hardening of the
IEN JAY GOULD
and hard versions
1937, just as Dobzhansky published the book that later generations would laud
as the foundation of the modern synthesis, the
a symposium on "supra-specific variation in nature and in classification."
Alfred C. Kinsey, who later became one of America's most controversial
intellectuals for his study of basic behaviors in another sort of WASP,1 led
off the symposium with a summary of his extensive work on a family of gall
wasps, the Cynipidae.
his article, Kinsey strongly advocated the central theme of the developing
synthesis: Evolution at all scales, particularly macroevolution, could be
explained by the genetic mechanisms observed in laboratories and local
populations. He first complained that some geneticists and naturalists were
still impeding a synthesis with their insistence upon causal separation of
levels: "Just as some of the geneticists have insisted that the laboratory
genetics may explain the nature and origin of Mendelian races, but not of
natural species, so others indicate that the qualities of higher categories
must be explained on bases other than those involved in species" (1937, p.
208). He then defended the central postulate:
the contrary, recent taxonomic studies and such experimental analyses as have
been made of natural species indicate that Mendelian genetics provides all the
Apologies to readers unfamiliar with American slang, but I coulelil't resist
the allusion. Yes, Kinsey the celebrated wasp taxonomist is the same man who
later published the famous "Kinsey Reports" on sexual behavior in thehuman male
and female - the first dispassionate studies, based on sufficiently large
samples, of what people really do do. WASP is a colloquial American acronym for
"white Anglo-Saxon Protestant," still, despite all our ethnicity, the largest
group in our country.
necessary for the evolution of species as well as for laboratory races; and I
shall undertake to show that the same genetics is all that is involved in the
origin and development of the characteristics of higher categories. [1937, p.
in his major monograph of 1930, the first "Kinsey Report" if you will, Kinsey
defended a position that would put him outside the synthesis by modern
definitions (and he continued to hold these views in 1937). In his section on
"mutation" in the origin of
he contrasted two points of view: the first, supported by "laboratory studies"
held that "changes in genes occur . . . as mutations which are complete as soon
as they have occurred"; and the second, defined as the "neo-Darwinian
conception of 'fluctuating variations' which, by being accumulated over many
generations and bent in a given direction by the force of natural selection,
would gradually give rise to the characters of new species" (1930, p. 25).
Kinsey was not merely talking about the origin of new features by sudden
mutation, but of new species. Later in the paragraph, he said so explicitly and
again contrasted his position with Darwinism:
this latter [neo-Darwinian] interpretation there may be incipient species of
the sort conceived by many taxonomists in their definitions of varieties and
subspecies; under the genetic interpretation the first mutant individual
embodies all that the new species will contain, and is the new species as soon
as it is given an opportunity to perpetuate its mutantcharacters through a
population of individuals. [1930, p. 25]
me emphasize that the "mutations" Kinsey defended for the origin of species
were generally of small - though discontinuous morphological effect, entirely
comparable to (indeed homologous with) standard laboratory mutations. Kinsey
did not ally himself with the true de Vriesian saltationists who refuted the
central syntheticist postulate by contrasting large-scale mutations that make
species all at once with small-scale variation that differentiates local
populations. Instead, Kinsey supported a unified genetics for all levels. His
"macro" mutants were relatively small, not discombobulations of entire genetic
systems a la Goldschmidt's "systemic mutants." They did not constitute a
different genetics acting above the level of processes that continually occur
within populations (the antisyntheticist claim). Rather, they represented the
standard kind of change that, according to Kinsey, occurred within populations
made new species.
then explicitly defended the "genetic" view for speciation in
p. 34) and promptly committed a second, and consequent, apostasy from later
conceptions of the synthesis: He denied
of the modern synthesis
most of the morphological expressions of species-forming mutations had any
seems no basis for believing the shortened wings or any of the concomitant
variations of any adaptive value to any of these insects. The short wings are
not confined to warmer or colder climates, and long- and short-winged forms of
various species are active at the same season in the same localities. The field
data suggest nothing as to the survival value of these outstandingly basic
modifications of structure. [1930, p. 34]
brief synopsis of Kinsey's views raises a dilemma: How could a man who seemed
so much "in" the synthesis (as later defined) by his defense of genetic
continuity in causation, be so far out of it in his explicitly
antiadaptationist views and his defense of discontinuous origin (however small
the steps) for new species. Consider, for example, Mayr's definition (1963, p.
