Escalation, retreat, and female indifference as alternative outcomes of sexually antagonistic coevolution.

Verbal and quantitative genetic models of sexually antagonistic coevolution suggest that coevolutionary arms races should be common. Sexual selection favors exaggeration of male persistence traits that are costly to females, and females, in turn, are selected to resist these traits. The heightened resistance by females is thought to then favor further exaggeration in the male trait, leading to an escalating coevolutionary arms race between persistence and resistance traits. Much of this theory, however, is based on an (implicit) assumption that there are tight constraints on how female resistance can evolve. We develop a theory that identifies and relaxes these constraints, allowing female resistance to evolve in a fashion that better reflects known empirical patterns in the evolution of female preference functions (the resistance trait). Our results suggest that evolutionary arms races that lead to the exaggeration of persistence and resistance will be much less common than formerly predicted. Females sometimes evolve indifference to male traits rather than resistance and can even evolve to discriminate against these traits. These alternative outcomes depend on the existence of genetic variance in the components of the female sensory system underlying female resistance and on the strength of natural selection acting on these components. Female indifference tends to evolve when natural selection on the sensory system is weak, and under these conditions, sexually antagonistic coevolution tends not to reduce female fitness significantly at equilibrium. When natural selection on the female sensory system is strong, however, then arms races are more likely, and female fitness is then sometimes significantly depressed at equilibrium. Sexually antagonistic coevolution is thus likely to have strong deleterious effects on population fitness only when female sensory traits are under strong natural selection to perform functions in addition to those involved with mating. Together, these results suggest that identifying the nature of genetic variation in and the strength of natural selection on female resistance should be a central goal of future studies of sexual conflict.