Fitness effects of local seasonal adaptation depend on the timing of reproduction in a range-expanding butterfly

Abstract Local seasonal adaptation across latitudes is ubiquitous, but its fitness consequences are rarely estimated in the field. Using a common-garden field experiment at the northern range margin of the butterfly Lasiommata megera , we show that fitness consequences of among-population, genetic differences in photoperiodism for diapause (dormancy) timing depend on when during the adult flight period eggs are laid. In early-developing cohorts, individuals from southern populations were more likely to avert diapause than locally adapted northern range margin populations, and this non-diapause development was strongly disfavoured by natural selection. However, for all populations, virtually all eggs laid only one week later entered diapause, limiting overall among-population phenotypic differences. This demonstrates how evolved genetic differences for a reaction norm interact with phenology to shape the developmental decisions of caterpillars in the wild. Rapid local adaptation of photoperiodism has likely occurred through natural selection acting on a limited part of each autumn generation, implying a smooth fitness landscape where mildly locally maladapted populations can establish and subsequently evolve towards the local fitness peak. Thus, latitudinal differences in daylength are unlikely to restrict climate change–driven range expansions.