Conditional relatedness, recombination, and the chromosome numbers of insects

If two polymorphic loci are out of phase equilibrium, a homozygote at one of these loci is more highly related to its kin, at the other locus, than is an equivalent heterozygote. As a result, selection can favor (1) phenotypic responses to relative heterozygosity, and (2) increased recombination between the loci inducing these responses. Selection is expected to have these consequences only to the extent that kin strongly affect each other's fitnesses. The chromosome numbers of social insects appear to be higher, on average, than those of allied solitary species, which is consistent with this model on the assumption that chromosome numbers are selected in part for their effects on recombination.