My question comes from the Barron's SAT Subject Test Practice Test #1 question 2
Two populations of rhododendron, R. ferrugineum, grow in the same region of Connecticut. Although rhododendrons are able to cross-pollinate, these two population never hybridize. Which of the following is the most likely reason for this phenomenon?
A) Genetic drift has caused one populatoin to be betteer adapted than the other.
B) Self pollination is more advantageous because it increases variation in the gene pool.
C) One population produces pollen in early June when the pistils of the other population are not ready to receive pollen.
D)The two populations demonstrate convergence of two separate species over time.
E) The two populations fill different niches
The Barron's book says that the correct answer is C. Why? If the it's the same species, shouldn't they produce pollen at the same time?