Western and Eastern meadowlarks look very similar and it is difficult to tell them apart, until they sing. Their mating songs are quite different. The Western meadowlark has a melodious song, whereas the song of the Eastern meadowlark sounds more like a whistle. Even though their ranges overlap in the central United States, they do not interbreed and are two different species. How would speciation most likely have occurred in these two species of meadowlarks?