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.

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 its 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.

Related Topics:
Fitness - Survival of the fittest

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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.)

Related Topics:
R.A. Fisher - 1930 - Positive feedback - Gene pool - Evolutionarily stable strategy

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Another, more recently developed theory, the Handicap theory 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.

Related Topics:
Handicap theory - Amotz Zahavi - Russell Lande - W.D. Hamilton - Disease

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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.

Related Topics:
Predator - Jousting - Competition

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The theories are not mutually exclusive; combinations of them may also be considered.

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