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

Abstract

Homokaryotypic laboratory populations [36] of D. pseudoobscura, all of which originated from the same freshly collected population, were maintained for 2 yr. The populations had 0, 1, 2 or 3 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 9 polymorphic enzyme loci. In going from 0 to 1 factor varying, all 3 factors increased the level of heterozygosity observed; i.e., populations experiencing 2 temperatures maintained more genetic variation than populations held at 1 temperature; populations which had 2 types of media present had higher levels of genetic variation than populations with only 1 medium; populations to which D. persimilis were added maintained more genetic variation than when D. pseudoobscura was alone. In going from 1 to 2 or 2 to 3 variables, increased genetic variation sometimes could and sometimes could not be detected. Population sizes were estimated. Genetic drift evidently cannot account for the pattern observed. Selection was apparently operating; at least some of the electrophoretically detectable alleles may themselves be parts of the units of selection. Such diversifying selection is apparently important in maintaining high levels of genic variation in natural population, especially in organisms such as Drosophila.