Elastic response of laboratory populations on artificial selection for positive and negative phototaxis. A population of Drosophila pseudoobscura was divided into two parts, one of which was subjected to long-term selection for positive phototaxis - movement towards light in a Y-maze (broken line) and the second to negative phototaxis - movement into darkness (solid line). During twenty generations, the population diverged substantially in this trait. After termination of selection, the average phenotypes in both populations returned to the original value during the subsequent twenty generations. Data from Dobzhansky and Spassky.