organs. By during these
that
is,
and
first
embryology had
two periods. The aim was
development occurs
—
mation of embryonic
that
is,
it
became
the
structures.
By
that result in the for-
controlled by specific genes
is
cells
was attributable not
but also to the cellular environment in which these
had
how
Beginning early in the twentieth
at different stages in
necessity, the analysis of genes
to
be reductionist
much
like
cells
development.
and gene-controlled biochemical
at the
realized that the genes interact with
environment,
what happens.
complex interactions occur between the parts of the
found themselves
processes
to discover
possible to investigate
embryo. Thus the behavior of developing just to genes
be largely descriptive
to
mechanisms
was shown that development
also that
in the eventual fate of
their contribution to the different structures
necessity,
During the third period,
it
was
Investigators during this time were especially interested in
each of these
and
it
and that the
element in the semen, the spermatozoon, was likewise a
the division of the fertilized egg into cells
century,
cell
and the
when
theory,
cell
discovered that the vertebrate egg was a single fertilizing
the father
beginning, but
it
was soon
one another and with the
cellular
musicians in an orchestra. The study of this
well-orchestrated interaction of genes and cells during the
making of
"How?" Questions: The Making of a New Individual an individual this
is
153
currently the frontier of developmental biology. But
study could not begin until after centuries of careful descriptive
work. Discovery was painfully slow.
The Beginnings of Developmental Biology Diversity is
is
the outstanding characteristic of the living world, and this
developmental processes. Yet related organisms usually
also true of
have similar developments. That the development of a chick in the incubated egg
is
also a vertebrate
mammalian embryo
a process akin to that of the
—was
already vaguely perceived by the Egyptians
perhaps 1,000 years bc. But what
little
was previously known was
completely eclipsed by Aristotle's great writings on descriptive and
comparative embryology in animals.
He
established the field of repro-
ductive biology, by discussing the nature of maleness
and femaleness,
the structure and function of the reproductive organs, viviparity (characterized
by
live births)
versus oviparity (characterized by eggs that
hatch outside the body), the form of copulation in different kinds of animals, the origin and characteristics of semen, and almost every
other conceivable aspect of reproduction and development.
Indeed, Aristotle already faced two major problems in the field of
reproduction that remained controversial until the very end of the nineteenth century.
One
is
the theory of pangenesis (that every
body contributes hereditary materials
in the
to the
germ
the other the debate over preformation versus epigenesis.
inconceivable
how
this
pioneer in the
cells)
It is
cell
and
almost
of animal development
field
could write an account of such completeness, based on such wide
comparative observation and governed by such excellent judgment, that
it
was not surpassed
until the nineteenth century.
Being human, however, Aristotle females in eggs,
also
it
all
might have
eggs. Instead,
Although the
him
that
mammalian
was long believed
females
he adopted the theory that the male
to the coagulate of the female's menstrual
mammalian embryo
Aristotle
a few mistakes.
other groups of animals observed by Aristotle produced
apparently never occurred to
semen gave form that the
made
originated from to have
made
a
this.
blood and
1
second error when he
THIS
154
BIOLOGY
IS
development, which so strongly
tried to explain the specificity of
impressed him. The egg of a frog invariably developed into a frog and not into a
or chicken, as
fish
would guide
it
toward
if it
contained some information that
intended goal. This specificity induced
its
Aristotle to postulate a "final cause" responsible for the unerring
development of the egg to the adult realized that Aristotle's
nothing
but what we
else
strictly explicable
egg
fertilized
eidos,
now
Only
stage.
refer to as the genetic
by physicochemical
factors.
guided by a genetic program.
is
in
our time was
the seemingly metaphysical agent,
it
is
program, hence
The development of
a
2
Although reproduction and the development of embryos were surely of fascination through the centuries, the discipline of developmental biology did not
make any
real progress after Aristotle until
Harvey
in
the seventeenth century carefully studied incubated hens' eggs with
and with the help of a simple
the naked eye a structure
on the yolk membrane of
lens.
He
clearly described
a chicken egg as the spot
from
which the embryo originated. Harvey further demonstrated that there
was no coagulated menstrual blood
in the uterus of a
serve as the female's contribution to the embryo,
the existence of a
mammalian
mammal
to
and he postulated
egg. Shortly afterward, the egg follicles
were discovered in the ovary by Stensen and de Graaf, although the
mammalian ovum was not
true
von
Baer.
the male
Much
It
became
discovered until 1827 by Karl Ernst
clear that the ovary
was the female equivalent of
testis.
detail
about the development of the chick was discovered in
the years after Harvey, particularly through the use of early
microscopes. First
it
was Malpighi,
Caspar Friedrich Wolff, details
who
later Spallanzani,
greatly
von
compound Haller,
and
expanded our knowledge of the
of chick development. All these investigators, however,
still
tried
to correlate the gradual development of the embryonic organs with Aristotle's physiological theories. This
into
which they attempted
By
contrast, nineteenth-century
entirely different spirit spirit.
In
all
was the conceptual framework
to squeeze their observations.
embryology was carried out
—one might almost
say in a
more
areas of functional biology, secure facts
indispensable basis for sound theories.
The
in
an
truly scientific
became the
three great representatives
"How?" Questions: The Making of a New Individual
155
of early nineteenth-century embryology, Christian Pander, Heinrich Rathke, and von Baer,
carefully described their findings,
first
mainly
based on the chick, and only then theorized about them. This included 3
the recognition of the notochord, the neural tube, tantly,
the three
germ
layers.
and most impor-
These embryologists compared their
findings in the chick with those in other vertebrates,
and eventually
even with the crayfish and other invertebrates.
The development of the chick (and the
rather similar development
of a frog), being readily available, has traditionally been considered the gold standard of embryology. Both are characteristic only of vertebrate development, however, while there
is
an endless number of
separate developmental pathways in the other phyla of organisms. 4
The pattern of cleavage of the developing strikingly in different groups.
When
ners of experimental embryology
may
egg, in particular,
differ
the nineteenth-century practitio-
compared the development of the
vertebrates with that in tunicates, echinoderms, molluscs, coelenterates,
and other invertebrate phyla, many
differences
became apparent. Most
of the generalizations in the following pages apply mainly to the vertebrates.
The Impact of
One
the Cell Theory
of the numerous unifying contributions of the
cell
theory, pro-
posed in the 1830s by Schwann and Schleiden, was to give new
meaning
to the terms eggs
and semen, which up
demonstrate that the egg
is
a
had discovered spermatozoa
cell.
But even
in the
semen,
after it
had
to that time
been rather formless concepts. Remak (1852) was the
first
person to
Leeuwenhoek
was widely held
in 1680
that they
were merely parasites in the semen. Others declared them to be the carriers of the father's contribution to the
realized that each this
spermatozoon
is
one
cell,
embryo, but
it
the male germ
was not cell,
until
was demonstrated by Kolliker (1841).
Curiously, as to the
up
to about 1880 considerable uncertainty
meaning of
fertilization.
For the
still
remained
physicalists, fertilization
was
merely the impulse or signal that initiated the cleavage divisions of the egg
cell.
This
is
how
Miescher, the discoverer of
DNA,
interpreted
THIS
156
BIOLOGY
IS
such as O. Hertwig
fertilization as late as 1874. Eventually cytologists
and van Beneden showed the egg than merely the it
also
cell.
its
Fertilization, thus,
to
haploid set of male chromosomes, enters the
to
cell,
set
of female
form the diploid nucleus of the
zygote.
not only restores diploidy but also combines in the
mother and
offspring the genes of
had discovered
as Koelreuter,
more
(gamete).
cell
These chromosomes combine with the haploid
chromosomes of the egg
far
to start the first cleavage division;
brought the nucleus of the male germ
This nucleus, with
egg
spermatozoon brought
that the
command
this
father.
The plant
hybridizers, such
long before.
EPIGENESIS OR PREFORMATION?
But
how
can
this
apparent blob of "unformed" material of the zygote
give rise to a chick or a frog or a fish? This puzzle led to a controversy in the seventeenth century that lasted until the twentieth century.
Eventually two major hypotheses developed, both of
good arguments and both of them now known right
and
them based on
to have
been partly
and partly wrong. These were the hypotheses of preformation
epigenesis.
The preformationists derived
their hypothesis
from the observation
that a fertilized egg unerringly produces the adult of the species that
had produced the a miniature
egg.
From
this
they concluded that at fertilization
form of the future organism
or sperm and
development
all
called "evolution"
—of
by the claim of the
when he looked
is
this original
earliest
is
already present in the egg
—which they
merely the unfolding
form. This theory was strengthened
pronounced preformationist, Malpighi,
at a fertilized hen's
that
egg he was able to see the earliest
stages of development, indicating to
him
that the
form of the future
organism was already preformed in the egg.
The
logical extension of the concept of
preformation was the as-
sumption that not only was an organism preformed but that in the preformed organism
all
of
its
descendants had to be present. This
extension of preformation was called the theory of emboitement.
