538 THE STRUCTURE OF EVOLUTIONARY THEORY
body proportions to natural selection. The former belief that proportions are
determined by 'built-in' allometry factors and change automatically with changes in
body size is not supported by these findings" (1963, p. 324).
Neutral genes become improbable, almost nonsensical in principle, once we
recognize the pervasive monitoring of nature by selection:
Entirely neutral genes are improbable for physiological reasons. Every gene
elaborates a "gene product," a chemical that enters the developmental
stream. It seems unrealistic to me to assume that the nature of the particular
chemical (enzyme or other product) should be without any effect
whatsoever on the fitness of the ultimate phenotype. A gene may be
selectively neutral when placed on a particular genetic background in a
particular temporary physical and biotic environment. However, genetic
background as well as environment change continually in natural
populations and I consider it therefore exceedingly unlikely that any gene
will remain selectively neutral for any length of time (1963, p. 207).Consequently, even the most apparently trivial features probably originated
by direct selection. "One can never assert with confidence that a given structure
does not have selective significance. The peculiar tarsal combs of the males in
certain species of Drosophila turned out to have an important function during
copulation; the color patterns of Cepaea snails have cryptic significance,
mitigating predator pressure" (1963, p. 190).
In 1963, Mayr repudiated all three major classes of nonadaptation that he had
defended in 1942: polymorphisms, clines, and much geographic variation in
general. Explicitly refuting his own former view, Mayr now argues (1963, p. 162)
that the ubiquity of selection must imply an adaptive basis for polymorphisms (see
also pp. 158 and 167): "Such neutral polymorphism, it was claimed, was
maintained by 'accident.' Now that the cryptic physiological effects of 'neutral'
genes have been discovered, it is evident that such genes are anything but
selectively neutral. It is altogether unlikely that two genes would have identical
selective values under all the conditions in which they may coexist in a
population."
In a remarkable statement, he then urges that polymorphisms and clines be
viewed as evidence for adaptation a priori: "Selective neutrality can be excluded
almost automatically wherever polymorphism or character clines are found in
natural populations... Virtually every case quoted in the past as caused by genetic
drift due to errors of sampling has more recently been reinterpreted in terms of
selection pressures" (1963, pp. 207-208).
As for geographic variation, what else could such a phenomenon represent but
adaptation to an altered environment, with selection as an efficient and
omnipresent watchdog: "The geographic variation of species is the inevitable
consequence of the geographic variation of the environment. A species must adapt
itself in different parts of its range to the demands of the local environment. Every
local population is under continuous selection pressure for maximal fitness in the
particular area where it occurs... Each local environment