A range‐expanding butterfly is susceptible to cold and long winters but shows no signs of local adaptation to winter conditions

Abstract

Numerous species shift or expand their ranges poleward in response to climate change. Even when expanding species follow their climatic niches, expanding range margin populations are likely to face unfamiliar environmental conditions and thus natural selection for local adaptation. The wall brown butterfly (Lasiommata megera) has expanded northward in Sweden in the years 2000–2020, most likely as a result of climate change, and has previously been shown to have evolved local adaptations to northern daylength conditions. This evolution has occurred despite hypothesised genetic constraints to adaptation at range margins. We studied local adaptation to winter conditions in four of the previously‐studied L. megera populations, using a common garden laboratory experiment with a warm and short, an intermediate, and a cold and long winter treatment. We compared the winter and post‐winter survival of caterpillars from two southern core range and two northern range margin populations in Sweden. During the experiment, we measured metabolic rates of a subset of diapausing caterpillars to test whether populations differ in metabolic suppression during diapause. Further, we measured supercooling points, which reflect lower lethal temperature in L. megera, of the same subset of caterpillars. We also compared supercooling points between L. megera and three closely related species with more northern distributions. Few individuals survived the coldest treatment all the way to successful adult emergence, so L. megera seems susceptible to cold winters. Individuals of northern descent did not survive cold winters any better than individuals from southern populations. Similarly, there were no signs of local adaptation in metabolic rates or supercooling points. The comparison among species did not reveal any clear relationship between geographical distribution and supercooling point. Although northern winters probably exert strong selection on L. megera, we provide comprehensive evidence for the lack of local adaptation to winter conditions. This contrasts with the previous finding of quickly evolved local adaptation in diapause timing, highlighting the need to consider how traits associated with different seasons differ in how they may evolve and facilitate climate change‐induced range expansions. Read the free Plain Language Summary for this article on the Journal blog.