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Epilogue
ERNST MAYR
Museum of Comparative <oology, The Agassiz Museum, Haruard University, Cambridge,
Massachusetts 02138, U.S.A .
One of the most striking, indeed almost puzzling, aspects of Darwin’s thought is the
difference in reaction that it has induced in different constituencies. For a biologist,
the concept of evolution by common descent caused by natural selection resulted
in a complete reorientation of his thinking, for “nothing in biology makes sense
except in the light of evolution” as Dobzhansky has said so rightly. Intellectual
historians likewise have recognized and emphasized the profound impact of the
Darwinian revolution. Yet, at the same time some historians have brushed off
Darwin as a weak intellect, and numerous authors have tended to agree with Louis
Agassiz’s judgement, that the Darwinian theory was a “scientific mistake, untrue
in its facts, unscientific in its methods, and mischievous in its tendency” (1860:154).
When a book with the title At the Deathbed of Daminism came into the hands ofV.
Kellogg in 1907, he wrote that “ever since there has been Darwinism there have
been occasional ‘deathbeds of Darwinism’ on the title pages of pamphlets,
addresses and sermons”. Curiously, this situation has not materially changed in the
ensuing 75 years even though the theories of Darwin have been strengthened in the
meantime through the researches of genetics, systematics, biogeography and
population biology. In this respect Darwinism is unique among major scientific
theories. Thus, it is necessary to explain why Darwinism has encountered a so
much greater and more broadly based opposition than any other well established
scientific theory. Let me try, therefore, to analyse historically the reasons for the
opposition to Darwin and to present the current consensus of leading evolutionists
concerning the validity of Darwin’s ideas.
It is rarely noticed how recent the Darwinian revolution is in terms of Man’s
intellectual history. The Origin was published only 45 years before I was born. And
yet Europe had been in the throes of a continuous intellectual upheaval during the
three preceding centuries, culminating in the Scientific Revolution of the 16th and
17th centuries and in the Enlightenment of the 18th century. Why did it take so
long for evolution to be seriously proposed? And why did Darwinism face such an
uphill battle after it was proposed? The reason, I contend, is that Darwin
challenged some of the basic beliefs of his age. Let me enumerate the more
important ones.
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0 1982 The Linnean Society of London
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E. MAYR
(1) A Beliefin a constant world. In spite of Lamarck and the Naturphilosophen it was
still widely, if not almost universally, accepted in 1859 that except for minor
perturbations (floods, volcanism, mountain building) the world had not changed
materially since creation. And in spite of Buffon, Kant, Hutton, Lyell, and the ice
age theory, the prevailing opinion was still that of a rather recently created world.
(2) A beliefin a created world. Species and other taxa were believed to be unchanging
and therefore the existing diversity of the living world could only be due to an act
of creation. This was a single creation as believed by the orthodox Christians or
repeated creations, either of whole biota as believed by the so-called progressionists
(for example L. Agassiz), or of individual species as proposed by Charles Lyell.
(3) A beliefin a world designed by a wise and benign Creator. Even though the world had
its imperfections, it was the best of the possible worlds (Leibniz). If things changed
they moved toward greater perfection. Teleological processes dominated the
working of all developments and the implementation of all laws. The adaptation of
organisms to their physical and living environment was perfect because it had been
designed by an omnipotent Creator.
(4)A belief in the philosophy of essentialism. The variable phenomena of this world
according to Plato and his followers, are the reflections (or outward manifestations)
of constant, sharply delimited essences. These essences (natures, ‘eide’) are
identical for all the members of a class or species. They are unchanging, all
deviations being ‘accidents’. This philosophy, of course, made evolution
impossible. The only seriously opposing philosophy, that of nominalism (names
bracketing individuals into classes), was only slightly less incompatible with
evolution. According to Lamarck, all individuals exposed to the same environment
would show identical reactions.
(5) A beliefin an interpretation of the causal processes ofnature as they had been elaborated iy
the physicists. During the Scientific Revolution as a result of the work of Galileo,
Descartes, Newton and their associates and followers, a theory of the world had
been elaborated, dominated by a belief in laws, determinism, rigid prediction,
classes, and a reductionist approach. This, as Darwin saw clearly, was quite
unsuited to explain historical processes such as are the cause for the diversity and
adaptedness of the living world.
