Frequency-dependent sexual selection.

Sexual selection by female choice is expected to give rise to a frequency-dependent sexual advantage in favour of preferred male phenotypes: the rarer the preferred phenotypes, the more often they are chosen as mates. This 'rare-male advantage' can maintain a polymorphism when two or more phenotypes are mated preferentially: each phenotype gains an advantage when it is rarer than the others; no preferred phenotype can then be lost from the population. Expression of preference may be complete or partial.

Sexual selection and fertility.

Genetic models are analyzed in which sexual selection is combined with fertility selection. In these models, the sexual selection acts on males, the fertility selection on either males, females or both sexes. The phenotypes thus selected may be determined either by dominant and recessive alleles or by each homozygous and heterozygous genotype.

A general analysis of genetic models with frequency-dependent mating. II. Sexual selection for heterozygotes.

Heterozygotes are assumed to mate with a frequency that is any general function, f(v), of their population frequency, v. Models are analysed in which the selection that determines the function f(v) acts either on one sex alone or on both sexes equally. The central equilibrium point v* = 1/2 always exists; it is stable if f(1/2) greater than 1/2.

Mating advantage of rare males in models of sexual selection.

Models of sexual selection in polygynous species of animals have been derived on the assumption that some females have preferences to mate with males with particular genotypes. The mating advantage gained by the males is always frequency-dependent because the preferred males take part in the same number of preferential matings when they are rare as when they are common; individually therefore, they mate more often when they are rare. Frequency-dependent sexual selection has been demonstrated in many experiments with Drosophila: rare males take part in a higher proportion of matings than their frequency as available mates.

A demographic analysis of the components of selection in a population of Arctic Skuas.

From previously published demographic data of the age distributions and reproductive rates of the pale, intermediate and dark phenotypes of the Arctic Skua, revised estimates are obtained of the intrinsic rates of increase and selective coefficients of the phenotypes in each sex. Two significant components of selection are variation in age of maturity and variation in reproductive success. Sexual selection is a component of the variation in reproductive success.

Estimation of assortative mating preferences in the Arctic skua.

Methods are described for the maximum likelihood estimation of mating preferences in models of assortative mating for monogamous and polygamous organisms. These methods are applied to data of matings of the three phenotypes, pale, intermediated and dark of the Arctic Skua. The data were obtained by exhaustive surveys of the Arctic Skua populations on the islands of Fair Isle and Foula.