The evolution of recombination in a heterogeneous environment

Most models describing the evolution of recombination have focused on the c ase of a single population, implicitly assuming that all individuals are eq ually likely to mate and that spatial heterogeneity in selection is absent. In these models, the evolution of recombination is driven by linkage diseq uilibria generated either by epistatic selection or drift. Models based on epistatic selection show that recombination can be favored if epistasis is negative and weak compared to directional selection and if the recombinatio n modifier locus is tightly linked to the selected loci. In this article, w e examine the joint effects of spatial heterogeneity in selection and epist asis on the evolution of recombination. In a model with two patches, each s ubject to different selection regimes, we consider the cases of mutation-se lection and migration-selection balance as well as the spread of beneficial alleles. We find that including spatial heterogeneity extends the range of epistasis over which recombination can be favored. Indeed, recombination c an be favored without epistasis, with negative and even with positive epist asis depending on environmental circumstances. The selection pressure actin g on recombination-modifier loci is often much stronger with spatial hetero geneity, and even loosely linked modifiers and free linkage may evolve. In each case, predicting whether recombination is favored requires knowledge o f both the type of environmental heterogeneity and epistasis, as none of th ese factors alone is sufficient to predict the outcome.