What happens when two populations of the same species are separated by distance dor a long period of time?

they diversify and adapt to their new environment

Explanation:

when a species separates they adapt to their new environment depending on what the species needs to survive. for example reference Darwin's studies on the Galapagos islands, the same species of a bird were spread out across different islands that had different habitats which forced the bird to adapt to the new conditions.