My question comes from the Barron's SAT Subject Test Practice Test #1 question 2

It states:

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?


1 Answer 1


Yes, they can. Consider an original population that produces pollen over a fairly long time, and then something happens to select earlier & later times for different populations. This is actually quite common for domestic plants such as fruit trees, where different varieties are selected (by humans) to produce fruit at different times.

As far as the question itself goes, it's more a case of the other answers being either obviously wrong, or not supported by the information in the question.

  • $\begingroup$ You are obviously right (+1). I do however find the answer peculiar, because I doubt how often this occurs in the wild. A much more logical explanation would be that the two populations are seperated by a mountain range or a desert (and afterwards the pollen time started to grow apart). $\endgroup$
    – RHA
    May 4, 2019 at 8:49
  • $\begingroup$ @RHA: I don't know how often it would happen naturally, though logically it could be caused by a random mutation, and like anything that produces reproductive isolation, would be the start of the two populations eventually evolving into different species. $\endgroup$
    – jamesqf
    May 4, 2019 at 15:57
  • $\begingroup$ given the number of imported or specially bred rhododendrons have two populations end up with different seasonal triggers is not unbelievable. Nowhere does it say they are native species. $\endgroup$
    – John
    May 4, 2019 at 16:39
  • $\begingroup$ @John: Good point! You got me curious. I don't recall rhododendrons as being native to the northeast, though apparently there are some. However, the R. ferrugineum of the question is native to Europe, so ones in Connecticut would almost certainly be garden varieties: en.wikipedia.org/wiki/Rhododendron_ferrugineum So now we can pretty well assume that the populations were selected for different bloom times by humans :-) $\endgroup$
    – jamesqf
    May 5, 2019 at 17:39
  • 1
    $\begingroup$ There are species native to the US appalachia en.wikipedia.org/wiki/Rhododendron_maximum but yes R. ferrugineum is invasive. Because of that the two populations could have two different origin events. $\endgroup$
    – John
    May 5, 2019 at 21:20

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