Sexual selection

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Illustration from The Descent of Man and Selection in Relation to Sex by Charles Darwin showing the Tufted Coquette Lophornis ornatus, female on left, ornamented male on right.

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 appear more ornate than utilitarian, 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. R.A. Fisher pointed out that this female preference could be under genetic control and therefore subject to natural selection just as much as the qualities of the males that are actually 'preferred'.

Sexual dimorphism

The Rhinoceros beetle demonstrates a classic case of sexual dimorphism. This plate is from Darwin's Descent of Man, with the male at top, female at bottom.

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 sex 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 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.

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 dimorphism, 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 sexual size dimorphism, with 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 individuals. 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 its tail, it is beautiful, but very obvious (though this may actually be advantageous to the survival of the male's offspring and the breeding population as a whole; see below). 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, the Handicap principle due to Amotz Zahavi, 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.

An example of an apparently maladaptive trait that can indirectly help offspring to reach reproductive age, thus resulting in increased reproductive fitness, is the bright coloration of male birds. While the peacock's bright colors make him more obvious to predators and his enormous tail feathers burden him with an unecessary hindrance to movement while simultaneously giving a persuing predator more to hang on to and catch him by, these features also make him far easier prey than his mate and offspring. By distracting predators from his offspring and giving said predators a satisfying meal, he greatly increases his offspring's chances of surviving until reproductive age, thereby giving him a distinct advantage in passing on his genes. And since males are far more expendible in terms of population dynamics than females, distracting the attention of predators away from females and attracting attention to males bennefits the gene-pool as a whole.

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. Darwin hypothesized that sexual selection could also be what had differentiated between different human races, as he did not believe that natural selection provided a satisfactory answer.

Geoffrey Miller, drawing on some of Darwin's largely neglected ideas about human behavior, has hypothesized that many human behaviors not clearly tied to survival benefits, such as humor, music, visual art, verbal creativity, and some forms of altruism, are courtship adaptations that have been favored through sexual selection.

History and application of the theory

Darwin argued that the female peahen chose to mate with the male peacock who had the most beautiful plumage in her mind.

The theory of sexual selection was first proposed by Charles Darwin in his book The Origin of Species, though it was primarily devoted to natural selection. A later work, The Descent of Man and Selection in Relation to Sex dealt with the subject of sexual selection exhaustively, in part because Darwin felt that natural selection alone was unable to account for certain types of apparently non-competitive adaptations, such as the tail of a male peacock. He once wrote to a colleague that "The sight of a feather in a peacock's tail, whenever I gaze at it, makes me sick!" His work divided sexual selection into two primary categories: male-male competition (which would produce adaptations such as a Bighorn Sheep's horns, which are used primarily in sparring with other males over females), and cases of female choice (which would produce adaptations like beautiful plumage, elaborate songs, and other things related to impressing and attracting).

Darwin's views on sexual selection were opposed strongly by his "co-discoverer" of natural selection, Alfred Russel Wallace, though much of his "debate" with Darwin took place after Darwin's death. Wallace argued that the aspects of it which were male-male competition, while real, were simply forms of natural selection, and that the notion of "female choice" was attributing the ability to judge standards of beauty to animals far too cognitively undeveloped to be capable of aesthetic feeling (such as beetles). Historians have noted that Wallace had previously had his own problem with "female choice": he had been left at the altar by a woman of a higher social class.

Wallace also argued that Darwin too much favored the bright colors of the male peacock as adaptive without realizing that the "drab" peahen's coloration is itself adaptive, as camouflage. Wallace more speculatively argued that the bright colors and long tails of the peacock were not adaptive in any way, and that bright coloration could result from non-adaptive physiological development (for example, the internal organs of animals, not being subject to a visual form of natural selection, come in a wide variety of bright colors). This has been questioned by later scholars as quite a stretch for Wallace, who in this particular instance abandoned his normally strict "adaptationist" agenda in asserting that the highly intricate and developed forms such as a peacock's tail resulted by sheer "physiological processes" that were somehow not at all subjected to adaptation.

Though Darwin considered sexual and natural selection to be two separate processes of equal importance, most of his contemporaries were not convinced, and sexual selection is usually de-emphasized as being a lesser force than, or simply a part of, natural selection.

The sciences of evolutionary psychology, human behavioral ecology, and sociobiology study the influence of sexual selection in humans, though these are often controversial fields. 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

• Andersson, Malte (1994). Sexual Selection Princeton U. Press. ISBN 0-691-00057-3
• Darwin, Charles (1871). The Descent of Man and Selection in Relation to Sex
• Fisher, Ronald A. (1930). The Genetical Theory of Natural Selection, chapter 6.
• Miller, G. F. (1998). How mate choice shaped human nature: A review of sexual selection and human evolution. In C. Crawford & D. Krebs (Eds.), Handbook of evolutionary psychology: Ideas, issues, and applications. Lawrence Erlbaum, pp. 87-129.[1]

See also: courtship, aggression, sperm competition