Genetics of Natural Populations. Xvi. Altitudinal and Seasonal Changes Produced by Natural Selection in Certain Populations of Drosophila Pseudoobscura and Drosophila Persimilis.

REVIOUS work has shown that populations of Drosophila pseudoobscura which inhabit localities only a few miles apart may differ in the incidence of certain genetic variants. Furthermore, the composition of a population may change with time; the changes are either cyclic, paralleling the annual succession of seasons, or exhibit more sustained trends which may last for several years. Thus, the populations of three localities on Mount San Jacinto, in California, differ very appreciably in the incidence of gene arrangements in the third chromosome. The localities in question lie a t different elevations, but the distances between them are not over 15 miles. In the populations of two of these three localities, the frequencies of some gene arrangements undergo cyclic seasonal changes. In the third locality, no seasonal changes have been recorded; instead, between the years 1939 and 1946, the population has shown a steady increase in the frequency of a certain gene arrangement, and decreases in the frequencies of two other arrangements (DOBZHANSKY 1943, 1947a, b). At least the cyclic changes are caused by natural selection, which favors possessors of certain chromosomal types a t some seasons, and of other types a t other seasons. Indeed, some of the changes which take place in nature can be reproduced experimentally in “population cages.” The experiments disclose that individuals heterozygous for two different gene arrangements possess the highest adaptive values, while the corresponding homozygotes are discriminated against to different extents. The superiority of the heterozygotes results in a balanced polymorphism, which acts as a sensitive mechanism enabling the species to adapt itself very rapidly to variations in the environment (WRIGHT and DOBZHANSKY 1946, DOBZHANSKY 1947a, c). The present article reports observations on altitudinal gradients and on seasonal changes in the frequencies of gene arrangements in the third chromosome of Drosophila pseudoobscura and Drosophila persimilis living in and near Yosemite National Park, in the Sierra Nevada of California. This region lies some 300 miles north of Mount San Jacinto. Experiments on the behavior of the Sierran chromosomes in population cages will be reported in another publication. The purpose of these observations and experiments has been not only to show whether the phenomena previously observed on Mount San