Testing the melanism-desiccation hypothesis: A case study in Darwinian evolution

Abstract

In ectotherms, the occurrence of melanism within populations, across populations or closely related species is pervasive in nature. Many hypotheses have been proposed to explain diverse patterns of body color variations or polymorphisms in insects (e.g. thermoregulation, cryptic coloration, sexual selection, pathogen resistance, etc.). Since the body temperature of ectotherms is dependent on their surrounding environment, melanisation should play an important role in these organisms. The thermal melanism hypothesis states that melanic or dark individuals should be at a fitness advantage under cold conditions while lighter individuals are better adapted under warmer conditions. There is overwhelming support for this hypothesis in diverse taxa (beetles, butterflies and flies) from the temperate parts of the world. However, it is not clear why melanics occur in the tropics? We tested the role of desiccation stress in the maintenance of color polymorphism in tropical populations of different Drosophila species on the Indian subcontinent. We tested several predictions which are most significant in the field of evolutionary ecology. First, if desiccation resistance evolves through changes in cuticular permeability, the target of natural selection might be cuticular components either cuticular lipids or cuticular melanisation. Thus, species specific contrasting levels of desiccation resistance might correspond with varying levels of body melanisation across species. Secondly, we tested whether a thicker cuticle (either due to melanisation or cuticular lipids) leads to lesser cuticular water loss and higher desiccation resistance across seasons (autumn vs. winter). Thirdly, we examined, mechanistic basis of evolving desiccation resistance on the basis of differential rates of cuticular water loss in assorted darker and lighter phenotypes from a given population. Finally, if body melanisation confers desiccation resistance, dark morph is expected to prevail under dry season while the reverse may occur for light morph under humid conditions/season. Further, humidity changes (dry vs. wet) impact mating propensity (assortative matings) among different body color morphs. Thus, there are several evidences in favor of melanism — desiccation hypothesis. In this short review, an attempt has been made to sum up the available information on the role of varying levels of humidity as natural selection agent in maintaining body color polymorphism in diverse Drosophila species populations on the Indian subcontinent.