A FURTHER STUDY OF CHROMOSOMAL POLYMORPHISM IN DROSOPHILA WILLISTONI IN ITS RELATION TO THE ENVIRONMENT

Abstract

The purpose of the present study has been to test further the working hypothesis that the genetic polymorphism in Mendelian populations enables them to exploit a greater variety of adaptive niches than could be within reach of genetically more uniform populations. The chromosomal variability has been used as an index of the degree of genetic polymorphism in populations of Drosophila willistoni. Samples of natural populations of D. willistoni have been collected in four regions in South America, for which expectations of the degrees of polymorphism to be found in the populations of the species in question were formulated on the basis of the above working hypothesis and of data secured earlier (by da Cunha, Burla, and Dobzhansky, 1950). The expectations were confirmed in three regions, and contradicted in the fourth. The diversity of adaptive niches occupied by D. willistoni in different regions has been evaluated in quasi-quantitative terms, following methods suggested by Dansereau (1952). The values obtained show a positive correlation with the degrees of polymorphism in the populations of the fly (figs. 2 and 3). This correlation would have been more pronounced than it is if it were not for the populations which inhabit the tropical coastal forests in the State of Bahia and adajacent territories. These exceptional populations are chromosomally more uniform than expected in the rich and diversified environments in which they live.