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Genetics of Natural Populations. XXXV. A Progress Report on Genetic Changes in Populations of Drosophila pseudoobscura in the American Southwest

1964; Oxford University Press; Volume: 18; Issue: 2 Linguagem: Inglês

10.2307/2406389

1558-5646

Th. Dobzhansky, W. W. Anderson, O. Pavlovsky, B. Spassky, C. J. Wills,

Insect Utilization and Effects

Some natural populations of Drosophila pseudoobscura undergo genetic alterations from year to year.Samples taken on different years on Mount San Jacinto, in southern California, showed statistically quite significant changes in the relative frequencies of gene arrangements of third chromosomes (Dobzhansky, 194 7; Epling and Lower, 19 57).Changes of a similar kind were observed in the Yosemite Park region of the Sierra Nevada of California (Dobzhansky, 1952(Dobzhansky, , 1956)).The causative agents which bring about these changes are obscure.One of the questions which suggests itself is how strictly local, or how widespread geographically, are the changes.An attempt to answer this question was made in the summer of 1957, when samples of the populations were taken in 21 localities in California, Nevada, Arizona, Utah, and Colorado.The frequencies of the different gene arrangements in these samples were compared with those observed in the same localities in 1940 or thereabouts.The results showed that the populations throughout California changed quite appreciably, and generally in the same direction, while, with some exceptions, the populations outside California have remained fairly stable.Observations were continued at Mather, in the Yosemite region (Dobzhansky, 1963), and on San Jacinto (Epling, unpublished).These observations disclosed new, and rather impressive, genetic changes at the two localities.The question of the territorial spread of these changes must again be faced.During the summer of 1963, we accordingly collected population samples in 18 localities 1 A work supported under Contract No. AT-(30-1)-3096, U. S. Atomic Energy Commission.in California, Nevada, and Arizona.In almost all these localities, the populations were also sampled in 19 57 or earlier.The results of the study and comparison of these samples are reported in the present article. MATERIAL. The collections of the population samples m 1 ?63 were made, so far as possible, in precrsely the same localities and neighborhoods as in 1957 and earlier, or at any rate in the immediate vicinities.A list of the localities, numbered as they are also in table 1 and in figs.1-6, is as follows (for some further data on these localities '