Two populations of birds with somewhat different coloration live on opposite sides of a peninsula. The habitat between 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 each other or with birds from either parent population, and all offspring of these pairings appear intermediate to various degrees.What keeps the two populations separate?A. temporal reproductive isolation B. lack of hybrid viability C. behavioral reproductive isolation D. spatial reproductive isolation

The spatial reproductive isolation can be defined as the reproductive isolation in which populations are separated by great distances, this can also occur when the members of the same population inhabit to different parts of the same area.

According to the given situation, the birds of the two populations have been separated by the peninsula. This is creating a spatial reproductive barrier between the two populations. When the members of the spatially separated populations brought together they will not be able to produce offspring. Due to the fact that the members of the population might have developed traits which get expressed in the progeny. Thus the offspring appears to be the intermediate of the two.