Sexual selection

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 and  Selection in Relation to Sex by Charles Darwin showing the Tufted Coquette Lophornis ornatus, female on left, ornamented male on right."> and  Selection in Relation to Sex by Charles Darwin showing the Tufted Coquette Lophornis ornatus, female on left, ornamented male on right." width="250" height="295" longdesc="../../../d/e/s/Image%7EDescent_of_Man_fig48.jpg_2682.html" />

Illustration from  and  Selection in Relation to Sex">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  a theory proposed by Charles Darwin that states that the frequency of traits can increase or  decrease depending on the attractiveness or  fertility of the bearer. Biologists today distinguish between male to male combat (it is  usually males who fight), mate choice (usually female choice of male mates) and  mate coercion (forced mating). Much attention has recently been given to cryptic female choice^[1], a phenomenon in internally fertilising animals such as mammals and  birds, where a female may simply dispose of a male's sperm without his knowledge.

The exact effect of sexual selection depends on the sex ratio, which is  usually slightly biased in favour of the "limiting" sex (typically females).

Male to male combat is  also classified as intrasexual competition, while mate choice and  mate coercion are  also known as intersexual competition.

Sexual selection can create two kinds of traits - weapons and  ornaments. Weapons are  used in male to male combat, that is, competition for mating opportunities. Sperm competition is  another way for males to compete.

Females often prefer to mate with males with external ornaments - exaggerated features of morphology. These can plausibly arise because an  arbitrary female preference for some aspect of male morphology initially increased by genetic drift, creating, in due course, selection for males with the appropriate ornament. This is  known as the sexy sons hypothesis. Alternatively, genes that enable males to develop great ornaments may simply show off greater disease resistance or  a more  efficient metabolism - features that also benefit females. This idea is  known as the good genes hypothesis.

[ and  base conditions">edit] Phylogeny of sexual selection and  base conditions

The success of an  organism is  not only measured by the number of offspring left behind, but by the quality or  probable success of the offspring: reproductive fitness. Sexual selection is  the expansion on the ability of organisms to differentiate each other at the species level, interspecies selection.

The grossest blunder in sexual preference, which we can conceive of an  animal making, would be to mate with a species different from its own and  with which hybrids are  either infertile or, through the mixture of instincts and  other attributes appropriate to different courses of life, at so serious a disadvantage as to leave no descendants. ..it is  no conjecture that a discriminative mechanism exists, variations in which will be capable of giving rise to a similar discrimination within its own species, should such a discrimination become at any time advantageous. —Ronald Fisher, 1930

The expansion of interspecies selection and  intraspecies selection is  a driving force behind species fission: the separation of a single contiguous species into multiple non-contiguous variants. Sexual preference creates a specialized tendency towards homogamy that provides a system by which a group constantly invaded by the diffusion of unfavourable genes may suppress ill effects.

Individuals in each region most readily attracted to or  excited by mates of the type there favored, in contrast to possible mates of the opposite type, will, in fact, be the better represented in future generations, and  both the discrimination and  the preference will thereby be enhanced. it  appears certainly possible that an  evolution of sexual preference due to this cause would establish an  effective isolation between two differentiated parts of a species, even when geographical and  other factors were least favorable to such separation. —Ronald Fisher, 1930

When it  is possible for it  to be exercised usefully, the general conditions of sexual discrimination appear to be (1) the acceptance of one mate precludes the effective acceptance of alternative mates, and  (2) the rejection of an  offer will be followed by other offers, either certainly, or  at such high chance that the risk of non-occurrence will be smaller than the chance advantage to be gained by selecting a mate.

[ and  intersexual selection">edit] Intrasexual and  intersexual selection

Sexual selection takes two major forms: intrasexual selection (also known as 'male–male competition') in which members of the less  limited sex (typically males) compete aggressively among themselves for access to the limiting sex, and  intersexual selection (also known as 'mate choice' or  '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

Intrasexual confrontation between deer

With intrasexual selection it  should be brought to mind that adorned males will gain reproductive advantage without the intervention of female preference and  intersexual selection. This advantage will be conferred by weapons used in the process of resolving disputes, such as those over territorial rights. The use of male sexual ornamentation is  primarily used in the search of asymmetries in rival males, contrary to what would seem most obvious (mortally wounding the opponent), since a high number of fatal combats over territory would result in a clear disadvantage. The use of sexual ornamentation is  used as a signaling device (signalling theory) amongst males to create a dominance hierarchy, also known as a pecking order, without unneeded detriment and  fatality. it  is predominantly when two opposing males are  so closely matched, as would be found in males not having established themselves in a dominance hierarchy, that asymmetries can not be found and  the confrontation escalates to a point where the asymmetries must be proved by aggressive use of ornamentation.

As a propagandist the cock behaves as though he knew that it  was advantageous to impress the males as the females of his species, and  a sprightly bearing with fine feathers and  triumphant song are  quite as well adapted for war-propaganda as for courtship. —Ronald Fisher, 1930

How often males will physically engage each other, and  in what manner, can best be understood by applying game theory developed for biology, most notably by John Maynard Smith^[2].