586; but see also 1980, p. 1, to appreciate his long advocacy of this account):
"The proponents of the synthetic theory maintain that all evolution is due to
the accumulation of small genetic changes, guided by natural selection, and
that transpecific evolution is nothing but an extrapolation and magnification
of the events that take place within populations and species."
dilemma can be resolved (ironically, since professional students of evolution
must face it) by recognizing a property of science that monolithic, textbookish
interpretations ignore at their peril: theories evolve, too. I shall argue that
the synthetic theory underwent a major change in intent between its formulation
in the 1930s and its hardening in the late 1940s. Most biologists are unaware
of this change because we only read the last editions of famous works
(Dobzhansky, 1951; and Simpson, 1953 for example) and ignore as superseded
historical curiosities the original presentations (Dobzhansky, 1937; and
Simpson, 1944, in these cases). Kinsey's work lay within the original
boundaries for its defense of ordinary genetics as the source of all
evolutionary change, while it clearly fell beyond the pale of the later,
hardened version in its non-Darwinian approach to the roles of adaptation and
natural selection in speciation.
change involved intent even more than content. During the 1930s, the main
concern of the primary builders (for a movement as yet unnamed) was
methodological. They knew the great Darwinian hope of the late nineteenth
century: The mechanism of heredity, when elucidated, would vindicate Darwin's
views on small-scale variation and the creative action of natural selection
working in the cumulative mode. They remembered the crushing of this hope during
Stephen J. Gould
early twentieth century when Mendelism, in its first evolutionary incarnation,
contributed yet another anti-Darwinian theory in the form of de Vriesian
saltationism. They rued the resulting anarchy of incommensurate, competing
theories that had driven so many great naturalists to the despair of nearly
total agnosticism, as in Bateson's famous declaration: "Less and less was heard
in genetical circles and now the topic is dropped. When students of other
sciences ask us what is now currently believed about the origin of species we
have no clear answer to give. Faith has given place to agnosticism" (1922, p.
genetics had sown confusion, so too (they hoped) might it eventually breed
concord by setting a unified causal base for all evolutionary change. Advances
in genetics had established the ground for such a hope. De Vries' systemic
saltations had proven to be, quite literally, a primrose path; a genetics of
smaller-scale change might now prove sufficient for all levels of evolution,
from the lowest (by observation and experiment) to the highest (by
extrapolation). Fisher had proven that continuous variation could have a
Mendelian underpinning. Morgan had abandoned his commitment to saltational
change and had, late in life, accepted the idea that micromutations underlay
evolutionary change. The time was ripe for causal unity.
just as geneticists were weeding out their confusion and settling upon a
consensus about how heredity worked, many evolutionists refused to recognize
the unifying potential of this emerging agreement, and continued to insist upon
a separation between smallscale (and, by implication, unimportant) evolution
produced by these known genetic mechanisms and a larger-scale evolution
produced by other genetic processes as yet unknown. Kinsey, as a synthesist of
the first period, expressed his frustration at this defeatist argument in
ridiculing (see quote on p. 71) those evolutionists who liked Mendelism for lab
stocks but not for natural populations, or for change within population but not
for transspecific evolution.
original version of the synthesis, therefore, sought to remove this idea of a
separate but unknown genetics for large-scale changes and to render all of
mechanisms that could be studied directly in field and laboratory. It was
primarily a plea for knowability and operationalism, for a usable and workable
evolutionary unity. It did not attempt to crown any particular cause of change,
but to insist that all permissible causes be based on known Mendelian
mechanisms. In particular, it did not insist that adaptive, cumulative natural
selection must underlie nearly all, or even most, change - though many
synthesists personally favored this view. Any theory of change would be
admitted, so long as its causal base lay in
Mendelian genetics. In the 1930s, for example, genetic drift was often granted
a predominant role in phenotypic change, not only at the level of demes, but
also in the origin of many species. (Dobzhansky's 1937 book may favor selection
slightly, but it comes far closer to asserting this role for drift than his
later work would lead one to expect.) Such views about the relative importance
of drift (and consequently diminished role for selection) were admitted to the
original synthesis because they invoked only known and small-scale genetic
change. Likewise, the original version admitted Kinsey's view that the primary
phenotypic changes of speciation were generally nonadaptive. For Kinsey
vigorously supported the key methodological claim of the original synthesis:
Small-scale genetic changes within natural populations are the stuff of all
evolution. I have called this original version "pluralistic" because it
admitted a range of theories about evolutionary change, Darwinian and
otherwise, and insisted only that explanations at all levels be based upon
known genetic causes operating within populations and laboratory stocks.
pluralistic version was slowly and subtly altered, primarily during the 1940s
(and perhaps with the 1947 Princeton conference as a focal point), as the
intent of explanation by known genetics shifted to the content of one
particular theory - neo-Darwinism and its insistence that cumulative natural
selection leading to adaptation be granted pride of place as
of evolutionary change. The synthesis hardened by elevating one theory to
prominence among the several that supported the primary methodological claim of
the original version - and eventually (particularly among many secondstring
votaries) by insisting to the point of dogma and ridicule that selection and
adaptation were just about everything. The hard version had no more room for
Kinsey, who was off doing something else by then anyway.
