SELECTION FOR GEOTAXIS IN MONOMORPHIC AND POLYMORPHIC POPULATIONS OF DROSOPHILA PSEUDOOBSCURA

Abstract

Using a classification maze constructed by Professor Jerry Hirsch, populations of Drosophila pseudoobscura were selected for a positive and a negative geotactic behavior. The initial populations were respectively mono-morphic for AR, monomorphic for CH, and polymorphic for AR and CH gene arrangements in their 3rd chromosomes. All the initial populations were about neutral to gravity, yet all responded both to plus and to minus selection. The plus selection favored AR chromosomes, while the negative selection favored the AR/CH heterokaryotype. Relaxation of the selection resulted in a partial relapse towards the original state. Reversal of the selection gave an almost complete return to the original state. In the polymorphic AR/CH population the back-selection favored an AR chromosome which was lethal in double dose.