Homeostatic drive counteracting selection for positive and negative phototaxis and geotaxis in Drosophila pseudoobscura.

Tested in Hirsch-Hadler mazes, Drosophila pseudoobscura is on the average geoneutral and photoneutral. Strongly geo- and photopositive and geo- and photonegative populations can be obtained by artificial selection, but upon relaxation of the selection they tend to relapse toward neutrality. This genetic homeostasis is due to natural selection favoring neutrality. Experiments are described in which artificial selection for positivity and for negativity was deliberately made so weak that it only counterbalanced the nautral selection or "homeostatic drive" and the effects of cross-migration. Under these conditions, the behavior of the population artificially selected for positivity diverges only slightly from that of the population selected for negativity, but, at least in females, both populations move closer to neutrality.