When populations of closely related pied flycatchers and collared flycatchers live in different geographic areas, the males of both species look a lot alike. However, when populations of the two species share a common geographic area, there are significant differences in the appearance of the two species. What conclusions might you draw from this situation?
This is a case where hybrid offspring of the two species are less fit than either of the parent species. In the geographic areas where the populations overlap, natural selection has reinforced color differences that serve as reproductive barriers between the two species.