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 proposed by Charles Darwin that states that the frequency of traits can increase or decrease depending on the attractiveness 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). Traits selected for by male combat are called "weapons", and traits selected by mate choice are called "ornaments". 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 equivalent in male-to-male combat is sperm competition. Figure 48 from The Decsent of Man and selection in relaton to sex by Charles Darwin. ... Figure 48 from The Decsent of Man and selection in relaton to sex by Charles Darwin. ... The Descent of Man and Selection in Relation to Sex by British naturalist Charles Darwin was first published in 1871. ... For other people of the same surname, and places and things named after Charles Darwin, see Darwin. ... Binomial name Lophornis ornatus (Boddaert, 1783) The Tufted Coquette ( Lophornis ornatus ) is a tiny hummingbird that breeds in eastern Venezuela, Trinidad, Guiana and northern Brazil. ... The word theory has a number of distinct meanings in different fields of knowledge, depending on their methodologies and the context of discussion. ... For other people of the same surname, and places and things named after Charles Darwin, see Darwin. ... Look up trait in Wiktionary, the free dictionary. ... This article needs additional references or sources for verification. ... Different types of sperm cells: A) spermatozoon (motile), B) spermatium (non-motile), C) fertilization tube with sperm nuclei The term sperm is derived from the Greek word spermos (Latin: sperma) meaning seed and refers to the male reproductive cells. ... Sperm competition is competition between sperm of two or more males for the fertilization of an ovum (Parker 1970). ...

The exact effect of sexual selection depends on the sex ratio, which is usually slightly biased in favour of the "limiting" sex (typically females). Sex allocation is allocation of resources to male versus female reproduction. ... This article does not cite any references or sources. ...

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

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 son 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. In population genetics, genetic drift is the statistical effect that results from the influence that chance has on the success of alleles (variants of a gene). ... The sexy son hypothesis is a concept from evolutionary biology, proposed by P. J. Weatherhead and R. J. Robertson in 1979. ... A scanning electron microscope image of a single neutrophil (yellow), engulfing anthrax bacteria (orange). ... A few of the metabolic pathways in a cell. ...

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. In biology, phylogenetics (Greek: phylon = tribe, race and genetikos = relative to birth, from genesis = birth) is the study of evolutionary relatedness among various groups of organisms (e. ... Reproductive success is defined as the passing of genes onto the next generation in a way that they too can pass those genes on. ...

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. Sir Ronald Aylmer Fisher, FRS (17 February 1890 – 29 July 1962) was a British statistician, evolutionary biologist, and geneticist. ...

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. Sir Ronald Aylmer Fisher, FRS (17 February 1890 – 29 July 1962) was a British statistician, evolutionary biologist, and geneticist. ...

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.

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 between 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. For other people of the same surname, and places and things named after Charles Darwin, see Darwin. ... Within evolutionary biology, signalling theory refers to the scientific theory around how organisms signal their condition to others. ... A dominance hierarchy or social hierarchy is an organizational form by which individuals within a community control the distribution of resources within the community. ... A hierarchy (in Greek hieros = sacred, arkho = rule) is a system of ranking and organizing things. ...

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]. Sir Ronald Aylmer Fisher, FRS (17 February 1890 – 29 July 1962) was a British statistician, evolutionary biologist, and geneticist. ... Game theory is often described as a branch of applied mathematics and economics that studies situations where multiple players make decisions in an attempt to maximize their returns. ... Professor John Maynard Smith[1], F.R.S. (6 January 1920 – 19 April 2004) was a British evolutionary biologist and geneticist. ...

In addition to conventional aggression, male–male competition may take the form of sperm competition. Sperm competition is competition between sperm of two or more males for the fertilization of an ovum (Parker 1970). ...

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. In biology, Batemans principle is the theory that the sex which invests the most in producing offspring becomes a limited resource over which the other sex will compete. ... It has been suggested that Biparental zygote be merged into this article or section. ...

The stochastic process of sexual selection, the combined effects of its subsets (intra and intersexual selection), create assortative mating and linkage disequilibrium. In the mathematics of probability, a stochastic process is a random function. ... Assortative mating (also called Assortative pairing) takes place when sexually reproducing organisms tend to mate with individuals that are like themselves in some respect (positive assortative mating) or dissimilar (negative assortative mating). ... Linkage disequilibrium (LD) is the non-random association of alleles at two or more loci on a chromosome. ...

