Sexual selection

The concept was first articulated by Charles Darwin who wrote of a "second agency" other than natural selection, in which competition between mate candidates could lead to speciation.

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.

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.

—Ronald Fisher, 1930[9]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.

[10] Richard Dawkins presents a non-mathematical explanation of the runaway sexual selection process in his book The Blind Watchmaker.

[10] 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.

[13][14] Alongside this, biologists have extended Darwin's formulation; Malte Andersson's widely-accepted[15] 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".

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.

[21] 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.

Sexually dimorphic traits, size, sex ratio,[27] and the social situation[28] 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.

A "bewildering"[34] range of models variously attempt to relate sexual selection not only to the fundamental[34] questions of anisogamy and parental roles, but also to mechanisms such as sex ratios – governed by Fisher's principle,[35] parental care, investing in sexy sons, sexual conflict, and the "most-debated effect",[34] namely mate choice.

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.

[44] Humans are sexually dimorphic; females select males using factors including voice pitch, facial shape, muscularity, and height.

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 eggs are large and reduce the ability of the male to fertilise other females and catch prey, and increases its predation risk.

[53] 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.

Males arrive at the water's edge first in large numbers, and produce a wide range of vocalizations to attract mates.

[63] The prepollex, which serves as a rudimentary digit, contains a projecting spine that may be used during this combat, leaving scars on the head and forelimbs of other males.

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; the differences such as in size and coloration are energetically costly attributes that signal competitive breeding.

Conflicts between an individual's fitness and signalling adaptations ensure that sexually selected ornaments such as coloration of plumage and courtship behaviour are honest traits.

Many bird species make use of mating calls, the females preferring males with songs that are complex and varied in amplitude, structure, and frequency.

painting of male and female birds of paradise
Sexual selection creates colourful differences between sexes in Goldie's bird-of-paradise . Male above; female below. Painting by John Gerrard Keulemans .
Victorian era cartoon of Darwin as a monkey looking at a woman in a bustle dress
Victorian cartoonists mocked Darwin's ideas about display in sexual selection. Here he is fascinated by the apparent steatopygia in the latest fashion.
Photograph of flour beetles
Sexual selection protected flour beetles from extinction in a ten-year experiment. [ 6 ]
Photograph of a bird with an exceptionally long tail
Male long-tailed widowbird
Photograph of a flying peacock
The peacock tail in flight, the proposed classic example of a Fisherian runaway
Photograph of a museum specimen of an Irish elk skull with large antlers
The enormous sexually-selected antlers of the Irish elk might have helped it on its way to extinction. [ 16 ]
Photograph of a large male gorilla
Male mountain gorilla , a species with very large males [ 26 ]
Artist's reconstruction of a proto-bird fossil as if it used its small wings in courtship display
Protarchaeopteryx was flightless, but had feathers, perhaps used in courtship, that pre-adapted it for flight.