Genetic models of sexual and natural selection in monogamous organisms

Abstract

Darwin's theory of sexual selection in monogamous organisms is described in terms of specific genetic models in which the sexually selected characters are either dominant and recessive or show no dominance in their effects. Mating preferences determine the sexual advantage of particular phenotypes or genotypes. The equilibrium frequencies and eigenvalues giving the rate of approach to equilibrium are derived from recurrence equations of genotypic frequencies in successive generations. When sexual selection alone acts on a population, the ultimate outcome of evolution is identical in both monogamous and polygynous organisms: the same equilibria are usually reached in both mating systems; but polygyny produces a faster rate of selection than mon ogamy. In some of the cases in which there is no dominance of the sexually selected characters, alternative equilibria may also be reached, as well as the single polygynous equilibrium. The advent of natural selection gives rise to different equilibria in monogamous and polygynous organisms. From data of the reproductive success of birds, the average fertility of the preferential matings can be calculated relative to the fertility of the matings that take place at random. From these values of relative fertilities, it can be shown that a sexually selected character will be maintained in balance with natural selection at a lower equilibrium frequency in a monogamous species than in a polygynous species. Observations show that sexual dimorphisms are more highly developed in polygynous organisms, thus supporting hypotheses of Darwin and Fisher that characters for sexual competition evolve until a point of balance is reached between natural and sexual selection.