Figure 3: Kin selection on a migration modifier. Sketches how a migration modifier evolves because of kin selection. As in Figure 2, the M allele causes maximal migration (1/2) and the m allele specifies zero migration). Before migration (step 1), consider two subpopulations. In one of them the M allele is frequent (1/2), but it is absent in the other. During migration, only M individuals move between habitats (step (2), the migrant is shown in red). Half of the M individuals move to the other habitat and the other half stays at home. Then reproduction occurs (step 3). All individuals produce say, two offsprings (note that all individuals have the same survival and reproduction). Finally, population regulation occurs: juveniles compete to repopulate each subpopulation with four adults and all have the same chance to get established. After this step, the M frequency has risen to (1/3 + 1/5)/2, which is greater than 1/4, the initial frequency. There is thus selection on M allele, which is traditionally explained in terms of “kin selection”: the migrating M allele sacrifices itself by competing in a more crowded population, but it leaves room behind that benefits the other M allele, which will compete in a less crowded population. The decreased chance of survival by the migrating M (1/5–1/4) is more than compensated by the increased chance that the remaining M allele will survive (1/3–1/4). This process requires only that the M alleles are concentrated in the same population at step 1 (i.e., it requires population structure or relatedness), which is easily generated by drift [54].