Buckeye butterfly (Junonia coenia) seasonal wing color polyphenism varies dramatically between populations in different climatic zones

In many species, individuals that are genetically similar can develop into one of several phenotypically distinct morphs in response to specific environmental cues. This taxonomically widespread mechanism, known as polyphenism, can allow fast adaptive matching of organisms with their environments without relying on genetic changes between generations. Seasonal wing color polyphenism in buckeye butterflies (Junonia coenia) provides a useful system for studying this phenomenon. Buckeyes are widely distributed across the western hemisphere, have relatively short generation times, and show seasonal wing color variation that is easily scored. In this study I address two questions. First, how does the distribution of J. coenia wing color phenotypes differ between populations in different climatic regions? Second, can these differences be explained by a response to environmental differences, or do they reflect genetic divergence of polyphenic response? Using over a century’s worth of museum specimen data I show that the response to environmental conditions during development produces a significantly larger shift in wing color phenotype for buckeye populations in the Eastern USA than the West. I further show with controlled incubator trials that the cause of this variation in polyphenic response is not due to a narrower range of temperature and day-length conditions experienced, but represents a genetically-based decrease in phenotypic response to polyphenism-inducing environmental cues. Together these experiments show that while polyphenic traits of individuals are influenced by environmental conditions, the range of this variation in different geographical regions can evolve to allow better adaptation to local conditions.