Finite populations, finite resources, and the evolutionary maintenance of genetic recombination.

Most previous models for the evolution of sex implicitly assume infinite population sizes and limitless resources. However, because favorable mutations are very rare and eukaryotic populations are finite, it has already been shown that multiple favorable mutants virtually never occur by chance. Therefore, sex is required to combine different favorable mutations into a single lineage. Second, we show that even when multiple favorable mutations do coexist, competition between genotypes can create negative epistasis for fitness, thus favoring recombination. Competition is especially effective when selection is at the level of viability in K-selected species living in a resource-limited environment. This means that recombination is advantageous both for incorporating new favorable mutations into the gene pool and for accelerating their increase to fixation. These advantages of recombination are diminished, however, as genome sizes decrease or as the amount of competition within the species is a less important component of selection.