Sexual selection

Darwin in the latest fashion.|alt=Victorian era cartoon of Darwin as a monkey looking at a woman in a bustle dress Sexual selection was first proposed by Charles Darwin in On the Origin of Species (1859) and developed in The Descent of Man, and Selection in Relation to Sex (1871), as he felt that natural selection alone was unable to account for certain types of non-survival adaptations. 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 male–male competition and female choice. These views were to some extent opposed by Alfred Russel Wallace, mostly after Darwin's death. He accepted that sexual selection could occur, but argued that it was a relatively weak form of selection. He argued that male–male competitions were forms of natural selection, but that the "drab" peahen's coloration is itself adaptive as camouflage. In his opinion, ascribing mate choice to females was attributing the ability to judge standards of beauty to animals (such as beetles) far too cognitively undeveloped to be capable of aesthetic feeling. s from extinction in a ten-year experiment. Only in the 21st century have they become more important in biology; the theory is now seen as generally applicable and analogous to natural selection. A ten-year study, experimentally varying sexual selection on flour beetles with other factors held constant, showed that sexual selection protected even an inbred population against extinction. Fisherian runaway Ronald Fisher, the English statistician and evolutionary biologist, developed his ideas about sexual selection in his 1930 book The Genetical Theory of Natural Selection. These include the sexy son hypothesis, which might suggest a preference for male offspring, and Fisher's principle, which explains why the sex ratio is usually close to 1:1. The Fisherian runaway describes how sexual selection accelerates the preference for a specific ornament, causing the preferred trait and female preference for it to increase together in a positive feedback runaway cycle. He remarked that: Fisher understood that female preference for long tails may be passed on genetically, in conjunction with genes for the long tail itself. Long-tailed widowbird offspring of both sexes inherit both sets of genes, with females expressing their genetic preference for long tails, and males showing off the coveted long tail itself. Females that prefer long tailed males tend to have mothers that chose long-tailed fathers. As a result, they carry both sets of genes in their bodies. That is, genes for long tails and for preferring long tails become linked. The taste for long tails and tail length itself may therefore become correlated, tending to increase together. The more tails lengthen, the more long tails are desired. Any slight initial imbalance between taste and tails may set off an explosion in tail lengths. Fisher wrote that: |alt=Photograph of a flying peacock The female widowbird chooses to mate with the most attractive long-tailed male so that her progeny, if male, will themselves be attractive to females of the next generation—thereby fathering many offspring that carry the female's genes. Since the rate of change in preference is proportional to the average taste amongst females, and as females desire to secure the services of the most sexually attractive males, an additive effect is created that, if unchecked, can yield exponential increases in a given taste and in the corresponding desired sexual attribute. Alongside this, biologists have extended Darwin's formulation; Malte Andersson's widely accepted 1994 definition is that "sexual selection is the differences in reproduction that arise from variation among individuals in traits that affect success in competition over mates and fertilizations". Despite some practical challenges for biologists, the concept of sexual selection is "straightforward". == Modern theory ==

Reproductive success might have helped it on its way to extinction. Sexual preference creates a tendency towards assortative mating or homogamy. 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 is followed by other offers, either certainly or at such high chance that the risk of non-occurrence is smaller than the chance advantage to be gained by selecting a mate. Bateman's principle states that the sex which invests the most in producing offspring becomes a limiting resource for which the other sex competes, illustrated by the greater nutritional investment of an egg in a zygote, and the limited capacity of females to reproduce; for example, in humans, a woman can only give birth every ten months, whereas a male can become a father numerous times in the same period. More recently, researchers have doubted whether Bateman was correct. Honest signalling The handicap principle of Amotz Zahavi, Russell Lande and W. D. Hamilton, holds that the male's survival until and through the age of reproduction with