DENSITY-DEPENDENT NATURAL-SELECTION IN DROSOPHILA - EVOLUTION OF GROWTH-RATE AND BODY-SIZE

Drosophila melanogaster populations subjected to extreme larval crowdi ng (CU lines) in our laboratory have evolved higher larval feeding rat es than their corresponding controls (UU lines). It has been suggested that this genetically based behavior may involve an energetic cost, w hich precludes natural selection in a density-regulated population to simultaneously maximize food acquisition and food conversion into biom ass. if true, this stands against some basic predictions of the genera l theory of density-dependent natural selection. Here we investigate t he evolutionary consequences of density-dependent natural selection on growth rare and body size in D. melanogaster. The CU populations show ed a higher growth rate during the postcritical period of larval life than UU populations, but the sustained differences in weight did not t ranslate into the adult stage. The simplest explanation for these find ings (that natural selection in a crowded larval environment favors a faster food acquisition for the individual to attain the same final bo dy size in a shorter period of lime) was tested and rejected by lookin g at the larva-to-adult development times. Larvae of CU populations st arved for different periods of time develop into comparatively smaller adults, suggesting that food seeking behavior in a food depleted envi ronment carries a higher cost to these larvae than to their UU counter parts. The results have important implications for understanding the e volution of body size in natural populations of Drosophila, and stand against some widespread beliefs that body size may represent a comprom ise between the conflicting effects of genetic variation in larval and adult performance.