Factors Determining Rates of the Evolution of Fitness in Laboratory Populations of DROSOPHILA PSEUDOOBSCURA.

EXPERIMENTS testing the evolution of fitness of genetic material can be performed by evaluating changes in the competitive abilities of laboratory genotypes that have occurred over relatively short periods of time. In Drosophila such experiments have been performed by utilizing tests for success in interspecific competition (MOORE 1952; STRICKBERGER 1963a) or by utilizing competitive tests between phenotypically or chromosomally marked strains of the same spec'es (BUZZATI-TRAVERSO 1955; LEWONTIN 1958; STRICKBERGER 1963a, 1965; SPIESS 1966). The utilization of unique gene arrangements as chromosomal markers in such competitive fitness tests offers the advantage that competing genetic material, through interbreeding, can be placed in what is otherwise the same randomized genetic background. Because of this, ecological differences between the competing chromosomal arrangements probably play a relatively small role, and the tests can be considered essentially confined to the competitive survival of different kinds of genetic material for a common environment. In addition, if the prior evolution of these chromosomal strains has occurred under the same selective conditions in which the tests for competitive success are taking place then the tests can also be considered to measure some of the actual components that are responsible for evolutionary adaptive changes under those selective conditions. Utilizing this approach, previous experiments ( STRICKBERGER 1965) have shown that the opportunity for recombination between genetically different isogenic strains of the Arrowhead third chromosome arrangement (AR) of Drosophila psezdoobscura in population cages enhances the evolution of competitive fitness of this arrangement compared to instances in which such recombination is absent. Also, the same experiments indicate that a single dose of 8,000r of X-ray irradiation does not add either the type or quantity of variability which can be used for the improvement of fitness of these Arrowhead third chromosome arrangements when the irradiated population contains four different isogenic but recombining Arrowhead arrangements. In the present experiments, laboratory cage populations carrying Arrowhead