Koinophilia is a specific form of mate selection, with, potentially, major evolutionary consequences. Koinophilia is a term used in genetics, meaning that sexual creatures prefer mates with a preponderance of common or average features. ...

An example of koinophilia: the female peahen chose to mate with the male peacock who had the most beautiful plumage in her mind (intersexual selection).

Since natural selection causes beneficial (or "fit") features to replace their disadvantageous counterparts, the beneficial features become increasingly more common with each generation, while the disadvantageous features become increasingly rare. A sexual creature, therefore, wishing to mate with a fit partner, would be expected to avoid individuals sporting unusual features, while being especially attracted to those individuals with a predominance of common or average features. This is termed Koinophilia. Koinophilia has, as an important side effect, that mates displaying mutant features (the result of a genetic mutations) are also avoided. This, in itself, is also advantageous, since the vast majority of mutations are disadvantageous. Since it is impossible to judge whether a new mutation is beneficial or not, koinophilic creatures will avoid them all with equal determination, even if this means avoiding the very occasional beneficial mutation. Thus, koinophilia, while not perfect or infallible in its ability to distinguish fit from unfit mates, remains far and away the best bet strategy when choosing a mate: it will be right far more often than it will be wrong. And, even when it is wrong, a koinophilic choice always ensures that the offspring will inherit a suite of tried and tested features. Image File history File links Download high-resolution version (2560x1920, 2000 KB) En: Peacock (Pavo cristatus), displaying male. ... Image File history File links Download high-resolution version (2560x1920, 2000 KB) En: Peacock (Pavo cristatus), displaying male. ... Peacock re-directs here; for alternate uses see Peacock (disambiguation). ... Peacock re-directs here; for alternate uses see Peacock (disambiguation). ... Koinophilia is a term used in genetics, meaning that sexual creatures prefer mates with a preponderance of common or average features. ... It has been suggested that mutant be merged into this article or section. ...

Koinophilia provides very simple and obvious explanations for such evolutionary conundrums as the process of speciation,^[3] evolutionary stasis and punctuated equilibria,^[3][4] sex and the affordability of males,^[5][6] and the evolution of cooperation,^[7][8] and ritualistic behavior. Most of the answers to these evolutionary problems stem from the evolutionary conservatism (i.e. resistance to change) that is the inevitable result of koinophilia. Evolutionary change can still take place, but generally not in large groups. It is presumably only when small groups of individuals become isolated from the main group that genetic drift will induce changes, which, as the isolate grows, become the new set of norms according to which the desirability of potential mates will be judged. If the new set of norms is inherently disadvantageous the isolate will die out. If, on the other hand, the isolate survives and comes into contact with other isolates with other norms, competition between the groups will determine which set of norms survives, and therefore what the new "species" will look like,^[3] and how it will behave.^[8] Charles Darwins first sketch of an evolutionary tree from his First Notebook on Transmutation of Species (1837) Speciation is the evolutionary process by which new biological species arise. ... Co-operation or co-operative behaviours are terms used to describe behaviours by biological organisms which are beneficial to other members of the same species. ...

The koinophilia model assumes that sexual creatures have senses with which they evaluate the various individual features of potential mates; and that they are capable of judging which features are common or usual, and which are unusual. Commonness is the only universally appropriate sign of fitness, or, at the very least, of an evolutionarily tried and tested phenotype. The link between all other mate attractants and fitness is both cirumstance-dependent, and ultimately fakable (i.e. the link between the mate attractant and fitness fades, or might even reverse). Commonness cannot be faked, nor is it circumstance-dependent in the way that the other mate attractants are. (Thus, rats and peacocks can both use commoness to judge the desirability of a potential mate, but a gaudy tail cannot be used likewise.) Fitness, by definition, causes commonness, and drives less fit traits into rarity. Koinophilia is therefore evolutionarily robust and likely to be widespread amongst sexual creatures.

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. Peacock re-directs here; for alternate uses see Peacock (disambiguation). ...

