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Th. Dobzhansky, W. W. Anderson, O. Pavlovsky, Genetics of Natural Populations. XXXVIII. Continuity and Change in Populations of Drosophila pseudoobscura in Western United States, Evolution, Vol. 20, No. 3 (Sep., 1966), pp. 418-427
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Dobzhansky et al. (1966) studied this question.
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