Adaptive evolution in the lab: unique phenotypes in fruit flies comprise a fertile field of study.

Laboratory selection for desiccation resistance, which has been imposed on five replicate populations of Drosophila melanogaster for >200 generations, has resulted in enhanced survivability during periods of extreme water stress. The ability of these populations to persistently resist the fatal effects of desiccation is correlated with evolved physiological traits, namely preferential storage of carbohydrates (associated with reduced lipid reserves) and a dramatic increase in blood volume, which has led to a significant increase in extracellular sodium and chloride content, as well as body mass. When compared to other populations of this drosophilid species, these adaptive traits are unique. While some may argue against the value of evolved traits that have not been found in natural populations, we counter that such traits are of considerable value to the analyses of physiological functions, as well as the underlying mechanisms and evolutionary trajectories of these functions. We propose that multiple physiological consequences almost certainly derive from the evolution of these singular traits; and, furthermore, we discuss future directions for the elucidation of such consequences.