Abstract
Natural populations of many species of Drosophila are mixtures of chromosomal types which differ in inversions of blocks of genes. This polymorphism is known to be in the main adaptive, and most of the chromosomal variants are maintained in the populations owing to a superiority of heterozygotes over homozygotes (heterosis). The amount of polymorphism is, however, variable from species to species, as well as in populations of the same species. Some species are chromnosomally tiniform, while in Drosophila willistoni, D. paulistorum, and D. subobscura about 40 different gene arangements per species are known (Patterson and Stone, 1952; Dobzhansky, 1951; Mainx, Koske, and Smital, 1953). In some species and populations inversion heterozygotes are rare or absent, while in populations of D. willistoni which inhabit central Goyaz (Brazil) the average number of heterozygous inversions per individual is about 9, and one individual heterozygous for 16 inversions has been encountered (da Cunha, Burla, and Dobzhansky, 1950). The reasons for these spectacular variations in the degree of polymorphism are a challenging problem. Da Cunha, Burla, and Dobzhansky (1950) and Dobzhansky, Burla, and da Cunha (1950) have advanced a working hypothesis, according to wvhich the amount of adaptive polymorphism carried in a population is, other
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