Sexual selection, natural selection and quality advertisement

Abstract

In Darwin's and Fisher's theory of sexual selection, females prefer ornamented males; the evolution of larger ornaments is limited by increased mortality, lor example through predation. An adornment of given size should often raise mortality more in low than in high quality phenotypes. Possible consequences for the evolution and optimal size of ornaments are here examined with mathematical models. Fisher suggested that an ormamem may evolve if it initially improves male survival. Female preference then spreads for adorned males. Their consequent mating advantage furthers propagation of the preferred trail. An alternative, the ‘handicap mechanism’, suggests that only those males best able to survive can do so with a large, handicapping ornament. Choosing adorned males, females might therefore bear offspring with high general survivorship, but sons also inherit the handicapping ornament. When it reduces survival more in low than in high quality phenotypes, the handicap mechanism (in conjunction with the Fisherian mating advantage) becomes powerful with lower heritability of fitness than previously supposed. It still requires that fitness has some heritability, for which there is indirect evidence, but field measurements are lacking. In Darwin's and Fisher's theory, both mating advantage and mortality disadvantage increase with ornament size, and balance at its optimal development. If a given adornment reduces survival most in low quality phenotypes, the optimum increases with phenotypic quality. Ornament size then may provide a measure of fitness, and can be used in mate choice. A male can usually not gain fitness by developing larger adornments than other males of the same quality. Ornaments can probably also evoke through direct competition between males, without female pREFERENCES for adorned males. Field experiments are needed to clarify this and other aspects of sexual selection. Sexual ornaments form part of the reproductive effort. Certain predictions from life-history theory should therefore apply. One is that ornaments will increase in size over the first few reproductive seasons; this is the case in many species.