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. This is an example of

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The separation of the original snake population by the dam is an example of genetic drift, where a random event (in this case, the construction of the dam) results in a significant change in the genetic makeup of the population. The small population of all black snakes to the west represents a genetic bottleneck, as only a small portion of the original population contributed to the gene pool of that subgroup. Additionally, the larger population to the east still maintains the same percentage of black and brown coloration as before, suggesting that there was no significant selective pressure for one color over the other.

A Population of Snakes Lives in a Valley