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