Allele surfing causes maladaptation in a Pacific salmon of conservation concern

The Anthropocene threatens worldwide biodiversity, with a potential to induce species decline and range contraction. In this context, it is important to quantify species adaptive potential and how demographic changes may impact selection efficacy and the burden of deleterious mutations in different populations. Here we show that key evolutionary processes, including variation in effective population size through postglacial change in demography and recombination rates have affected the efficacy of selection and impacted the load in Coho salmon (Oncorhynchus kisutch), a widely distributed salmonid species on the west coast of North America. Using whole genome resequencing (30x coverage) data from populations at different latitudinal distances from their southern glacial refugium, we found support for postglacial gene surfing, with reduced Ne at the range recolonization front, thus inducing both a reduction of the adaptive potential and a surf of deleterious alleles in populations evolving at low Ne. This inference was robust to various proxy of the load, namely ratios of non-synonymous to synonymous diversity, the number of missense and loss of function mutations. In addition, comparing residual tetrasomic and re-diploidizing regions of the salmon genome, we found support for a prime role of recombination rates in shaping the within-genome variation of the load. Overall, our empirical results are remarkably consistent with expectations under the nearly neutral theory of molecular evolution. We discuss the fundamental and applied implications of these findings for evolutionary and conservation genomics. ### Competing Interest Statement The authors have declared no competing interest.