Drosophila willistoni is the commonest and most wide-spread of the four sibling species of the willistoni group in Brazil; D. paulistorum is less, and D. tropicalis and D. equinoxialis are least common and widespread. In D. willistoni 40 different inversions have been recorded, compared to 34 in D. paulistorum, and 4 each in D. tropicalis and D. equinoxialis. In D. willistoni, the mean numbers of heterozygous inversions per individual vary in different populations from 0.8 to between 9 and 10 (in females); in D. paulistorum from 0.6 to 1.8; in D. tropicalis 0.14 and in D. equinoxialis 0.11. These facts confirm the hypothesis, advanced on the basis of comparative studies of different populations of D. willistoni, that the chromosomal polymorphism present in a species or a population is positively correlated with the diversity of habitats occupied or exploited.
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Burla et al. (1950) studied this question.