Goats and sheep belong to the same family but different genera. While they often live together in the same pastures, the hybrid offspring that are occasionally produced between the two species rarely survive. When such a hybrid does survive, it is usually sterile. Which of the following best explains the mechanism that maintains reproductive isolation between goats and sheep? a. Gene flow is prevented because the two species belong to different trophic levels and therefore do not share a food source.
b. Habitat isolation creates a prezygotic barrier between the two species.
c. The males of one species and the females of the other species are fertile at different times.
d. The two species have a different number of chromosomes, resulting in a postzygotic barrier.