The
further question arose as to the location of the preformed individual:
Was
it
in the egg, as claimed
by the
ovists, or
was
it
in the
sperm, as
"How?" Questions: The Making of a
Numerous
claimed by the animalculists?
during
in the literature
New
descriptions
period showed a
this
157
Individual
little
and
illustrations
man (homunculus)
enclosed in the spermatozoon. Koelreuter's hybridization experiments (1760) with plants clearly
refuted both preformationist theories
by showing that hybrids were
and mother. There could not have
equally determined by both father
been a preformed adult of the species
in a
germ
cell
the parents. Perhaps because his experiments were this decisive
of only one of
done with
plants,
disproof of preformation was long ignored. But so was
the intermediacy of mules
and other animal hybrids. Equally ignored
were the findings of regeneration, which showed that when major parts of certain organisms were removed, as in the hydra or in certain
amphibians and sentially
The
reptiles,
they could be regenerated by what was es-
an epigenetic process.
epigenesists,
who opposed
the preformationists, thought that
development started from an entirely unformed mass that was given
form by some extraneous C.
F.
force, a vis essentialis, as
Wolff. 5 But epigenesis could not explain
produced chickens and those of a differentiation of tissues
why the
frog, frogs,
it
was
called
by
eggs of a chicken
nor could
it
explain the
and embryonic structures during ontogeny.
Moreover, belief in epigenesis meant that every species had to have its
own
vis essentialis,
by the
forces described sists
something quite different from the universal such as
physicists,
could explain what the
vis
gravity.
essentialis
None of the
was and why
epigene-
it
was so
specific.
Nevertheless, epigenesis after
won
out in the controversy, particularly
improved microscopic techniques could not find any
preformed body to this puzzle
in the
newly
was not found
came from the
field
fertilized egg.
trace of a
But the ultimate solution
until the twentieth century.
The
first
step
of genetics, which distinguished between a geno-
type (the genetic constitution of an individual) and a phenotype (the totality
of the observable characteristics of an individual) and showed
that during
becoming
development the genotype, by containing the genes for
a chick, could control the production of a chick phenotype.
By thus providing the information
for development, the genotype
is
THIS
158
IS
BIOLOGY
the preformed element. But by directing the epigenetic development
of the seemingly formless mass of the egg,
also played the role of
it
the vis essentialis of the epigenesists. Finally,
molecular biology removed the
that the genetic
DNA
last
unknown by showing
program of the zygote was
this vis essentialis.
The introduction of the concept of a genetic program terminated the old controversy. The answer was thus, in a way, a synthesis of epigenesis and preformation. The process of development, the unfolding phenotype,
is
epigenetic.
However, development
also preformationist be-
is
cause the zygote contains an inherited genetic program that largely
determines the phenotype.
That the ultimate answer in a long-lasting controversy combines elements of the two opposing camps
men
are like the proverbial blind
phant.
They have part of the
lations
from these
truth,
partial truths.
typical in biology.
is
Opponents
touching different parts of an
ele-
but they make erroneous extrapo-
The
final
answer
is
achieved by
eliminating the errors and combining the valid portions of the various
opposing theories.
DIFFERENTIATION, THE DIVERGENCE OF DEVELOPING CELLS
One
of the most wonderful, and for a long time totally inexplicable,
aspects of development
is
the gradual differentiation
descending from the single
become
The problem of the 1870s
and 1880s, when If the
become
it
cell
was
as
How
the cells
does a nerve
cell
of the intestinal tract?
became even more puzzling
in
finally realized that genetic deter-
nucleus, and
nucleus of every
same genetic determinants, cells
of the zygote.
cells
cell differentiation
mination resided in the
chromosomes.
cell
from the
so different
among
more
cell in
the
specifically in the
body contained the
was claimed by Weismann, how could
so different during the course of development?
The simplest solution was to assume that during mitotic cell division, when the chromosomes divided, a somewhat different assortment of chromosomes with different genetic elements would go to the two daughter
cells,
and
cell differentiation
would depend on the
specific
genetic elements that the cell received. This theory of unequal division was,
no doubt, the majority opinion from the 1880s
cell
until at
"How?" Questions: The Making of a least 1900.
But
were
if this
New
159
Individual
true, then the elaborateness of mitosis, as
observed by the cytologists, would make no sense. Roux (1883) asked quite rightly
why
the nucleus did not simply divide along
its
equatorial
plane with both half-nuclei becoming the nuclei of the two daughter
What
cells.
is
chromosome into one This makes sense, as Roux pointed
single very long string of
mitosis each tin?
mechanism converting during
the sense of this elaborate
chroma-
out, only if the nucleus consists
of highly heterogeneous material, perhaps of uniquely different particles.
In that case, an equal distribution of these particles into the
two daughter
cells is
possible only
if
and then
a single thread, so to speak,
these particles are strung
thread
this
is
up on
sliced longitudinally.
This would guarantee a completely equal distribution of the hetero-
geneous contents of the nucleus to the two daughter
We now know most
brilliant
seemed
that Roux's theory
was
cells.
essentially correct
and was
deduction from his observations of mitosis. Alas,
to be refuted
by some observations made
and Roux himself eventually gave up
a it
in the ensuing years,
his valid original theory
and
accepted instead unequal mitotic division. The reasons for this conversion were studies which divisions, the
descendant
different
and gave
possibly
happen
if
rise to
showed
cells in
that after the earliest cleavage
some organisms were exceedingly
very different organ systems.
How
could this
the genetic elements were divided equally?
Other findings deepened the mystery. Experiments by Roux, Driesch,
Morgan, and Wilson showed that the early cleavage animal groups had different "potencies." Cleavage
when
of different
an ascidian,
separated, produced a lineage of descendant cells that
have the same properties as cells
cells
cells in
produced by the
ascidian larvae. This
first
mode
if
would
they had not been separated; the two
cleavage division
would produce two
half
of differentiation has been referred to as
mosaic or determinate development. But
when
the two cells of the
first
cleavage division of a sea urchin are separated, these two cells eventually
produce two near normal
very different
mode
larvae,
although of reduced
of differentiation came to be
size.
This
called regulative
development. To complicate matters even further, development in
many
groups turned out to be somewhat intermediate between these two
modes.
THIS
160
The more the studied, the principles.
BIOLOGY
of development in different organisms were
difficult
The processes
to be different
seemed
details
more
IS
it
became
to establish clear-cut general
one kind of organism often turned out
in
from those
in another one.
Some
be impervious to influences from their
to
reprogrammed by
others could be completely
developing
cellular it.
cells
environment;
Some
cells
stayed
which they were first laid down; others went on more or less extensive migrations within the embryo. At the conclusion of numerous experiments, the nature of the relation between genotype, on the one hand, and differentiation of cleavage cells, on the other, right in the tissue in
riddle. 6
remained long a Eventually,
particularly through the
century molecular biology,
it
was
contributions of twentieth-
realized that
all
cells
undergo a
process of differentiation and that at any particular time only a small fraction of the genes in the nucleus of a given cell are active. Regulatory
mechanisms turn on or turn its
gene product
is
regulatory activity
needed is
in part
determined by neighboring
Weismann was unable
off a given gene, depending
in that cell at that time.
programmed cells.
in the genotype
and he opted,
in part as
an elabo-
therefore, for the erro-
neous solution of unequal nuclear division. Even today,
how
and
this
Even so sophisticated a biologist
to conceive of the possibility of such
rate capacity of the genotype,
understood
on whether
The timing of
the regulatory genes "know," that
is,
it
sense,
is
poorly
when
to
activate other genes. It
was further discovered that control of the
many
zygotes, particularly yolk-rich ones,
factors in the cytoplasm. This
is
is
early cell divisions in
entirely
due to maternal
what had misled Roux. Only
after
the earliest stages of development have been completed do the nuclear
genes of the
new
zygote take over.
How
the ovary determines what
material to place in the different parts of the egg yolk and transfers this material appropriately
In the
nematode Caenorhabditis,
several different cell lineages
cytoplasm which, origin.
By
it is
is
is still
a
how
it
cell
of
deep mystery.
for example, the
founder
provided with a specific sector of egg
assumed, contains regulatory factors of maternal
contrast, in taxa with regulative development, as in the
"How?" Questions: The Making of a New Individual
no
vertebrates, there are
fixed early cell lineages; there
is
161
extensive
on the
migration; and induction (the influence of existing tissues
development of other
tissues) largely
Profound differences
in the
cell
determines the specificity of
cells.
pathways of differentiation can be found
not only between nematodes and vertebrates but even between species of
more
closely related phyla
vertebrates) patterns,
tal
—example, between chordates (including
and echinoderms. There
is
a great variety of
and some proceed independently of
influences while others are greatly affected
all
developmen-
environmental
by them.
FORMATION OF THE GERM LAYERS The students of development
in the eighteenth century,
a primitive methodology, thought that the heart to appear in
by the developing embryo. C.
and von Baer showed, however, Rather, through the
of
first
structure
ontogeny and that other organs appeared when they were
functionally needed
ball
was the
working with
cells is
first
that this
was not
F.
Wolff, Pander,
at all the case.
