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

Abstract

During range expansion, differences can evolve between populations at the core and expanding edge of a range. While theory and experimental work has focused on range expansions across uniform environments, natural range expansions often occur over environmental gradients, which present novel selection pressures. We seek to understand how genetic variation expressed in different environments may constrain adaptation during range expansion across environmental gradients, by testing whether long-established populations are better adapted to their local environments than newly established populations in the expanding edge. We study the timing of winter dormancy in a beetle introduced for biological control (Diorhabda carinulata), expanding from areas with cold winters to areas with milder, shorter winters. In a reciprocal environment experiment, core populations showed a pattern of local adaptation, but only some edge populations showed a similar pattern, indicating these populations vary in their degree of adaptation. Expressed genetic variation of dormancy timing in a core population was high in a local (core) environment but disappeared in a novel (edge) environment. These results show that adaptive evolution has been rapid, likely fueled by high heritability, but long-distance movement may hinder adaptation by reducing the heritable genetic variation on which selection can act.