Sexual Dimorphism - Evolution

Evolution

In 1871 Charles Darwin advanced the theory of sexual selection, which related sexual dimorphism with sexual selection.

It has been proposed that the earliest sexual dimorphism is the size differentiation of sperm and eggs (anisogamy), but the evolutionary significance of sexual dimorphism is more complex than that would suggest. Anisogamy and the usually large number of small male gametes relative to the larger female gametes usually lies in the development of strong sperm competition, because small sperm enable organisms to produce a large number of sperm, and make males (or male function of hermaphrodites) more redundant. This intensifies male competition for mates, and promotes the evolution of other sexual dimorphism in many species, especially in vertebrates, including mammals. However, in many species the females can also be larger than males, irrespective of gametes, and in some species females (usually of species in which males invest a lot in rearing offspring and thus no longer considered as so redundant) even compete for mates in ways more usually associated with males.

In many non-monogamous species, the benefit to a male's reproductive fitness of mating with multiple females is large, whereas the benefit to a female's reproductive fitness of mating with multiple males is small or non-existent. In these species, there is a selection pressure for whatever traits enable a male to have more matings. The male may therefore come to have different traits from the female.

These traits could be ones that allow him to fight off other males for control of territory or a harem, such as large size or weapons; or they could be traits that females, for whatever reason, prefer in mates. Male-male competition poses no deep theoretical questions but female choice does.

Females may choose males that appear strong and healthy, thus likely to possess "good alleles" and give rise to healthy offspring. However, in some species females seem to choose males with traits that do not improve offspring survival rates, and even traits that reduce it (potentially leading to traits like the peacock's tail). Two hypotheses for explaining this fact are the sexy son hypothesis and the handicap principle.

The sexy son hypothesis states that females may initially choose a trait because it improves the survival of their young, but once this preference has become widespread, females must continue to choose the trait, even if it becomes harmful. Those that do not will have sons that are unattractive to most females (since the preference is widespread) and so receive few matings.

The handicap principle states that a male who survives despite possessing some sort of handicap thus proves that the rest of his genes are "good alleles". If males with "bad alleles" could not survive the handicap, females may evolve to choose males with this sort of handicap; the trait is acting as a hard-to-fake signal of fitness.