From Local Adaptation to Speciation: Specialization and Reinforcement

Figure 1

Indirect selection on a selfing/assortative mating modifier with local adaptation [53]. Sketches how a selfing or assortative mating modifier evolves in presence of local adaptation (for the sake of illustration, the S allele causes maximal assortment and s specifies random mating). Here assortment can be produced by selfing or assortative mating based on the genotype at the local adaptation locus (A mates with A and a with a). Before migration (step 1), consider two habitats with haploid individuals. On the left A allele is favored at a local adaptation locus, whereas a is favored on the right. To make things simple, we consider these alleles to be fixed where they are beneficial. At step 2, migration occurs between habitats (with m = 1/2, migrants in red). Then syngamy occurs in each habitat. The small circles represent diploid individuals. In each habitat, one can distinguish the subpopulation with full S-assortment allele (4 individuals on the left) and random mating s allele (4 individuals on the right). Importantly, at this step the S allele becomes positively associated to extreme aa and AA homozygotes (thus, variance in fitness is greater in the subpopulation with the S allele). Finally, selection occurs (favoring A on the left and a on the right, very strongly but in a codominant way in the illustration). At the end of this generation, the modifier has not changed in frequency (it is still 1/2). Yet, selection has generated LD between the random mating s allele and the locally inferior allele locally (the inferior allele is only found on the same chromosome as s in each habitat, orange dot). At the next generation, this LD will persist (it is decreased at most by one half by a round of free recombination) and cause indirect selection in favor of S. Note that if the locally beneficial allele is recessive (all heterozygotes eliminated on the right and the left), we see that direct selection occurs favoring S (its frequency rises to 2/3 on the sketch), but that less LD is generated. Exactly the opposite occurs if the local beneficial allele is dominant.