As a result, a population of sexually reproducing organisms can evolve faster than a population of asexual organisms of similar size. Under conditions of high rate of favorable mutation, imagine a sexual and an asexual population each of which has alleles a and b fixed at two different loci. Mutations a' and b' arise in two different individuals and are advantageous. The asexual population will eventually consist of a'b and ab' individuals because the mutations cannot spread to other clones. Thus, an a'b' individual cannot arise unless the b gene mutates to b' in the a'b clone, or the a gene mutates to a' in the ab' clone.
Evolution proceeds more rapidly in the sexual population.
After arising in separate individuals the two favorable mutations can be combined into one individual through sexual reproduction. The two mutations do not have to occur twice to be combined in one individual, and a population can go from ab to a'b' more rapidly than if asexual reproduction were occurring. This idea about the relative rates of evolution was first presented by Fisher. He concluded that sexual populations evolve more rapidly than equivalent asexual populations. If mutations are rare and one becomes fixed in the sexually reproducing population before another one occurs, sexual and asexual populations would be expected to evolve at the same rate.