AYR (1954, 1959), CARSON (1955,1956,1959), BROWN (1957), DOBZHANMs~~ (1957 and other publications), TOWNSEND (1952) and others have pointed out that the genetic structures of populations of the same species inhabiting the central and the peripheral parts of the distribution area are likely to be characteristically different on the average. Central populations have, by and large, mastered a greater range of ecological niches, and have thus achieved a more secure adaptedness to the environment than the peripheral ones, which may well be able to survive in only a restricted adaptive zone. The form of natural selection which predominates in central populations has been termed by CARSON (Zoc. cit.) heteroselection; it favors polymorphism, restricted recombination, and a general adaptedness to a multiplicity of conditions. Marginal populations are dominated by homoselection, which favors a relative homozygosity, random genetic drift, free recombination, and adaptive specialization. Evolutionary novelties, such as new species, are, however, more likely to arise, in the opinion of most writers, from the marginal rather than from the central populations. The population of Drosophitila pseudoobscuru discovered in the Andes of Colombia around Bogota occurs in an apparently small area isolated from the main body of the species by extensive territories in which the species in all likelihood does not occur. The results of a cytological and genetic analysis of this population are reported in the pages that follow. MATERIALS AND METHODS During the years 1960-1962, populations of Drosophila have been sampled in different parts of Colombia, especially in the highlands in the vicinity of the city of Bogota. The flies were attracted to bait consisting of fermenting bananas. Drosophila pseudoobscura has been encountered only in the highlands, at elevations of 2,200 to 3,280 meters, The species has been encountered in gardens (near the University), in forests (Pine Woods, Villavicencio Road and El Hato Road,