546 THE STRUCTURE OF EVOLUTIONARY THEORY
and yet at some times brings about such forms of integration of the gene pool of
the population which lead to the sacrifice of some of the individual members of the
population. The phenomenon of balanced polymorphism, with highly fit
heterozygotes contrasting with less fit homozygotes, is one of such forms of
genetic integration of Mendelian populations."
Mayr's most authoritative book (1963) provides an excellent illustration of
organismic orthodoxy amidst statements, lacking clear definition, about selection
at higher levels. Mayr surely recognizes the usual form of proper Darwinian
argument—that apparent benefits to populations should be explained, whenever
possible, as effects of selection upon organisms: "The solution usually proposed
for the difficulty raised by the conflict between a benefit for the individual and one
for the population is to make the population rather than the individual the unit of
selection.... It would seem preferable to search for solutions based on the selective
advantage of individual genotypes, such as Fisher's explanation of an even sex
ratio" (1963, pp. 198-199).
In rereading Mayr's 1963 book with the hindsight of thirty years, however, I
was struck by the number of passages and arguments that either speak loosely
about explicit advantages to groups and populations (rather than fortuitous
beneficial effects arising as side consequences of selection on organisms), or seem
to state an explicit claim for selection at the population level. Most of these
statements focus on the virtues of genetic variability. Mayr asks (1963, p. 158):
"Why are not all individuals of a population identical in appearance? Is it because
diversity is of selective advantage to the population?" He then argues (1963, p.
308) that the primary function of chromosomal variation lies in the flexibility thus
accorded to populations: "They appear to have, as primary function, either the
increase of adaptability and adaptedness of these populations through balanced
polymorphism of entire chromosome sections or the regulation of the amount of
recombination." In fact, Mayr's main justification for regarding polymorphism as
adaptive—a major shift in his own adaptationist hardening from the examples used
to support non-selectionist claims in his 1942 book—focuses on advantages to
populations (1963, p. 251).
Polymorphism is based on and produced by definite genetic mechanisms,
such as genes for differential niche selection and the heterosis of
heterozygotes. A population that has not responded to selection for such
mechanisms and therefore lacks polymorphic diversity is more narrowly
adapted, more specialized, and therefore more vulnerable to extermination.
The widespread occurrence of genetic mechanisms that produce and
maintain polymorphism is directly due to selection and is in itself a
component of adaptiveness. It seems appropriate, therefore, to speak of
"adaptive polymorphism."A rally around the flag of organismic selection, and an explicit (and
vociferous) denial of higher levels, became a major movement in evolutionary
theory during the 1960's. The hardening of adaptationism had occurred largely