Genetic consequences of gene surfing and their relationship to fitness

Populations expanding their ranges experience unique evolutionary dynamics, with perhaps the most ubiquitous being an increased role for genetic drift. The increase in genetic drift during range expansion is predicted to increase the frequency of deleterious alleles along the expansion edge, termed expansion load, and therefore reduce fitness at the edge of expansions. While theoretical predictions of expansion load are well established, direct links between whole-genome estimates of load and decreases in an expanding population's fitness remain scarce. We quantified expansion load during experimental range expansions of the red flour beetle (Tribolium castaneum) and then regressed observed population growth rates against estimated genetic loads to characterize if and how much expansion load decreased fitness at expansion edges. As predicted by theory, gene surfing resulted in the fixation of an increased number of putatively deleterious alleles. However, metrics of whole-genome load displayed relatively weak relationships with fitness. We suggest this discrepancy may partly be due to the recessive nature of some deleterious variation, which our data were not able to robustly assess. Genetic diversity, in contrast, was strongly associated with fitness and may provide a robust, easy-to-assess metric for expanding populations.