seemingly maladaptive traits is taken by the female as a signal of his overall fitness. Such handicaps might prove he is either free of or resistant to disease, or that he 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 and several new 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. Male intrasexual competition , a species with very large males|alt=Photograph of a large male gorilla Male–male competition occurs when two males of the same species compete for the opportunity to mate with a female. Sexually dimorphic traits, size, sex ratio, and the social situation may all play a role in the effects male–male competition has on the reproductive success of a male and the mate choice of a female. Larger males tend to win male–male conflicts. Males take many risks in such conflicts, so the value of the resource must be large enough to justify those risks. Winner and loser effects further influence male behaviour. Male–male competition may also affect a female's ability to select the best mates, and therefore decrease the likelihood of successful reproduction. Multiple models More recently, the field has grown to include other areas of study, not all of which fit Darwin's definition of sexual selection. A "bewildering" parental care, investing in sexy sons, sexual conflict, and the "most-debated effect", Elaborated characteristics that might seem costly, like the tail of the Montezuma swordfish (Xiphophorus montezumae), do not always have an energetics, performance or even survival cost; this may be because "compensatory traits" have evolved in concert with the sexually selected traits. Toolkit of natural selection '' was flightless, but had feathers, perhaps used in courtship, that pre-adapted it for flight.|alt=Artist's reconstruction of a proto-bird fossil as if it used its small wings in courtship display Sexual selection may explain how characteristics such as feathers had survival value at an early stage in their evolution. The earliest proto-birds such as Protarchaeopteryx had well-developed feathers but could not fly. The feathers may have served as insulation, helping females incubate their eggs, but if proto-bird courtship combined displays of forelimb feathers with energetic jumps, then the transition to flight could have been relatively smooth. Sexual selection may sometimes generate features that help cause a species' extinction, as has historically been suggested for the giant antlers of the Irish elk (Megaloceros giganteus) that became extinct in Holocene Eurasia (although climate-induced habitat deterioration and anthropogenic pressure are now considered more likely causes). It may, however, also do the opposite, driving species divergence—sometimes through elaborate changes in genitalia—such that new species emerge. Sexual selection often interacts with natural selection to drive speciation. Sex role reversal Darwin addressed sex role reversal in The Descent of Man, with females undergoing selection by male mating partners. Darwin and then others described reversed sex roles in the barred buttonquail (Turnix suscitator). Females of these species are generally larger, more colorful, and more aggressive than males. == In different taxa ==

Sexual selection is widely distributed among the eukaryotes, occurring in plants, fungi, and animals. Since Darwin's pioneering observations on humans, it has been studied intensively among the insects, spiders, amphibians, scaled reptiles, birds, and mammals, revealing many distinctive behaviours and physical adaptations. Humans are sexually dimorphic; females select males using factors including voice pitch, facial shape, muscularity, and height. Among the many instances of sexual selection in mammals is extreme sexual dimorphism, with males as much as six times heavier than females, and male fighting for dominance among elephant seals. Dominant males establish large harems of several dozen females; unsuccessful males may attempt to copulate with a harem male's females if the dominant male is inattentive. This forces the harem male to defend his territory continuously, not feeding for as much as three months. Also seen in mammals is sex-role reversal, as in the highly social meerkats, where a large female is dominant within a pack, and female–female competition is observed. The dominant female produces most of the offspring; the subordinate females are nonbreeding, providing altruistic care to the young. In arthropods Sexual selection occurs in a wide range of spider species, both before and after copulation. Post-copulatory sexual selection involves sperm competition and cryptic female choice. Sperm competition occurs where the sperm of more than one male competes to fertilise the egg of the female. Cryptic female choice involves the expelling of a male's sperm during or after copulations. Many forms of sexual selection exist among the insects. Parental care is often provided by