(6) A belief in the unique position of M a n in the Creation. This was an anthropocentric
world in the eyes of the Christian religion as well as in that of the foremost
philosophers. Man had a soul, something animals did not have. There was no
possible transition from animal to Man.
How did Darwin challenge these traditional views?
The theory of evolution proposed by Darwin challenged all six of these
traditional and well entrenched views. Actually, Darwin’s comprehensive theory
of evolution was an entire set of theories. We see this more clearly when we try to
define what we mean by organic evolution. It is necessary to reject some
reductionist definitions that have been rather in vogue in recent decades and adopt
instead, the definition “organic evolution is change in the adaptation and in the
diversity of populations of organisms”. The important emphasis here is on the dual
nature of evolution. It deals, so to speak, both with the ‘vertical’ phenomenon of’
adaptive change and with the ‘horizontal’ phenomenon of the diversity of
populations, incipient species and new species.
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The origin of adaptation and the origin of diversity are two entirely separate
problems and the study of these two sets of problems represents two separate
traditions in biology. Lamarck consistently concentrated on vertical evolution.
Darwin, particularly early in his career, was far more interested in the origin of
diversity, that is in horizontal evolution. The two fathers of evolution thereby
established two different traditions that are still with us, and that to a considerable
extent have lived side by side, often almost without any contact. The leaders of the
new systematics, for instance, were almost entirely concerned with the origin of
diversity, while the palaeontologists concerned themselves almost entirely with
phyletic evolution and its adaptive trends and shifts.
Darwin’s interest in the origin of diversity had two sources. On one hand he was
trying to find the answer to the great problem posed by Charles Lyell, that of the
source of the new species that according to Lyell are continuously introduced on
earth to replace extinct species. The second source was his experience in the
Galapagos Islands, where he encountered geographic variation and geographic
speciation in an almost diagrammatic manner. The most important aspect of
geographical evolution is that it reflects the properties of a multidimensional
system, in other words that it leads from the study of a single population to that of a
set of related populations replacing each other in space. Many of the controversies
of the Post-Darwinian period were due to the fact that the experimental biologists
ignored the new horizontal tradition inaugurated by Darwin and dealt with
species in a typological manner. Eventually this led to a split of the biologists into
two feuding camps, that of the experimental geneticists and that of the
taxonomists-na turalists.
The comprehensiveness of Darwin’s theory of evolution, dealing both with
vertical and horizontal evolution, is made apparent when we dissect it into five
major components or subtheories.
(1) Evolution as such. Like Lamarck, Darwin denied that the world is constant, that
it is a world of short duration, and that it is a world that was created at one time.
For these claims Darwin provided such an overwhelming documentation that
within a few years after 1859, evolution was no longer denied by any serious
student.
(2) Evolution b~ common descent. This component was, curiously, missing in
Lamarck’s theory, and Darwin was the first to propose it in a consistent manner.
Darwin had seen that the various species of mockingbirds (flesomimus) on the
Galapagos Islands were evidently descendants of a parental species on the South
American mainland, and as soon as he had grasped this important principle, he
realized that all related species could be the result of speciation by an ancestral
species. By following this principle backward to ever more remote ancestors,
Darwin realized it might be possible to derive all life on earth from a single origin.
The principle of common descent was extraordinarily productive since it supplied
at once the answer to the previously puzzling existence of the Linnaean hierarchy,
it also explained why the members of the higher taxa had so many characters in
common, and it explained many previously puzzling facts of animal and plant
distribution. At first, it was merely a working hypothesis, but the studies of the
comparative anatomists and taxonomists were able to prove the common descent
of more and more groups eventually even of animals and plants and, within the last
25 years through the discovery of the genetic code, even of the prokaryotes and
eukaryotes.
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More than anything else, it was the heuristic power of the principle of common
descent that led the theory of evolution to such a rapid victory among biologists in
the period after 1859. And yet it was this same principle that led to the most
determined opposition against Darwinism among non-biologists. Carrying
common descent to its logical conclusion forces us to include Man in the line of
descent by deriving him from apes and lower primates. This was an unacceptable
proposal for orthodox Christians because it seemed to make it impossible to explain
the origin of the human soul.
