Asymmetric gene flow maintains an abundant range center

Abstract

The abundant center hypothesis postulates that population density is the highest in the center of the range and declines towards the periphery. While many evolutionary hypotheses have been offered to explain what generates this pattern, ecologically it's assumed the pattern emerges due to increasing distance from a species ideal niche with edge populations occupying a marginal niche of low suitability. However, the ubiquity of the abundant center pattern has recently been questioned. Furthermore, it has proven difficult to assess both the pattern of the abundant center and the potential generative scenarios (e.g., gene swamping, migration limitation) due to the inherent complexity of species ranges. In this study, we evaluate the suitability of the abundant center hypothesis to explain patterns of genetic diversity and inbreeding in shovel bugs, small, sand-burrowing coastal crustaceans. In addition, we evaluate potential biases in migration direction due to the dominant current regimes in the North Atlantic using a common method for querying the site frequency spectrum, the directionality index (\psi). While patterns of genetic diversity are consistent with an abundant center, we find a strong signature of recent central colonization from the range periphery. Using simulations, we evaluate the performance of \psi under different migration regimes, finding that, in the presence of asymmetric migration, it strongly infers an edge-to-center colonization dynamic. We conclude that asymmetric migration towards the center can lead to signatures of an abundant center without necessarily implying a reduction in niche quality towards the range periphery.