Eco-evolutionary Dynamics of Colonization: When Does Source Selection Matter?

Ancestral balancing selection may facilitate evolutionary rescue following migration into a new habitat by providing founder populations with adaptive alleles, even when those alleles were not the fittest in the ancestral habitat. However, recent studies emphasize the roles of immigration history and admixture on colonization success. The extent to which balanced polymorphism may contribute to colonization success and the conditions under which it matters remain unclear. We use computer simulations to investigate how the mode of selection in the source population influences colonization success in new habitats under eco-evolutionary population dynamics, different invasion scenarios, and different selection regimes in the new habitat. We model source populations under balancing selection, directional selection, and neutrality, and allow founder groups of different sizes to invade a novel stable or perturbed habitat. We find that balancing selection in ancestral populations leads to higher rates of establishment in new habitats than are observed under source neutrality or directional selection when founder populations are relatively small, which would occur with a population bottleneck. This trend is observed regardless of whether invaders experience stable or variable conditions in a new habitat and persists under increasingly harsh conditions or greater environmental perturbations. When the colonizing populations are large, selection in the source population has little effect on invasion success. This study elucidates the interaction between genetic diversity and propagule pressure on colonization success, highlighting the number of adaptive alleles as a unique metric.