(3) The origin ofdiuersity. Darwin had adopted the theory of common descent by
extrapolating from the process of speciation. Diversity, for him, was the origin of
new species; and new species for him did not originate as new individuals, that is
by a process of saltation, but rather by the modification of populations. New
species started as new populations which in time became so different that they
finally reached species status. Similar species would be in competition with each
other for the available resources of nature and this component of the struggle for
existence would lead to character divergence and thus to the origin of different
higher taxa. These taxa were not the result of an intrinsic perfecting force, as in
Lamarck’s theory, but merely a product of variation and the struggle for existence
that would force competing species to search out different niches in nature.
(4)Gradualness. Like Lamarck, but for very different reasons, Darwin insisted on
the gradual nature of evolution. He dogmatically denied the claims of essentialism
that all living nature consists of constant and discontinuously separated types.
Instead, said Darwin, organisms consist of populations, each individual of a
population being uniquely different from all others. We now refer to this type of
thinking as population thinking. T o have replaced essentialism by population
thinking was one of the greatest conceptual innovations proposed by Darwin. The
shift from thinking in terms of an evolutionary change of individuals or types to
such of populations permitted at once the resolution of all sorts of difficulties.
Speciation was no longer a problem, since a population can gradually be
reconstructed, whereas a type can change only by saltation. There are indications
that Darwin applied population thinking only half-heartedly and that he often
slipped back into typological thinking. This aspect of Darwin’s theory deserves
further study. Nor is it quite clear how Darwin came to be so insistent on the
gradualness of evolution and what the sources of his population thinking were.
(5) Theprinciple of natural selection. Let Darwin speak for himself “This preservation
of favourable variations and the rejection of injurious variations, I call Natural
Selection” ( 1859:81). “. . . unless profitable variations do occur, natural selection
can do nothing” (1859:82). “Natural selection can act only through and for the
good of each being” (1859:84). “If variations useful to any organic being do occur,
assuredly individuals thus characterized will have the best chance of being
preserved in the struggle for life; and from the strong principle of inheritance they
will tend to produce offspring similarly characterized” (1859:127). “I should
premise that I use the term Struggle for Existence in a large and metaphorical
sense, including dependence of one being on another, and including (which is
more important) not only the life of the individual, but success in leaving
offspring” ( 1859:62).
The theory of natural selection was Darwin’s most daring theory. There was
nothing like it in science, nothing like it in any of the existing systems of philosophy,
all of which took essentialism more or less for granted. It was a remarkable theory
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postulating not a single event, not a single force, not a single process, but a two-step
process. The first step consisted in the production of a tremendous amount of
variation while the selection of the few founders of the next generation took place
at the second step. None of Darwin’s ideas encountered anywhere near the
opposition generated by natural selection. It proposed no less than a replacement
of the wise hand of a Creator by a purely mechanical process, and, at that, by a
process that contained a large chance element. How could this produce perfection?
Everyone at that time, at least in England, was a natural theologian, a believer in
the argument from design and a believer in the perfection of adaptations and the
mutual relation of organisms. Indeed, having been raised in this tradition himself,
Darwin saw in natural selection a process potentially able to produce perfection.
Darwin’s opposition
When a new scientific theory is proposed, it usually has to battle one or two
competing theories in the same scientific field. Although ideological commitments
are often at the bottom of the arguments (see for instance Roe, 1981) for the
controversy of preformation u. epigenesis), nevertheless only a single front is usually
involved. By proposing his five-pronged theory of evolution, Darwin had opened a
battle on about half a dozen fronts. Several of these are still raging. The camps of
his opponents may be characterized (quite tentatively) as follows.
(1) Orthodox Christians. This group corresponds more or less to what we would now
call fundamentalists, i.e. those who reject anything that is in conflict with the
literal interpretation of the Bible, as far as Creation and a constant world of short
duration is concerned.