the rest of this essay, I shall illustrate this shift by discussing the
increasing commitment to adaptationist explanations by three key figures:
Dobzhansky, who was certainly
the whole movement; Simpson, who brought the most disparate field of
paleontology under the aegis of the synthesis; and Wright, the great geneticist
who did so much to create the original version and then largely stood outside
its later hardening (his own shift to adaptationism is especially interesting
in this light). If this subject strikes some readers as a bit arcane or
unimportant, I can only assert an insider's view, admittedly partisan (Gould
and Lewontin, 1979; Gould and Vrba, 1982): The rather strict Darwinism of the
hard version established a research program that has directed (and in some ways
restricted) the field for 30 years. Adaptationist commitments extend
Stephen J. Gould
work on the origin of life to sociobiology. I am also painfully aware that the
"triumph of adaptationism" is an exceedingly complex historical subject. I do
not wish to fall into the typological traps for history that all evolutionists
are trained to avoid for nature. Variation abounded at all times, and it was as
unconstrained as human intellects are broad. I speak of no lock step affecting
all evolutionists together as an imposed essence upon a field of variation, but
only of general tendencies that, like evolutionary trends generated by species
selection within clades, often sneak up on you and get established while
virtually no one notices what is happening.
examples of increasing commitment to adaptation
Increasing emphasis on selection and adaptation between the first (1937) and
last (1951) edition of Dobzhansky's
and the Origin of Species.
original probe (1937) toward synthesis was more a methodological claim for
knowability than a strong substantive advocacy of any particular genetic
argument - though his general Darwinian preferences are clear enough. It held,
contrary to his own mentor Filipchenko in Russia or H. F. Osborn in America,
that the methods of experimental genetics can provide enough principles to
encompass evolution at all levels, but it did not play favorites among the
admitted set of legitimate principles. It did not, in particular, proclaim the
pervasive power of natural selection leading to adaptation as. a mechanism of
evolutionary change. Macroevolution, it stressed, is not a thing apart,
unknowable in principle from experimental work on laboratory and natural
populations and requiring different (and perhaps unfathomable) modes of genetic
change. Dobzhansky writes cautiously, emphasizing the hope for complete
knowability based on microevolutionary genetics:
seems to show, however, that there is no way toward an understanding of the
mechanisms of macroevolutionary changes, which require time on a geological
scale, other than through a full comprehension of the microevolutionary
processes observable within the span of a human lifetime and often controlled
by man's will. For this reason we are compelled at the present level of
knowledge reluctantly to put a
I have written the Dobzhansky and Simpson examples in other contexts before
(Gould, 1980 and 1982a) and much of the material in this discussion is taken
from my previous work. William Provine knows so much more about Sewall Wright
than anyone else does that I have based my discussion upon his excellent
analysis in the manuscript cited in the references.
of the Modern synthesis
of equality between the mechanisms of macro- and microevolution, and,
proceeding on this assumption, to push our investigations as far ahead as this
working hypothesis will permit. [1937, p. 12]
inkling of the chaotic and depressed state of evolutionary theory before the
synthesis can be glimpsed in a simple list of previously popular arguments that
Dobzhansky regarded as sufficiently important to refute - claims that denied
his hope for synthesis by suggesting that Mendelian processes observed in the
laboratory do not represent the genetic style of "important" evolutionary
change in nature. Dobzhansky rebuts explicitly the following arguments:
Continuous variat,ion in nature is non-Mendelian and different in kind from
discrete mutational variation in laboratory stocks (p. 57); Mendelian variation
can only underlie differences between taxa of low rank (races to genera), while
higher taxa owe their distinctions to another (and unknown) genetic process (p.
68); chromosomal changes are always destructive and can only lead to
degeneration of stocks (p. 83); differences between taxa of low rank are
directly induced by the environment and have no genetic or evolutionary basis
(p. 146); Johannsen's experiments on pure lines showed the ineffectiveness of
natural selection as a mechanism of evolutionary change (p. 150); selection is
too slow in large populations to render evolution, even in geological time (p.
178); genetic principles cannot account for the origin of reproductive
isolation (p. 255).
fifth chapter, on "variation in natural populations," stresses the pluralism of
the early synthesis. Observable genetic phenomena are the source of
we find continuity from studies in the laboratory, to variation within natural
populations, to formation of races and species:
is now clear that gene mutations and structural and numerical chromosomal
changes are the principal sources of variation. Studies of these phenomena have
been of necessity confined mainly to the laboratory and to organisms that
aresatisfactory as laboratory objects. Nevertheless, there can be no reasonable
doubt that the same agencies have supplied the materials for the actual
historical process of evolution. This is attested by the fact that the organic
diversity existing in nature, the differences between individuals, races, and
species, are experimentally resolvable into genic and chromosomal changes that
arise in the laboratory. [1937, p. 118]
what forces shape and preserve this variation in nature? Dobzhansky stresses
natural selection (p. 120) as he does throughout the book, but he does not
grant it the dominant role that later "hard"
and 79 are missing!