8 to 12 cleavage divisions of a frog egg a
formed, the so-called blastula. Into the hollow of
this
blastula, part of the outer layer of cells "invaginates," resulting in the
double-layered gastrula. Finally, a median layer develops (by a
of different processes), called the mesoderm. The the three
become
germ
layers
form the outside of the
number
cells that will
blastula.
form
Those that
will
the ectoderm are the upper hemisphere; those in the equatorial
region are the mesoderm.
Most of the
endoderm. Pander (1817)
first
ventral hemisphere will
three cell layers in the developing chick,
and
(1828) showed that the production of three
development in
all
a particular set of
system, the
become
demonstrated the existence of these
classes of vertebrates.
a few years later
germ
von Baer
layers characterized
Each germ
layer gave rise to
organ systems: the ectoderm to the skin and nervous
endoderm
to the intestinal system,
and the mesoderm
to
muscles, connective tissue, and the blood system. After the 1830s, the application of the tigators'
cell
theory increased inves-
understanding of the development of the germ
soon realized that an ectoderm and endoderm
layers. It
also exist in
all
was
groups
of invertebrates, particularly the coelenterates. Also, the formation of
THIS
162 the
germ
layers
IS
was the same
in
BIOLOGY all
groups of organisms, consisting
of an invagination of the ectoderm of the blastula resulting in the
formation of the gastrula. 7
By
the end of the 1870s, considerable doubts
whether the same germ
layers gave rise in all
structures, and, in particular,
germ
the other two
layers.
had
about the relation of the mesoderm to
Experiments with regeneration, treatment
with various chemicals, and the analysis of pathologies that
germ
layers
arisen as to
organisms to the same
indicated
all
could adopt roles that were different from their
normal one.
A new era in the study of the potential of the germ layers began when surgical methods were introduced into experimental embryology, particularly transplantation experiments. They showed that when pieces of a germ layer were transplanted to a new location in the embryo or cultured from that
at the
in tissue culture,
normal
development was often
location. For instance, isolated
to differentiate nerve tissue in tissue culture;
when deprived of tissues of early cavity,
of
normally produced by the other germ
these experiments
all
of the germ
layers,
when
layers
seemed
The
germ
in their usual relation to other
could
normal
layers or cell
when
the normal
was disturbed.
Moreover, integrity
last century,
to have a
complexes, but revealed additional potentialities relationship
differ-
layers.
was that the doctrine of the absolute
widely adopted in the
no longer be upheld. The germ potentiality
failed
When
the influence of cells of the other layers.
ectodermal as well as endodermal tissues were able to
specificity
it
was discovered that the germ
throughout development. Instead,
dertake long migrations.
from
ectoderm
formed only epidermis
amphibian embryos were implanted into the abdominal
entiate structures result
it
different
cells that
and neurons
The mesoderm,
layers
do not
retain their
many embryonic cells unmay be formed
for instance,
migrate from the ectoderm or endoderm. Pigment
in vertebrate
embryos undertake long migrations from
their place of origin in the neural crest. In cells definitely are
cells
attracted
some
cases, the
migrating
by chemical stimuli emanating from the
target area, in a process called induction.
"How?" Questions: The Making of a
New
Individual
163
made by Roux, between
tissues or
INDUCTION
Around
1900, the distinction,
structures that
seem
first
to develop strictly according to a fixed genetic
program (determinate development) and others
that are affected
by
adjacent tissues or structures (regulative development) eventually led to a
new concept
term
in experimental
refers to all cases in
development of another
The phenomenon was for the eye of the frog
and
yet
anlage)
fails
it
is
embryology
which one
tissue affects the
subsequent
tissue. first clearly
demonstrated by Spemann (1901)
embryo. The lens
is
formed by the
to develop if the underlying
destroyed or removed.
the formation of the lens.
called "induction." This
It
lens ectoderm,
mesodermal
tissue (eye
was said that the eye anlage induces
Spemann
tested his findings
by transplanting
the eye anlage to other parts of the body, to see whether the ectoderm
of different body regions had the same lens- forming capacity.
indeed
it
had. Finally, he removed the local ectoderm of the eye region
and replaced
it
by ectoderm from other parts of the body, and again
a lens
was formed. Subsequently, other authors obtained
results,
primarily
there
was
And
when working with
"free-lens
removed. Spemann
development" even finally
different
other species of frogs. Sometimes
had been
after the eye anlage
concluded that a large region of head
ectoderm did indeed possess a lens-forming predisposition. In another series of transplantation experiments,
Spemann showed
that a portion of the dorsal blastopore lip induced neural tube tissue in the roof of the primitive gut.
was responsible
He
for this effect. This
hypothesized that an "organizer"
paper
—co-authored with Hilde
Mangold, who had done most of the technical work, and published in
1924
activity
shown
— caused among
quite a sensation
the experimental embryologists.
that even "dead" organizers,
stances, are
and resulted and
in almost feverish
Eventually
was
sometimes able to induce neural tube formation.
Spemann himself and many
others in the field either stopped work-
ing altogether or turned to other problems, and yet that he
it
in fact even inorganic sub-
had been on the
it
right track. Recently a protein
is
now
was
that seems to have the capacity to induce neuronal tissue.
clear
isolated
A
review
THIS
164
of a
all
experiments in the
BIOLOGY
IS
Spemann
field led
induction as
later to see
complex interplay between the inducing and induced
tissue. 8
Regardless of the nature of the chemical signal sent by the inducing
induced one,
tissue to the
it
well established that induction plays
is
an important role in the development of organisms with regulative
development (such of
and
cells
tissues
ent behavior of
as the vertebrates).
The study of the
during ontogeny, in particular the position-depend-
cells,
has
now become an
independent
(topobiology), in which the properties of
out for special analysis. of
cells
nearly
and
all
interaction
It
tissues plays
has
become
cell
of biology
field
membranes
are singled
quite clear that the interaction
an important role in the development of
organisms, except perhaps in a few with
strictly
determinate
development.
RECAPITULATION Naturalists
all
the
way back
to Meckel-Serres
and von Baer have been
interested in the evolutionary implications of development. In the
mid- 1820s Rathke discovered the onic birds and
mammals
gill slits
and pouches of the embry-
—an observation which
fitted excellently into
the thinking of the period of the "great chain of being" (scala naturae). If
adult organisms could be arranged in a series of ever greater
perfection, series
why should not
their
embryos go through an equivalent
of stages, reflecting the preceding archetypes of
perfection? Surely, the
gill slits
indicated a fish stage,
embryonic stages represented recapitulations of
still
less
and
advanced
still
more
earlier
primitive
types.
Thus, the recapitulation theory was born, also referred to as the Meckel-Serres law: organisms recapitulate during their ontogeny the phylogenetic stages through which their ancestors had passed. Evolutionary thinking in the pre-Darwinian period was
but recapitulation
fit
in the scale of being
in with a
still
rather confused,
widespread idea that organisms "higher"
went through
earlier phylogenetic stages
during
their ontogeny.
Von
Baer, even
though confirming the
similarity of
some ontoge-
netic stages with those of "lower" types, categorically rejected the
evolutionary interpretation. For
him
the earlier stages merely were
"How?" Questions: The Making of a New Individual
165
more homogeneous and the later stages more specialized, more heterogeneous; all ontogeny was a move from simple to more complex (this was designated "von Baer's law"). Teleological interpretations were quite acceptable to von Baer, but anything like Darwin's simpler,
common
theory of
The
situation
anyone
else,
descent was not.
was
different with Ernst Haeckel. Haeckel,
more than
emphasized a recapitulationary aspect of development,
proposing that the gastrula stage corresponded to the evolution of the invertebrates
and
that later stages of
development corresponded to the
evolution of "types" of "higher" organisms. Soon after the publication
of Darwin's Origin, Haeckel proclaimed "the fundamental biogenetic law" as "Ontogeny recapitulates phylogeny." At once this raised enor-
mous
interest in
comparative embryology, and students of ontogeny
thought that they found confirmation of Haeckel's claims wherever they looked. For a few years in the late nineteenth century, embryology
became the search from
common
for
ancestors with the help of evidence
recapitulation.
But on the whole embryologists have tended to recapitulation, particularly in
von
Baer's law.
The reason
its
more extreme
for this choice
could not think of any convincing cause
through ancestral gression
from simple
embryos adults. gill
stages,
and they
theory of
versions, in favor of
largely theoretical.
why an embryo should
more comfortable with
to complex, as claimed
by von
They pass
a pro-
Baer. Indeed,
are usually simpler, less differentiated than the resulting
However, the supporters of von Baer neglected the
arches
and other manifestations of recapitulation
than the resulting development. lation
felt
was
reject the
under the rug;
it
Von
fact that
are never simpler
Baer's law merely swept recapitu-
did not explain
it.
Developmental Genetics In the last quarter of the nineteenth century, development was also
came to be called was not homogeneous. The students of
studied by a branch of biology that eventually genetics.
But
this
new
field
inheritance soon realized that their field consisted of two branches,
one
later called
transmission genetics, the other one developmental or
THIS
166
BIOLOGY
IS
physiological genetics. Mendelian genetics,
which
dealt with the
mode
of transmission of the genetic factors from one generation to the next,
was pure transmission
genetics.
