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 Darwin's original distinction between traits that aid survival and those that send signals to potential sexual partners. Others emphasise the distinction between the two because sexual selection, which pertains to the survival of the genes, can conflict with survival of the individual, as with the cumbersome peacock tail used mainly in courtship display.
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 potentially 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.) These extreme differences between a male and female of a species are referred to as sexual dimorphisms. These can be as subtle as a size difference or as extreme as horns and color patterns. Examples of sexually dimorphic species can be seen in many instances throughout nature, notably male deer with their antlers and many male birds with their brighter coloration than females.
These traits are caused by secondary sexual characteristics. Secondary sexual characteristics distinguish the sexes of a species, yet have no direct role in reproduction. As mentioned above, these dimorphic traits sometimes become overdeveloped relative to other members of a species. Sometimes these exaggerated traits prove to be a hindrance to the animal, thereby lowering their fitness. These animals are still selected sexually and are therefore able to reproduce. Through this process, the exaggerated trait is passed on to future generations. Large antlers such as that of a moose are bulky and heavy and slow the creature down when running from a predator as well as run the risk of becoming entangled in low hanging tree branches and shrubs. No doubt this trait has lead to the demise of a creature in more than one instance. Bright colorations, such as those seen in many male birds, capture both the eye of a female and that of a predator; when a male peacock spreads it tail, it is beautiful, but very obvious. These traits also represent a costly energetic investment for the animal that bears them.
The question then arises as to why in a natural world in which survival of the fittest is the rule of thumb, can a seemingly detrimental physical trait be allowed to persist; the answer must be that it provides some unseen benefit. Many theories abound as to why. The fact that the male of the species is able to survive until and through the age of reproduction with such a seemingly maladaptive trait is a testament to his fitness in other areas. This might prove he is either free of or resistant to disease, or it might demonstrate that this animal possesses more speed or a greater physical strength that is used to combat the troubles brought on by the exaggerated trait. Other theories highlight an intrinsically useful quality. Antlers, horns and the like can be used in physical defense from a predator, and also in show “jousting” or competition among males in a species. The winner, who typically becomes the dominant animal in the population, is granted access to females, and therefore increases his reproductive output. Antlers are not the only mechanism that can be used to prevent predation. Predators typically look for the eyes of their prey so they can avoid attacking that end of the creature. The conspicuousness of eyespots on many species of butterflies and fishes confuses predators and prevents them from feeling that an attack can be made.
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 resemblance 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. The field of epigenetics is broadly concerned with the competence of adult organisms within a given sexual, social, and ecological niche, which includes the development of mating competences, e.g. by mimicking adult behavior.
K- and r-selection
One of the most famous types of sexual selection is selection by number of offspring.
Some animals, like human (sexually mature after adolescence) and Northern Gannet (5-6 years), produce few offspring. Others reproduce quickly, but unless raised in an artificial environment, most offspring do not survive to adults. A rabbit (mature after 8 months) produces 10 - 30 offsprings per year, a Nile Crocodile (15 years) produces 50, and a fruit fly (10-14 days) produces up to 900. Both strategies can be favoured by evolution: animals with few offspring can spend time nurturing and protecting them, hence greatly decreasing the need to reproduce; on the other hand, animals with many offspring do not need to spend parental energy on nurturing, allowing more energy to be devoted to survival and more breeding.
These two strategies are known as K-selection (few offspring) and r-selection (many offspring). (The letters "r" and "K" derive from the names used in the mathematical formulae in the original theory). Which strategy is favoured depends on a wide range of circumstances.
See also: courtship