than to those occupied by wolf, coyote, and jackal. The feline adaptive peaks
form a group different from the group of the canine "peaks." But the feline,
canine, ursine, musteline, and certain other groups of peaks form together the
adaptive "range" of carnivores, which is separated by deep adaptive valleys
from the "ranges" of rodents, bats, ungulates, primates, and others. In turn,
these "ranges" are again members of the adaptive system of mammals, which are
ecologically and biologically segregated, as a group, from the adaptive systems
of birds, reptiles, etc. The hierarchic nature of the biological classification
reflects the objectively ascertainable discontinuity of adaptive niches, in
other words, the discontinuity of ways and means by which organisms that
inhabit the world derive their livelihood from the environment. [pp. 9-10]
Dobzhansky renders the hierarchical structure of taxonomy as a fitting of
clades into ecological spaces. Discontinuity is not so much a function of
history as a reflection of adaptive topography. But this cannot be, for surely
the cluster of cats exists primarily as a result of homology and historical
constraint. All felines are alike because they arose from a common ancestor
shared with no other Lade. That ancestor was well adapted, and all its
descendants may be. But the cluster and the gap reflect history, not the
current organization of ecological topography. All feline species have
inherited the unique cat
cannot deviate far from it as they adapt, each in its own particular (yet
superficial) way. Genealogy, not current adaptation, is the primary source of
clumped distribution in morphological space.
The shift in G. G. Simpson's explanation of "quantum evolution" from drift and
nonadaptation (1944) to an exemplar of strict adaptation (1953).
Simpson, probably more than Dobzhansky, personally favored selectionist
arguments in the first version of his seminal work (1944), he was equally
pluralistic and nonrigid. Indeed, he developed an explicitly nonadaptationist
theory to resolve his problem, and he considered this theory of "quantum
evolution" as the crowning achievement of his book.
Dobzhansky, Simpson viewed consistency of all evolutionary change with
principles of modern genetics as his primary claim. The major challenge to
unity and consistency arose from the famous "gaps" or discontinuities in the
fossil record - the appearance of new
fossil intermediates. He wrote:
of the modern synthesis 81
most important difference of opinion, at present, is between those who believe
that discontinuity arises from intensification or combination of the
differentiating processes already effective within a potentially or really
continuous population and those who maintain that some essentially different
factors are involved. This is related to the old but still vital problem of
micro-evolution as opposed to macro-evolution . . . If the two proved to be
basically different, the innumerable studies of micro-evolution would become
relatively unimportant and would have minor value to the study of evolution as
a whole. [1944, p. 97]
explain discontinuities, Simpson relied, in part, upon the classical argument
of an imperfect fossil record, but he concluded that such an outstanding
regularity could not be entirely artificial. He also recognized that his
favored process of gradualistic Darwinian selection in the phyletic mode would
not suffice, and he, therefore, framed the hypothesis of quantum evolution. He
was clearly quite pleased with this formulation, for he ended the book with a
twelve-page defense of quantum evolution, calling it "perhaps the most
important outcome of this investigation, but also the most controversial and
hypothetical" (p. 206). Faced with the prospect of abandoning strict selection
in the gradual, phyletic mode, he framed a hypothesis that stuck rigidly to his
more important goal - to render macroevolution by genetical models operating
within species and amenable to study by neontologists. Thus, he focused upon
the one major phenomenon in the literature of population genetics that granted
control of direction to a phenomenon other than selection - Sewall Wright's
envisaged the major transition as occurring within small populations (where
drift might be effective and preservation in the fossil record virtually
inconceivable). He chose the phrase "quantum evolution" because he envisioned
the process as an "all-or-none reaction" (p. 199) propelling a small population
across an "inadaptive phase" from one stable adaptive peak to another. Because
selection could not initiate this departure from the ancestral peak, he called
upon drift to carry the population into its unstable intermediary position,
where it must either die, retreat, or be drawn rapidly by selection to a new
stable position. Simpson felt that with quantum evolution he had carried his
consistency argument to completion by showing that the genetical models of
neontology could encompass the most resistant and mysterious of all
evolutionary events. Quantum evolution, he wrote, "is believed to be the
dominant and most
process in the origin of taxonomic units of relatively high rank, such as
families, orders, and classes. It is believed to include circumstances that
explain the mystery that hovers over the origins of such major groups" (p.
206). Simpson could, therefore, conclude: "The materials for evolution and the
factors inducing and directing it are also believed to be the same at all
levels and to differ in megaevolution only in combination and in intensity" (p.
pressured for a new edition of
realized that too much had happened in the intervening ten years to permit a
reissue or even a simple revision. The field that he pioneered had stabilized
and flourished: "It was [in the late 1930s] to me a new and exciting idea to
try to apply population genetics to interpretation of the fossil record and
conversely to check the broader validity of genetical theory and to extend its
field by means of the fossil record. The idea is now a commonplace" (1953,
p.ix). Thus, Simpson followed the outline of
wrote a new book more than double the length of its ancestor -
Major Features of Evolution,
two books differ in many ways, most notably by Simpson's increasing confidence
in selection within phyletic lineages as the only important cause of change.