Developmental
genetics,
on the other
hand, dealt with the activity of these factors in organisms during ontogeny. The failure of some biologists, such as Weismann, to separate these two aspects of genetics
was responsible
misunderstandings. The achievement of
and
separate the two
T.
for
much
of the early
H. Morgan was to
to confine himself strictly to
clearly
an elucidation of
transmission genetics. In the
same period other authors concentrated on developmental
genetics, a field in first
major
which Richard Goldschmidt (1938) produced the
Much
text.
of what was stated in this
pure speculation, and
it
was not
field at that
time was
until after the rise of molecular
biology that developmental genetics began to mature. Yet earlier publications,
such as those of Waddington and Schmalhausen, had already
outlined most of the problems that are the object of
A new era in
demonstrated that
DNA is
modern
research.
developmental genetics was opened when Avery (1944)
DNA
was the
carrier of the genetic information.
controls the production of the proteins of which an organism
composed. Development, then,
the elaboration of different kinds
is
of proteins during ontogeny and of the very specific combination of proteins that are characteristic of the different organ systems. Although the founders of
modern
genetics were fully aware of the connection
between genes and development, they did not succeed did not even seriously attempt
—
to
—indeed they
produce a synthesis of genetics
and development.
The emphasis
in classical genetics
was on individual genes. But
that time a given gene's contribution to
mined only by the study of mutations, lethal mutations.
at
development could be deter-
particularly deleterious or even
There was no way to study the contribution to
development of a normal
(or,
as
it
was
called, "wild type")
Indeed, the analysis of deleterious genes was the preferred
developmental genetics from the 1930s on.
It
gene.
method of
produced modest
results,
very often pinpointing the particular tissue or even germ layer involved in the mutation.
The
analysis also
showed
that
most mutations con-
"How?" Questions: The Making of a New Individual sisted of a failure to
produce a needed gene product, but
167 it
failed to
help in determining the biochemical nature of the deficiency.
Even though the chemical nature of the gene product remained
unknown,
these studies clearly demonstrated that a given gene
is
usually active during development only in particular tissues and
at
particular stages of the development.
one could describe development
On
as
the basis of this recognition,
an ordered sequence of gene
expressions.
THE IMPACT OF MOLECULAR BIOLOGY The
realization,
provided by molecular biology, that the gene
is
not a
protein and does not itself form one of the building blocks of the
developing embryo, but that the genotype tions
is
simply the
set
of instruc-
needed for the construction of the embryo, had a profound
impact on the methodology and conceptualization of developmental genetics.
When
the 1960s
and
schemes had
Not only
the details of gene action began to be elucidated in 70s,
became evident why our previous explanatory
are genes composite, consisting of exons that are tran-
and introns
scribed
it
fallen short.
that are excised prior to protein synthesis, but in
addition to the enzyme-producing structural genes there are regulatory
genes and flanking sequences.
It
finally
became
clear, as
had been
cautiously suggested since the 1880s, that a gene can be turned
and
off
whenever
its
on
product was needed. Furthermore, the molecular
revolution helped us appreciate the fact that cells are characterized by the proteins they produce. 9
The
entire system,
to polypeptides
and
from the nuclear proteins,
whole apparatus with
its
DNA through
messenger
RNAs
and the continuous interaction of
cellular
environment, turned out to be
this far
more complex than had been realized before. The ideal achievement of developmental biology would be to discover every last gene involved in
development, to determine each gene's exact contribution, including
the chemical nature of the relevant gene product and the role this
molecule plays in development, and to analyze the regulatory machinery which controls the timing of the activity of each gene. Amazingly,
THIS
168
developmental
BIOLOGY
IS
scientists are well
on
way toward
their
this goal in
certain organisms.
The
been made
greatest progress has
in those
with rigidly determi-
nate development, such as nematodes and Drosophila. In the nematode
Caenorhabditis elegans, for instance,
more than 100 genes with over
mapped. Furthermore, the
1,000 mutations have been
DNA
of
many
of these genes has been sequenced, and the exact sequence of base pairs has
been established. The adult nematode has a
of 810 nongonadal
cells,
and through
fixed
number
a study of cell lineages
it
been possible to determine which organs are derived from which
has cells
of the early cleavage divisions. Drosophila (fruit
fly),
another organism with determinate develop-
ment, has some disadvantages as a case study, such
number of
genes, but this
is
as the
much
larger
more than compensated for by genetic First of all, when the modern de-
and morphological advantages.
velopmental studies began there was already a huge inventory of Drosophila mutations available. Moreover, their position on the chro-
mosomes had been determined. Also, Drosophilds giant salivary chromosomes often allow the nature of the mutations to be elucidated. But most importantly, Drosophila
is
a
metameric organism, and
through genetic analysis investigators can determine which genes contribute to the
development of which segment. There are
head
five
segments, three thoracic segments, and eight to eleven abdominal
segments; numerous genes are
now known
to affect either particular
segments or groups of segments. To a large extent
it
has been discov-
ered which of these genes do what. Particularly interesting
parison of the effects of different
same gene Far
less
alleles (versions
is
a
com-
of a gene) at the
locus.
progress has been
made
in the genetic analysis of
organisms
with regulative development, such as vertebrates. In these species, are not yet committed until the 16 to 32
cell
cells
stage of development.
Perhaps the greatest contribution to the understanding of development in
humans has been made by
that
is,
the study of
human
genetic diseases,
of mutations resulting in deleterious changes in the phenotype.
This has allowed investigators to assign a high percentage of mutations to particular
chromosomes.
No
doubt, through the
human genome
"How?" Questions: The Making of a New Individual project, all
mutations
will eventually
169
be localized. But considering the
regulative nature of development, the frequency of induction,
extensive
be
amount of migration of
difficult to establish a
and
specific aspects
certain cell complexes,
it
and the
will often
one-to-one relation between specific genes
of phenotypic development. The developmental
systems of organisms with regulative development are considerably
more complex than those of
species with determinate development.
One may have to be satisfied with generalized conclusions. One of the most exciting developments in molecular embryology has been the discovery that certain clusters of genes are widely dis-
among only distantly related groups of animals. These soHox genes were first discovered in Drosophila but, through
tributed called
sequence analysis, were also found in the mouse, in an amphibian, in a
nematode, and in other animals. There
mologous encode
clusters of
Hox genes
relative position
structure.
are, for
in vertebrates.
example, four ho-
These clusters seem to
within the organism rather than any specific
Homologous Hox genes were
also discovered in
most phyla
of invertebrates, from the coelenterates and flatworms to arthropods,
A certain number of the Hox cluster genes, number of other development-controlling genes, are widely distributed among the animal phyla that it has been sug-
molluscs,
and echinoderms.
together with a so
gested by Slack et
al.
(1993:491) that this set of genes (they
call
it
the
"zootype") reflects part of the genotype of the ancestral metazoan.
Unquestionably, this assembly of genes
Which of animals
is
of great phylogenetic age.
these genes are also found in the protist ancestors of the
is still
unknown.
Development and Evolutionary Biology For a while, a
when most
geneticists
thought that evolution was merely
change in gene frequencies, the role of development in macroevo-
lutionary changes was neglected. In recent years, particularly following the developmental biologists' rather reluctant acceptance of Darwinism, legitimate stress has again been placed
on
this
very interesting
aspect of development.
The
individual, the principal target of selection,
is
the product of
THIS
170
BIOLOGY
IS
the interaction during development of
and with the environment, and
this interaction sets
permissible evolutionary changes. This fact
uniformity of most species.
Any
genes with one another
all its
is
narrow
shown by
limits
on
the phenotypic
deviation from the standard
morpho-
type of the species will be eliminated through stabilizing or normalizing selection
constraints in
Chapter
(see
9).
10
A
study of these developmental
on evolution has become one of the major
modern developmental Different genes
and
areas of interest
biology.
sets
of genes are active at different stages in the
development of the zygote. Developmental biologists have long believed that the genes active near the
acquired
earliest in
are the ones
development are the "oldest" genes of an organism. Any
change in a recent gene, so change
end of development
during phylogeny and, conversely, that the genes active
last
it
was
believed,
in the phenotype, let us say,
dimorphism or by
would
effect
only a minor
by changing the degree of sexual
affecting a behavioral
component of an
isolating
mechanism, while a mutation of one of the early genes may lead a fundamental change in the whole process of development therefore
Many
most
likely
ing that
it is
and
be deleterious.
objections have been raised against too
tion of this concept,
to
and
yet there are
literal
an interpreta-
numerous observations
perhaps valid in principle.
