Answer: The populations become adapted to different environments and eventually become so different that they cannot interbreed to produce fertile offspring.
Step-by-step explanation:
Speciation is a process of development of new species from the parent species. This occurs when the members of the population get separated due to geographical barriers such as river, mountains and others. Or if the members of the same species diverge on the basis of morphological, physiological, reproductive and behavioral changes.
The isolated members in the end in the cases of geographical separation becomes diverge or different species as they cannot interbreed with the members of the parent species due to the adaptations according to the new environment.