Consider this addition to the 1953 book, a speculative comment on trends in
titanothere horns and too rapid a dismissal (in my view) of the venerable
argument that incipient stages of useful structures may have no evident
long seemed an extremely forceful argument, but now it can be dismissed with
little serious discussion. If a trend is advantageous at any point, even its
earliest stages have
thus if an animal butts others with its head, as titanotheres surely did, the
slightest thickening as presage of later horns already reduced danger of
fractures by however small an amount. [pp. 270-1]
the most dramatic difference between the two books lies in his demotion to
insignificance of the concept that was once his delight and greatest pride -
quantum evolution. It had embodied the pluralism of his original approach -
reliance on a
genetical models. For he had advocated genetic drift to propel very small
populations off adaptive peaks into an ultimately untenable inadaptive phase.
And he had explicitly christened quantum evolution as a mode different in
only in rate, from phyletic transformation within lineages. But now, as the
adaptationist program of the synthesis hardened, Simpson decided that genetic
drift could not trigger any major evolutionary event: "Genetic drift is
certainly not involved in all or
of the modern synthesis
most origins of higher categories, even of very high categories such as classes
or phyla" (p. 355).
an "intermediate stage" - his presentation to the Princeton conference -
Simpson (1949, p. 224) had emphasized the dominance of selection in quantum
evolution, but had not denied other factors. But by 1953, he had completed his
own transition. In 1953, quantum evolution merits only four pages in an
enlarged final chapter on modes of evolution. More importantly, it has now
become what Simpson explicitly denied before - merely a name for phyletic
evolution when it proceeds at its most rapid rates, a style of evolution
differing only in degree from the leisurely, gradual transformation of
populations. Quantum evolution, he now writes, "is not a different sort of
evolution from phyletic evolution, or even a distinctly different element of
the total phylogenetic pattern. It is a special, more or less extreme and
limiting case of phyletic evolution" (p. 389). On page 385, quantum evolution
is listed as one among the four styles of phyletic evolution - and all four are
characterized by "the continuous maintenance of adaptation." The bold
hypothesis of an absolutely inadaptive phase has been replaced by the semantic
notion of a relatively inadaptive phase (an intermediary stage inferior in
design to either ancestral or descendant
is no threat to the adaptive paradigm, for it matters little that an
intermediate is not so well suited for its environment as its ancestor was for
a different habitat (because they are not in competition). And it matters even
less that the intermediate is not so well designed as its descendent will be
(for there is even less opportunity for competition here!). In short,
relatively inadaptive populations are fully adaptive to their own environments
in their own time - and quantum evolution moves comfortably under the umbrella
of the adaptationist program. Simpson even suggests that quantum evolution may
be more rigidly controlled by selection than other modes of evolution (though
he still invokes inadaptation for the initial trigger): "Indeed the relatively
rapid change in such a shift is more rigidly adaptive than are slower phases of
phyletic change, for the direction and the rate of change result from strong
selection pressure once the threshold is crossed" (p. 391).
Sewall Wright's early change on the role of adaptation in the formation of
Wright, when interviewed today (as both W. Provine and I can attest), complains
bitterly that his views on the evolutionary role of genetic drift, once called
the "Sewall Wright effect," have been consistently misrepresented. Since
genetic drift is incontestably about
Stephen J. Gould
change in gene frequencies via sampling error alone, one might assume (as many
have, given the lamentable "scholarly" tradition of pronouncement without
reading) that Wright had advocated a radically non-Darwinian approach to
evolutionary change by demoting selection and adaptation in favor of accident.
Not so, Wright insists. He argues, with evident justice apparent to anyone who
studies his works of the past thirty years, that his theory of "shifting
balance," which does specify an important role for genetic drift, is strongly
adaptationist - but that adaptation arises at a level higher than the
traditional Darwinian focus on individuals.
brief, Wright believes that most evolutionary change within species occurs by
the differential success of some demes over others (rather than by strictly
individual selection within a panmictic population). Wright views this
"interdemic selection" as adaptive and argues that his theory lies within
Darwinian traditions because it identifies (higher-level) selection as the
cause of evolutionary change, and adaptation as its result.
an appeal to higher-level or demic selection poses another problem: How does a
species get divided into demes in the first place? In other words, what process
supplies the raw material (differences among demes) that higher-level selection
must utilize to produce adaptive change? Wright invokes genetic drift to
resolve this problem, and argues that drift acts as a generator of higher-level
variability among demes, not as an agent of evolutionary change.
