Geographic Variation of Tolerance to Environmental Stress in Drosophila pseudoobscura

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Abstract
Geographic variation was studied for resistance to 3 environmental stresses (heat, cold and desiccation) in 7 populations of D. pseudoobscura from areas of extreme climate. The surival 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 1 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. Adaptive differentiation among populations is more pronounced for pupae than for adults, perhaps because pupae have no behaviorial 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.