(2) Natural theologians. Many of them were reasonable, liberal deists, who
nevertheless continued to believe in the argument from design and who saw the
evidence of teleology everywhere in nature. This group included some of the
outstanding scientists in Darwin’s day, like Sedgewick in England and K. E. von
Baer on the Continent.
(3) Luy-persons. Darwin’s theories seemed to be contradicted by the evidence
encountered by any lay-person. How can one believe in gradual speciation when
every species is sharply separated from every other one in our gardens, fields and
woods? How could the gaps between birds, mammals and reptiles be ever bridged,
not to mention those between animals and plants, or any of the higher taxa of
organisms? Intermediate stages between these types are quite unthinkable, it was
said. How can selection lead to perfection when there has been so much extinction
in the earth’s history? Many similar questions were raised in the post-Darwinian
battles where ‘common sense’ clearly seemed to contradict Darwin.
(4) The philosophers. With a few exceptions (Baldwin, Dewey, Goudge and a
contemporary group of young philosophers), philosophers have been essentialists
and physicalists. In spite of sincere efforts by some of them, they have been quite
unable to adopt population thinking, the concept of historical narratives, the
distinction between proximate and ultimate causations (Mayr, 1961), the absence
or at least irrelevance of laws (as defined by the physicists) in evolutionary biology,
the invalidity of much of reductionism, the necessity to partition teleology, and
various other basic concepts of evolutionary biology. The anti-Darwinian
arguments of certain philosophers (including some contemporary ones) are to such
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a degree beside the point that, to a competent evolutionist, it is almost embarrassing
to read them.
(5) Physical scientists. Most physicists believed that all phenomena in nature,
whether living or inanimate, have to obey the same laws and have to be
investigated by the same methods. All of them were essentialists and more or less
strict determinists. They strongly inclined to an atomistic reductionism and
considered experimentation to be the only true scientific method. At the time of
Darwin the physical scientists considered themselves the only true natural
philosophers and were convinced that they had the necessary expertise to pass
judgement on anything in science. The usually appallingly ignorant reviews of the
Origin by physical scientists and mathematicians are eloquent testimony of “the
arrogance of the physicists” (Hull, 1973). Yet, their prestige and authority was so
great that in any argument between a physical scientist and an evolutionist, as in
that between Lord Kelvin and Darwin on the age of the earth, everyone
automatically assumed that the physicist (being a ‘true scientist’) had to be right.
Darwinism greatly suffered, particularly in the first 100 years, from demands
that evolutionary biology should be a science like a physical science, with its set of
absolute laws and an ability to supply ‘proofs’. Such a demand overlooks that
evolutionary events are unique historical events that almost never could have been
‘predicted’. The causation ofsuch events can usually only be inferred. And laws, in
the sense of the physicists’ laws, hardly exists in biology, least of all in evolutionary
biology. Most biological systems are large and extremely complex. In such systems
stochastic processes (‘chance’) often play a large role.
(6) Non-Darwinian biologists. Although the biologists accepted evolution and
common descent almost unanimously, most of them had reservations with respect
to natural selection. Here, one must realize that there were two classes of
opponents. A radical minority denied that natural selection played any
constructive role whatsoever in evolution, while the majority of evolutionists did
not dehy the existence of selection but denied (as had Darwin himself) that
selection alone could account for all adaptations and evolutionary changes. All
these partial selectionists (Plate called them the “old Darwinians”, to distinguish
them from the neo-Darwinians) invoked some other evolutionary force, be it
inheritance of acquired characters, finalistic forces, or saltations. Only the neoDarwinians, beginning with Weismann, rejected any or all auxiliary agents and
relied exclusively on natural selection.
None of the opposition to Darwin was as serious as that coming from his own
profession. In the 80 years after 1859, the non-Darwinian biologists were decidedly
in the majority. In fact in some countries, France for instance, there were virtually
no true Darwinians. But even in the English- and German-speaking worlds, the
flowering of Mendelism after 1900 seemed to spell the defeat of Darwinism to such
an extent that books on the ‘death of Darwinism’ could be published.
