Genetic diversity in introduced populations with Allee effect
The demographic Allee effect, a reduction in per-capita population growth rate at small population sizes, is of key importance for the dynamics and persistence of newly introduced populations. Because it shapes population-size trajectories, the Allee effect should also strongly influence the strength of genetic drift that introduced populations are exposed to and hence their genetic diversity and evolutionary potential in the new environment. In this study, we focus on populations that have successfully overcome a strong demographic Allee effect, a scenario where the per-capita growth rate is negative below a certain critical population size. We take a stochastic modeling approach to investigate how population-size trajectories, conditioned on success, differ from those of successful populations without Allee effect and how this leads to different levels of genetic diversity.
Our results indicate that compared to successful populations without Allee effect, successful Allee-effect populations can have either a higher or a lower expected amount of genetic diversity, depending on the typical size of founder populations and on the variability of the number of surviving offspring produced by individuals or families. With increasing offspring-number variability, the Allee effect increasingly promotes genetic diversity in successful populations, especially if the founder population size is small relative to the critical population size. An important and somewhat counter-intuitive mechanism contributing to these results is that in order to avoid extinction, Allee-effect populations must rapidly escape the region of small population sizes where genetic drift is strongest; in fact they tend to grow faster than successful populations without Allee effect. Finally, we explore how the observed genetic patterns might be employed to solve a challenging problem with relevance for invasion biology: the statistical inference of the critical population size from genetic data.