Two populations of birds with somewhat different coloration live on opposite sides of a peninsula. The habitatbetween the populations is not suitable for these birds. When birds from the two populations are brought together, they produce young whose appearance is intermediate between the two parents. These offspring will breed with eachother or with birds from either parent population, and all offspring of these pairings appear intermediate to variousdegrees. What keeps the two populations separate?

A. temporal reproductive isolation
B. lack of hybrid viability
C. behavioral reproductive isolation
D. spatial reproductive isolation—the only reason these two species don’tmate is because of the inability of the middle land to be occupied