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An Experimental Study of Interaction between Genetic Drift and Natural Selection

Theodosius Dobzhansky and Olga Pavlovsky
Evolution
Vol. 11, No. 3 (Sep., 1957), pp. 311-319
DOI: 10.2307/2405795
Stable URL: http://www.jstor.org/stable/2405795
Page Count: 9
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An Experimental Study of Interaction between Genetic Drift and Natural Selection
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Abstract

Twenty replicate experimental populations of Drosophila pseudoobscura were kept in a uniform environment for approximately 18 months. The foundation stocks of all the populations consisted of F2 hybrids between flies of Texas origin which had the PP gene arrangement in their third chromosomes, and flies of California origin with the AR gene arrangement in the same chromosome. In ten of the populations the founders numbered 4,000 individuals; the other ten populations descended from only 20 founders each. The frequencies of PP and AR chromosomes in all the populations were originally 50 per cent. Eighteen months later, the frequencies of PP varied from about 20 to 35 per cent in the populations descended from the large numbers of founders, and from 16 to 47 per cent in those descended from small numbers of the founders. The heterogeneity of these frequencies of PP chromosomes observed in the replicate populations is statistically highly significant. More important still, the heterogeneity is significantly greater in the populations descended from small numbers of founders than in those descended from large numbers of founders. Heterozygotes which carry a PP and an AR third chromosome are superior in adaptive value to the PP and AR homozygotes. Therefore, the frequencies of PP and AR chromosomes in the experimental populations are controlled by natural selection. However, the heterogeneity of the results in the replicate populations is conditioned by random genetic drift. Only some of the possible combinations of the genes of the Texas and California genomes are actually realized in the populations. The segments of the gene pool which arise from race hybridization are smaller, and therefore less uniform, in the populations descended from small than in those descended from large numbers of founders. It may reasonably be inferred that evolutionary changes involving interactions of natural selection and random drift of the kind observed in our experiments are not infrequent in nature.

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