Question: Do invasive species adapt during range expansion? Data are few and many expect adaptation to be constrained by low genetic variation in invaders, which frequently experience population bottlenecks during colonization.

Experimental results: We used a common-garden experiment to show that the invasive hemlock woolly adelgid (Insecta: Hemiptera: Adelgidae: Adelges tsugae) has evolved greater resistance to cold shock as it has expanded its range northward. This insect feeds on exposed twigs in winter and is vulnerable to extreme cold.

Modelling results: Adelges tsugae has grown to a sufficiently large population size that its adaptive evolution appears unconstrained by the availability of new mutants, despite its parthenogenetic reproduction. Conservatively, its population size likely exceeds 1/(2u) within 40 ha of forest, where u is the haploid per-base mutation rate, so that on average a mutation arises each generation at every base pair in the entire genome within this area.

Conclusion: This escape from genetic constraint is likely to be found in many species that have invaded successfully, facilitating their adaptation to novel conditions.