Why does geographic isolation cause speciation?

A. It allows two populations to evolve separately

B. It causes adaptive radiation

C. It increases gene flow between populations

D. It increases interbreeding between populations

A. Is correct. It allows two populations to evolve separately

A. It allows two populations to evolve separately.

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.