The correct answer is speciation.
Speciation is the term that describes the evolutionary process causing populations to evolve into different species. There are four different types of speciation: allopatric, peripatric, parapatric and sympatric. In this case, the process described is an allopatric speciation, the most common of these four types. Allopatric speciation happens when populations are geographically separated, an event that obstructs the gene flow. Since the gene flow stops, the populations evolve in response to the different environments that they live in.
