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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 lead males to extreme efforts to demonstrate their fitness to be chosen by females, producing sexual dimorphism in secondary sexual characteristics , such as the ornate plumage of birds such as birds of paradise and peafowl , or the antlers of deer , or the manes of lions , caused by a positive feedback mechanism known as a Fisherian runaway , where 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 almost without exception. 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 Sex , 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! 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. Ronald Fisher , the 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 exponentially , or in geometric progression.
This causes a dramatic increase in both the male's conspicuous feature and in female preference for it, resulting in marked sexual dimorphism , until 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 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:. 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 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. 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 the 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 intersexual selection have been hypothesized with Bateman's principle , which 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.
Hubbell and Johnson suggested that variance in reproductive success can be influenced by the time and allocations of mating. In , Gowaty and Hubbell suggested that mating tendencies depend on the choice of strategy; in some cases, males can be more selective than females, whereas Bateman suggested that his paradigm would be "almost universal" among sexually reproducing species. Critics proposed that females might be more subject to sexual selection than males, but not in all circumstances.
Darwin's ideas on sexual selection were met with scepticism by his contemporaries and not considered of great importance until in the s biologists decided to include sexual selection as a mode of 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.
Hamilton , holds that the fact that the male is able to survive 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 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.
In , 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. 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 cuckoldry , nuptial gifts, sperm competition , infanticide especially in primates , physical beauty , mating by subterfuge, species isolation mechanisms, male parental care, ambiparental care, 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 in a tug of war. Besides, the sexual propaganda theory only argued that mates were opportunistically led, 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 Archaeopteryx 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 bird 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.
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. As a result of these factors, males can be expected to be more willing to mate than females, while females are expected to be the ones doing the choosing except in cases of forced copulations , which can occur for example in certain primates and ducks.
The effects of sexual selection are thus often 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 colour patterns.
Sexual dimorphisms abound in nature. Examples include the possession of antlers by only male deer , the brighter coloration of many male birds in comparison with females of the same species, or even more distinct differences in basic morphology, such as the drastically increased eye-span of the male stalk-eyed fly. The peacock , with its elaborate and colourful tail feathers, which the peahen lacks, is often referred to as perhaps the most extraordinary example of a dimorphism.
Male and female black-throated blue warblers and Guianan cock-of-the-rocks also differ radically in their plumage. Early naturalists even believed the females to be a separate species.
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 and birds of prey. 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 due to their sheer strength and ability to ward off other males from taking over their females. For instance, in the fly Dryomyza anilis , size shows the strongest correlation to the outcome of male-male conflicts over resources like territory and females.
There are multiple types of male-male competition that may occur in a population at different times depending on the conditions. Competition variation occurs based on the frequency of various mating behaviours present in the population. However, all techniques are not equally successful when in competition for reproductive success. Disruption results in a shorter copulation period and can therefore disrupt the fertilization of the eggs by the sperm, which frequently results in lower rates of fertilization and smaller clutch size.
Another factor that can influence male-male competition is the value of the resource to competitors. Male-male competition can pose many risks to a male's fitness, such as high energy expenditure, physical injury, lower sperm quality and lost paternity. A male is more likely to engage in competition for a resource that improves their reproductive success if the resource value is higher.
While male-male competition can occur in the presence or absence of a female, competition occurs more frequently in the presence of a female. A third factor that can impact the success of a male in competition is winner-loser effects. Male-male competition can both positively and negatively affect female fitness. When there is a high density of males in a population and a large number of males attempting to mate with the female, she is more likely to resist mating attempts, resulting in lower fertilization rates.
However, group mating in Japanese medaka has been shown to positively affect the fitness of females due to an increase in genetic variation, a higher likelihood of paternal care and a higher likelihood of successful fertilization. Sexual selection has been observed to occur in plants, animals and fungi. Today, biologists say that certain evolutionary traits can be explained by intraspecific competition —competition between members of the same species—distinguishing between competition before or after sexual intercourse.
Before copulation , intrasexual selection —usually between males—may take the form of male-to-male combat. Also, intersexual selection , or mate choice , occurs when females choose between male mates. Due to their sometimes greatly exaggerated nature, secondary sexual characteristics can prove to be a hindrance to an animal, thereby lowering its chances of survival.
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 colourations 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. 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.