If so,
it
suggest-
would explain many
evolutionary phenomena, such as the exuberance in the production
of
new
structural types in the Precambrian
the metazoan genotype was
still
and
early
Cambrian when
young, in contrast with the
stability
of structural types ever since.
instance,
why
It
would explain
relative
also,
for
evolutionary innovations are often due to a change of
function of a structure that had been gradually acquired step by step for a different function.
that
it
The
requires only a
Such
a shift of function has the advantage
minimal restructuring of the genotype.
realization that every individual
is
a developmental system
more or less as an integrated system explains also two evolutionary phenomena that long puzzled developmentalists. The first is the existence of vestigial structures. Most genes and groups of genes have widespread effects, and even when one of the phenotypic
which
reacts to selection
manifestations of such a group of genes,
let
us say the presence of a
"How?" Questions: The Making of a vestigial digit,
is
New
no longer supported by natural
Individual
171
selection, this vestigial
character will not be lost as long as the controlling genes
other functions, so,
it
will
still
us say in the maintenance of the other
let
have
digits. If
be maintained by natural selection. The second evolutionary
phenomenon
is
recapitulation.
RECAPITULATION RECONSIDERED
modern
In order to explain recapitulation in terms acceptable to a biologist,
one must
was proposed
at a
start
time
on
when
a
new
basis.
The Meckel- Serres
morphology was
idealistic
principle
ruling. Haeckel
and other proponents of recapitulation knew perfectly well that no bird or
mammal went They did not
like a fish.
that the
through an embryonic stage that was exactly claim, as they were accused
embryonic stages of a
as the "adult" stages
mammal
of amphibians or
by their opponents,
or bird were exactly the same
fish.
Rather, they claimed that
the embryonic stages resembled the "permanent" stages of their ancestors.
What
they meant by "permanent" was that the earlier onto-
genetic stages represented the antecedent archetypes. 11 Indeed, these recapitulationists pointed out that the earlier ontogenetic stages often
had advanced further evolutionarily than the adult which the
particularly true for organisms in for special
modes of
stages.
larval stages
living, as for instance the larvae
This
is
were adapted
of some marine
organisms and parasites. In evaluating the theory of recapitulation one sets
of questions: (1)
Do
of ancestral types? That so,
why
does
it
occur?
is,
distinguish
case of the second question,
there such a
one
is
permanence of the
first
question
is
yes.
justified in asking:
If
ancestral
But in the
Why
does a
not develop the neck region directly instead of roundabout
through the
phenotype
gill
is
arch stage?
not
The answer
strictly, exclusively,
is
and
that the
development of the
directly controlled
by genes
but by the interaction between the genotype of the developing
and
two
does "recapitulation" actually occur? (2)
Why is
ontogenetic stages? The answer to the
mammal
must
ontogenetic stages sometimes resemble those
their cellular environment.
cells
At any stage of ontogeny, the next
by the genetic program of the genotype and by a "somatic program" consisting of the embryo at this stage of
development
is
controlled both
THIS
172
To apply
stage.
that the
gill
IS
BIOLOGY the
this, for instance, to
arch system
the somatic
is
gill
arch problem,
program
means
it
for the subsequent
development of the avian and mammalian neck region (Mayr 1994). In spite of the
new term
more than one hundred
"somatic program," this interpretation
years old.
is
has long been one of the
It
fundamental ideas of developmental biology that any stage of devel-
opment
is
in part controlled
by the previous
nothing mysterious about recapitulation except that vorced from the typological thinking of Despite the
many
There
stages.
idealistic
it
is
thus
must be
di-
morphology.
complexities and variations from group to group
of organisms, the early development of animals, as reflected in the
formation and development of the germ layers (gastrulation), shows great similarity throughout feeling that this stage
may
all
phyla.
Somehow
I
Haeckel's extravagant theories have
condition.
cannot suppress the
represent the recapitulation of an ancestral
made
this
thought
highly unpopular, but even a hard-nosed look at the facts does not
me
lead
to a different
and superior
interpretation.
HOW EVOLUTIONARY ADVANCES OCCUR The developmental system
is
so tightly knit that biologists often speak
of the "cohesion" of the genotype. For evolutionists the problem
how new
this
cohesion developed, and
how
it
is
is
broken to make major
evolutionary advances possible.
According to a model
I
proposed in 1954, evolution progresses
rather slowly in large, populous species, while
most rapid evolutionary
changes occur in small, peripherally isolated founder populations. 12 Expressed in terms of development, this suggests that large populous species are developmentally stable, while small founder populations
may
lack this stability, enabling
them
to shift quickly to a
new phe-
notype through rapid genetic restructuring. Eldredge and Gould (1972), using the phrase "punctuated equilibria," accepted this
and proposed last
this
through millions of is
indeed true for
years.
many
why the
may
Subsequent research has confirmed that
species. This
model
quite clearly stresses
the importance of development in macroevolution. However,
not explain
model
that the developmental stasis of populous species
it
does
genotypes of certain species are highly stable while
"How?" Questions: The Making of a New Individual
1
73
those of other species can undergo rapid evolutionary change. This difference
is
unexplained even today.
This model Fisher
is
almost the exact opposite of the one proposed by
and Haldane
rate of evolutionary
in the early 1930s.
change
is
According to their view, the
correlated with the
amount of
genetic
variance in a population or species, and therefore the larger and
populous a species
is,
more
the
rapidly
it
evolves. All subsequent
researches have clearly refuted the Fisher-Haldane thesis. interpretation
is
more populous
that the
interactions occur
and the longer
it
more
a species
is,
will take for a
the
My opposing
more
epistatic
new mutation
or
recombination to spread through the entire species and therefore the slower evolution will proceed.
A
founder population, with
cealed variation because of having fewer individuals, can shift to
another genotype,
adaptive peak.
Change
or, to
at
now
con-
readily
use another metaphor, to another
in evolutionary rates in populations
caused by mutation or genetic recombination, It is
less
more
is
and
species,
called "heterochrony."
well understood that considerable genetic variation exists
every stage in the hierarchy of developmental processes.
(1961) showed beautifully
how much
Milkman
cryptic genetic variation there
can be in a natural population for the expression of a single phenotypic character.
Such variation permits natural selection to
mental processes.
Many
affect develop-
morphological properties are evidently closely
correlated with physiological processes. Selection pressure pleiotropic physiological processes
is
on
these
often responsible for otherwise
inexplicable morphological changes.
By comparing changes graphical races
and
in
developmental processes in different geo-
closely related species, developmental biologists
should be able to show what kind of developmental changes are possible in close relatives
and what others are
not. Unfortunately for
studies such as these, developmental biologists' traditional methodol-
ogy has permitted,
if
not actually favored, typological thinking. Dar-
winian population thinking was rarely required in their researches.
few of them, such ation,
as
Waddington, appreciated the existence of
but the gradual acceptance of population thinking
A
vari-
among
de-
velopmental biologists has been a slow process. Developmental biologists in the past
have tended to go for their analysis to model systems
THIS
174
—the
in the laboratory directly
IS
BIOLOGY
chick, the frog, Drosophila
from the phenotype to the gene
to take advantage of the
pathway that
most macroevolutionary
initiation of
level. is
—and
have gone
Until recently they failed
truly responsible for the
events, namely, geographic vari-
ation.
Yet in
no other branch of biology
aspects of the
are the different explanatory
sciences represented in such exemplary fashion as
life
in developmental biology. This discipline
is
highly analytical (often
misleadingly called reductionist), with the goal of determining the contribution that each gene makes to the developmental process. At the
same time
it
is
conspicuously
holistic, since viable
depends on the influence of the organism interaction
among
development
as a whole, reflected
by the
genes and tissues. The decoding of the genetic
program represents the proximate causation of ontogenetic
processes,
while the contents of the genetic program are the result of ultimate (evolutionary) causations. that
is
the fascination
It is this
richness of factors
and beauty of the
living world.
and causations 13
CHAPTER NINE "Why?" Questions: The Evolution of Organisms
the Middle Ages
In
to Darwin's time, the
world was
and of short duration. But the
credibility
and almost up
believed to be constant
of this Christian worldview had already been weakened in some quarters by a series of scientific developments.
the Copernican Revolution, which
human
inhabitants from the center of the
literally.
first
cosmos and
had demonstrated that not every statement interpreted
The
of these was
had removed the earth and
its
in the process
in the Bible
had
to
be
Second, the researches of the geologists had re-
vealed the great age of the earth, and, third, the discovery of extinct fossil
faunas had refuted the theory that the earth's biota was un-
changed since the Creation.
much more
Despite this and
evidence which undermined the theory
of a constant world of short duration (and even though doubts were expressed in the writings of Buffon, Blumenbach, Kant, Hutton, and Lyell, as well as in
the
more or
popular not only naturalists
Lamarck's full-fledged theory of gradual change),
less biblical
among
worldview lay people
and philosophers.
quired before evolutionism
long duration
—was
A
still
prevailed
but also
up
among
to 1859.
It
was
the majority of
long series of developments was re-
—which
posits
fully established. It
an ever-changing world of
may seem
strange to us today,
but the concept of evolution was alien to the Western world.
THIS
176
IS
BIOLOGY
The Manifold Meanings of "Evolution'
The word "evolution" was introduced
into science
by Charles Bonnet
for the preformational theory of embryonic development (see Chapter
but developmental biology no longer uses the word in
8),
this sense.
