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Although the major morphological polymorphism is rare in the genus Drosophila, the chromosomal polymorphism is very common. The chromosomal variability is mainly due to inverted sections in the chromosomes. The degree of chromosomal variability differs in different species of Drosophila and within the species in different populations. It has been shown by several authors that inversion polymorphism is adaptive (for review see da Cunha 1960). Dobzhansky et al. (1950) and da Cunha et al. (1950) studied the natural populations of Drosophila willistoni in Brazil and reported that the mean number of heterozygous inversions per individual varies in different populations. To account for such findings they advanced the working hypothesis that the popu lations which exploit a greater variety of ecological niches in the territory which they inhabit, are more polymorphic than the populations restricted to a narrow range of ecological opportunities. This hypothesis has been tested further by several workers and the results obtained tend to support it (da Cunha et al. 1953, da Cunha and Dobzhansky 1954, Carson 1955, Salzano 1955, da Cunha et al. 1959). There are, however, a number of exceptions to the above ecological niche hypothesis. A prominent exception is the presence of a very low degree of chromosomal polymorphism in most of the cosmopolitan species of the genus Drosophila (Carson 1955). Drosophila ananassae, although a cosmopolitan and domestic species, shows a high degree of chromosomal polymorphism (Kaufmann 1936, Kikkawa 1938, Dobzhansky and Dreyfus 1943, Shirai and Moriwaki 1952, Freire-Maia 1955, 1961, Seecof 1957, Futch 1966, Ray-Chaudhuri and Jha 1966).

Quantitative Variation of Chromosomal Polymorphism in Natural Populations of <i>Drosophila ananassae</i>