Effects of selection and migration on geotactic and phototactic behaviour of Drosophila . II

Abstract

Genetic effects of selection and migration have been studied in populations of Drosophila pseudoobscura kept for twenty generations in plastic population cages. The experimental procedure can be seen in figure 1. Four donor populations are selected for positive or for negative phototaxis or geotaxis. The donor populations yield also migrants, which are selected for phenotypes opposite in sign to the selection in the donor populations themselves. The recipient populations are perpetuated by selecting parents with phenotypes close to the average in the respective populations, and adding to them the migrants from the donor populations. The migration, is, thus, unidirectional-from the donors to the recipient populations. The donor populations have, as expected, responded to the directional selection by becoming photo- or geopositive or negative. The recipient populations showed little change for several generations, but eventually changed in the same directions as did the donors. This result seems at first sight paradoxical, because the migrants were selected for phenotypes opposite in sign to the selection in the donor populations. It is shown, however, that the result is explicable when the characteristics concerned have very low heritabilities. The migrants came from genetically improving populations, meaning by 'improvement' simply that these populations were changing in the direction for which they were being selected. Such migrants may transfer genetic improvements even if their own phenotypes do not manifest them.