Sexual selection is a mode of natural selection where members of one biological sex choose mates of the other sex to mate with intersexual selectionand compete with members of the same sex for access to members of the opposite sex intrasexual selection.
These two forms of selection mean that some individuals have better reproductive success than others within a populationeither from being more attractive or preferring more attractive partners to produce offspring. The females then arrive and choose the males with the deepest croaks and best territories. Generalizing, males benefit from frequent mating and monopolizing access to a group of fertile females.
Females have a limited number of offspring they can have and they maximize the return on the energy they invest in reproduction.
The concept was first articulated by Charles Darwin and Alfred Russel Wallace who described it as driving species adaptations and that many organisms had evolved features whose function was deleterious to their individual survival,  and then developed by Ronald Fisher in the early 20th century. Sexual selection can, typically, lead males to extreme efforts to demonstrate their fitness to be chosen by females, producing sexual dimorphism in secondary sexual characteristicssuch the ornate plumage of birds such as birds of paradise and peafowlor the antlers of deeror the manes of lionscaused by a positive feedback mechanism known as a Fisherian runawaywhere the passing-on of the desire for a trait in one sex is as important as having the trait in the other sex in producing the runaway effect.
Although the sexy son hypothesis indicates that females would prefer male offspring, Fisher's principle explains why the sex ratio is 1: Sexual selection is also found in plants and fungi. Many non-exclusive hypotheses have been proposed,  including the positive impact of an additional form of selection, sexual selection, on the probability of persistence of a species.
Sexual selection was first proposed by Charles Darwin in The Origin of Species and developed in The Descent of Man and Selection in Relation to Sexas 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 Sexual selection evolutionary force tail, "Sexual selection evolutionary force" I gaze at it, makes me sick!
These views were to some extent opposed by Alfred Russel Wallacemostly 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 "Sexual selection evolutionary force" 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 Sexual selection evolutionary force as beetles far too cognitively undeveloped to be capable of aesthetic feeling.
Ronald Fisherthe English statistician and evolutionary biologist developed a number of ideas about sexual selection in his book The Genetical Theory of Natural Selection including the sexy son hypothesis and Fisher's principle. 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.
In a remark that was not widely understood  for another 50 years he said:. 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 exponentiallyor in geometric progression. This causes a "Sexual selection evolutionary force" increase in both the male's conspicuous feature and in female preference for it, resulting in marked sexual dimorphismuntil practical physical constraints halt further exaggeration.
A positive feedback loop is created, producing extravagant physical structures in the non-limiting sex. A classic example of female choice and potential runaway selection is the long-tailed widowbird. While males have long tails that are selected for by female choice, female tastes in tail length are still more extreme with females being attracted to tails longer than those that naturally occur. 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.
Richard Dawkins presents a non-mathematical explanation of the runaway sexual selection process in his book The Blind Watchmaker. 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 Sexual selection evolutionary force 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.
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. The female widow bird 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 "Sexual selection evolutionary force" created that, if unchecked, can yield exponential increases in a given taste and in the corresponding desired sexual attribute.
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.
Since Fisher's initial conceptual model of the 'runaway' process, Russell Lande  and Peter O'Donald  have provided detailed mathematical proofs that define the circumstances under which runaway sexual selection can take place. The reproductive success of an organism is measured by Sexual selection evolutionary force number of offspring left behind, and their quality or probable fitness.
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.
The conditions determining which sex becomes the more limited resource in Sexual selection evolutionary force have been hypothesized with the Bateman's principlewhich states that the sex which invests the most in producing offspring becomes a limiting resource over which the other sex competes, illustrated by the greater nutritional investment of an egg in a zygoteand the limited capacity of females to reproduce; for example, in humans, a woman can only Sexual selection evolutionary force birth every ten months, whereas a male can become a father numerous times.
The sciences of evolutionary psychologyhuman behavioural ecologyand sociobiology study the influence of sexual selection in humans. Darwin's ideas on sexual selection were met with scepticism by his contemporaries and not considered of great importance in the early 20th century, until in the s biologists decided to include sexual selection as a mode of natural selection.
Research in indicates that sexual selection, including mate choice"improves population health and protects against extinction, even in the face of genetic stress from high levels of inbreeding" and "ultimately dictates who gets to reproduce their genes into the next generation - so it's a widespread and very powerful evolutionary force.
Hamiltonholds that the fact that the male is able to until and through the age of reproduction with such a seemingly maladaptive trait is taken by the female to be a testament to his overall fitness.
Such handicaps might prove he is either free of or resistant to diseaseor 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 Sexual selection evolutionary force, which has produced an ever-accelerating number of theories. InHamilton 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.
InMichael Ryan and A. Rand, working with the tungara frogproposed the hypothesis of "Sensory Exploitation", where exaggerated male traits may provide a sensory stimulation that females find hard to resist. Rice have been added. In the late 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 other areas of study, not all of which fit Darwin's definition of sexual selection. These include cuckoldrynuptial gifts, sperm competitioninfanticide especially in primatesphysical beautymating by subterfuge, species isolation mechanisms, male parental care, ambiparental "Sexual selection evolutionary force," mate location, polygamy, and homosexual rape in certain male animals.
Sexual conflict leads to an antagonistic co-evolution in which one sex tends to control the other, resulting Sexual selection evolutionary force a tug of war. Besides, the sexual propaganda theory only argued that mates were opportunistically lead, on the basis of various factors determining the choice such as phenotypic characteristics, apparent vigour of individuals, strength of mate signals, trophic resources, territoriality etc.
Several workers have brought attention to the fact that elaborated characters that ought to be costly in one way or another for their bearers e. One possible explanation for the apparent lack of costs is that "compensatory traits" have evolved in concert with the sexually selected traits.
Sexual selection may explain how certain characteristics such as feathers had distinct survival value at an early stage in their evolution. Geoffrey Miller proposes that sexual selection might have contributed by creating evolutionary modules such as Sexual selection evolutionary force feathers as sexual ornaments, at first.
Some have suggested that the feathers served as insulation, helping females incubate their eggs. But perhaps the feathers served as the kinds of sexual ornaments still common in most Sexual selection evolutionary force species, and especially in birds such as peacocks and birds-of-paradise today.
If proto-bird courtship displays combined displays of forelimb feathers with energetic jumps, then the transition from display to aerodynamic functions could have been relatively smooth. Sexual selection sometimes generates features that may help cause a species' extinction, as has been suggested  for the giant antlers of the Irish elk Megaloceros giganteus that became extinct in Pleistocene Europe.