Concept
The sexual selection concept arises from the observation that many animals develop features whose function is not to help individuals survive, but help them to maximize their reproductive success. This can be realized in two different ways:
- by making themselves attractive to the opposite sex (intersexual selection, between the sexes); or
- by intimidating, deterring or defeating same-sex rivals (intrasexual selection, within a given sex).
Thus, sexual selection takes two major forms: intersexual selection (also known as 'mate choice' or 'female choice') in which males compete with each other to be chosen by females; and 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. The limiting sex is the sex which has the higher parental investment, which therefore faces the most pressure to make a good mate decision.
For intersexual selection to work, one sex must evolve a feature alluring to the opposite sex, sometimes resulting in a "fashion fad" of intense selection in an arbitrary direction. Or, in the second case, while natural selection can help animals develop ways of killing or escaping from other species, intrasexual selection drives the selection of attributes that allow alpha males to dominate their own breeding partners and rivals.
Sexual selection sometimes creates monstrously absurd features that, in harder times, could 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. However, sexual selection can also do the opposite, driving species divergence - sometimes through elaborate changes in genitalia - such that new species emerge.
Although the driving force for both sexes is reproductive success, the two genders have different concerns: males may seek to monopolize access to a group of fertile females, while the females want to maximize return on the energy they invest in reproduction, seeing their offspring grow into healthy adults - and especially into alpha males with well-developed, sexually attractive features that sire them many descendants. Because of their limited number of breeding opportunities (due to seasonal breeding cycles, limited litter sizes, and the amount of food available to bring up the offspring) females have much more reason to be "picky". They need a way to choose the males that are most capable. Male and female investments in rearing offspring are not equal, females' energy expenditures on gestation and parental care being much higher. In contrast, males often use every opportunity they have to mate, as they are less invested in each individual offspring.
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