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Sexual selection

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Theory of Evolution > Selection > Sexual selection

The 'Bird of Paradise' as an extreme example of phenotype caused by strong sexual selection.

In addition to simple survival, organisms must reproduce for their genes to be passed on to later generations. There are many genes which control the processes involved in sexual reproduction, and this includes the choosing of partners. In sexual selection genes are selected for because genes for a particular phenotype and genes for finding that phenotype desirable in a mate are passed on to offspring such that those genes spread throughout the population.

Sexual selection can be, but does not have to be, exclusive of natural selection. In English slang the word "fit" is often used to describe an attractive body, because attractive bodies are fit and healthy. When humans were hunters and gatherers, people who were fit and healthy were better at providing food for their families, so that their offspring, who carried their genes, did not starve but grew up to pass on their genes.

Sexual selection can occur against the pressures of natural selection, however. Or, at least, against many of the obvious pressures of natural selection. Peacocks and male Birds of Paradise have large bright feathers, which provide no advantage to flying, and might actually impede flight, thus attracting predators. These spectacular feathers have evolved because they attract potential mates. We can imagine a population of these birds' ancestors, with an optimum tail length dictated by natural selection, somewhere between the optimum length for flying, and the optimum for avoiding predators. There will be some males with longer than average tails, and some with shorter than average, but most close to the optimum. A gene may be introduced, by the processes of variation, that causes female carriers to prefer males with long tails. The carriers will therefore mate more often with males with long tails, and the offspring will carry the genes for both long tails and prefering long-tailed mates. While tail length is not an issue to non-carriers when choosing mates, the carriers choose only other carriers, so that the average tail length slowly increases. Any new gene that causes a further increase in tail length will spread through the population for the same reason, until tail length reaches an optimum between the pressures of sexual selection and the pressures of natural selection (predation, flight).

Several theories exist on the development of such hindering characteristics by sexual selection. The theory described above is that of R.A Fisher, where a mutation sets of a positive feedback loop which leads to exponential growth in that selected characteristic, continuing until it reaches a balance with natural selection. Another theory, the 'Handicap Theory' proposed by Israeli biologist Amotz Zahavi, and also others such as British evolutionary biologist W. D. Hamilton, proposes that surviving with a maladaptive trait is a sign of survival skill, because only a very healthy animal could spare the energy to grow such a sign of his health. In this way, it is much like rich pepole buying expensive cars to show the extent of their disposable income.

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