Answer:
A. It allows two populations to evolve separately.
Step-by-step explanation:
Geographic isolation is theorized to have catalyzed the formation of new species. Let’s say groups A and B of a bird species get separated by something, and they can’t cross between to interbreed or exchange alleles.
We describe this as no gene flow, which is the opposite of choices C and D. Because of this, they may diverge if given enough time due to the difference in environmental pressures, because they’re now in different environments.
B is incorrect because it doesn’t apply.