SEXUAL SELECTION AND THE EVOLUTION OF FEMALE CHOICE

A paradox for evolutionists has been the development in one sex, usually females, of preferences for mates possessing characters that impair survival. Darwin (187 1) extensively documented cases of sexual selection in animals and showed that many of the most dramatic examples are attributable to female mating preferences operating in polygynous populations. But while he was untroubled to explain the evolution of adaptations for male-male sexual competition, Darwin did not provide a hypothesis for the origin or maintenance of female mating preferences. Fisher (1958, p. 150-153) provided a subtle solution to this puzzle. He reasoned that the initial evolution of a female mating preference would require "bionomic conditions in which such preference shall confer a reproductive advantage. " He showed that evolution of the preferred male trait could then "proceed, by reason of the advantage gained in sexual selection, even after it has passed the point in development at which its advantage in Natural Selection has ceased." Because females with stronger preferences mate with males bearing more exaggerated traits, a genetic correlation between the preference and trait is maintained by sexual selection. If the -more extreme males are increasing in frequency, the genetic correlation results in the evolution of stronger preferences and so causes further selection for the extreme males. Fisher dubbed this a "runaway process." O'Donald (1967, 1980) used simulations to verify several essential features of Fisher's argument. He differed with Fisher, though, in his belief that females must respond to supernormal mating stimuli for a male trait to evolve to a point where it decreases viability. O'Donald (1977, 1980) pointed out that a small increase in the male trait might result in a small additional viability loss but a large gain in mating advantage by this mechanism. The primary conclusion of the present paper is that the initial selective advantages for the female preference assumed by Fisher, O'Donald, and many later authors are not necessary for either the origin or subsequent elaboration of mating preferences for traits associated with reduced survivorship. Using a two-locus analytic model that follows evolution of both the preference and the trait in a polygynous population, I will show that such mating preferences are neither selected for nor against. A sufficiently strong mating preference can, however, maintain in the population a male trait that causes greatly reduced viability. The result can be a dramatic deterioration of the average survivorship, as Fisher and O'Donald concluded, but no particular assumptions about behavioral mechanisms (e.g., response to supernormal mating stimuli) are