SELECTION AND INVERSION POLYMORPHISM IN EXPERIMENTAL POPULATIONS OF DROSOPHILA PSEUDOOBSCURA INITIATED WITH THE CHROMOSOMAL CONSTITUTIONS OF NATURAL POPULATIONS

Natural populations of Drosophila pseudoobscura, with an area of distribution ranging from British Columbia to Guatemala and the highlands of Colombia, are widely polymorphic for structural rearrangements of the third chromosome. Sturtevant and Dobzhansky (1936) first showed how the different gene arrangements could be related by a stepwise process of inversion from one ancestral chromosome pattern. Thus a "phylogeny" of chromosomal types was constructed (Dobzhansky and Sturtevant, 1938). The maintenance of the inversion polymorphisms by natural selection was inferred from the seasonal cyclic changes which occur in some localities (Dobzhansky, 1943), and was confirmed in studies of experimental populations maintained in the laboratory (Wright and Dobzhansky, 1946). The chromosome frequencies are delicately balanced, shifting with changes in the environment and with changes in the genetic composition of the populations. There have been long term (of the order of years) changes in chromosomal frequencies in some parts of the distribution area of the species, changes which show a pattern over thousands of miles of habitat (see Dobzhansky, Anderson, and Pavlovsky, 1966 and references therein). Pavlovsky and Dobzhansky (1966) recently studied experimental populations containing gene arrangements from a locality in California; gene arrangements from this same locality had been used to establish similar experimental populations twenty years previously. The different courses of selection in the two sets of populations, observed twenty years apart, reflect in a broad way the changes in the gene arrangements in nature during the time between the samples. The process of selection in experimental populations with chromosomes from a single locality is repeatable. For instance, four replicate populations were set up, carrying the ST and CH gene arrangements from California (Dobzhansky and Pavlovsky, 1953). Seven samples were taken over a period slightly longer than a year, at which time the gene arrangements had approached the equilibrium frequencies. There was no statistically significant heterogeneity among the samples from the replicate populations. But experimental populations initiated with chromosomes of mixed geographic origin behave quite differently. The course of selection is not predictable, and replicate populations may diverge widely. This led Dobzhansky (1949) to conclude that the gene contents of the chromosomes within each population are coadapted, i.e., mutually adjusted by natural selection. There are broad patterns of distribution of the third chromosome gene arrangements over the territory inhabited by Drosophila pseudoobscura (Dobzhansky, 1944). With the demonstration of the selective differences among the various gene arrangements, it became probable that the chromosomal races are products of natural selection. This selection occurs over large areas, and is on a geographically greater scale than the coadaptation within local populations. 1 Present address: Department of Biology, Yale University, New Haven, Connecticut. 2 Present address: Department of Anatomy, The University of Texas, Southwestern Medical School, Dallas, Texas.