A certain species of frog comes in two different colors, green and brown. A population of these frogs live in and along a river. In 1980, a very large drought occurred, causing most of the river to dry up. Most of the plant life around the river died as well. Why did the green frog population decrease after the drought?

After the drought, there was very little water and many plants died, so the remaining environment was mostly brown. Because of this, having brown coloration was a favorable adaptation for frogs. The brown frogs are the ones that survived and reproduced, causing the number of brown frogs to increase. Having green coloration, on the other hand, was an unfavorable adaptation because they were unable to blend in with the surrounding environment. They became less likely to survive and reproduce, causing the number of green frogs to decrease.

The green frog's population decreases because they cannot blend as easily into the mostly brown environment of the dry river bed as their brown counter parts. There are not as many green plants for them to hide against, making it easier for predators to see them.