Genetic variation of body size along latitudinal clines is found globally in Drosophila melanogaster, with larger individuals encountered at higher latitudes. Temperature has been implicated as a selective agent for these clines, because the body size of laboratory populations allowed to evolve in culture at lower temperatures is larger. In this study, we investigated the hypothesis that larger size is favoured at lower temperature through natural selection on adult males. We measured life‐span and age‐specific fertility of males from lines of flies artificially selected for body size at two different experimental temperatures. There was an interaction between experimental temperature and body size selection for male fitness; large‐line males were fitter than controls at both temperatures, but the difference in fitness was greater at the lower experimental temperature. Smaller males did not perform significantly differently from control males at either experimental temperature. The results imply that thermal selection for larger adult males is at least in part responsible for the evolution of larger body size at lower temperatures in this species. The responsible mechanisms require further investigation.
The aim of this study was to examine the latitudinal variation in preadult competitive ability of Drosophila melanogaster. Two pairs of populations from Queensland and Tasmania, Australia, were examined. Queensland flies are genetically smaller and develop more slowly than the Tasmanian flies. Survival and body size of flies raised at different temperatures and densities were compared when larvae were challenged with a common competitor. No latitudinal variation in larval survival was detected. Body size (measured as wing length) decreased with increasing temperature and larval density. Flies from the Tasmanian populations were more sensitive to the effects of temperature and density and to the joint effect of increased temperature and density. This could explain the evolution of greater growth efficiency and larger body size at lower temperatures.
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