Competitive fitness in experimental populations of Drosophila willistoni

Abstract

Experiments are reported in which genetically different strains of Drosophila willistoni compete with D. pseudoobscura. The competition was studied at three temperatures, 20°, 22°, and 25°C. The outcome of the competition depends on the genetic constitution of the competing species, but at 25° and 22°C D. willistoni flies are generally stronger competitors than D. pseudoobscura, while at 20°C D. pseudoobscura generally has a competitive advantage. There is a significant interaction between genotype and temperature; the strain RP3 is the weakest competitor of all D. willistoni strains at 22° and 25°C, but not at 20°C; the strain M18 is the best competitor at 20° and 22°C but not at 25°C.

The performance of the four strains of D. willistoni was measured in two more ways. First we estimated their Darwinian fitness relative to other genotypes of the same species. Second, we measured the average population size of each strain in pure culture. There is no significant correlation between population size in pure culture and either competitive fitness or Darwinian fitness. There is, however, a strong positive correlation between Darwinian fitness and interspecific competitive fitness.

It is pointed out that natural selection leads to an increase in the average Darwinian fitness of a population but not necessarily to an increase in its adaptedness to the environment. Yet the synthetic theory of evolution assumes that the genes and genotypes favored by natural selection are usually those which increase the adaptedness of their carriers to the environments where they live. The correlation between Darwinian fitness and adaptedness needs to be studied experimentally.