A population of snakes lives in a valley. This population of snakes is 30% black and 70% brown in coloration. A dam is built to flood part of the valley, separating the snakes into two populations. West of the human-made lake, there's a very small population of all black snakes, while east of the lake, the population is much larger and retains the same percentage of black and brown coloration as before the dam was built. What process applies to this example?