GILCHRIST, G. W.; HUEY, R. B.; College of Wiliam & Mary; University of Washington: Is plasticity an adaptation? Body size as a function of temperature within and among parallel clines in Drosophila subobscura.

The European fruit fly, Drosophila subobscura, exhibits striking clinal variation in wing size, with overall size steadily increasing with latitude. About 25 years ago, a small number of Mediterranean D. subobscura colonized South and North America and rapidly expanded on both continents, resulting in two independently evolving invasive populations. We have shown that females (but not males) in the New World have evolved wing size clines parallel to the ancestral Old World cline and that the geographic position of the cline on each continent is correlated with environmental temperature. Wing and body size also exhibit plastic variation in response to temperature; like most drosophilids, rearing these flies at low temperature produces a plastic increase in size. Here we examine the reaction norms of wing and body size (in response to rearing at 15, 20 and 25C) in various populations along the clines. Specifically, we will test the hypothesis that rapid evolution of wing size clines has resulted from the conversion of plastic variation into fixed population differences. This hypothesis predicts a reduction in plasticity in the colonizing populations, especially at high latitudes. If the rapidly evolved changes in wing size have occurred without a significant reduction in plasticity, it suggests that the genetic basis of size is independent of the genetic basis of size plasticity. We will also discuss plastic and genetic differences in sexual dimorphism within and among continents.