EXTINCTION DUE TO EVOLUTION OF A COMPETITOR

When Drosophila simulans and Drosophila funebris were cultured together in population cages over many generations, there was a prolonged period of apparently stable coexistence followed by a rapid exclusion of D. funebris. As both species maintained large population sizes in monocultures it follows that the extinctions of D. funebris in the mixed-species cultures must have been caused by D. simulans. The time to extinction of D. funebris ranged from 26 weeks in one cage, to between 40 and 48 weeks in the other five. To test the idea that an evolutionary increase in competitive ability of D. simulans had occurred during the course of its interaction with D. funebris, a single-generation experiment was set up. In this experiment the interspecific competitive ability of a population of D. simulans that had been in competition with D. funebris for 44 weeks was compared to that of a stock population that had had no previous contact with D. funebris. In this experiment both stock and precompeted populations of D. simulans increased the egg-to-adult development time of D. funebris. However, precompeted D. simulans caused a significantly greater increase in the development time of D. funebris than did stock D. simulans. Thus D. simulans had evolved an increase in competitive ability as a result of its interaction with D. funebris. Development time is important because in the population cages the resource bottles-in which the larvae reside-were replaced every three weeks. An increase in development time of D. funebris in the multigeneration experiment similar to that observed in the single-generation experiment would lead to a rapid decrease in adult population size, resulting in the extinction of this species, as was observed to happen.