Two frog populations (same species) living in two neighboring lakes sing slightly different courtship songs. Increased irrigation makes the land between the two lakes wetter, allowing frogs to expand their ranges to the area between the lakes. Females in both populations prefer loud frogs to quieter frogs but do not distinguish between the two slightly different songs. Assuming that courtship song differences have a genetic basis, predict what will likely happen to the songs of the two frog populations.

A. You cannot predict a change in the courtship songs at the two lakes.
B. Disruptive selection will cause the songs to differentiate even more.
C. The songs will become more similar to each other.
D. Males will become louder.
E. Genetic drift will cause the songs to differentiate even more.

To answer this question, first, you need to remember that both populations belong to the same species, so they can mate because there is no reproductive barrier. The only limiting physical factor impeding individuals of both groups to cross was the piece of land before the land got wetter. So once populations got together, individuals can easily mate.

One population´s males exhibit a courtship song that slightly differs from the one of the other group. This difference has a genetic basis. But females hardly notice the difference. All of them prefer loud frogs to quieter frogs, independently of the population to which they belong.

So when the two populations get together, individuals get mixed. Females do not recognize males of their group, and during the breeding season, all females will mate the most with louder males of any of the population. So females from population A might mate with males from population B, and females from population B might mate with males from population A. So after mating season, there is also a genetic mixture between populations.

The new generations will carry elements of both genotypes. So males´ songs will be even more similar to each other. Songs will become more similar to each other because individuals of both populations are mixed and mated, and their genes intercrossed, producing new individuals with genes of both populations.