The results of the study indicate that a balance in the frequency of the two inversions and the gene arrangements they contain is superior to either inversion by itself. The equilibrium reached in the experiment presumably represents the highest mean fitness in the population under controlled laboratory conditions. This interpretation of the experiment suggests that natural selection is the driving force maintaining the diversity in chromosome 3 inversions.

In a more extensive study, Dobzhansky and his colleagues sampled D. pseudoobscura populations over a broad geographic range. They found 22 different chromosome arrangements in populations from 12 locations. In Figure 26-7, the frequencies of five of these inversions are shown according to geographic location. The differences are largely quantitative, with most populations differing only in the relative frequencies of inversions. Collectively, Dobzhansky's data show that the genetic structure of D. pseudoobscura populations changes from place to place and from one time to another. At least some of this variation in population genetic structure is the result of natural selection.

Figure 26-7. Relative frequencies (percentages) of five chromosomal inversions in D. pseudoobscura in different geographic regions.