534 THE STRUCTURE OF EVOLUTIONARY THEORY
the focus of the first phase of the Synthesis (see pp. 503-508). Thus, Mayr
explicitly rejects such fallacies as Larmackian inheritance, and the idea that higher
taxa arise by different and mysterious routes—thereby invoking an argument by
elimination to make evolutionary change at all levels fully consistent with
principles of genetics at work in modern populations and subject to experiment in
the laboratory or observation in the field. Whereas Mayr's major themes remain
Darwinian, he still invokes the full panoply of legitimate genetic forces. Note in
particular that selection (leading to adaptation), while listed first, represents only
one force in an array collectively responsible for the formation of species.
Adaptation holds no exclusivity, or even any particular pride of place: "First, there
is available in nature an almost unlimited supply of various kinds of mutations.
Second, the variability within the smallest taxonomic units has the same genetic
basis as the differences between the subspecies, species, and higher categories.
And third, selection, random gene loss, and similar factors, together with isolation,
make it possible to explain species formation on the basis of mutability, without
any recourse to Lamarckian forces" (1942, p. 70).
Mayr reemphasizes this pluralistic theme at the end of his book in asserting
the essential integrative claim that all phenomena of macroevolution can also be
subsumed by the Synthesis. Inclusion within the Synthesis implies explanation by
principles of modern genetics, not a commitment to any particular mode of genetic
change: "It is feasible to interpret the findings and generalizations of the
macroevolutionists on the basis of the known genetic facts (random mutation)
without recourse to any other intrinsic factors" (1942, p. 292). Mayr then lists the
eight key principles of modern genetics that he regards as necessary for
accomplishing the integration. Only one, number seven on the list, mentions
selection and adaptation (p. 293).
As a more positive argument against adaptationist exclusivity, Mayr's own
taxonomy of "factors involved in speciation" (p. 216) grants explicit and equal
weight to adaptation and nonadaptation as the two primary categories of
divergence. He writes (p. 216): "We may classify these factors as (1) those that
either produce or eliminate discontinuities and (2) those that promote or impede
divergence. The latter may be subdivided further into adaptive (selection) and non-
adaptive factors."
Within this important category of nonadaptation, Mayr includes many
prominent phenomena that he would later ascribe to selection.
- Nearly all polymorphism within species:
There is, however, considerable indirect evidence that most of the
characters that are involved in polymorphism are completely neutral, as far
as survival value is concerned. There is, for example, no reason to believe
that the presence or absence of a band on a snail shell would be a noticeable
selective advantage or disadvantage. Among the many species of birds,
which occur in several clear-cut color phases, there is, with one or two
exceptions, no evidence for selective mating or any other advantage of any
of the phases (p. 75).