Contrasting Types of Population Structure in Drosophila

The known facts concerning population structure in the single species Drosophila robusta and the many "wild" members of the Drosophila virilis species group, both of which are widespread in North America, are reviewed and discussed. Although the information is incomplete, the suggestion is made that the recent and rapid speciation in the virilis group, as contrasted with the much more conservative tendencies of D. robusta, has been conditioned by the close ecological restriction of the former to a riparian habitat which imposes on the species of this group a fundamentally linear distribution, following river valleys. Populations of these species may be viewed as being relatively discontinuous, due to the nature of the habitat and the apparent low motility of the flies. D. robusta, on the other hand, displays strongly contrasting features of population structure which appear to be less conducive to speciation. So far as is known, the patterns of genetic variability within the compared forms fit the above hypothesis.