Fitness and Adaptedness of Polymorphic and Monomorphic Populations of Drosophila melanogaster

Abstract

X chromosomes containing the mutant, lozenge 63i, and Muller-5 X chromosomes each behave as single units and establish a stable equilibrium in experimental populations because heterozygous females have a strong advantage. I have attempted to compare fitness and adaptedness of monomorphic and polymorphic populations in the lozenge-Muller-5 system. The average fitness of populations was calculated on the basis of estimates of viability and fertility of all genotypes, while adaptedness was measured on the basis of the populations' abilities to maintain certain sizes, to expand, and to compete with another species. Introduction of the above heterotic system into the monomorphic M-5 populations increased the the average fitness of these populations almost twofold. Corresponding increase in adaptedness was observed only when adaptedness was measured by the abilities of the populations to expand under uncrowded conditions and by their abilities to compete with D. simulans. In monospecific D. melanogaster populations expansion ability of a polymorphic populations decreased more rapidly with the increase in density than that of the monomorphic ones. The polymorphic populations failed also to maintain higher population sizes than the monomorphic populations. Consequently, the relationship between Darwinian fitness and adaptedness is not direct and depends on the ecological conditions under which a given population exists.