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Sex creates novel genotypes more rapidly

This diagram illustrates how sex might create novel genotypes more rapidly. Two advantageous alleles A and B occur at random. The two alleles are recombined rapidly in (a), a sexual population, but in (b), an asexual population, the two alleles must independently arise.

Sex could be a method by which novel genotypes are created. Since sex combines genes from two individuals, sexually reproducing populations can more easily combine advantageous genes than can asexual populations. If, in a sexual population, two different advantageous alleles arise at different loci on a chromosome in different members of the population, a chromosome containing the two advantageous alleles can be produced within a few generations by recombination. However, should the same two alleles arise in different members of an asexual population, the only way that one chromosome can develop the other allele is to independently gain the same mutation, which would take much longer.

Ronald Fisher also suggested that sex might facilitate the spread of advantageous genes by allowing them to escape their genetic surroundings, if they should arise on a chromosome with deleterious genes.

But these explanations depend upon the rate of mutation. If favourable mutations are so rare that each will become fixed in the population before the next arises (bearing in mind that mutation is a Poisson process), then sexual and asexual populations would evolve at the same rate.

Additionally, these explanations depend upon group selection, which is a weak selective force relative to natural selection – sex is still disadvantageous to the individual due to the twofold cost of sex. Therefore, these explanations do not explain why heterogonic species choose to adopt sexual reproduction, as George C. Williams pointed out in his balance argument, and hence are insufficient to explain the evolution of sex.

Supporters of these theories respond to the balance argument that the individuals produced by sexual and asexual reproduction may differ in other respects too – which may influence the persistence of sexuality. For example, in water fleas of the genus Cladocera, sexual offspring form eggs which are better able to survive the winter.

Source from : http://en.wikipedia.org/wiki