The extreme case of variable optima is what is known
as negative frequency-dependent selection. This describes
the situation in which the relative fitness of a trait is high as
long as it is rare in the local population but declines as it
becomes widespread. It has long been recognized that
negative frequency dependency can in theory lead to the
maintenance of polymorphism (Maynard-Smith, 1982),
though it has been harder to demonstrate empirically and
unequivocally that such a mechanism is in operation. One
such demonstration comes from the bluegill sunfish, Lepomis
macrochirus (Gross, 1991). Males of this species can
be either parental or cuckolding, with the former delaying
reproduction and building nests and the latter maturing
small and early and sneaking into nests built by other
males. Gross (1991) showed that the reproductive success
of the cuckolding males is high when they are rare in a
colony but declines steeply as they become more common.
Thus, selection must produce a dynamic equili