Speciation is gradual as isolated populations diverge. Populations may still be theoretically able to interbreed but cannot meet, so the boundaries between species can be partly arbitrary.
Evolution Canyon is a steep sided rocky valley near Mount Carmel in Israel. There are populations of fruit flies (Drosophila melanogaster) living on the north facing and south facing slopes of the canyon. Fruit flies are sensitive to drought conditions and die if they become desiccated.
Twenty samples of fruit flies were collected from both sides of the canyon. They were subjected to the same conditions of drought stress and the life span (in hours) of the adult flies was measured.
Discuss whether the populations on the two slopes could evolve into two species.