Relative fitness of genetically open and closed experimental populations of Drosophila robusta.

HEN a genetically variable gene pool faces a new environmental challenge, Wmicroevolutionary change may be expected to occur. To understand this change, it would be ideal to know the details of the structure and integration of the gene pool both before and after the change. Although such conditions can hardly be met in any study of natural populations, an approach, at least, can be made in laboratory experiments. In a number of species of Drosophila, genetic control is now such that the artificial construction of various kinds of gene pools presents no particular difficulty. The classical methods for the experimental study of gene pools in the laboratory ( L’H~RITIER, NEEFS and TEISSIER 1937; WRIGHT and DOBZHANSKY 1946; REED and REED 1948; BUZZATI-TRAVERSO 1955) provide an opportunity for the application of stringent natural selection and afford some control of population size. These experimental designs, however, provide no means for comparing the over-all biological performance of one gene pool with another when both are maintained under similar environmental conditions. By providing exact amounts of food and space and exact change intervals to various genetically different experimental populations of D. nelanogaster, CARSON (1958~; 1961), BERT (1960) and SMATHERS (1961) developed methods for making weekly population size measurements under the arbitrarily imposed conditions. CARSON (1961 ) has proposed that population size, measured for comparable gene pools under these conditions, may be used as a measure of the performance of the population, or, in short, its relative population fitness. The gene pool which maintains the larger size is judged the relatively more fit. The present paper concerns itself with the extension of this method to laboratory populations of Drosophila robusta. This species is a common native faunal element of the deciduous forest of the eastern United States, having a large number of different gene arrangements which generally show clinal distributions and a strong tendency towards marginal homozygosity (CARSON 1955, 1958a, 1959). Data are presented in this paper on laboratory populations derived from various geographic sources, both marginal and central. Some of the populations studied are genetically “closed” throughout the course of the experiment. Others have been kept genetically “open” by maintaining a continual inflow of genes; still others have been subjected to mass hybridization. The performance of these various populations and their controls has been compared, and the fate of chromosome polymorphism carried by them has been followed.