the founding deme of a new species. It occupies one adaptive peak of the
potential landscape. Other peaks are available for habitation, but how can they
be populated? Other peaks cannot be reached by selection alone, because
organisms descending the slopes of the original peak - a prerequisite to any
climbing of a new slope - will be eliminated by selection, even though adjacent
peaks may be higher than the original habitation. Wright invokes genetic drift
as a mechanism for permitting small demes to descend slopes and cross adaptive
valleys. When a small deme has, by accident, entered a valley and approached
another slope, selection can then draw it up to the new peak. In this way, many
peaks can be inhabited and the raw material for interdemic selection can be
theory is rooted in the conceptually unfamiliar notion of hierarchy (see Gould,
1982b). Indeed, Wright told me that he originally thought of naming his
shifting balance theory the "two-level" theory for its emphasis on the
concerted interaction of ordinary Darwinian selection upon organisms and the
higher-level process of interdemic selection. In this context, we can
understand the major
of the modern synthesis
of the misrepresentation that Wright so bitterly deplores: Few evolutionists
are used to thinking in terms of hierarchically superposed and interacting
levels of selection, for the strict Darwinian tradition advocates nearly
complete explanation by selection acting upon organisms within a population. If
evolutionists tend to translate everything they hear into their favored terms
of organisms within populations, then what else can genetic drift be but a
mechanism of evolutionary
it indisputably alters gene frequencies within demes - and this is the stuff
(by extension) of all evolution. Genetic drift, in this limited view, must,
therefore, be a non-Darwinian agent and opposed to selection. Hierarchical
thinking is a prerequisite to understanding Wrigsht's true intent, for drift
can then act (as he believes) as an agent of variation, supplying raw material
in the form of differentiated demes to the higher-level process of interdemic
we must also acknowledge a different and historical (or ontogenetic) reason for
the confusion. Wright's later shifting balance theory is adaptationist as he
claims and does invoke drift only as a source of variation. But Wright, though
in many ways estranged from the developing synthesis, followed the same trend
toward increasingly exclusive emphasis upon adaptation in evolutionary change.
The version of shifting balance that he has advocated for 30 years did not
arise by sudden creation, complete in its final form. It had antecedents of
differing tenor in Wright's earlier work, and these articles, written during
the pluralistic phase of the synthesis, granted a much greater role to
nonadaptation in evolutionary change. In short, Wright did sometimes invoke
drift as a non-Darwinian agent of change in articles written during the early
pluralistic phase of the synthesis.
Provine, who is writing a complete scientific biography of Wright, has
catalogued Wright's ambiguities and multiple intents during the crucial period
1929-32. The later selectionist view is already in the wings, but several key
passages also advocate the nonadaptationist role for drift that Wright would
later reject. Wright wrote in 1931 (p. 158) that shifting balance "originates
new species differing for the most part in nonadaptive respects." In the
following year, he stated (1932):
evolution involves nonadaptive differentiation to a large extent at the
subspecies and even the species level is indicated by the kinds of differences
by which such groups are actually distinguished by systematists. It is only at
the subfamily and family levels that clear-cut adaptive differences
Stephen J. Gould
the rule. The principal evolutionary mechanism in the origin of species must
then be an essentially nonadaptive one.
concludes "The careful reader in 1932 would almost certainly conclude that
Wright believed nonadaptive random drift was a primary mechanism in the origin
of races, subspecies, species, and perhaps genera. Wright's more recent view
that the shifting balance theory should lead to adaptive responses at least by
the subspecies level is found nowhere in the 1931 and 1932 papers" (ms., p. 58).
hardening a parochial American phenomenon?
community of American evolutionists, although large enough, was itself
sufficiently focused and hierarchical that the changed opinion of a few key
leaders might have swayed an entire field for no particular good reason beyond
authority. We must, therefore, ask whether the hardening of the synthesis was
simply a parochial American phenomenon, traceable to one or a few key people,
and without general significance.
provides a good test for the generality of hardening because adaptationist
traditions have been so strong in Darwin's own land (though Darwin himself was
a pluralist). Perhaps the synthesis was "hard" in Britain from-the first, and
all the change I have documented merely represents a few recalcitrant Americans
finally falling into line. After all, England has produced a long line of
hyperselectionists, from Wallace in Darwin's own day, through the purity of R.
A. Fisher's adaptationism, to the convictions of E. B. Ford and the
launched by A. J. Cain.
we can demonstrate that hardening also affected the work of several prominent
British evolutionists. Consider, for example, Julian Huxley who, after all,
gave the synthetic theory its name (1942). Huxley, to be sure, and as a good
Englishman (but unlike his famous grandfather), was always more committed to
adaptation than, say, the early Dobzhansky. Yet his early views had the
pluralistic cast so characteristic of the 1930s and early 1940s, and he
undertook what many biologists regard as his most important research in a
largely nonadaptationist context. Throughout the 1920s, and culminating in his
book of 1932, Huxley developed and largely founded the study of allometric
growth expressed by power functions. Though this theme could have been
developed in a strictly adaptationist context (see next paragraph), Huxley took
a radically different approach and used the allometric insight primarily to
argue that major features of
modern synthesis 87
organism's phenotype are probably not adaptations ill
complex nonadaptive consequences of inherited growth tendencies, when selection
works on one feature and "drags along" a large set of correlated traits.