Evolutionary synthesis
The prestige of Darwinism was perhaps never lower than in the first quarter of
the 20th century. Far more than 50% of the biologists, in fact even of the
evolutionary biologists, subscribed to neo-Lamarckism, to various orthogenetic
theories, or to saltationism. And yet it was in this period that the foundation was
laid for a synthesis of seemingly conflicting evolutionary schools, a synthesis that
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took place in the 1930s and 40s. The delay was due to the fact that the two groups
of biologists who supplied the information and the set of concepts that made the
synthesis possible, were hardly communicating with each other and had a onesided approach.
On one side were the experimental geneticists concerned with the level of the
gene, preoccupied with the study of single gene pools, strongly reductionist in
approach, concerned only with the vertical components of evolution (adaptation)
and dealing with discontinuous entities (genes, chromosomes). In spite of these
handicaps, the geneticists supplied the answer to the problem that had always
eluded Darwin, that is the nature and the origin of the genetic variation that is
necessary for natural selection to be successful. Also the geneticists had a major
share in refuting soft inheritance (inheritance of acquired characters). In addition,
they were unable to find any evidence for the existence of any orthogenetic
processes.
The other group consisted of the naturalists-systematists who worked with
populations, who were interested in horizontal evolution (populations, incipient
species), who thoroughly believed in gradual evolution and who appreciated that
the individual is the true target of natural selection. Their major contribution was
that they saw that evolutionary processes and phenomena involve not only
mechanisms at the level of the gene but also whole individuals and, what is most
important, that the divergence of populations (geographic variation and
geographic speciation) and of species (macroevolution) were major components of
evolution. Only by including populations and species in the evolutionary analysis
was it possible to explain the major problems of evolution, such as the
multiplication of species, the origin of higher taxa, the origin of evolutionary
novelties, and other manifestations of evolution in nature. These phenomena can
not be explained at the level of genes. The synthesis occurred when actual students
of evolution, like Huxley, Rensch, Simpson, Mayr and Stebbins agreed that the
evolutionary phenomena studied by them are fully explicable through gradual
evolution by means of natural selection (Mayr & Provine, 1980). The synthesis
and what happened afterwards was in many respects (except for the rejection of
soft inheritance) a return to a purer Darwinism. For Darwin, it had always been
clear that the individual and not the gene is the target of selection and that
evolution has two components, the vertical one (adaptation) and the horizonal one
(diversity).
Post-synthesis developments
Neither the discovery of numerous new facts relating to evolution nor the
development of new concepts of speciation and genetic variation have required
any essential revision of the picture of evolution as developed during the
evolutionary synthesis. I emphatically deny the claims of various authors that these
recent developments have led to an end of Darwinism, or of neo-Darwinism, or of
the evolutionary synthesis. They are simply a filling-in of missing pieces in the
edifice that the evolutionary synthesis had constructed. If one wanted to
characterize the post-Synthesis developments in a few sentences, one might
mention the following items :
The reductionist concept introduced by the mathematical population
geneticists, that evolution is a change in gene frequencies in populations, was
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rejected and Darwin’s old concept that evolution is a change in the components of
the phenotypes, of whole individuals, populations, species and higher taxa, was
restored. A new theory of speciation was proposed (Mayr, 1954) in which the role
of peripherally isolated founder populations was emphasized. This formulation
calls attention to the fact that the rate of evolutionary change is often inversely
correlated with population size. A better understanding of the chemical and
structural nature of the genetic material was achieved through the work of
molecular genetics. And finally, while the evolutionary synthesis was preoccupied
with phenomena at the level of populations and species, much work in the postSynthesis era has been devoted to an understanding of the phenomena of
macroevolution. The results of these studies considerably expand the framework
laid down during the evolutionary synthesis by Rensch and Simpson. None of these
developments have led, however, to a rejection of the basic principles established
during the evolutionary synthesis.
Danuin’s theories in t h light of current thinking
In conclusion, I would like to analyse Darwin’s views on evolution in the light of
current thinking. I am careful not to say “to analyse where Darwin was right and
where he was wrong”, because there are still sufficient uncertainties concerning
certain aspects of the evolutionary process to make it perilous to assert too
dogmatically what is right and what is wrong.
