EXPERIMENTAL POPULATIONS OFDROSOPHILA PERSI-MILISFROM AN ALTITUDINAL TRANSECT OF THE SIERRA NEVADA

Abstract

Populations of Drosophila pseudoobscura and D. persimilis throughout their respective distribution areas in western North America exhibit a very extensive and intricate pattern of gradients, or clines, in relative frequencies of the gene arrangements of the third chromosome -(Dobzhansky and Epling, 1944). These gradients may be very gradual over a wide area from north to south or west to east, or they may be rather steep in slope across an altitudinal transect of relatively short distance in a mountain range (Dobzhansky, 1948a). Within certain populations of D. pseudoobscura temporal fluctuations of less amplitude than the more extensive geographic differences have been observed (Dobzhansky, 1943, 1947a) while in some other populations of the same species long term trends were found (Dobzhansky, 1947b). It was by studying the behavior of these chromosomal types in artificial populations (Dobzhansky, 1947a, Wright and Dobzhansky, 1946) and later by statistical analysis of wild populations (Dobzhansky and Levene, 1948) that these gene arrangements were proved to have selective values and were shown to be-maintained at the observed frequencies by natural selection under both wild and artificial conditions. This information together with evidence of physiological differences between carriers of each arrangement -(Heuts, 1947b, 1948) leaves no doubt as to the importance of variation