524 THE STRUCTURE OF EVOLUTIONARY THEORY
differentiation to a large extent at the subspecies and even the species level is
indicated by the kinds of differences by which such groups are actually
distinguished by systematists. It is only at the subfamily and family levels that
clear-cut adaptive differences become the rule. The principal evolutionary
mechanism in the origin of species must then be an essentially nonadaptive one"
(pp. 363-364). Provine (1986) concludes: "The careful reader in 1932 would
almost certainly conclude that Wright believed nonadaptive random drift was a
primary mechanism in the origin of races, subspecies, species, and perhaps genera.
Wright's more recent view that the shifting balance theory should lead to adaptive
responses at least by the subspecies level is found nowhere in the 1931 and 1932
paper."
INCREASING EMPHASIS ON SELECTION AND ADAPTATION
BETWEEN THE FIRST (1937) AND LAST (1951) EDITION OF
DOBZHANSKY'S GENETICS AND THE ORIGIN OF SPECIESDobzhansky's original probe (1937) toward synthesis operated more as a
methodological claim for the sufficiency of genetics than a strong substantive
advocacy of any particular causal argument—although he clearly states his general
Darwinian preferences in this first edition. Dobzhansky held, contrary to his own
Russian mentor Filipchenko, that the methods of experimental genetics can provide
enough principles to encompass evolution at all levels. But Dobzhansky did not
play favorites among the admitted set of legitimate principles. He did not, in
particular, proclaim the pervasive power of natural selection leading to adaptation
as a predominant style and outcome of evolutionary change.
Some inkling of the chaotic and depressed state of evolutionary theory 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 treating Mendelian processes observed in the laboratory as
different from the genetic modes for regulating "important" evolutionary change in
nature. Dobzhansky rebuts the following arguments explicitly: Continuous
variation in nature is non-Mendelian and different in kind from discrete mutational
variation in laboratory stocks (p. 57); Mendelian variation can only generate
differences between taxa of low rank (races to genera), while higher taxa owe their
distinctions to another (and 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 prove the ineffectiveness of natural selection as a mechanism of evolutionary
change (p. 150); selection is too slow in large populations to render evolution, even
in geological time (p. 178); genetic principles cannot account for the origin of
reproductive isolation (p. 255).
Dobzhansky's fifth chapter, on "variation in natural populations," stresses the
pluralism of the early synthesis. Observable genetic phenomena provide