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A comparative study of chromosomal and enzyme polymorphisms in Drosophila pseudoobscura and in D. persimilis was conducted from April 1971 to October 1972 by taking samples at monthly intervals in two different localities. Seasonal cyclic changes occur in the chromosomal polymorphisms. Significant changes occur in the frequencies of the enzyme polymorphisms; preliminary evidence indicates that some of these changes may also be cyclic. Natural selection is the main process responsible for the changes in both kinds of polymorphisms.
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Dobzhansky et al. (1973) studied this question.
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