say this with personal diffidence because I am, in some measure, responsible
for the reinterpretation of this work along hard adaptationist lines (Gould,
1966). In one of my first papers, written with all the fervor of a graduate
student asked to compose a review article, I argued that virtually all
allometries should be recast as adaptations because parameters of power
functions are as subject to selection as the morphology of phenotypes. I well
remember, in my unquestioning adaptationist commitment of the time, feeling
that Huxley had somehow "betrayed the cause" in departing from the straight and
narrow of strict Darwinism when an adaptationist interpretation for the same
phenomena was available. My general contention about selection on growth rates
was not wrong, but I now rather suspect that Huxley's original perspective is
the more important theme for allometricians.
continued his active pluralism into the book that gave the synthesis its name
(Huxley, 1942). He emphasized adaptation and the revival of Darwinism, but held
throughout that differences among taxa of lower rank were largely nonadaptive
and that Wright's genetic drift supplied an explanation. Provine (ms.) has
counted the references and documented that Huxley gives about equal time to
Fisher's strict adaptationism and to the nonadaptationist interpretation of
Wright's genetic drift.
by the 1950s, Huxley had adopted the hard version of strict adaptationism. At
the major symposium of the 1959 Darwin centennial - a three volume paean to the
hard version (Tax, 1960) - Huxley presented the keynote address (Huxley, 1960)
and defended panselectionism with an explicit renunciation of his former view
that nonadaptation regulates small-scale diversification. He quotes some lines
of Darwin (slightly out of context I think), and then comments: "The first
sentence refers to small-scale processes and makes intelligible the
omnipresence of detailed adaptation" (1960, p. 11), thus recalling Weismann's
famous line, the rallying cry of strict Darwinism at the turn of the century,
about the omnipotence
is also reflected through the several versions of what may be the most
celebrated empirical study of the time - David Lack's work on the Galapagos
finches. (I thank Malcolm Kottler for calling my attention to this example and
for documenting it in a long letter of May 4, 1981.) Lack published his first
monograph on the finches
Stephen J. Gould
1945, though a first draft had been written just before war broke out in 1939.
In this work, Lack strongly supported the common view that small scale
differences were largely nonadaptive. Kottler writes:
this first monograph, Lack took the position that all subspecific differences
and almost all differences between closelyrelated species were non-adaptive.
The major exceptions in the latter case were the beak differences. But at the
time Lack explained them entirely in terms of reproductive isolation. He
explicitly rejected the view that closely-related coexisting species had to be
ecologically isolated and denied that their beak differences were correlated
with significant dietary differences.
his major work of 1947 (Darwin's Finches), Lack continued to defend
nonadaptation for many small-scale differences, but now advanced an adaptive
interpretation for several others. He also introduced the famous, and now
well-established, claim that beak differences are adaptive responses to diet
and act to prevent competition.
1960, as the Darwin centennial marked the heyday of the hard version, Harper
Torchbooks reissued Lack's 1947 monograph. In the major point of a one-page
preface, Lack explicitly renounced his nonadaptationist view, citing the
general change in concept that I have labeled as the hardening of the synthesis:
reader may therefore be reminded that this text was completed in 1944 and that,
in the interval, views on species-formation have advanced. In particular, it
was generally believed when I wrote the book that, in animals, nearly all of
the differences between subspecies in the same genus, were without adaptive
significance. I therefore specified the only exceptions then known and reviewed
the various ideas as to how nonadaptive differences might have been evolved.
Sixteen years later, it is generally believed that all, or almost all,
subspecific and specific differences are adaptive, a change of view which the
present book may have helped to bring about. Hence it now seems probable that
at least most of the seemingly nonadaptive differences in Darwin's finches
would, if more were known, prove to be adaptive.
all this documentation, I fear that the conclusion will seem a bit flat. I now
arrive at the point where I should be giving a conclusive and erudite
explanation of why the synthesis hardened. Yet truly, I do not know. Some
guidelines and factors seem clear enough. First
of Modern Synthesis
all, the community of evolutionary biologists is sufficiently small and
sufficiently stratified - a few lead and many follow, as in most human
activities - that we need not seek some deep and general scientific or societal
trend to render a change in opinion of so many evolutionists of different
nations. A change by a few key people, themselves in close contact and with
mutual influence, might trigger a general response.
aspects certainly influenced the hardening. Once the pluralistic version had
reemphasized classical Darwinism as a respectable alternative, the search for
actual measures of selection and adaptation in nature intensified and
succeeded. The British panselectionist scho;ol, headed by Ford and Cain (see
Ford, 1963, for example), presented many examples from butterflies and snails.
More importantly, Dobzhansky, the key figure in the transition, discovered that
his favorite example of potential nonadaptation needed to be reinterpreted in
selectionist terms. In 1937, he had attributed differences in inversion
frequencies within natural populations of Droso
genetic drift, but he then discovered (see 1951) that these frequencies
fluctuate in a regular and repeatable way from season to season, and decided
(with evident justice) that they must be adaptive. Still, we surely cannot
attribute such a major change as the hardening of the synthesis entirely to
induction from a few empirical cases. After all, no pluralist had doubted that
selection regulated many examples, so the elegant display of a few should not
have established a generality. After all, the arch antisynthesist Richard
Goldschmidt argued forcefully (1940, see Gould, 1982c) that most changes within
populations had a selectionist base (that could not be extrapolated to explain
importantly, most evolutionists brought up wholly within the generation of the
synthesis do not know that an "official" (though admittedly not exclusive)
viewpoint among taxonomists during the early 1930s identified as nonadaptive
the great majority of phenotypic differences separating subspecies and species.
