Response of Drosophila persimilis to phototactic and geotactic selection.

The initial experimental population of Drosophila persimilis was photopositive and slightly geonegative. In this respect, the initial population of D. persimilis differed from the experimental population of D. pseudoobscura, described by others, which was close to photo- and geotactic neutrality. In D. persimilis as in D. pseudoobscura, photo- and geotactic selection was efficient in both positive and negative directions. In D. persimilis, unlike in D. pseudoobscura, the responses to geotactic and phototactic selection were clearly asymmetrical. As a result of such responses, selection practically eliminated interspecific difference in geotaxis and almost eliminated interspecific difference in phototaxis. The realized heritabilities of photo- and geotaxis over the first ten generations of selection in D. persimilis are between 6 and 7 percent. Practically, these heritabilities in D. persimilis have the same value, whereas in D. pseudoobscura the heritabilities are clearly different. These findings suggest that D. persimilis differs considerably from D. pseudoobscura in the composition of the genes determining photo- and geotactic behavior, most probably reflecting adaptations of these sibling species to different ecological niches. However, both species are capable of reacting quickly to external stresses by reorganizing their gene pools and by correspondingly changing their behavior.