Studies in the Dynamics of Genetically Variable Populations. I. Frequency- and Density-Dependent Selection in Experimental Populations of DROSOPHILA MELANOGASTER.

Changes in genetic composition and in fitness of karyotypes in synthetic populations of Drosophila melanogaster carrying fourth chromosomes marked with recessive mutants in repulsion were measured as functions of initial population composition and density. There was no detectable influence of initial population density on the magnitude of chromosome frequency change or on adaptive values of karyotypes, but both measures proved sensitive to population composition. That fitness be measured zygote-to-zygote is argued to be of dubious necessity in all contexts except demonstration of frequency-dependent selection. Some prior reports cited as evidence of the operation of density-dependent selection, like the present study, contain no evidence that the rate of natural selection is influenced by population crowding. Reports that do demonstrate density-dependent selection suggest that the relationship between fitness and crowding differs from that assumed in most theoretical treatments. Further work is needed to elucidate the interaction between fitness, population crowding, and population composition.