Abstract
Of the Nearctic species of Drosophila not affected in their distribution by the activities of man (as D. melanogaster, D. simulans, D. funebris as well as a few other species certainly are), the longest and most intensive studies from the standpoint of population genetics have been carried out on D. pseudoobscura. Beginning in 1937 (Dobzhansky and Queal, 1938) and continuing to the present (see, for example Dobzhansky, 1963, and Dobzhansky et al., 1964) Dobzhansky and collaborators have shown seasonal, altitudinal, geographic, and temporal variation in cryptic characters throughout the distribution of this species. In contrast, variation in morphological characters has seldom been investigated, and then only to enable the investigator to distinguish D. pseudoobscura. from its sibling species, D. persimilis, by other than cytological methods. Mather and Dobzhansky (1939) found that the two species (then considered races of the same species) differed in the number of teeth in the sex combs in the male. Reed et al. (1942) calculated a special wing index by means of which these species could be identified. Rizki (1951) showed that a dependable index, which distinguishes males of the two species, is obtainable from the length and width of the penis lamina. Spassky (1957) found additional morphological differences which enable one to classify etherized males to species without dissecting the flies. The purpose of the present paper is to fill a gap in information regarding geographic variation in quantitative characters in Drosophila pseudoobscura. This species is eminently suitable for such studies: except for an isolated population occurring near Bogota, Colombia (Dobzhansky et al., 1963) its distribution is continuous from British Columbia to Guatemala, and in the United States from the West Coast eastward to a point near Austin, Texas. Furthermore, as Dobzhansky and Epling (1944) have emphasized, this is a truly euryokous species (tolerant of a wide variety of environmental factors) since it can be found in low desert areas as well as at timberline.
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