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Variation in Genetic Response to Interspecific Competition in Laboratory Populations of Drosophila

Populations of Drosophila melanogaster begun as samples of varying sizes from relatively enriched and impoverished gene pools were exposed to competition for 10 generations with D. simulans. The data are relevant to questions relating to the probability of a sufficiently rapid genetic change in competitive properties to slow the process of competitive exclusion, the kinds of changes in competitive properties expected to occur, and the nature of the genetic variance for traits affecting the outcome of competition. In only 10 of 28 comparisons of control populations with those exposed to competition was evidence obtained of divergence with respect to the proportion of D. melanogaster emerging from a crowded mixed brood, or with respect to the total number of flies or the biomass of a mixed brood. An apparent variety of responses was observed, explanation of which appears to require invoking qualitatively different changes in different populations. The observations are consistent with the hypothesis that the genetic variance for traits affecting the outcome of competition is non-additive in large part, which suggests that some ecologically important characters may be subject to genetic homeostasis that prevents rapid response to environmental change. The relevance of these observations to the dynamics of changes in community composition is discussed.