ADAPTATION AND SPECIALIZATION IN A 2-RESOURCE ENVIRONMENT IN DROSOPHILA SPECIES

We assayed two components of performance (development time and survivo rship), on food medium with and without ethanol, in laboratory populat ions of Drosophila simulans and D. melanogaster for which ethanol-medi um was a novel food resource. These assays were done before and after 12 generations of rearing in either one-(regular medium only) or two-r esource (regular medium and ethanol medium) environments. Initially, D . simulans was highly susceptible to ethanol, whereas D. melanogaster was relatively unaffected. After 12 generations in the two-resource en vironment, D. simulans showed significantly improved mean performance on ethanol medium; mean performance of D. melanogaster did not signifi cantly change. Variation among families for both traits was higher on ethanol medium in D. simulans. Variation in D. melangaster was not sig nificantly affected by ethanol level, suggesting that resource quality was more important than novelty per se. In both species, the least va riation was seen in populations after 12 generations in the two-resour ce environment. For development time in D. simulans, the decrease in v ariation was largely due to reduced variation within families, suggest ing the evolution of canalization. Development time on the two media w as not negatively correlated. In D. simulans, correlations measured be fore and after the experiment were not heterogeneous, suggesting that trade-offs in performance did not block diet expansion. In D. melangas ter, correlations became significantly less positive after 12 generati ons in the two-resource environment, supporting the view that correlat ions between performance on different resources may become less positi ve over time through selection.