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. Peacock re-directs here; for alternate uses see Peacock (disambiguation). ...

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

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^[10] & P.O'Donald. Sir Ronald Aylmer Fisher, FRS (17 February 1890 – 29 July 1962) was a British statistician, evolutionary biologist, and geneticist. ... Sir Ronald Aylmer Fisher, FRS (17 February 1890 – 29 July 1962) was a British statistician, evolutionary biologist, and geneticist. ... Sir Ronald Aylmer Fisher, FRS (17 February 1890 – 29 July 1962) was a British statistician, evolutionary biologist, and geneticist. ...

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 (such as in courtship) without being directly involved in reproduction, are called secondary sex characteristics. Female (left) and male Common Pheasant, illustrating the dramatic difference in both color and size, between the sexes Sexual dimorphism is the systematic difference in form between individuals of different sex in the same species. ... Image File history File links Descent_of_Man_-_Figure_16. ... Genera See text. ... The Descent of Man and Selection in Relation to Sex by British naturalist Charles Darwin was first published in 1871. ... A sex organ, or primary sexual characteristic, narrowly defined, is any of those parts of the body (which are not always bodily organs according to the strict definition) which are involved in sexual reproduction and constitute the reproductive system in an complex organism; namely: Male: penis (notably the glans penis... For other uses, see Reproduction (disambiguation) Reproduction is the biological process by which new individual organisms are produced. ... A peacock displays his long, colored feathers, an example of his 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 (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 forced copulations, which can occur in certain species of primates, ducks, and others). The effects of sexual selection are thus held to typically be more pronounced in males than in females. The word male has the following meanings: In biology, it refers to one half of a heterogamous reproduction system, where the female is the other half. ... Female is a sex that denotes an animal which produces egg cells in order to reproduce. ... In game theory, the Nash equilibrium (named after John Forbes Nash, who proposed it) is a kind of solution concept of a game involving two or more players, where no player has anything to gain by changing only his or her own strategy unilaterally. ... A pregnant woman near the end of her term Pregnancy is the carrying of one or more offspring in an embryonal or fetal stage of development by female mammals, including humans, inside their bodies, between the stages of conception and birth. ... For other meanings of bird, see bird (disambiguation). ... Orders  Crocodilia - Crocodilians scary crocodiles. ... Genera See text. ... In monogamy (Greek: monos = single/only and gamos = marriage) a person has only one spouse at a time (as opposed to polygamy). ... Sexual conflict is a form of evolutionary conflict where males and females do not share interests over reproduction. ... Families 15, See classification A primate is any member of the biological order Primates, the group that contains all the species commonly related to the lemurs, monkeys, and apes, with the latter category including humans. ... Subfamilies Dendrocygninae Oxyurinae Anatinae Aythyinae Merginae Duck is the common name for a number of species in the Anatidae family of birds. ...

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. Female (left) and male Common Pheasant, illustrating the dramatic difference in both color and size, between the sexes Sexual dimorphism is the systematic difference in form between individuals of different sex in the same species. ... Peacock re-directs here; for alternate uses see Peacock (disambiguation). ... Peacock re-directs here; for alternate uses see Peacock (disambiguation). ... Typical classes Petromyzontidae (lampreys) Placodermi - extinct Chondrichthyes (cartilaginous fish) Acanthodii - extinct Actinopterygii (ray-finned fish) Actinistia (coelacanths) Dipnoi (lungfish) Amphibia (amphibians) Reptilia (reptiles) Aves (birds) Mammalia (mammals) Vertebrata is a subphylum of chordates, specifically, those with backbones or spinal columns. ... Genera Altolamprologus - A few varieties Lamprologus Lepidiolamprologus - May be invalid Neolamprologus Telmatochromis Pseudotropheus - P. lanisticola Shell dwellers are a group of cichlids from Lake Tanganyika with one definite example from Lake Malawi. ... Genera Cichlids (pronounced “sick-lids”) are fishes from the family Cichlidae in the order Perciformes. ... For other uses, see Fish (disambiguation). ... Families Suborder Mesothelae     Liphistiidae (primitive burrowing spiders) Suborder Mygalomorphae     Atypidae (atypical tarantula)     Antrodiaetidae (folding trapdoor spider)     Mecicobothriidae (dwarf tarantulas)     Hexathelidae (venomous funnel-web tarantula)     Dipluridae (funnel-web tarantula)     Cyrtaucheniidae (wafer trapdoor spider)     Ctenizidae (trapdoor spider)     Theraphosidae (tarantula) Suborder Araneomorphae     Hypochilidae (lampshade spider)     Filistatidae (crevice weaver)     Sicariidae (recluse spider)     Scytodidae (spitting...

