THE DARWIN-FISHER THEORY OF SEXUAL SELECTION IN MONOGAMOUS BIRDS

Males of monogamous birds often show secondary sexual traits that are conspicuous but considerably less extreme than those of polygynous species. The authors develop a quantitative-genetic model for the joint evolution of a male secondary sexual trait, a female mating preference, and female breeding date, following a theory proposed by Darwin and Fisher. Good nutritional condition is postulated to cause females to breed early and to have high fecundity. The most-preferred males are mated by early-breeding females and receive a sexual-selection advantage from those females' greater reproductive success. Conspicuous male traits that decrease survival can evolve but suggest that the extent of maladaptive evolution is greatly limited relative to what is possible in a polygynous mating system for 2 reasons. First, in the absence of direct fitness effects of mate choice on the female, the equilibria for the male trait and female preference form a curve whose shape shows that the maximum possible strength of sexual selection on males (and hence the potential for maladaptive evolution) is constrained. Under certain conditions, a segment of the equilibrium curve may become unstable, leading to 2 alternative stable states for the male trait. Second, male parental care will often favor the evolution of mating preferences for less conspicuous males. Sexual selection can appear in the absence of the nutritional effects emphasized by Darwin and Fisher. -from Authors