Explanation:
A species becomes separated as a result of a geographical situation – earthquake, continental drift, etc.
The two species each have different variations or mutations which may or may not be suited to their new environment.
Natural selection occurs; the ones who have the most suited genes pass them on.
Over time, two different species evolve.
An example: Hawaii was once populated by one species of finch-like birds. The species was separated onto different islands and developed different feeding methods to survive. As such, new species were developed.