1030 THE STRUCTURE OF EVOLUTIONARY THEORY
shortage of building stones can slow, or even derail, the construction of a well
designed house.)
In the "consensus paper" of Maynard Smith and eight prominent colleagues
associated with a wide range of views from Darwinian orthodoxy (Lande and
Maynard Smith himself) to serious structuralist heterodoxy (Raup, Goodwin and
Kauffman), "limitations on phenotypic variability" (1985, p. 269) became the
nucleating point of agreement in their remarkable exercise in intellectual diplomacy.
Therefore, "developmental constraint" in this sense of limitation in necessary raw
material to fuel the workings of natural selection, has become (in a minimalist
interpretation that I do not challenge) the canonical "base-line" or "common ground"
definition for this important structural component in Darwinian theory. (In fairness,
Maynard Smith et al., do acknowledge positive meanings of constraint as both
legitimate and more interesting (see p. 1037), while advocating this "negative"
definition as a minimal standard that all evolutionists can embrace, and that no
Darwinian need regard as dangerously debilitating.)
Thus, to return to my previous and facetious example, zebra wings would not
work for the reason cited above, but natural selection will presumably never
encounter an opportunity even to attempt their construction because sufficient
variability for a supernumerary pair of limbs presumably does not exist in the genetic
and developmental systems of tetrapods. In a more meaningful category—
representing a frustrating and unresolved issue that has permeated Darwinian
discussion ever since the eponym himself—the limited range of realized phenotypes
in some clades (with domestic breeds of cats vs. dogs as the classic example) may
reflect a structural limit in variation, rather than a lack of selective opportunity or
advantage.
An important functionalist principle of natural selection, frequently (and quite
explicitly) emphasized by Darwin, holds that we may, in operational terms as the
"null hypothesis" of our initial assumptions for empirical testing, treat populations as
though they always possess sufficient variation to permit natural selection an
unimpeded range of action. As a practical expression of this basic Darwinian belief,
rates of evolutionary change fall under the control of natural selection, not of
limitations (or superfluities) in raw material. Clades that either change slowly, or fail
to generate many species, should be regarded as subject to little selective pressure,
not as limited by intra-populational variation. If cats have developed far fewer
varieties than dogs, then the differential selective efforts of human breeders, rather
than any disparity in the ranges of available raw material, should explain the striking
difference. *
To cite just one among his many explicit statements of this crucial claim,
*To make a personal statement, I was a dyed-in-the-wool adaptationist during my un-
dergraduate and graduate years, but this particular claim—advanced by all my teachers as
an article of faith—always bothered me. I saw no reason beyond an overweening faith in the
strength and ubiquity of selection (Weismann's Allmacht all over again) to assume that
variation in supply of raw material should exert so little effect on rates of evolutionary