Polymorphisms maintained by sexual selection in monogamous species of birds

Abstract

A computer model has been developed of sexual selection in monogamous birds as a result of variations in fitness at breeding time. In the model, the females may prefer to mate with any of the three male genotypes, AA, Aa and aa, in a simple genetic polymorphism such as the different colour phases of the Arctic Skua. Results are given for two different distributions of breeding times and fitnesses: (i) the distribution observed in the Arctic Skua, a polymorphic sea bird; (ii) normal distribution of breeding times with fitnesses decreasing as the square of the deviations from the mean. In the computer simulations, values chosen for the female mating preference were either (i) additive, (ii) over-dominant, or (iii) “disruptive” with homozygotes preferred to heterozygotes. When breeding times and fitnesses are distributed as in the Arctic Skua, both additive and over-dominant mating preferences give rise to selection for stable polymorphisms with similar gene frequencies at equilibrium to those in the corresponding models of selection in polygamous species. Disruptive mating preferences generally produce unstable equilibria, unlike the model for polygamy. When breeding times are normal, the rarer allele is usually at a disadvantage and eliminated. But when mating preferences are very slight, the same equilibria are sometimes reached as in the polygamy model. © 1973, The Genetical Society of Great Britain.