Enriched Pools Research Articles

RAPID EVOLUTION OF COMPETITIVE ABILITY IN LARVAL MIXTURES OF THE HOUSEFLY.

The purpose of this paper is to establish whether natural selection in larval mixtures of two strains of the housefly, Musca domestica L., can alter fitness (as measured by developmental viability, developmental time from egg to adult, and dry weight of emerged adults) of one or both of the competing types within a few generations. Previous studies addressing this same question on a variety of organisms have yielded variable results. Moore (1952), Seaton and Antonovics (1967), Ayala (1969) and Van Delden (1970) found significant improvements in fitness in mixed populations of Drosophila. Similarly, Harper (1968) found that later growth stages of two plant species showed improved weights in mixtures over pure stands. However, Park and Lloyd, (1955) and Sokal et al. (1970) found no responses in competitive ability in mixtures of species of Tribolium and of strains of Tribolium and Musca, respectively. Futuyma (1970) found changes in 10 of 28 experimental lines of Drosophila melanogaster; half of these showed improvements and the remaining showed decreases. Since the responses were indeterminate Futuyma concluded that variance for traits affecting the outcome of competition is nonadditive in large part and thus, changes in fitness shohl be slow and unnredictable. Discrepancies among these separate studies could have resulted from (1) differences in the initial genetic variability among the competing population in different studies. (2) Multiple fitness strategies in different species (and strains) that may have resulted in little additive genetic variance remaining in some cases (Levins, 1965). (3) Differences in the experimental designs among studies. The only attempt to separate these possibilities was Futuyma's. He found no differences in responses of lines from relatively enriched and impoverished gene pools, arguing against possibility (1). The slow rates of changes in those lines that did respond to selection in his study seemed to indicate nonadditive genetic effects, however this does not eliminate (2) since a large variability in response occurred, which would be consonant with this hypothesis or that of Futuyma. A posteriori, it would be difficult to assess (2) for the various studies; therefore, we were led operationally to (3). The principal difference between our earlier study (Sokal et al., 1970), that of Futuyma (1970) and the successful one of Seaton and Antonovics (1967) lay in a head-start given to the less fit mutant strain in the latter study. One of us (Bryant, 1971) recently studied the effect

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.