Which of the following would NOT characterize allopatric selection?

I'm not sure what the answer choices are, exactly, but looking up the question I found several other questions with answers, so I'm guessing based on those the choices would be:

1) physical barrier within population

2) extensive gene flow

3) chromosomal changes within population.

Which, in this case, the answer would most likely be 3) chromosomal changes within population.