Explain how two populations of organisms may develop into two separate species.

The first thing to identify here is that this is a question about evolution. Students should look to give a comprehensive explanation of how organisms evolve via natural selection, while framing the answer within the context of two populations that are evolving to become sufficiently different from one another to be considered separate species.
For example:In order for two populations to undergo speciation (technical term not included in the question title, able students should be able to link the question to this subject) and become different species, they must reach reproductive isolation. Reproductive isolation is where the populations are unable to successfully breed with each other and produce fertile offspring, therefore satisfying the 'biological species concept' definition of two different species. Populations can become isolated from one another via either physical (allopatric) or (sympatric) ecological barriers. Once isolated, these populations will be subjected to different selection pressures. Genetic mutations will occur in each population, generating more variation between individuals, and natural selection will favour those mutations that convey a selective advantage, thus allowing individuals with such mutations to survive and breed to pass the mutated genes into the next generation. Each population will accumulate a different set of advantageous mutations (provided the two environments they now occupy differ enough) that eventually mean that their phenotypes are sufficiently different, preventing them from interbreeding. They have now become two different species.