The Modern Synthesis as a Limited Consensus 525
a source for all evolution; we can trace full continuity from studies in the
laboratory, to variation within natural populations, to formation of races and
species:
It is now clear that gene mutations and structural and numerical
chromosome changes are the principal sources of variation. Studies of these
phenomena have been of necessity confined mainly to the laboratory and to
organisms that are satisfactory 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 genie and
chromosomal elements, which resemble in all respects the mutations and
the chromosomal changes that arise in the laboratory (1937, p. 118).But what forces mold and preserve this variation in nature? Dobzhansky
stresses natural selection (p. 120), but he does not grant this process the dominant
role that later "hard" versions of the synthesis would confer. He emphasizes
genetic drift (which he calls "scattering of the variability") as a fundamental mode
of evolutionary change in nature, not as an odd phenomenon occurring in
populations too small to leave any historical legacy. He argues that local races can
form without influence from natural selection, and he supports Crampton's (1916,
1932) interpretation of the nonadaptive and indeterminate character of substantial
racial differentiation in the Pacific land snail Partula. He emphasizes that
evolutionary dynamics depend, in large measure, upon the size of populations
because selection does not always control the outcome (and we therefore need
information about numbers of individuals and their mobility in order to assess the
effects of drift, migration, and isolation). He coins the term "microgeographic
race" and argues that most group distinctions at this level may be both nonadaptive
and genetically based, contrary to the opinions of many naturalists who then
regarded such races as adaptive and nongenetic.
The sixth chapter then treats natural selection explicitly. Dobzhansky begins
by clearing away some early Mendelian misconceptions about the impotence of
natural selection (logical errors in interpreting Johannsen's experiments on pure
lines, for example). He then poses a central question: Darwin devised the theory of
natural selection to explain adaptation; admitting Darwin's success in this area,
may we then extrapolate and argue that selection controls the direction of all
evolutionary change (p. 150)? Dobzhansky answers that we cannot defend such an
extension of selection's power. He then criticizes the strict selectionism of Fisher
(p. 151), and praises a book that would later be castigated by all leading synthesists
as a remnant of older and unproductive ways of thought—Robson and Richards
(1936), with their defense of a nonadaptive origin for most subspecific and even
interspecific differences in closely related forms.
A long concluding section (pp. 185-191) supports Wright's "island model"