The basic theory of evolution has been confirmed so completely that most
modern biologists consider evolution simply a fact. How else except by the word
evolution can we designate the sequence of faunas and floras in precisely dated
geological strata? And evolutionary change is also simply a fact owing to the
changes in the content of gene pools from generation to generation.
Darwin’s theory of common descent has also been gloriously confirmed by all
researches since 1859. Everything we have learned about the physiology and
chemistry of organisms supports Darwin’s daring speculation that “all the organic
beings which have ever lived on this earth have descended from some one
primordial form, into which life was first breathed” (1859:484). The discovery that
the prokaryotes have the same genetic code as the higher organisms was the most
decisive confirmation of Darwin’s hypothesis.
Darwin’s theory of gradualism, unpalatable even to his close friends Huxley and
Galton, has triumphed decisively and makes the more sense, the more clearly we
realize that evolution is a process involving populations. The only apparent
exceptions are the occasional abandonment of sexual reproduction and certain
chromosomal processes such as polyploidy.
Darwin’s great emphasis on the development of diversity as an important
component of the evolutionary process, undeservedly neglected during the first
third of this century, is again at the forefront of interest, particularly in
palaeontologyand ecology. As far as speciation is concerned-the process that serves
as the source of new diversity-Darwin
was somewhat confused (Kottler,
1978 : Sulloway, 1979). Although supporting geographic speciation on islands,
Darwin believed in a widespread occurrence of sympatric speciation on continents.
Many recent researches have clearly demonstrated that geographic speciation is a
frequent process on continents (owing to vegetational or minor physiographic
barriers) and that the phenomena that puzzled Darwin can be explained
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historically (Mayr, 1982a). To what extent various forms of non-allopatric
speciation also occur is still controversial (Mayr, 1982a). There has been, however,
a rather drastic revision of the concept of allopatric speciation (Mayr, 1954). In
addition, the genetic basis of speciation is still very poorly understood. Here is an
area where molecular genetics is likely to produce some exciting findings in the
near future.
The greatest triumph of Darwinism is that the theory of natural selection, for 80
years after 1859 a minority opinion, is now the prevailing explanation of
evolutionary change. It must be admitted, however, that it has achieved this
position less by the amount of irrefutable proofs it has been able to advance than by
the default of all the opposing theories. It must further be admitted that the
modern theory is not quite the same as that of Darwin. First of all, Darwin was
afraid that natural selection by itself was not quite sufficient to achieve the
perfection of adaptation, and he therefore invoked, to some extent, inheritance of
acquired characters and direct induction by the environment, both of which are
now unanimously rejected by evolutionists. All of Darwin’s teachers and most of
his friends were natural theologians, and inevitably Darwin conceived of the idea
of selection as something able to produce the same perfection as design by the
Creator. He believed that “natural selection is daily and hourly scrutinizing,
throughout the world, every variation even the slightest” (1859:84). “And as
natural selection works solely by and for the good of each being, all corporeal and
mental endowments will tend to progress toward perfection” (1859:489). As I shall
presently show, we are now less euphoric about the perfection-giving capacity of
natural selection than a writer in the era of natural theology.
The process of natural selection is a two-step process. The first step consists of the
production of genetic variation. No one prior to 1900 knew how this came about.
When de Vries and others demonstrated the occurrence of mutations, the emphasis
at first was on drastic mutations. And as late as the 1940s Goldschmidt,
Schindewolf and Willis proposed theories of evolution based on macromutations.
In this counterproductive climate it was important to emphasize, as was done by
virtually all geneticists, that evolution is due to ‘small genetic changes’ as had been
maintained also by Darwin. Darwin, of course, had also invoked soft inheritance to
give him a more abundant supply of genetic variation-and gradual genetic
variation at that. The latest findings ofmolecular genetics indicate that the picture
is not as simple as we thought 25 years ago. There are many varieties of DNA that
seem to have different functions. That regulatory genes (whatever they are) play a
different role from that of the enzyme genes is now generally accepted. It is my
conviction that in this area (the study ofdifferent kinds of DNA) discoveries will be
made that may lead to the most important modification of the currently accepted
views.
