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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 selection depends on the success of certain individuals over others of the same sex, in relation to the propagation of the species; while natural selection depends on the success of both sexes, at all ages, in relation to the general conditions of life. --Charles Darwin, 1871.
Ambiguous combinations of both types of selection acting on the same traits is usually referred to as natural selection. Consistent with Charles Darwin's usage, some present-day accounts refer to natural selection as strictly ecological, and distinct from sexual. However, since sexual reproduction is natural, this is inaccurate and, further, fails to distinguish combinations of the two processes from other "natural" concepts of selection, such as of societies, or of genes.
Typically, traits held to be due to sexual selection conflict with the strict survival fitness of the individual, and this has often been considered to be somewhat paradoxical; various theories have been proposed to account for it.
Intrasexual and Intersexual Selection
Sexual selection takes two major forms: intrasexual selection (aka Male-male competition) in which members of the less limiting sex (typically males) compete aggressively between themselves for access to the more limiting sex, and intersexual selection (aka female choice) in which males compete with each other to be chosen by females.
The sexual struggle is of two kinds: in the one it is between the individuals of the same sex, generally the males, in order to drive away or kill their rivals, the females remaining passive; while in the other, the struggle is likewise between the individuals of the same sex, in order to excite or charm those of the opposite sex, generally the females, which no longer remain passive, but select the more agreeable partners. --Charles Darwin, 1871.
In addition to direct aggression, male-male competition may take the form of sperm competition.
Typically however, sexual selection refers to the process of female choice.
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.
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 differnce 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.
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.
The question regarding viability of the theory
Due to their sometimes greatly exaggerated nature, secondary sexual characteristics can prove to be a hindrance to the animal, thereby lowering its fitness. For example, the large antlers of a moose are bulky and heavy and slow the creature's flight from predators; they also can become entangled in low-hanging tree branches and shrubs, and undoubtedly have led to the demise of many. Bright colorations and showy ornamenations, such as those seen in many male birds, in addition to capturing the eyes of females, also attract the attention of predators; when a male peacock spreads it tail, it is beautiful, but very obvious. Some of these traits also represent energetically costly investments for the animals that bear them. Because traits held to be due to sexual selection often conflict with the survival fitness of the individual, the question then arises as to why, in nature, in which survival of the fittest is considered the rule of thumb, such apparent liabilities are allowed to persist.
An often-cited theory, published by R.A. Fisher in 1930, that attempts to resolve the paradox, posits that such traits are the results of explosive positive feedback loops that have as their starting points particular sexual preferences for features that confer a survival advantage and thus "become established in the species." Fisher argued that such features advance in the direction of the preference even beyond the optimal level for survival, until the selection pressure of female choice is precisely counterbalanced by the resultant disadvantage for survival. Fisher further argued that the strength of the female preference tends to grow exponentially (leading to 'explosive' evolution of the characteristic) until finally checked by ecological selection, since the offspring of those females with the strongest preference typically fare better in reproducing than the offspring of females with weaker preferences. Any mutations for the preference opposite to the given characteristic, though tending to promote survival against ecological selection, nevertheless tend not to survive in the gene pool because male offspring that result from matings based on the preference are less sexually attractive to the majority of the females in the population, and thus infrequently chosen as mates. An equivalent way of expressing this is that if most females are looking, for example, 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. (The females do not actually have this thought process; this kind of decision is an evolutionarily stable strategy.)
Another, more recently developed theory, due to Russell Lande and W.D. Hamilton, holds that 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 effectively considered by the female to be a testament to his overall fitness. Such handicaps 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 intrinsically useful qualities of such traits. 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 counteract 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.
The theories are not mutually exclusive; combinations of them may also be considered.
Proposed human examples
Charles Darwin conjectured that the male beard, as well as the relative hairlessness of humans compared to nearly all other mammals, are results of sexual selection. He reasoned that since, compared to males, the bodies of females are more nearly hairless, hairlessness is one of the atypical cases due to its selection by males at a remote prehistoric time, when males had overwhelming selective power, and that it nonetheless affected males due to genetic correlation between the sexes.
History and application of the theory
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. Alfred Russel Wallace, a co-discoverer of the principle of natural selection, while allowing for the role of male-male competition, steadfastly argued against the possibility that female choice had any significant force.
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.
References
See also: courtship, aggression, sperm competition
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