Abstract
Photo-preferences of the sibling species, Drosophila simulans and D. melanogaster, were studied using an apparatus and population cage having a gradient of light intensities. D. simulans preferred to stay and to lay eggs at light places whereas D. melanogaster did not show such a strong light preference. Selections of eggs in the light gradient cages made it possible to segregate a mixed species population into different species populations; selection for photo-positive flies soon eliminated D. melanogaster and slection for photo-negative or photo-neutral flies finally eliminated D. simulans. The difference of the photo-preference seems to be a cause of coexistence of the sibling species in nature.
