RESOURCE PARTITIONING AMONG GENOTYPES OF DROSOPHILA PSEUDOOBSCURA

It is appealing to think that competition among genotypes within a population is essentially like that among species within a community. Important for the community are interspecific resource specialization and partitioning, which contribute to its species richness and stability. It is possible that resource specialization and partitioning among genotypes also contribute to the richness and stability of genetic polymorphisms. Evolutionists have long believed that habitat specialization by the genetic variants in a population maintains genetic variability (e.g., Darwin, 1872; Mayr, 1945; daCunhaetal., 1950). A wide variety of models for such selection in heterogeneous environments has now been studied (see Felsenstein, 1976; Hedrick et al., 1976; and Powell and Taylor, 1979, for reviews). The consequences predicted by these models are diverse and frequently seem in conflict. For example, some models predict that environmental heterogeneity should increase polymorphism (Gillespie, 1976; Taylor, 1976), others that it should have little or no effect (Fisher, 1958; Maynard Smith and Hoekstra, 1980), and others that it should decrease genetic variability (Templeton, 1977). Consequences for genetic loads and net biomass are equally diverse. Just what is expected from environmental diversity rests on the assumptions that go into the model. It has proven difficult to determine which of these models is appropriate for natural populations. This has usually been because genotypes were difficult to identify (e.g., Van Valen, 1965) or because fitnesses in the different microhabitats could not be measured (e.g., Taylor and Powell, 1977). In the study reported below we attempted to construct a model situation of heterogeneous environments in the laboratory; these were small vials that contained two different media. We then tried to determine what set of assumptions were most appropriate for describing competition and natural selection in such an environment. We examined: (1) whether there were genotype x environment (G x E) interactions for fitness components (Do some genotypes fare better in one environment while others do better elsewhere?); (2) whether larvae preferred the microenvironments in which they were more fit or if they moved at random; and (3) if there was resource specialization and partitioning by genotypes within the species. Our observations were then compared to the assumptions used in models of selection in heterogeneous environments. The mathematical models alluded to above all make one or another assumption that affects the conclusions in important ways and that is inconsistent with our laboratory measurements. It therefore seemed appropriate to develop a model of natural selection in such a system. This is described and some of its consequences deduced. It leads to the conclusions that: (1) there is no necessary relation between genetic variation and environmental diversity; (2) genetic loads are not necessarily less in heterogeneous environments; (3) additional genetic variation does not necessarily lead to greater mean fitness (i.e., there is no "genetic facilitation" or "overcompensation"); but (4) all else being equal,

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