The second step in the process of natural selection is the differential reproduction
of different genotypes (Berry, 1979). Darwin here saw much more clearly than his
contemporaries-indeed than almost anybody for the next 100 years-that there
are two kinds of reproductive advantage. One is due to what might be called
efficiency selection, that is a premium on any characteristic of an individual that
makes it more efficient in the ordinary struggle for existence, whether this be
greater tolerance towards the physical characteristics of the environment, a better
utilization of food, an improved capacity to cope with predators or pathogens, and
so forth. A second component of reproductive success is due to what some modern
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authors have called ‘selfish selection’, that is selection for pure reproductive
success, including what Darwin had called sexual selection. The sociobiological
literature contains abundant illustrations of this type of selection.
Perhaps the most important departure of modern thinking on natural selection
as compared to Darwin, is the much greater role attributed by us to chance. To be
sure, the importance of chance in the production of variation and in the
production of the zygotes that are exposed to selection had long been recognized
(for a tabulation of such chance factors see Mayr, 1962). However, selection itself
was usually considered a more or less deterministic antichance factor. To what
large extent chance plays a role even in the process of selection is a more recent
insight, although some evolutionists had stressed the importance of chance from
1872 (Gulick) on. Since the targets of selection are not single genes but whole
individuals, there are numerous constraints on natural selection, as stressed by
Gould & Lewontin (1979) and Mayr (1982b).The larger a population or the more
numerous the local populations in a species, the greater is the probability that
chance deviations from the average constitution of the species will be reversed in
due time. No one has described the inertia of large, populous species better than
Haldane ( 1957). In contradistinction, the smaller a population, the greater the
probability that chance events will become irreversible and will have a
determining influence on subsequent evolution. It is this consideration that has led
to my theory of peripatric speciation (Mayr, 1954).
When reviewing all the additions to our knowledge and all the changes in our
concepts of the last 50 years I do not have the feeling that they constitute any
decisive change in the overall view of evolution as composed during the
evolutionary synthesis. The numerous claims to the contrary in the recent
literature, were made without any supporting evidence (Stebbins & Ayala 1981).
There is perhaps no better way to characterize Darwin’s impact on our thinking
than by listing the concepts that he refuted and those that he newly introduced into
our thinking, or at least to whose acceptance he contributed materially.
CONCEPTS REFUTED BY DARWIN
That the diversity of organisms found on the earth was created. (See also
Gillespie, 1979.)
That the earth is relatively new. In the battle with Lord Kelvin, Darwin
clearly emerged as the victor and his estimate of organic life being several thousand
million years old is now well established.
Cosmic teleology. All the phenomena that previously had been ascribed to design or
to finalistic causes Darwin was able to explain in terms of natural selection.
Anthropocentrism. Darwin and his followers showed conclusively that man is not a
separate creation but the product of common descent.
NEW CONCEPTS INTRODUCED OR BOLSTERED IN A DECISIVE FASHION BY DARWIN
Population thinking. Living nature does not consist of types but of variable
populations in which each individual is unique.
Nuturul selection. Owing to the vast reproductive surplus, there is an intense
competition among individuals of the same population for survival and
reproduction. Certain genotypes have a greater probability to leave offspring than
others.
Geogruphic speciation. Different populations undergo different genetic modifications
and if such populations are isolated, the genetic changes may be compounded into
species differences.
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Evolutionary progress. Since there is no cosmic teleology, there is no necessary organic
progress. Whatever seeming progress we find between the first origin of life and the
existing biota is due to competition among species and to character divergence.
There are numerous processes, such as change of function of a structure, that
facilitate the acquisition of new capabilities of advantage in species competition.
It was through Darwin that we have come to realize that every biological
phenomenon and process requires at least two explanations, a purely functional
one and an evolutionary one, and that evolution has two major components,
changes in adaptation and changes in diversity. But Darwin’s work has had an
impact far transgressing the domain of biology. No other scientific theory has
challenged and, in fact, refuted so many commonly held beliefs as Darwin’s theory
of evolution by natural selection. But more than that : Darwin introduced entirely
new ways of thinking and of carrying out scientific research. In short, no other
philosopher or scientist has had as great an impact on the thinking ofmodern Man
as Darwin.
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