(Kinsey's position, cited in the first section, reflects this professional
consensus; the highly influential book of Robson and Richards, 1936, advanced
this view as its primary contention.) Deference to experts is an important
phenomenon in science - all the more so in a diffuse and largely
nonexperimental field like evolutionary biology, where no one can have direct
experience in all subdisciplines, where simple laboratory repetition cannot
validate many conclusions, and where deference to expert opinion, therefore,
plays a larger role than usual. One cannot read Wright and Huxley, in
particular, during their pluralistic period without getting the definite
impression that they personally rooted
Stephen J. Gould
a stricter form of Darwinism, but hesitated to advance it because taxonomists
proclaimed so certainly that the required frequency of adaptation did not exist
at low levels in nature.
the theoretical basis for neo-Darwinism grew, a new generation of systematists
(Mayr, 1942, in particular) asserted a much wider role for selection and
adaptation, and the earlier view of Robson and Richards eventually faded. One
might give two different interpretations to such a scenario. The "heroic"
version holds that the prevalence of adaptation is an empirical truth whose
recognition had been impeded by an inadequate theoretical base. The supply of
proper theory opened the eyes of systematists to an empirical reality that, in
turn, reinforced the theory. The more "cynical" version holds that we still do
not have a proper assessment of the relative importance of adaptation in
small-scale differences. Since the world is so full of a number of things,
cases of both adaptation and nonadaptation abound, and enough examples exist
for an impressive catalogue by partisans of either viewpoint. In this light,
historical trends in a science might reflect little more than mutual
reinforcement based on flimsy foundations - in this case, an assertion of
neo-Darwinian theory might lead taxonomists to emphasize the cases of
adaptation that manifestly exist, and theorists might then feel sufficiently
strengthened to assert a harder version on the false assumption that its
empirical base had been independently and securely established.
of relative frequency are among the hardest to resolve in science, and natural
history abounds with them. No crucial experiment is possible, nor can we (given
the multitude of potential cases) establish an adequate relative frequency by
simple enumerative induction (What, after all, is a "random" sample in a world
of protean taxonomic diversity?) We still do not know the relative frequency of
adaptation in small-scale differences. The adaptationist context of current
theory leads us to focus upon established examples, but how often do we find
any discussion, or even acknowledgment, of the hundreds of other unexplained
differences that separate most taxa?
a methodological point in conclusion. Darwin's assertion of evolution was an
event of such unrivaled importance in the history of science and human society
that we tend to view it as a watershed for all concepts in biology - as though
everything should be discussed primarily in terms of before or after
evolutionary theory. Such a perspective is inadequate, for many traditions of
thought persist through evolutionary theory, emerging with a reinterpretation
of causality to be sure, but intact nonetheless. In particular, certain
"national styles" persisted from the eighteenth century, through Darwin's era,
and into our own time. Views on adaptation provide a good example.
of the modern synthesis 91
have said nothing about German biology because it has generally held a view of
adaptation outside the scope of this essay (Rensch and other synthesists
notwithstanding). Adaptation is seen as real but superficial, a kind of
jiggling and minor adjustment within a
by some other mechanism - not, in any case, a general mechanism (by
extrapolation) for evolutionary change at higher levels. This viewpoint is firm
in the nonevolutionist transcendental morphology of Goethe and many of the
affects the entire "laws of form" tradition, underlies the pre-Darwinian
evolutionism of Etienne Geoffroy de Saint-Hilaire, and persists to our time
among such German evolutionists as Remane, Schindewolf, and Goldschmidt. ;
adaptationist tradition, on the other hand, has been an English pastime for at
least two centuries. If continental thinkers glorified God in nature by
inferring the character of his thought from the laws of form linking his
created species, or incarnated ideas (as Agassiz maintained), then Englishmen
searched for him in the intricate adaptation of form and function to
environment - the tradition of natural theology and Paley's watchmaker. Darwin
approached evolution in a quintessentially English context - by assuming that
adaptation represented the main problem to be solved and by turning the
traditional solution on its head. Few continental thinkers could have accepted
such a perspective, since adaptation, in their view, was prevalent but
superficial. The centrality of adaptation among English-speaking evolutionists
in our own times, and the hardening of the synthesis itself, owes much to this
continuity in national style that transcends the simple introduction of
evolutionary theory itself. One might say that adaptation is nature's truth,
and that we had to overthrow the ancient laws-of-form tradition to see it. But
one might also say that twentiethcentury panselectionism is more a modern
incarnation of an old tradition than a proven way of nature. It may now be
impeding a more pluralistic account that the early synthesists groped toward
and then let slip away. The only honest answer at the moment is that we do not
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