In addition to conventional aggression, male–male competition may take the form of sperm competition.

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

However, 'sexual selection' typically refers to the process of choice (the limiting factor, which is  typically females) over members of the opposite sex (the non-limited factor, typically males). This process is  known as intersexual selection. Fisher pointed out that preference could be under genetic control and  therefore subject to a combination of natural and  sexual selection just as much as the qualities of the ornamentation 'preferred'.

The conditions determining which sex is  the limiting factor in intersexual selection can be best understood by way of Bateman's principle which states that the sex which invests the most in producing offspring becomes a limiting resource over which the other sex will compete. This can be most easily illustrated by the contrast in nutritional investment into a zygote between egg and  sperm, and  the limited reproductive capacity of females compared to males.

The stochastic process of sexual selection, the combined effects of its subsets (intra and  intersexual selection), create assortative mating and  linkage disequilibrium.

[edit] Geometric progression

In species which the reproductive success of one sex depends heavily on winning the concession of the other, as is  evident with many polygamous birds, sexual selection will act by increasing the degree of preference to which it  is due, with the consequence that both the trait preferred and  the intensity of preference will be increased together with ever-increasing velocity. This process causing a fervent and  rapid evolution of both the conspicuous ornamentation and  the preference for such, until so arrested directly or  indirectly by bionomic Natural Selection reasons. Thus, in many cases a positive feedback loop of sexual selection is  created, resulting with exorbitant physical structures in the non-limited sex. The most notorious example being the Peacock (shown above). it  is important to note that while a Peacock may have  exorbitant plumage, the Peahen has equally exorbitant taste for such.

Initially to start the process, there would be a correlation between the trait and  higher fitness. Two previously isolated species, A and  B, could come to inhabit the same area resulting in some hybridization. In this situation reproductive isolation will be favored. If the mean value of a trait i.e. tails, in species A, is  larger than those of species B, selection would favor females of species A with preference for large tails. Once started the process could continue past the need for species isolation.

The Peahen will desire to copulate with the most attractive Peacock so that her progeny, if male, will be attractive to females in the next generation. Additionally the Peacock will desire to copulate with a Peahen that finds him attractive so that if the progeny is  female, preference for his degree of ornamentation remains present in the next generation. Since the rate of change in preference is  proportioned according to the highest average degree of taste amongst females, and  that females desire to best other members of the sex, it  creates an  additive effect in the cyclical process that will yield exponential increases, in both sexes, if unchecked.

R.A.Fisher in The Genetical Theory of Natural Selection was the first to articulate this process in a game theoretic style treatment.

plumage development in the male, and  sexual preference for such developments in the female, must thus advance together, and  so long as the process is  unchecked by severe counterselection, will advance with ever-increasing speed. In the total absence of such checks, it  is easy to see that the speed of development will be proportional to the development already attained, which will therefore increase with time exponentially, or  in geometric progression. —Ronald Fisher, 1930

The exponential element, which is  the kernel of the thing, arises from the rate of change in hen taste being proportional to the absolute average degree of taste. —Ronald Fisher, 1932 [3]

It is  important to notice that the conditions of relative stability brought about by these or  other means, will be far longer duration than the process in which the ornaments are  evolved. In most existing species the runaway process must have  been already checked, and  we should expect that the more  extraordinary developments of sexual plumage are  not due like most characters to a long and  even course of evolutionary progress, but to sudden spurts of change. —Ronald Fisher, 1930

Since R.A.Fishers initial conceptual model of the 'run-away' process, various others have  continued the work on modeling an  accurate mathematical proof. Notably R.Lande^[4] & P.O'Donald.

[edit] Sexual dimorphism

 is  from Darwin's Descent of Man, with the male at top, female at bottom."> is  from Darwin's Descent of Man, with the male at top, female at bottom." width="247" height="265" longdesc="../../../d/e/s/Image%7EDescent_of_Man_-_Figure_16.jpg_a99f.html" />

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 (such as 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 (pregnancy in mammals or  the production of the egg in birds and  reptiles) , 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 pipefish, and  Wilson's phalarope. Also, unlike a female, a male (except in monogamous species) 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 to raise offspring that may or  may not be related to him. As a result of these factors, males are  typically 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.

[edit] The question regarding viability of the theory

Due to their sometimes greatly exaggerated nature, secondary sexual characteristics can prove to be a hindrance to an  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.)

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 attack that end of the creature. The conspicuousness of eyespots on many species of butterflies and  fishes confuses predators and  helps to prevents the prey from suffering serious damage.

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.

