Deleterious mutations and selection for sex in spatially structured, diploid populations

Genetic drift is potentially an important component of selection for sex, as it is a source of statistical associations between alleles at selected loci. By increasing local drift, population structure may thus amplify the evolutionary advantage of sex. However, most previous models have focused either on haploid populations or on diploid populations without spatial structure. In this article, we use two- and three-locus analytical models and multilocus simulations to explore selection for sex in a diploid population structured according to the island model, in the presence of recurrent deleterious mutations. Our results show that selection generally favors an intermediate rate of sex that decreases as the direct cost of sex increases and increases moderately as the degree of population structure increases. Selection for sex is generated by multiple effects involving genetic associations within and between loci. When selection occurs at many loci, it is generally dominated by interference effects involving deleterious alleles at different loci, captured by our three-locus model. In our multilocus simulations, we observed an irreversible spread of asexual mutants under strong costs of sex, and when deleterious mutations are partially recessive. However, population structure may prevent this spread of asexual mutants when dispersal rates are sufficiently small.