Geographic Variation of Tolerance to Environmental Stress in Drosophila pseudoobscura

Abstract

We studied geographic variation for resistance to three environmental stresses (heat, cold, and desiccation) in seven populations of Drosophila pseudoobscura from areas of extreme climate. The survival of adults and pupae in stress tests was compared to a priori predictions that survivorship would reflect local adaptation to climate. Six tests of adult flies showed significant heterogeneity among populations, but in only one case was this in an apparently "adaptive" direction. Two tests on pupae (heat and cold stress) showed less differentiation among populations, but in both cases in an apparently "adaptive" direction. These results suggest that adaptive differentiation among populations is more pronounced for pupae than for adults, perhaps because pupae have no behavioral response to extreme climate. Such adaptive differentiation and the extensive long-distance movement previously observed in this species may indicate that other geographic variation of morphology, physiology, and chromosomal inversion frequencies in D. pseudoobscura reflect local selection.