Sexual selection

Sexual selection is the theory that competition for mates between individuals of the same sex (typically males) drives the evolution of certain traits. Within a species, one sex (typically females) acts as a limiting resource for the other (typically males). Competition over the limiting sex results in sexual selection. It is distinct from ecological selection which is the competition for other limiting resources within the species' ecological niche.

Sexual dimorphism

Sex differences directly related to reproduction and serving no direct purpose in courtship are called primary sexual characteristics. Traits amenable to sexual selection, which give an organism an advantage over its rivals in courtship without being directly involved in reproduction, are called secondary sexual characteristics.

Related Topics:
Primary sexual characteristics - Reproduction - Secondary sexual characteristics

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In most sexual species the males and females have different equilibrium strategies, due to a difference in relative investment in producing offspring. As formulated in Bateman's principle, females have a greater initial investment in producing offspring, and this difference in initial investment creates differences in variance in expected reproductive success and bootstraps the sexual selection processes. Classic examples of reversed sex-role species include the seahorse, and Wilson's phalarope. Also, unlike a female, a male has some uncertainty about whether or not he is the true parent of a child, and so will be less interested in spending his energy helping a child who may or may not be related to him. As a result of these factors, males are typically much more willing to mate than females, and so females are typically the ones doing the choosing (except in cases of rape, which occurs in certain primate species, as well as in some species of ducks). The effects of sexual selection are thus held to typically be more pronounced in males than in females.

Related Topics:
Males - Females - Equilibrium - Bateman's principle - Seahorse - Rape - Primate - Ducks

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Differences in secondary sexual characteristics between males and females of a species are referred to as sexual dimorphisms. These can be as subtle as a size difference (sexual size dimporhism, often abbreviated as SSD) or as extreme as horns and color patterns. Sexual dimorphisms abound in nature. Examples include the possession of antlers by only male deer, and the brighter coloration of many male birds, in comparison with females of the same species. The peacock, with its elaborate and colorful tail feathers, which the peahen lacks, is often referred to as perhaps the most extraordinary example of a dimorphism. The largest sexual size dimorphism in vertebrates is the shell dwelling cichlid fish Neolamprologus callipterus in which males are up to 30 times the size of females. Extreme reversed sexual size dimorphism (females larger than males) is quite common in spiders.

Related Topics:
Sexual dimorphism - Peacock - Peahen - Vertebrates - Shell dwelling - Cichlid - Fish - Neolamprologus callipterus - Spiders

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