My teacher says I'm wrong... WHY!? (Isolation of Species)?

Divergence only occurs when populations are reproductively isolated, meaning they don't mate with members of another population. If they are mating between populations regularly, then the allele frequency of both populations will tend to remain similar. That's not to say that the overall allele frequency of both populations won't change. It will, but they will change together as long as they continue to interbreed.