526 THE STRUCTURE OF EVOLUTIONARY THEORY
of selection among semi-isolated demes occupying different peaks of an adaptive
landscape. Dobzhansky pleads for more study of "the physiology of populations"
since Wright's model proclaims three factors as important in different ways, while
not granting inherent predominance to any: genetic drift, migration, and natural
selection: "Since evolution as a biogenic process obviously involves an interaction
of all of the above agents, the problem of the relative importance of the different
agents unavoidably presents itself. For years this problem has been the subject of
discussion. The results of this discussion so far are notoriously inconclusive; the
'theories of evolution' arrived at by different investigators seem to depend upon the
personal predilections of the theorist" (p. 186). Dobzhansky does, however,
suggest that Wright's model may validate the common conviction of naturalists
that the morphological differentia of races and species must often be nonadaptive.
Genetics and the Origin of Species went through three editions (1937, 1941,
and 1951). As in the successive versions of Darwin's Origin, the differences among
these editions cannot be dismissed as trivial or cosmetic, for they convey a major
change in emphasis—an alteration that set the research program for most
evolutionary biologists until the past few years. As the Synthesis developed, the
adaptationist program grew in influence and prestige, and other modes of
evolutionary change fell into disrepute, or became redefined as locally operative
but unimportant in the overall picture.
Dobzhansky's third edition (1951) clearly reflects this hardening. He still
insists, of course, that not all change can be called adaptive. He attributes the
frequency of some traits to equilibrium between opposed mutation rates (p. 156)
and doubts the adaptive nature of racial variation in blood types. He asserts the
importance of genetic drift (pp. 165, 176) and does not accept as proof of
panselectionism one of the centerpieces of the adaptationist program—A. J. Cain's
work on frequencies of banding morphs in the British land snail Cepaea (p. 170).
But inserted passages and shifting coverage convey, as their common focus,
Dobzhansky's increasing faith in the scope and power of natural selection, and in
the adaptive nature of most evolutionary change. He deletes the two chapters that
contained most material on nonadaptive or nonselected phenomena (polyploidy
and chromosomal changes, though he includes their material, in much reduced
form, within other chapters). He adds a new chapter on "adaptive polymorphism"
(pp. 108-134). Moreover, he now argues that anagenesis, or "progressive"
evolution, works only through the optimizing, winnowing agency of selection
based on competitive deaths; species adapting by increased fecundity in
unpredictably fluctuating environments do not contribute to anagenesis (p. 283).
But the most remarkable addition occurs right at the beginning. I label these
passages remarkable because I doubt that Dobzhansky really believed what he
literally said; I feel confident that he would have modified his words had anyone
pointed out how his increasing fascination for adaptationism had led him to
downgrade the deepest and oldest of evolutionary themes to effective