Theodosius Dobzhansky and Boris Spassky demonstrated the working of normalising selectioion a behavioural trait in two populations of Drosophida pseudobscura. The two populatiors were subject to artificial selection. One population was selected for positive phofobctic behaviour and the other negative phototactic behaviour Flies were placed in a container from where they could choose to go towards light or darkness. The photopositive flies or those that moved towards light were collected and bred, as also tljie ones which exhibited photonegative (movement away from light) behaviour. The brezding experiments were repeated for several generations and for each generation the artifical selection was maintained. At the end of several generations of artificial selection, it was observed that two distinct populations resulted, one increasingly photopositive and the other increasingly photonegative.