Worldwide latitudinal clines for the alcohol dehydrogenase polymorphism in Drosophila melanogaster: what is the unit of selection?

Geographical clines may reflect the action of natural selection on genetic polymorphisms. In Drosophila melanogaster several latitudinal clines occur for many characters like allozymes, inversions and quantitative traits. The identical nature of these clines on the various continents, both on the Northern and Southern Hemispheres strongly suggests adaptation to specific stress factors. The alcohol dehydrogenase (Adh) polymorphism shows high frequencies of the S allele in tropical regions and declines with latitude. The reasons for this cline are difficult to determine because of the entanglement with other polymorphisms varying with latitude. In this paper the tentative connections with other polymorphisms like alpha-Gpdh, In(2L)t, body size and development time are reviewed with respect to the possible environmental stress factors involved. It is concluded, also from recent experiments, that the (2L)t inversion plays a dominant role in resistance to high temperature and is partly responsible for the Adh cline. Further research is aimed at the specific selective forces acting on Adh, focussing on the physiological and life history aspects.