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When individual organisms select mates for sexual reproduction, their choice is based upon certain characteristics in the opposite sex. This leads to competition for mates. The term sexual selection was coined by Charles Darwin to describe his theoretical ideas concerning the influence of natural selection on the choice of mating partner. He suggested that competition could exist between the sexes of a species for the opportunity to mate with the fittest individuals, that is, those with the best genes. This strategy would give offspring the best genes and thus enhance their fitness. A consequence of this phenomenon is that certain characteristics, called secondary sexual characteristics, are selected because they play a role in sexual selection. Thus peacocks have evolved oversized, colourful tail feathers as a secondary sexual characteristic, and are chosen by peahens on the basis of their ability to produce an impressive display. The phenomenon of competition for mates is seen in a wide variety of animals and the time, energy and risk which it involves suggests that there is an important issue at stake. Secondary sexual characteristics such as stag antlers require large amounts of energy to develop and may increase visibility to predators, while the energy expense and risk of injury during the rut (the battle between stags for dominance and the right to mate) can result in death.
It is difficult to quantify the link between sexual selection and fitness of offspring because of the great number of factors involved. Natural selection will result in the accentuation of any characteristics which infer fitness, so that a characteristic may be selected because it is attractive to the opposite sex. Thus male walruses may have become very large in order to attract females; however, they may also have become large in order to win fights with other males and gain mates in this way. Studies in the laboratory have shown that there is a correlation between the ability of the male fruitfly to perform a courtship dance and his fertility. RB
Further reading Richard Dawkins The Selfish Gene; , John Maynard-Smith, The Theory of Evolution. |
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