female insects, as in bees, but male parental care is found in belostomatid water bugs, where the male, after fertilizing the eggs, allows the female to glue her eggs onto his back. He broods them until the nymphs hatch 2–4 weeks later. The eggs are large and reduce the ability of the male to fertilise other females and catch prey, and increases its predation risk. Among the fireflies (Lampyrid beetles), males fly in darkness and emit a species-specific pattern of light flashes, which are answered by perching receptive females. The colour and temporal variation of the flashes contribute to success in attracting females. Among the beetles, sexual selection is common. In the mealworm beetle, Tenebrio molitor, males release pheromones to attract females to mate. Females choose mates based on whether they are infected, and on their mass. In molluscs Postcopulatory intersexual selection occurs in Idiosepius paradoxus, the Japanese pygmy squid. Males place their spermatangia on an external location on the female's body. The female physically removes spermatangia of males she is presumed to favour less. In amphibians and reptiles Many amphibians have annual breeding seasons with male–male competition. Males arrive at the water's edge first in large numbers, and produce a wide range of vocalizations to attract mates. Among frogs, the fittest males have the deepest croaks and the best territories; females select their mates at least partly based on the depth of croaking. This has led to sexual dimorphism, with females larger than males in 90% of species, and male fighting to access females. Spikethumb frogs are suggested to engage in male-male competition with their elongated prepollex to maintain their mating site. The prepollex, which serves as a rudimentary digit, contains a projecting spine that may be used during this combat, leaving scars on the heads and forelimbs of other males. Many different tactics are used by snakes to acquire mates. Ritual combat between males for the females they want to mate with includes topping, a behaviour exhibited by most viperids, in which one male twists around the vertically elevated fore body of its opponent and forcing it downward. Neck biting is common while the snakes are entwined. In birds Birds have evolved a wide variety of mating behaviours and many types of sexual selection. These include intersexual selection (female choice) and intrasexual competition, where individuals of the more abundant sex compete with each other for the privilege to mate. Many species, notably the birds-of-paradise, are sexually dimorphic. Males with the brightest plumage are favoured by females of multiple species of bird. Many bird species make use of mating calls, the females preferring males with songs that are complex and varied in amplitude, structure, and frequency. Larger males have deeper songs and increased mating success. In plants and fungi Flowering plants have many secondary sexual characteristics subject to sexual selection including floral symmetry if pollinators visit flowers assortatively by degree of symmetry, nectar production, floral structure, and inflorescences, as well as sexual dimorphisms. Fungi appear to make use of sexual selection, although they also often reproduce asexually. In the Basidiomycetes, the sex ratio is biased towards males, implying sexual selection there. Male–male competition to fertilise occurs in fungi including yeasts. Pheromone signaling is used by female gametes and by conidia, implying male choice in these cases. Female–female competition may also occur, indicated by the much faster evolution of female-biased genes in fungi. File:Gorillafamily.JPG|Among mammals, the male gorilla is much larger than the female. File:Phidippus putnami male.jpg|Males of many spiders, such as this Phidippus putnami, have elaborate courtship displays. File:Toe-Biter.jpg|A male Abedus indentatus belostomatid bug carries eggs on its back. File:Fireflies, Georgia, US (detail).jpg|Each firefly species attracts mates with its own flash pattern. File:Dendropsophus microcephalus - calling male (Cope, 1886).jpg|Male Dendropsophus microcephalus calling File:Indian rat snake,Ptyas mucosa, Territorial Fight.jpg|Territorial fight in the Indian rat snake, Ptyas mucosa File:Victoria's Riflebird courtship - Lake Eacham - Queensland S4E8070 (22198704599) (cropped).jpg|Male Victoria's riflebird displaying to a female File:Satin Bowerbird nest.jpg|A male satin bowerbird guards its bower from rival males in the hope of attracting females with its decorations. File:MacquarieIslandElephantSeal.JPG|Male southern elephant seals fighting on Macquarie Island for the chance to mate File:Lily Lilium 'Citronella' Flower.jpg|Citronella flower's symmetry may have been subject to sexual selection by its pollinators. File:RanaArvalisBlueMale3.jpg|Male moor frogs become blue to signal their fitness to females. == References ==