Zahavi's work spurred a re-examination of the field, which has produced an  ever-accelerating number of theories. In 1984, Hamilton and  Marlene Zuk introduced the "Bright Male" hypothesis, suggesting that male elaborations might serve as a marker of health, by exaggerating the effects of disease and  deficiency. In 1990, Michael Ryan and  A.S. Rand, working with the túngara frog, proposed the hypothesis of "Sensory Exploitation", where exaggerated male traits may provide a sensory stimulation that females find hard to resist. In 1991, Anders Pape Møller, working with the tails of male barn swallows, introduced fluctuating asymmetry to the field. Fluctuating asymmetry, a concept previously invoked under natural selection, is  based on the observations that healthier specimens have  more left-to-right sided symmetry than less  healthy specimens. Subsequently the theories of the "Gravity Hypothesis" by Jordi Moya-Larano et al. and  "Chase Away" by Brett Holland and  William R. Ricehave have  also been added. In addition, in the late 1970's Janzen and  Mary Willson, noting that male flowers are  often larger than female flowers, expanded the field of sexual selection into plants.

In the past few years, the field has exploded to include many additional areas of study, not all of which are  clearly included under Darwin's definition of sexual selection. These include cuckoldry, nuptial gifts, sperm competition, infanticide, physical beauty, mating by subterfuge, species isolation mechanisms, male parental care, ambiparental care, mate location, polygamy, and  mechanisms that can only be called bizarre, including homosexual rape in certain male animals, cementing of females' vaginal pores by males in some lepidopteran insects, and  insect penises specialized to remove any sperm packets from females which may have  been deposited by previous suitors.

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

Noting, however, that this proliferation of theories and  the widening confusion about the definition of the field matches the patterns of a Kuhnian crisis; Joe Abraham published two papers questioning whether the problems of sexual selection might be explained under natural selection. In 1998, he published the Female Sabotage hypothesis, pointing out that mating provides females with the opportunity to sabotage polygamous males, by only mating with males who exhibit life-threatening traits. As males increasingly die as a result of their elaborations, fighting, and  mating exertions, their declining numbers leave more  food and  other resources for females and  offspring, and  relieve them of predation pressure. In 2005 he published a companion paper looking at sexual dimorphism in flower sizes, resurrecting an  older theory by Hermann Müller, that larger male flowers may simply serve to attract pollinators to pollen donors, before they visit pollen acceptors. Abraham's experimental data showed strongly that this is  the case, and  flower dimorphisms may also be a function under natural selection rather than sexual selection.

[edit] 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. However, Darwin also doubted that characteristics of races he considered to be "inferior" would have  been attractive to potential mates.

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.

[ and  application of the theory">edit] History and  application of the theory

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,  and  Selection in Relation to Sex">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.

[edit] Criticism by Roughgarden et al.

A group of authors led by Joan Roughgarden have  criticised sexual selection theory,^[5] claiming there was evidence that individuals did not compete strongly for mating opportunities, but that the function of sex was mostly social. The evidence used in the paper, however, was heavily criticised by many other notable authors for its factual accuracy.^[6]

[edit] Notes

1. ^ Eberhard, WG. Sexual selection by cryptic female choice. Princeton, Princeton University Press. 1996.
2. ^ Maynard Smith, J (1982)  and  the Theory of Games">Evolution and  the Theory of Games. Cambridge University Press, pp. 131-137. ISBN 0-521-28884-3
3. ^ Ronald Fisher in a letter to Charles Galton Darwin, 22 November 1932, cited in Fisher, R. A., Bennett, J. H. 1999. The genetical theory of natural selection: A complete variorum edition, Oxford University Press, Oxford, p. 308
4. ^ Lande, R. (1981) Models of speciation by sexual selection on polygenic traits. PNAS 78:3721-3725.
5. ^ J, Roughgarden, Oisho, M & Akçay, E (2006). "Reproductive social behaviour: cooperative games to replace sexual selection". Science 311: 965-969..  news  abstract.
6. ^ (2006) "Debating sexual selection and  mating strategies". Science 312: 689-697.

[edit] References

• Abraham, J.N. (1998) La Saboteuse: an  Ecological Theory of Sexual Dimorphism in Animals. Acta Biotheoretica 46 23-35 [1]
• Abraham, J.N. (2005) Insect Choice and  Floral Size Dimorphism: Sexual Selection or  Natural Selection? J. Insect Behavior, 18 743-756 [2]
• Andersson, M (1994) Sexual selection Princeton University Press. ISBN 0-691-00057-3
• Darwin, C (1871)  and  Selection in Relation to Sex">The Descent of Man and  Selection in Relation to Sex
• Fisher, RA (1930) The Genetical Theory of Natural Selection, Oxford University Press, ISBN 0-19-850440-3, Chapter 6 [3]
• Lande, CF R (1981). Models of speciation by sexual selection on polygenic traits. Proc. Nat. Acad. Sci., USA, 78 3721-5 [4]
• Miller, GF (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.[5]
• Miller, GF (2000) The Mating Mind: How sexual choice shaped the evolution of human nature. London: Heinemann. ISBN 0-434-00741-2

[edit] External Links

• Sexual Selection: Stanford University
• The Great Debate: Sexual Selection
• An Introduction to Sexual Selection: University of Siegen
• Intralocus Sexual Conflict Diminishes the Benefits of Sexual Selection

[edit] See also

• Aggression
• Assortative mating
• Beauty
• Courtship
• Evolutionary arms race
• Irish elk
• Physical attractiveness
• Sexual attractiveness
• Sexual conflict
• Sperm competition
• Ugliness