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

During range expansion, differences in traits can evolve between populations at the core and expanding edge of a range. While theory and experimental work have focused on range expansions across uniform environments, natural range expansions often occur over environmental gradients, which present novel selection pressures. We study phenotypic evolution at the core and edge of an active range expansion across an environmental gradient, and how adaptation may be constrained if the expression of genetic variation in novel environments is reduced. 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, shorter winters. We examine, first, the pattern of evolution of winter dormancy timing in core and edge environments, and second, how heritable genetic variation of a core population is expressed in local and edge environments. Phenotypes of core populations are consistent with adaptation to northern environments and maladaptation to southern ones. However, phenotypes of edge populations varied, indicating potential adaptation to more variable conditions across the southern sites. Clear shifts in phenotype at the expanding edge relative to the core suggest rapid evolution at the edge 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, likely fueled by high heritability, but that long-distance movement into novel environments may reduce the heritable genetic variation on which selection can act, and thus hinder adaptation.