Diploidy, population structure, and the evolution of recombination.

In diploids, sex affects genetic variation through segregation and recombination. Several recent models on the advantage of recombination have focused on the effect of interaction between selection and drift in finite or structured populations; however, these models considered haploid organisms. In this article, I present a three-locus model of the evolution of recombination in structured diploid populations, including dominance and epistatic effects among alleles. This model shows that dominance generates a selective force against recombination due to the fact that recombination reduces correlations in homozygosity that are generated by population structure. This result is confirmed by multilocus simulations (representing deleterious mutations occurring over a whole genome), showing that when mutations are sufficiently recessive, the population evolves to zero recombination. In the presence of epistasis, the same effect of recombination on correlations in homozygosity generates an advantage for recombination under negative dominance by dominance epistasis (e(d x d)). Additive by additive epistasis (e(a x a)) favors recombination when it is negative and sufficiently weak, while additive by dominance epistasis has less effect. Some of these effects, however, are reversed when the deleterious mutation rate U is sufficiently high: in that case, strongly negative (e(a x a)) can favor recombination, while negative (e(d x d)) disfavors it. Interpretation of these results is given.