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Recently there has been an increasing awareness of the need to study the genus Drosophila in the wild, to assess the various ecological and behavioral parameters determining its distribution (1,2). It must be stressed that the classic work of Dobzhansky and his collaborators has over the years provided much information (3,4) especially in D. pseudoobscura and closely related species such as D. persimilis. It is one of the curiosities of this sort of scientific endeavor, that the species D. melanogaster, perhaps best known genetically of all species in the genus, has been studied extensively under laboratory conditions for over 60 years, with little thought given to the situation in the wild. In this review, some environmental stresses, natural and artificial, for which genetic variability has been found within and between species of Drosophila will be examined, as they are important in determining the distribution of species of the genus in the wild. Stresses to be considered are temperature extremes, desiccation, anoxia as assessed by resistance to long-term exposure to CO2, 6OCo y-radiation, various chemical stresses (some of importance in nature because of the activities of man), and ethyl alcohol. Genetic heterogeneity for many metrical traits iq wild populations of D. melanogaster has been detected by setting up cultures from single inseminated females collected in the wild, and following the trait over a number of generations. A good example is the incidence of flies with more than the normal
P. A. Parsons (Sat,) studied this question.