CHROMOSOMAL FITNESS CHANGES IN EXPERIMENTAL POPULATIONS OFDROSOPHILA PERSIMILISFROM TIMBERLINE. II. RE-EXTRACTED CHROMOSOMES

Abstract

Adjustment of a population's gene pool to maintain maximum fitness is to be expected following changes either in selective forces, in genetic variability, or in both. New genetic combinations may evolve when genic complexes are derived from unrelated genomes as in laboratory populations of Drosophila with chromosomes of diverse origins (mesoevolution of Dobzhansky, 1954; Dobzhansky and Pavlovsky, 1957; Dobzhansky and Spassky, 1962). But the fact that no laboratory population is static and fixed in its genetic composition (except an isogenic strain) is demonstrated by the evolution of new adaptive complexes when natural selection has an opportunity to act on any genetic variation which has not yet attained equilibrium, even when that variation is a sample from a single welladapted natural population (Buzzati-Traverso, 1955; Lewontin, 1958; Strickberger, 1963). These experiments on laboratory populations derived from natural populations serve to illustrate clearly the dynamic poise of those populations and the rapid deterministic nature of the selective process in establishing new genetic equilibria. On the basis of laboratory population results from earlier work, Spiess (1961) described the outcome of gene arrangement frequency changes with strains of Drosophila persimilis from Timberline (10,000 feet elevation in the Yosemite area) as unexpected responses indicating the extreme novelty of population cage conditions for Timberline genotypes compared with the more expected responses of flies