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Genetics of Natural Populations. XXV. Genetic Changes in Populations of Drosophila pseudoobscura and Drosophila persimilis in Some Localities in California

Theodosius Dobzhansky
Evolution
Vol. 10, No. 1 (Mar., 1956), pp. 82-92
DOI: 10.2307/2406099
Stable URL: http://www.jstor.org/stable/2406099
Page Count: 11
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Genetics of Natural Populations. XXV. Genetic Changes in Populations of Drosophila pseudoobscura and Drosophila persimilis in Some Localities in California
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Abstract

Populations of Drosophila pseudoobscura in the Yosemite region of the Sierra Nevada of California show seasonal changes in the relative frequencies of ST and AR chromosomes. During the summer of 1954 significant changes in the frequencies of certain chromosomal types have been observed also in the population of D. persimilis in the same region, although such seasonal changes were not observed in this species in previous years. During the period of observation (1945-1954 with 4 years missing), the frequencies of AR chromosomes first increased and later decreased from year to year in D. pseudoobscura, while ST chromosomes showed the opposite change. In D. persimilis during the same period WT chromosomes first increased and then decreased in frequency, KL chromosomes showing the opposite change. These changes may have been produced by climatic fluctuations, namely by a succession of dry and wet years. This tentative explanation is not regarded as proven. During the same period of frequency of chromosomes with PP gene arrangement rose in D. pseudoobscura populations of the Yosemite region from an ostensible zero per cent in 1945 to 2.8 per cent in 1950 and to 11 per cent in 1954. The gradual rise of PP was at the expense chiefly of CH gene arrangement. The causes which brought about this spectacular increase of the incidence of a genetic variant in the population both in the Yosemite region, and also on Mount San Jacinto, farther to the south, are unknown. Arguments are presented which show that the phenomenon cannot be explained by mutational origin or by introduction of PP chromosomes into California from elsewhere.

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