Viability and variations 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. Fitness (often denoted in population genetics models) is a central concept in evolutionary theory. ... Herbert Spencer coined the phrase, survival of the fittest. ...

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.) Sir Ronald Fisher Sir Ronald Aylmer Fisher, FRS (February 17, 1890–July 29, 1962) was an extraordinarily talented evolutionary biologist, geneticist and statistician. ... Year 1930 (MCMXXX) was a common year starting on Wednesday (link will display 1930 calendar) of the Gregorian calendar. ... Positive feedback is a feedback system in which the system responds to the perturbation in the same direction as the perturbation (It is sometimes referred to as cumulative causation). ... The gene pool of a species or a population is the complete set of unique alleles that would be found by inspecting the genetic material of every living member of that species or population. ... In game theory, an evolutionarily stable strategy (or ESS; also evolutionary stable strategy) is a strategy which if adopted by a population cannot be invaded by any competing alternative 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 prevent the prey from suffering serious damage.[2] A juvenile Red-tailed Hawk eating a California Vole In ecology, predation describes a biological interaction where a predator organism feeds on another living organism or organisms known as prey. ... Jousting is a staple entertainment at Renaissance Fairs. ... Competition is the act of striving against others for the purpose of achieving gain, such as income, pride, amusement, or dominance. ...

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. The handicap principle is an idea proposed by the Israeli biologist Amotz Zahavi. ... Amotz Zahavi is an Israeli Evolutionary Biologist from Tel-Aviv University, and one of the founders of the Israeli Society for the Protection of Nature. ... W. D. Hamilton William Donald Bill Hamilton, F.R.S. (1 August 1936 — 7 March 2000) was a British evolutionary biologist, considered one of the greatest evolutionary theorists of the 20th century. ... The term disease refers to an abnormal condition of an organism that impairs function. ...

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. Facial symmetry is one of a number of traits associated with health, physical attractiveness and beauty of a person or animal. ...

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. A cuckold is a married man whose wife has sex with other men. ... Sperm competition is competition between sperm of two or more males for the fertilization of an ovum (Parker 1970). ... In sociology and biology, infanticide is the practice of intentionally causing the death of an infant of a given species, by members of the same species - often by the mother. ...

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. Paradigm shift is the term first used by Thomas Kuhn in his 1962 book The Structure of Scientific Revolutions to describe a change in basic assumptions within the ruling theory of science. ... This article should belong in one or more categories. ... Hermann Müller (1829-1883), German botanist who provided important evidence for Darwins theory of evolution. ... Darwins illustrations of beak variation in the finches of the Galápagos Islands, which hold 13 closely related species that differ most markedly in the shape of their beaks. ...

Proposed human examples

Darwin befouls a lady with lecherous monkey tricks.

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. Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ...

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. For other uses, see Race (disambiguation). ...

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. Geoffrey Miller Geoffrey Miller is a widely recognised evolutionary psychologist, whose work is in the tradition of scientists such as Richard Dawkins, Daniel Dennett, and Steven Pinker. ...