Evolution has also been used for three concepts of the history of
on earth and
life
used for one of them.
is still
Transmutational evolution (or transmutationism) refers to the sud-
den origin of a new type of individual through a major mutation or saltation; this individual
becomes the progenitor of
through his descendants. Saltational
ideas,
a
new
species
although not under the
designation evolution, had been proposed from the Greeks to
Even
pertuis (1750). tional theories
after the publication
were adopted by
win's friend T. H.
Huxley
many
of Darwin's Origin,
evolutionists
Mausalta-
—including Dar-
—who could not accept the concept of natural
selection.
Transformational evolution, by contrast, refers to the gradual change
of an object, such as the development of a fertilized egg into an adult. All stars experience transformational evolution, as
red
star.
Nearly
all
struction
by erosion, are of
for the
a yellow to a
changes in the inanimate world, such as the
of a mountain range owing to tectonic forces or
As
from
this nature, if
its
rise
subsequent de-
they are directional at
all.
animate world, Lamarck's theory of evolution, which pre-
ceded Darwin's, was transformational. According to Lamarck, evolution consists of the origin
by spontaneous generation of a simple new
organism, an infusorian, and
its
gradual change into a higher,
more
perfect species. Lamarck's theory of transformational evolution, as
presented in his Philosophie Zoologique (1809), although at one time
widely adopted, has been replaced in most parts of the world by
Darwin's theory. Variational evolution
is
the concept represented by Darwin's theory
of evolution through natural selection. According to this theory, an
enormous amount of genetic
variation
is
produced
in every generation,
but only a few survivors of the vast number of offspring will themselves reproduce. Individuals that are best adapted to the environment have the highest probability of surviving
and producing the next generation.
"Why?" Questions: The Evolution of Organisms
Owing
to
(1)
177
the continuing selection (or differential survival) of
genotypes best able to cope with the changes of the environment, (2)
competition
(3)
stochastic
among
new genotypes of
the
and
the population,
(chance-based) processes affecting the frequency of
genes, there will be a continuous change in the composition of every
population, and this change
is
called evolution. Since
all
changes take
place in populations of genetically unique individuals, evolution necessity gradual
and continuous,
by
is
as populations are genetically re-
structured.
Darwin had been
In his earlier writings (the Notebooks),
well aware
of two evolutionary dimensions: time and space. Transformation in
time (phyletic evolution) deals with changes in adaptedness, as a given species acquires
new
characteristics.
But
this
when
concept alone
can never explain the extraordinary diversification of organic because
it
does not allow for the
number of
life
species to increase.
Transformation in space (speciation and multiplication of lineages)
new
deals with the establishment of multiple
populations outside the
range of the parental population, and with their change into species is
and eventually
called speciation.
Lamarck had had absolutely nothing
to say
about the geographical
and indeed, being
(speciational) aspect of evolution, ist
new
into higher taxa. This multiplication of species
a transformation-
and having accepted spontaneous generation, he seems not
been aware that the question
"How do
be asked. Even Darwin neglected the subject in his Paleontologists in Darwin's time
and
to have
needed to
species multiply?"
later writings.
for decades afterward continued
to adhere to the idea that phyletic evolution
was the only kind of
evolution that mattered. Only in the 1930s and 40s was
it
finally
emphasized, in the works of Dobzhansky and Mayr, that evolution as
much
transformation in space as
that the origin of organic diversity
it is
is
transformation in time, and
through speciation was
as
important
a concern of evolutionary biology as are adaptive changes within a lineage.
Darwin's Origin of Species established
five
different aspects of variational evolution:
evolve over time (this
we might
( 1
major theories )
relating to
that organisms steadily
designate as the theory of evolution
THIS
178 as such),
common
BIOLOGY
IS
that different kinds of organisms descended
(2)
ancestor (the theory of
common
from a
descent), (3) that species
multiply over time (the theory of the multiplication of species, or speciation), (4) that evolution takes place
through the gradual change
of populations (the theory of gradualism), (5) and that the mechanism
of evolution
the competition
is
among
vast
numbers of unique
indi-
viduals for limited resources, which leads to differences in survival
and reproduction
(the theory of natural selection).
Darwin s Theory of Evolution In the Origin
Darwin presented
as Such
a great deal of evidence in favor of
the theory that animals evolve over time. In the following decades biologists searched for
—evidence
a century
and
and found abundant favorable
—and no con-
more than evidence has become
that evolution as such has occurred. In the
trary
a quarter since Darwin's time this
so overwhelming that biologists
theory but consider
it
a fact
earth rotates around the sun
As Dobzhansky has
said:
—
no longer speak of evolution
as well-established as the fact that the
and
that the earth
is
round and not
flat.
"Nothing in biology makes sense except
the light of evolution." Considering evolution to be an established
no
as a
in
fact,
evolutionist any longer wastes time looking for further evidence.
only
It is
when
refuting creationists that
one may bother
to assemble
the powerful evidence that has accumulated in the last 130 years
proving evolution.
THE ORIGIN OF LIFE
One
objection to the theory of evolution
made by Darwin's
early
opponents was that though he may have explained the derivation of organisms from other organisms, he had not explained the origin of life itself
from inanimate matter. The researches of Louis Pasteur and
others demonstrating the impossibility of spontaneous generation in
an oxygen-rich atmosphere seemed to strongly support the idea that life
cannot
arise
from natural causes but requires some supernatural
origin, a Creator. It
has since been discovered that, unlike today, there was no oxygen
"Why?" Questions: The Evolution of Organisms (or only traces of originated.
1
in the early
it)
179
atmosphere of the earth, when
life
Experiments carried out by Stanley Miller (1953) showed
through a gaseous mixture of methane,
that electrical discharges sent
ammonium,
hydrogen, and water vapor in a flask would result in the
production of amino acids, urea, and other organic molecules. Such organic molecules could have accumulated
when our atmosphere was
devoid of oxygen, and, indeed, similar molecules have since been found in meteorites
There are proteins
and
in interstellar space.
now numerous
hypotheses to explain
how life,
particularly
and RNA, might have emerged from a combination of these
organic molecules. Several of these prebiotic scenarios are quite convincing, but in the absence of any chemical fossils of the intermediate stages
we may never be
right one.
that
is,
It
able to prove
would seem
that the
first
which of the scenarios
macromolecules such
The organisms had
as proteins
the
organisms were heterotrophic,
compounds
they utilized prebiotically produced organic
able in the environment.
is
and nucleic
avail-
to build the larger
but they did not
acids,
have to synthesize de novo the amino acids, purines, pyrimidines, and sugars.
The simplest
naturally
formed organic compounds reacted
to
form polymers and eventually compounds of greater and greater complexity.
The
subject of
mystery is
it
life's
origin
is
it is
no longer the
once was, in the early post-Darwinian period. In
no longer any fundamental
physical
highly complex, but
and chemical
difficulty in explaining,
laws, the origin of
life
fact,
on the
there
basis of
from inanimate matter.
Darwin s Theory of Common Descent from
After returning
his
Beagle voyage in the 1830s, Darwin had
concluded that the three species of mockingbirds on the Galapagos Islands
must have been derived from
the South
descendant species. postulate that
and
likewise
finally,
all
a single
American mainland. Thus, a
all all
life.
It
mockingbird species on
species could produce multiple
was only a small step from
this discovery to the
mockingbirds were derived from a songbirds,
all
common
birds, all vertebrates,
all
ancestor,
animals, and
Every group of organisms had descended from one
THIS
180
common
BIOLOGY
IS
What was
ancestral species.
novel in Darwin's theory was
proposed a branching phylogenetic
that he
tree, in contrast
single linear ladder of the scala naturae that
with the
had been so widely
supported in the eighteenth century. Darwin's theory was persuasive because
numerous
for
biological
it
supplied an explanation
phenomena which up
to that time
had
to
be
recorded as simply curious aspects of the world or as evidence for the
planning of the Creator.
First,
Darwin's theory of
common
descent
supplied the explanation for the findings of the comparative anatomists, particularly Cuvier
and Owen,
that organisms
defined groups that are constructed according to a (or structural type or
fall
into well-
common
bauplan
morphotype) and that permit the reconstruction
of a definite archetype for each group. The theory of evolution through
common and
it
descent also explained the origin of the Linnaean hierarchy,
explained,
most convincingly, the pattern of geographical
tribution of the biota all
owing
dis-
to the gradual spread of organisms onto
continents and their adaptive radiation in the newly settled areas.
Common
descent has
become
the theoretical backbone of Darwin-
ian evolutionary thinking since the publication of the Origin, not
surprisingly so because
it
has such extraordinary explanatory powers.
Indeed, the manifestations of
common
descent, as revealed
anatomy, comparative embryology, systematics, and bio-
parative
geography, were so convincing that evolution through scent
by com-
common
was accepted by the majority of biologists within the
first
de-
decade
of the publication of the Origin.
How versial,
far
one could extend the
common
origin
was
even though Darwin himself suggested that
"all
at first contro-
our plants and
animals [have descended] from some one form into which first
breathed." Soon, indeed, protists were discovered that
animal and plant characteristics, so
some of these intermediates of
common
biologists
is still
when they
and
debated.
was
combined
so that the classification of
The capstone
in the theory
descent was provided in this century by the molecular discovered that even bacteria, which have no
nucleus, nevertheless have the
animals,
much
life
same genetic code
as protists, fungi,
plants.
