Phenotypic plasticity in tropical butterflies is linked to climatic seasonality on a macroevolutionary scale

Phenotypic plasticity can be adaptive in fluctuating environments by
providing rapid environment-phenotype matching and this applies
particularly in seasonal environments. African Bicyclus butterflies have
repeatedly colonized seasonal savannahs from ancestral forests around the
Late Miocene and many species now exhibit seasonal polyphenism. On a
macroevolutionary scale, it can be expected that savannah species will
exhibit higher plasticity due to them experiencing stronger environmental
seasonality than forest species. We quantified seasonality using
environmental niche modelling, and surveyed the degree of plasticity in a
key wing pattern element (eyespot size) using museum specimens. We show
that species occurring in highly seasonal environments display strong
plasticity, while species in less seasonal or aseasonal environments
exhibit surprisingly variable degrees of plasticity, including strong to
no plasticity. Furthermore, eyespot size plasticity has a moderate
phylogenetic signal and the ancestral Bicyclus likely exhibited some
degree of plasticity. We propose hypotheses to explain the range of
plasticity patterns seen in less seasonal environments, and generate
testable predictions for the evolution of plasticity in Bicyclus. Our
study provides one of the most compelling cases showing links between
seasonality and phenotypic plasticity on a macroevolutionary scale and the
potential role of plasticity in facilitating the colonization of novel
environments.