Selection for geotaxis in Drosophila melanogaster: heritability, degree of dominance, and correlated responses to selection.

Selection for geotaxis was carried out with flies from a natural population of Drosophila melanogaster; geotactic behavior was measured by means of a Hirsch classification maze. The population was initally almost neutral to gravity, and it responded to both positive (downward) and negative (upward) selection with a realized heritability of about 0.13. Stabilizing selection toward neutral gravity was carried out simultaneously. At generations 6, 9, and 10, all possibly hybrid crosses between pairs of the selected populations were generated and tested. The geotactic scores of hybrids in generations 6 and 9 were not significantly different from the midparent values, while the scores of hybrids in generation 10 deviated significantly from the midparent values in the direction of positive geotaxis. The frequencies of polymorphic inversions declined in every population during selection, but the population under neutral selection seemed to maintain a higher chromosomal polymorphism than those under positive or negative selection. There was no significant depression of productivity, measured as number of progeny, in any population during nine generations of selection.