The theory of common descent had an enormously stimulating
"Why?" Questions: The Evolution of Organisms
on taxonomy
influence
(see
Chapter
try to find the nearest relative of every
and
isolated ones,
more
group of organisms, particularly
common
suggestive for animals than for plants,
in the
was the
post-Darwinian period. In particular,
possibility that
that of another
putative immediate
common
groups was established by
as
when such
for the
was considered
if
both stemmed phylocharacteristic of the
the relationship of two
as in the case of reptiles
Not
indicates
was found
and
was necessarily a
stages the transition
for great
in the fossil record,
was part
in 1861, a fossil that
that Archaeopteryx
by what
was an occasion
like. It
a "missing link"
happened with Archaeopteryx
it
stimulated comparative
structure
When
method,
would have looked
ancestor
part reptile.
but
A
organism
ancestor.
this
certainly the con-
example, investigators attempted to predict what the com-
birds, for
rejoicing
it
from a corresponding structure or
genetically
was
a corresponding structure in
a related or possibly ancestral organism.
homologous with
and
and organ was studied
was homologous with
it
ancestor. This
favorite preoccupation of zoologists
researches in which every structure
mon
suggested that one should
7). It
to reconstruct their
struction of phylogenies
181
bird,
direct ancestor,
might have occurred.
These studies were extended to the comparative study of embryos,
and
it
was soon found,
as
emphasized particularly by Ernst Haeckel,
that the course of individual (ontogenetic)
development often went
through stages similar to corresponding stages in an ancestral group. Hence, for instance,
through a
opment of
gill
all terrestrial
tetrapods during their ontogeny go
arch stage, thus recapitulating (so to speak) the devel-
gills
in their fish ancestors.
good deal of
recapitulation has a
A
mild version of a theory of
validity,
though
it
is
not true that
animals in their ontogeny recapitulate the adult stages of their ancestors (see
Chapter
In due time,
it
8).
was possible
to reconstruct a credible phylogenetic
tree of the animals, while botanists, with the help
of molecular evi-
now on the way to doing the same for plants. Ultimately, method was applied also to the prokaryotes, which were shown by Woese to consist of two major branches, the eubacteria and the dence, are this
archaebacteria. These findings have permitted the proposal of a classification for all
organisms (see Chapter
7).
new
THIS
182
IS
BIOLOGY
THE ORIGIN OF HUMANS Perhaps the most important consequence of the theory of
common
descent was the change in the position of man. For theologians and
philosophers
alike,
man was
Aristotle, Descartes,
they
may
Origin
a creature apart
and Kant agreed on
from the
rest
of
life.
no matter how much
this,
have disagreed on other aspects of their philosophies. In the
Darwin confined himself
on the
"Light will be thrown
to the cautiously cryptic remark,
origin of
Haeckel (1866), Huxley (1863), and onstrated conclusively that
Man and
(in 1871)
humans must have
his history."
But
Darwin himself dem-
evolved from an apelike
ancestor, thus putting our species into the phylogenetic tree of the
animal kingdom. This effectively ended the anthropocentric tradition that
had been maintained by the Bible and by most philosophers.
Darwin s Theory of
the Multiplication of Species
According to the biological species concept, species are defined as aggregates of populations that are reproductively isolated from one another. This reproductive isolation
including
acteristics,
which are
is
sterility barriers
traditionally referred to
effected
by certain species char-
or behavioral incompatibilities, as
isolating
mechanisms. They
prevent the interbreeding of different species in areas where their ranges overlap.
The problem of
tions acquire such isolating gradually. 2
It
is
now
process of speciation
speciation
is
to explain
how
popula-
mechanisms and how they can evolve
almost universally agreed that the prevailing is
geographical, or allopatric, speciation
genetic divergence of geographically isolated populations.
It
—
the
occurs in
two forms: dichopatric speciation and peripatric speciation. In dichopatric speciation, a previously continuous range of populations
arm of
is
disrupted by a newly arisen barrier (a mountain range, an
the sea, or a vegetational discontinuity). Either strictly
chance, as in the case of
chromosomal
incompatibilities, or
by
by a change
of function in behavior as a consequence of sexual selection (see below), or as an incidental byproduct of an ecological separated populations will
become
genetically
shift,
more and more
the two different
"Why?" Questions: The Evolution of Organisms
183
in time and, as a correlate of this difference, will acquire isolating
mechanisms later,
they
that will cause
come
most
certain that
them
isolating
mechanisms evolve prior
the neospecies resume contact.
when,
to behave as different species
again into contact with one another.
The
isolation
may
It is
almost
to the time
when
some
addi-
receive
tional fine-tuning after the secondary contact has
now
been established,
but the basic isolating factor originated prior to the contact. In peripatric speciation, a founder population the periphery of the previous species' range.
founded by
a single inseminated female or
contain only a small percentage of,
of,
established
is
Such a population,
by a few
new and
biotic environment.
a drastic genetic modification
base and drastic genetic restructuring, position to undertake
may
be exposed
new
is
its
Such a founder population
and may
Furthermore, such a founder population, owing to
that
will
it
frequently severe set of selection pressures owing to
changed physical and
may undergo
individuals, will
and often an unusual combination
the genes of the parent species. Simultaneously,
to a
beyond
speciate rapidly.
its
narrow genetic
in a particularly favorable
evolutionary departures, including those
lead to macroevolutionary developments.
In addition to these two forms of allopatric speciation, other sce-
narios have been proposed,
The most origin,
likely
owing
and some of them may
of these processes
is
actually occur.
sympatric speciation, that
to ecological specialization, of a
new
is,
the
species within the
cruising range of the individuals of the parental species. Highly im-
probable
is
so-called parapatric speciation, the
development of a bor-
der between two species along an ecological escarpment within a species' range.
Darwin s Theory of Gradualism Throughout
his
tionary change. Lyell's
life,
Darwin emphasized the gradual nature of evolu-
Not only was gradualness
a necessary consequence of
uniformitarianism, but a sudden origin of
have seemed for Darwin too
be sure,
at a given locality
much
new
species
would
of a concession to creationism. To
every species was sharply demarcated against
THIS
184
other species, but
BIOLOGY
IS
when comparing
geographically representative popuDarwin saw everywhere the evidence of
lations, varieties, or species,
gradualness. Eventually, to us perhaps even
more
so than to Darwin,
became
it
evident that evolution occurs in populations, and that sexual populations can change only gradually, never are
some exceptions such
major
as polyploidy,
sudden
saltation.
There
of the most frequently raised objections to Darwin's gradualism
was that
it
was unable
new
to explain the origin of entirely
structures, physiological capacities,
it
a
but they have never played a
role in macroevolution.
One
how
by
and behavior
organs,
patterns. For instance,
can a rudimentary wing be enlarged by natural selection before
can perform the functions of flying? Darwin proposed two processes
by which such an evolutionary novelty can be acquired. One of them is
what Severtsoff (1931) has
called
an intensification of function. Let
us take as an example the origin of eyes.
How
could such a complex
organ be created by natural selection? Eventually,
it
was shown that
the earliest photoreceptor organs were simple light-sensitive spots
and
the epidermis, mis,
and
all
on
that pigment, a lens-like thickening of the epider-
the other accessory properties of eyes were gradually added
in the course of evolution.
Many
of the intermediate stages are
in existence in various kinds of invertebrates.
Such an
still
intensification
mammalian
of function accounts for the various modification of the
forelimbs in moles, whales, and bats, to mention just one other
example.
However, another entirely different and
by which evolutionary
much more
novelties can be acquired
is
function of a structure. Here an existing structure,
dramatic way
by
change in
a
let
us say the
antennae of Daphnia, acquires the additional function of a swimming paddle and, under modified.
The
new
selection
feathers of birds
pressure,
reptilian scales serving for heat regulation
on the forelimbs and During a stage
tails
become enlarged and
presumably originated but acquired a
as
modified
new
function
of birds in connection with flying.
a succession of functions, a structure always passes
when
it
through
can simultaneously perform both jobs. The antennae
of Daphnia are a sense organ and a
swimming
paddle.
Some of
the
"Why?" Questions: The Evolution of Organisms
most
interesting examples
patterns, such as
when
of
of function relate to behavior
shift
the preening of feathers
the courtship display of certain ducks.
mechanisms
in animals
in isolated populations
185
is
Many
incorporated into
behavioral isolating
probably originated through sexual selection
and assumed
their
new
function only after the
species established contact with a related species.
MASS EXTINCTIONS
The discovery of mass
extinctions
was the second objection raised
against Darwin's theory of gradualism. Prior to Darwin, the catastrophists,
beginning with Cuvier, insisted that there had been a number
of mass extinctions in which the then-ruling biota was decimated,
not totally exterminated, only to be replaced by a record suggested a considerable
from the Permian
The major
number of such
to the Triassic or
objective of Lyell's
new
biota.