A theory about the evolution of the human brain argues that the human brain is actually more cumbersome than the advantages it confers and it actually evolved through sexual selection in an exaggerated way similar to that of the peacock's plumage.^[11]

The zoologist Richard Dawkins pointed out in 1989 that the loss of the penis bone in humans, when it is present in our nearest related species the chimpanzee demands some form of evolutionary explanation. He speculates that its loss is probably a form of sexual selection by females looking for signs of good health in prospective mates. The reliance of the human penis solely on hydraulic means to achieve a rigid state makes it particularly vulnerable to blood pressure variation. Poor erectile function betrays, not only physical states such as diabetes and neurological disorders, but mental states such as stress and depression. ^[12] Clinton Richard Dawkins (born March 26, 1941) is a British ethologist, evolutionary biologist and popular science writer who holds the Charles Simonyi Chair for the Public Understanding of Science at Oxford University. ... The baculum (also penis bone, penile bone or os penis) is a bone found in the penis of most mammals. ... Type species Simia troglodytes Blumenbach, 1775 distribution of Species Pan troglodytes Pan paniscus Chimpanzee, often shortened to chimp, is the common name for the two extant species in the genus Pan. ... Hydraulics is a branch of science and engineering concerned with the use of liquids to perform mechanical tasks. ...

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, 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). Charles Darwins Origin of Species (publ. ... The Descent of Man and Selection in Relation to Sex by British naturalist Charles Darwin was first published in 1871. ... Peacock re-directs here; for alternate uses see Peacock (disambiguation). ... Binomial name Shaw, 1804 Synonyms Desmarest Cuvier[1] Bighorn Sheep (Ovis canadensis)[2] is one of three species of mountain sheep in North America and Siberia; the other two species being Ovis dalli, that includes Dall Sheep and Stones Sheep, and the Siberian Snow sheep Ovis nivicola. ...

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. For the Cornish painter, see Alfred Wallis. ... Suborders Adephaga Archostemata Myxophaga Polyphaga See subgroups of the order Coleoptera Beetles are the most diverse group of insects. ...

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. Peacock re-directs here; for alternate uses see Peacock (disambiguation). ... Countershaded Ibex are almost invisible in the Israeli desert. ... Maynard Smith was a proponent Adaptationism is the view that all or most traits are adaptations. ...

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. This article includes a list of works cited or a list of external links, but its sources remain unclear because it lacks in-text citations. ... Human behavioral ecology (HBE) or human evolutionary ecology applies the principles of evolutionary theory and optimization to the study of human behavioral and cultural diversity. ... This article or section includes a list of works cited or a list of external links, but its sources remain unclear because it lacks in-text citations. ... Epigenetics is a term in biology used today to refer to features such as chromatin and DNA modifications that are stable over rounds of cell division but do not involve changes in the underlying DNA sequence of the organism. ...

Criticism

A group of authors led by Joan Roughgarden have criticised sexual selection theory,^[13] 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 inaccuracy.^[14] Joan E. Roughgarden (b. ...

See also

In psychology and other social and behavioral sciences, aggression refers to behavior that is intended to cause harm or pain. ... Assortative mating (also called Assortative pairing) takes place when sexually reproducing organisms tend to mate with individuals that are like themselves in some respect (positive assortative mating) or dissimilar (negative assortative mating). ... This article does not cite any references or sources. ... This article does not cite any references or sources. ... An evolutionary arms race is an evolutionary struggle between a predator species and its prey (including parasitism) that is said to resemble an arms race. ... Binomial name Megaloceros giganteus (Blumenbach, 1799) Megaloceros redirects here. ... // When applied to comparative data, conventional statistical methods assume, in effect, that all species are completely unrelated, as if they descended from a big bang of special creation. ... Features such as a symmetrical face, full lips, and low waist-hip ratio, are commonly considered physically attractive because they are thought to indicate physical health and high fertility to a potential mate. ... Sex allocation is allocation of resources to male versus female reproduction. ... Sex ratio by country for total population. ... In species which reproduce sexually, sexual attraction is attraction to other members of the same species for reproduction. ... Sexual conflict is a form of evolutionary conflict where males and females share different interests. ... Sperm competition is competition between sperm of two or more males for the fertilization of an ovum (Parker 1970). ... This article or section does not adequately cite its references or sources. ...

