From Local Adaptation to Speciation: Specialization and Reinforcement

Figure 4

Indirect selection on a recombination modifier with local adaptation. Sketches how a recombination modifier evolves in presence of local adaptation (for the sake of illustration, the R allele causes maximal recombination, and the r allele specifies zero recombination). Before migration (step 1), consider two habitats with haploid individuals. On the left, the A and B alleles are favored at two different loci; the a and b alleles are favored on the right. To simplify the illustration, we consider the alleles to be fixed where they are beneficial. After migration between habitats (step (2) with m = 1/2, migrants shown in red), strong LD is generated between the selected loci: in each habitat there are AB and ab but no Ab and aB haplotypes. Then random mating and meiosis occur (step 3). In each habitat, one can distinguish the subpopulation with the full R recombination allele and the zero r recombination allele (groups of four individuals on the left and right, resp.). Within the former, LD between selected loci has been much reduced (illustrated at zero, in fact even full recombination only halves LD at each meiosis), whereas in the latter it stayed intact. Importantly, at this step the r recombination modifier becomes positively associated to the extreme AB and ab haplotypes. Note also that in each habitat, variance in fitness is greater in the subpopulation with the r allele. Finally, selection occurs (favoring A and B on the left and a and b on the right, very strongly on the illustration) and takes the r allele along because of the linkage disequilibrium generated at the previous step (the overall frequency of r has risen from 1/2 to 2/3 in the illustration). Note that the case illustrated involves positive epistasis (only the extreme genotypes AB and ab survive, on the left and right, resp.). However, the r allele is favoured even if epistasis is zero, because selection is more efficient in subpopulations carrying the r allele since variance in fitness is greater in these subpopulations.