Adaptation at the edge: Patterns of local adaptation and genetic variation during a contemporary range expansion

During range expansion, differences can evolve between populations at the core and expanding edge of a range. Theory and experiments often focus on range expansions across uniform environments, but in nature, many range expansions occur over environmental gradients that present novel selection pressures. We study phenotypic evolution at the core and edge of a range expansion across a gradient, and the expression of genetic variation of core populations in novel environments. We focus on the timing of winter dormancy in a beetle (Diorhabda carinulata), expanding from northern areas with cold winters to southern areas with milder winters. Phenotypes of core populations are consistent with adaptation to northern environments and maladaptation to southern ones. However, phenotypes of edge populations vary, indicating potential adaptation to more variable conditions across the southern sites. Clear shifts in phenotype at the edge relative to the core suggest rapid evolution in response to southern climates. Heritability in a core population was high in a local environment but undetectable in a novel (edge) environment. These results show that core populations have adapted to their local environments, and that long-distance movement into novel environments may reduce heritable genetic variation on which selection can act, and thus hinder adaptation.