Tempo and mode of winter diapause evolution in butterflies.

Quantifying the tempo and mode via modern phylogenetic comparative methods can provide key insights into how selection and constraints shape trait evolution on a macroevolutionary time scale. Here, we elucidate the evolution of hibernation (winter) diapause, a complex and defining life-history trait that allows temporal escape from harsh winters in temperate regions for many insects, including our model system, butterflies. Butterflies can diapause in all major life stages, and the availability of global-scale phylogenies makes them an ideal model system for studying diapause evolution. First, using a thorough literature survey, we scored the developmental stage of hibernation diapause (egg, larva, pupa, adult) vs. absence of diapause. We find that larval diapause is most common, while pupal, egg, and adult diapause are relatively rare. Next, we determined that the loss of diapause occurred at a much higher rate and that gains primarily occurred from the non-diapause state. While ancestral state estimation at deeper nodes remained uncertain, we found consistent patterns for some families and strong evidence for extensive convergence in diapause evolution. Contrary to expectations, we find no support for increased gain of diapause during the Eocene-Oligocene glaciation (~35 million years ago). Overall, the evolution of diapause in butterflies has a complex history, has evolved convergently, and has likely predated the major glaciation event consistent with the deep history of diapause evolution in insects. This study advances our understanding of the evolution of a complex and important life-history trait and establishes a macroevolutionary foundation for future studies on the ultimate and proximate basis of diapause evolution.