Asymmetric gene flow maintains range edges in a marine invertebrate

Species are restricted to finite ranges, and when they are in demographic equilibrium with the environment this implies a failure to adapt at the range periphery. There are several competing evolutionary hypotheses for the maintenance and persistence of range edges. One hypothesis is gene swamping of edge populations, in which migrants from the center carrying alleles adapted to the range center but maladaptive along the edge hamper local adaptation. A competing explanation is dispersal limitation, in which reduced migration to the periphery keeps edge populations small, causing drift to dominate over selection. It has proven difficult to assess these potential generative scenarios due to the inherent complexity of species ranges. In this study, we leverage range-wide genomic sequencing data to investigate the evolution of range edges in an empirical system - Haustorius canadensis, the Canadian shovel bug. We find increased genetic drift and inbreeding on the range edges and detect signatures of asymmetric migration towards the range center. The inferred directionality of migration is consistent with the direction of the dominant current regimes in the North Atlantic. We conclude that anisotropic dispersal towards the range center leaves progressively fewer migrants to replenish peripheral populations, consistent with the migration limitation hypothesis, and generates the appearance of an abundant center.