If One Species Is Split into Two Populations in Two

If one species is split into two populations in two different places, natural selection may drive them to become two different species. How?

A) They are isolated and adapting to two different environments.
B) It won't actually, because one species never splits into two.
C) Because everything evolves over time.
D) A single species is never perfected so it will always eventually branch off.
E) Natural selection makes every population perfectly suited to its environment.