The
if
fossil
drastic changes, as
from the Cretaceous
Principles of Geology
to the Tertiary.
was
to refute
catastrophism and to substantiate Hutton's thesis of gradual change in the history of the earth. Darwin's gradualism
Thus,
it
came
as a rather
tions were firmly
mirrored
Lyell's view.
unexpected development when mass extinc-
documented
exactly
where the catastrophists had
postulated them.
Mass
extinctions are rare cataclysmic events superimposed
normal Darwinian
cycle of variation
and
on the
selection leading to gradual
change. Darwin was fully aware that the extinction of individual species
and
their replacement
history of
life.
by new species
But in addition to
were definite periods
—which —when
all
between geological ages simultaneously.
Permian,
The most
this
is
continuous throughout the
background
along had served as demarcation lines
a large part of the biota
drastic of these
when more than 95
extinction, there
was
at
became
extinct
the end of the
percent of all species died out completely.
The cause of mass extinctions is still debated today. The one at the end of the Cretaceous, which wiped out the dinosaurs, was almost and the climatic and caused. This was first postulated by
surely the consequence of an asteroid impact
other environmental changes
it
the physicist Walter Alvarez in 1980, but a great deal of supporting
evidence has since been found. Indeed, the impact crater
itself
has
THIS
186
been identified near the attribute the other
tip
IS
BIOLOGY
of the Yucatan peninsula. Endeavors to
mass extinctions to asteroid impacts have been
Most of them seem,
unsuccessful.
rather, to
be connected either with
plate tectonic events that affected the size of shelf seas
lation of
and the
ocean currents or with other climatic changes. There
is
circu-
some
regularity in the sequence of these extinctions, and some authors have
postulated extraterrestrial causes, such as fluctuations in solar radiation
—
a plausible theory. However,
for extrater-
explanations has not withstood critical analysis.
restrial
The
most of the evidence
been lucky enough to survive a catastrophe
species that have
mass extinction are
that led to a
They have an
population.
new
can enter
the
members of
a
founder
environment and
evolutionary pathways. The most spectacular illustration
of this possibility
is
provided
an explosive radiation of the
more than one hundred dinosaurs
like
entirely different biotic
beginning of the Tertiary,
at the
mammals
—which had been on
when
earth for
million years before the extinction of the
—occurred. Darwin s Theory of Natural
Selection
For a long time after Darwin's composite theory of the gradual evolution of species
from a
common
ancestor had been widely accepted,
number of competing theories attempted to answer the question by what mechanism evolutionary change was effected. For some 80 years, a
the defenders of these theories were arguing with one another until
during the evolutionary synthesis (see below)
all
the non-Darwinian
explanations were so thoroughly refuted that Darwin's theory of natural selection
was
left as
the only serious contender.
COMPETING THEORIES OF EVOLUTIONARY CHANGE The
three major non- or anti-Darwinian theories were saltationism,
ideological theories,
Saltationism in the
a
and Lamarckian
among
Vries,
theories.
consequence of the typological thinking prevailing
pre-Darwinian period
Kolliker
De
—
—was
supported by
T.
H. Huxley and
Darwin's contemporaries, by the Mendelians (Bateson,
Johannsen), and by a few others (Goldschmidt, Willis,
"Why?" Questions: The Evolution of Organisms
187
Schindewolf) right into the period of the evolutionary synthesis.
was
finally
It
abandoned when population thinking was more widely
adopted and when virtually no evidence for such a process of specia-
A
tion could be found.
saltational origin of
new
species occurs in
sexually reproducing organisms only through polyploidy
and some
other forms of chromosomal restructuring, and these are relatively rare forms of speciation.
Teleological theories claim that there
nature which leads
The
all
is
an
intrinsic principle in
evolutionary lineages to ever greater perfection.
so-called orthogenetic theories, such as Berg's nomogenesis, Os-
borns
aristogenesis,
and Teilhard de Chardins omega
examples of teleological theories. They eventually
when
principle, are
lost all
the haphazardness of evolutionary change (including
versals)
adherents
many
re-
was demonstrated and when no mechanism could be found
that could effect consistent progressive changes.
According to Lamarckian and neo-Lamarkian theories, organisms are slowly transformed during evolution through an inheritance of
acquired characteristics. These
due
to the effects of use
new
and disuse
characteristics or,
more
were believed to be
directly,
induced by forces
of the environment. Since Lamarckism explained gradual evolution
much
better than the saltationism of the Mendelians,
it
was reasonably
popular prior to the evolutionary synthesis. Indeed, right up to the
more Lamarckians than Darwinians. lost favor when the geneticists demonstrated
1930s there were probably
Lamarckian theories
that inheritance of acquired characters ("soft inheritance")
cannot
occur, because newly acquired characteristics of the phenotype cannot
be transmitted to the next generation. The
final
demise of the theory
of soft inheritance in the twentieth century was due to the finding of
molecular biologists that the information contained in the proteins (the phenotype) cannot be transmitted to the nucleic acids (the genotype). This so-called central
Lamarckism of the that to
last
dogma of molecular
remnants of
credibility.
some microorganisms (perhaps up
biology deprived
There
the
DNA
it
would never be true
of the genotype
is
a possibility
to the protists) have the ability
mutate in response to external conditions, but even
be confirmed,
is
for
if this
should
complex organisms, where
too far removed from the phenotype.
THIS
188
IS
BIOLOGY
NATURAL SELECTION Darwinian natural selection biologists as the
is
today almost universally accepted by
mechanism responsible
for evolutionary change.
best visualized as a two-step process: variation
The
first
step
is
and
It is
selection proper.
the production of massive genetic variation in every
generation owing to genetic recombination, gene flow, chance factors,
and mutation. Variation,
clearly,
was the weakest point
thinking. In spite of a great deal of study
in Darwin's
and hypothesizing, he never
understood what the source of the variation was.
He
clearly
had some
erroneous ideas about the nature of variation, errors that were subsequently corrected by
know
We
Weismann and by
that genetic variation
know
also
that
fertilized
"hard," not "soft," as
Mendelian inheritance
genetic contributions of the is
is
is
Darwin thought.
particulate
—
that the
two parents do not blend when the egg
but remain discrete and constant.
since 1944 that the genetic material
Finally,
we have known
(composed of nucleic
not directly converted into the phenotype but information (the "blueprint" or program) which proteins
We now
post- 1900 genetics.
is is
acids)
is
merely the genetic translated into the
and other molecules of the phenotype.
The production of
variation turned out to be a
complex process.
Nucleic acids can mutate (by changes in the base-pair composition)
and do so
copiously. Furthermore, during the formation of the gametes
(meiosis) in sexually reproducing organisms, a process takes place
by
which the parental chromosomes are broken and reassembled. The resulting
enormous amount of
genetic recombination of the parental
genotypes ensures that each offspring
is
unique. During this process
of recombination, as well as in mutation, chance reigns supreme. There is
a
whole
series
of consecutive steps during meiosis where the assort-
ment of genes is largely random and contributes a huge chance component to the process of natural selection. The second step in natural selection is selection proper. This means the differential survival
and reproduction of the newly formed
indi-
viduals (zygotes). In every generation only a very small percentage of individuals in individuals,
most
owing
species of organisms will survive,
to their genetic constitution, will
and
certain
under the pre-
vailing circumstances have a greater probability of surviving
and
re-
"Why?" Questions: The Evolution of Organisms
189
producing than others. Even in species where the two parents during their reproductive
for oysters
them
are
phase produce millions of offspring, as
is
the case
and other marine organisms, on the average only two of
needed
quency; and even
to maintain the population at if
its
steady-state fre-
chance factors make a major contribution to the
survival of these few progenitors of the next generation, there
question that over time genetic properties
make
a
is
no
major contribution
manner, the adaptedness of the population
is
maintained from generation to generation, and the population
is
to survival. In this
enabled to cope with environmental changes because certain genotypes
among
the vastly variable offspring are favored.
CHANCE OR NECESSITY?
From
the Greeks to the nineteenth century there was a great contro-
versy over the question whether changes in the world are due to chance
or necessity.
It
was Darwin who found a
conundrum: they
are
due
brilliant solution to this old
to both. In the production of variation
chance dominates, while selection
itself
operates largely by necessity.
Yet Darwin's choice of the term "selection" it
suggests that there
is
some agent
was unfortunate, because
in nature
who
deliberately selects.
Actually the "selected" individuals are simply those after all the less well
adapted or
removed from the population.
It
less
who remain
alive
fortunate individuals have been
has therefore been suggested that the
term selection should be replaced by the phrase "nonrandom elimination."
Even those who continue to use the word
presumably that
it
will
really
selection,
which
be the majority of evolutionists, should never forget
means nonrandom
selection force in nature.
We
elimination,
and
that there
is
no
use this term simply for the aggregate
of adverse circumstances responsible for the elimination of some individuals.
And, of course, such a "selection force"
is
a composite of
environmental factors and phenotypic propensities. Darwinians take this for granted,
but their opponents often attack a
literal interpreta-
tion of these terms.
Only
in recent years have evolutionists fully
tically different
was from
understood
how
dras-
Darwin's theory of evolution through natural selection
earlier essentialistic or teleological theories.
When Darwin
.
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