Physiological responses to selection for desiccation resistance in Drosophila melanogaster

Comparative physiologists and physiological ecologists have striven to eluc idate the physiological adaptations which eliminate or mitigate environment al stress. Stress tolerance is thought to influence the distribution of spe cies as well as the fitness of individuals within various habitats. Differe ntial stress tolerance depends in large part on physiological mechanisms wh ich mitigate the deleterious effects of stress. Very little information is available, however, regarding the mechanisms and pathways by which such phy siological adaptations arose and were modified. We point out two methods by which one can investigate the evolution of stress tolerance: phylogenetic studies and selection studies. Phylogenetic studies have the advantage that they can be used to study wild populations, with the drawback that species numbers and distribution may be limiting. In addition, for many physiologi cally interesting clades, the phylogenetic relationships have yet to be det ermined, Selection studies have the advantage that the evolution of physiol ogical systems can be studied in response to very specific forms of stress. In addition, the phylogeny of the organisms can be experimentally manipula ted and replication permits rigorous statistical analysis. The results of s tudies of the evolution of increased desiccation resistance in Drosophila a re presented as an example of the methods by which insights can be obtained regarding the variables which respond to selection, the rate of evolutiona ry change and the process by which physiological performance changes over e volutionary time, Selection studies can be designed to provide models regar ding the mechanisms, timing and directions of physiological evolution.