Notes

1. ^ Eberhard, WG. Sexual selection by cryptic female choice. Princeton, Princeton University Press. 1996.
2. ^ Maynard Smith, J (1982) Evolution and the Theory of Games. Cambridge University Press, pp. 131-137. ISBN 0-521-28884-3
3. ^ ^{a b c} KOESLAG, J.H. (1995). On the engine of speciation. J. theor. Biol. 177, 401-409
4. ^ KOESLAG, J.H. (1990). Koinophilia groups sexual creatures into species, promotes stasis, and stabilizes social behaviour. J. theor. Biol. 144, 15-35
5. ^ KOESLAG, P.D., KOESLAG, J.H. (1994). Koinophilia stabilizes bi-gender sexual reproduction against asex in an unchanging environment. J. theor. Biol. 166, 251-260
6. ^ KOESLAG, J.H., KOESLAG, P.D. (1993). Evolutionarily stable meiotic sex. J. Heredity 84, 396-399
7. ^ KOESLAG, J.H. (1997). Sex, the prisoner's dilemma game, and the evolutionary inevitability of cooperation. J. theor. Biol. 189, 53--61
8. ^ ^{a b} KOESLAG, J.H. (2003). Evolution of cooperation: cooperation defeats defection in the cornfield model. J. theor. Biol. 224, 399-410
9. ^ 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
10. ^ Lande, R. (1981) Models of speciation by sexual selection on polygenic traits. PNAS 78:3721-3725.
11. ^ [1]
12. ^ Dawkins, Richard [1978] (2006). The Selfish Gene, 30th anniversary edition, p158 endnote. ““It is not implausible that , with natural selection refining their diagnostic skills, females could glean all sorts of clues about a male’s health, and robustness of his ability to cope with stress, from the tone and bearing of his penis.”” 
13. ^ Roughgarden, J., Oisho, M. and Akçay, E. (2006). "Reproductive social behaviour: cooperative games to replace sexual selection". Science 311: 965-969.  news  abstract.
14. ^ Dall, S. R. X.; McNamara, J.M., Wedell, N. and Hosken, D.J. (2006). "Debating sexual selection and mating strategies". Science 312: 689-697. 

Professor John Maynard Smith[1], F.R.S. (6 January 1920 – 19 April 2004) was a British evolutionary biologist and geneticist. ... Book cover Evolution and the Theory of Games is a 1982 book by the British evolutionary biologist John Maynard Smith on evolutionary game theory. ... Sir Ronald Aylmer Fisher, FRS (17 February 1890 – 29 July 1962) was a British statistician, evolutionary biologist, and geneticist. ... Sir Charles Galton Darwin. ... Clinton Richard Dawkins (born March 26, 1941) is a British ethologist, evolutionary biologist and popular science writer who holds the Charles Simonyi Chair for the Public Understanding of Science at Oxford University. ...

References

• Abraham, J.N. (1998) La Saboteuse: An Ecological Theory of Sexual Dimorphism in Animals. Acta Biotheoretica 46 23-35 [3]
• Abraham, J.N. (2005) Insect Choice and Floral Size Dimorphism: Sexual Selection or Natural Selection? J. Insect Behavior, 18 743-756 [4]
• Andersson, M (1994) Sexual selection Princeton University Press. ISBN 0-691-00057-3
• Darwin, C (1871) 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 [5]
• Lande, CF R (1981). Models of speciation by sexual selection on polygenic traits. Proc. Nat. Acad. Sci., USA, 78 3721-5 [6]
• 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.[7]
• Miller, GF (2000) The Mating Mind: How sexual choice shaped the evolution of human nature. London: Heinemann. ISBN 0-434-00741-2

For other people of the same surname, and places and things named after Charles Darwin, see Darwin. ... The Descent of Man and Selection in Relation to Sex by British naturalist Charles Darwin was first published in 1871. ... Sir Ronald Aylmer Fisher, FRS (17 February 1890 – 29 July 1962) was a British statistician, evolutionary biologist, and geneticist. ... The Genetical Theory of Natural Selection is a book by Ronald Fisher. ...

Further reading

Judson, Olivia (2003) Dr.Tatiana's Sex Advice to All Creation: Definitive Guide to the Evolutionary Biology of Sex. ISBN: 978-0099283751

Jolly, Allison (2001) Lucy's Legacy - Sex and Intelligence in Human Evolution. ISBN: 978-0674005402

Diamond, Jared (1997) Why is Sex Fun? The Evolution of Human Sexuality. ISBN: 978-0465031269

External links

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Look up sexual selection in
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• 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
• Review of GF Miller's The Mating Mind