The Effect of Heterogeneous Environments and a Competitor on Genetic Variation in Drosophila

Abstract

Thirty-six homokaryotypic laboratory populations of Drosophila pseudoobscura, all of which originated from the same freshly collected population, were maintained for 2 yr. The populations had zero, one, two, or three of the following factors variable: medium, temperature, and/or presence of a competitor, D. persimilis. Twelve, 18, and 24 mo after the populations started, estimates of levels of heterozygosity of each population were made by measuring the amount of electrophoretically detectable genetic variation at nine polymorphic enzyme loci. In going from zero to one factor varying, all three factors increased the level of heterozygosity observed; i.e., populations experiencing two temperatures maintained more genetic variation than populations held at one temperature; populations which had two types of media present had higher levels of genetic variation than populations with only one medium; populations to which D. persimilis were added maintained more genetic variation than when D. pseudoobscura was alone. In going from one to two or two to three variables, increased genetic variation sometimes could and sometimes could not be detected. Estimates of population sizes were made. From these it was concluded that genetic drift cannot account for the pattern observed. Therefore, it is concluded that selection was operating; at least some of the electrophoretically detectable alleles may themselves be parts of the units of selection. It is argued that such diversifying selection is important in maintaining high levels of genic variation in natural population, especially in organisms such as Drosophila.