Sexual selection

Sexual selection is the theory that competition for mates between individuals of the same sex drives the evolution of certain traits. It is distinct from ecological selection which is the competition for food within the species' ecological niche. Many traits, e.g. smooth skin or fur, strong muscles, fluid motions, appear not only to enable hunting or gathering but also to be important sexual attractors, especially in the more intelligent species. For these, ecological and sexual selection both operate on a trait.

Ambiguous combinations of both types of selection acting on the same traits is usually referred to as natural selection. Some accounts refer to natural selection as strictly ecological and as distinct from sexual, but this appears to be a holdover from Victorian sexual prudery, and further fails to distinguish combinations of the two "natural" processes from other concepts of evolution, e.g. evolution of societies or moral evolution[?].

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

In most sexual species (the seahorse is a notable exception), the males and females have different equilibrium strategies, due to a difference in time investment: the male can potential spend five or ten minutes impregnating the female, and have a chance at producing offspring. The female, by comparison, may spend as long as a year pregnant (and unable to produce other children during that time). A male is also much less able than a female to be certain 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 much more willing to mate than females, and so females are the main ones doing the choosing (except in cases of rape, which occurs in certain primate species). This causes sexual selection often to be more pronounced in males than in females: for example, the peacock has elaborate and colorful tail feathers which the peahen lacks.

The peacock's tail is also an example of a trait that, despite decreasing the apparent biological fitness of the organism, still persists. Sexually selected traits tend to become exaggerated: if most females are looking for long-tailed males, then each female individually does better to select a long-tailed male, since then her male children are more likely to succeed. (Of course, the females do not actually have this thought process; this kind of decision is simply an evolutionarily stable strategy.)

An example of sexual selection in human evolutionary history is humans' hairlessness relative to the other great apes. This is part of a general physiological resemblence between adult humans and adolescent chimpanzees (adult humans resemble young chimpanzees to a greater extent than they resemble young humans or adult chimpanzees). This youthful appearance may have evolved because males prefer young-looking mates (a young female is more likely to survive pregnancy). Blond hair lasting into adulthood is another example of a trait that makes a human look younger.

The theory of sexual selection was first proposed by Charles Darwin in his book The Origin of Species. A later book, The Descent of Man, and Selection in Relation to Sex[?] dealt with the subject exhaustively.

The sciences of evolutionary psychology and sociobiology study the influence of sexual selection in humans, though this is often a controversial field.