Final answer:
Habitat isolation occurs when a population moves to a new area without other populations of the same species, potentially leading to allopatric speciation. Therefore, the correct option is A.
Step-by-step explanation:
The phenomenon that occurs when one population moves to an area that does not contain other populations of the same species is known as habitat isolation. This form of isolation can lead to allopatric speciation, where the separated population starts to evolve independently from the parent species due to different environmental pressures, genetic drift, and mutations. An example of habitat isolation would be a cricket population that gets divided after a flood, preventing the two groups from interacting and eventually causing them to diverge into two separate species.