Timing of reproduction modulates fitness effects of local seasonal adaptation in a range expanding butterfly

Climate change is pushing species northward, where they will encounter novel abiotic conditions, such as novel daily light cycles and seasonal lengths, to which they will have to adapt. Despite the diversification of these adaptations being well described, the direct fitness consequences of the natural variation is rarely estimated in the field. To test the fitness effects of local adaptations to daylength and season length, we studied diapause induction and growth rate in the range expanding butterfly Lasiommata megera (wall brown). Using a common garden field-experiment (conducted near the northern range margin) where we manipulated the start of the last generation in the year that typically enter diapause, we compared populations from the southern-Swedish core range with populations from the northern-Swedish margin. Our results show differences between populations in diapause response and growth rate. In line with adaptive predictions caterpillars from the northern populations entered diapause earlier in the season compared to the southern populations. However, this difference was only present in larvae produced by the earliest individuals in the last adult generation. Consequently, the early laid eggs of the southern populations were more likely to produce an additional generation that turned out to be highly maladaptive. Additionally, caterpillars of northern origin grew faster compared to caterpillars from the southern populations, even though we found no clear evidence of prewinter larval mass affecting winter survival. The wall brown butterfly showed local evolution of seasonal timing traits, but just at a specific time of the season. This highlights the importance of local adaptations in northern-Swedish populations, during the early stages of the last annual generation. Despite that the additional generation is presently maladaptive, our fitness estimates suggests that a warmer climate is likely to favour the production of an additional generation.