Speciation

Speciation occurs when a group within a species separates from other members of its species and develops its own unique characteristics.

The demands of a different environment or the characteristics of the members of the new group will differentiate the new species from their ancestors.

For example in Allopatric speciation (aka geographic speciation), populations that are completely separated by a physical barrier, such as a mountain range, river, or desert, resulting in the development of new species. 

Geographic isolation has led to the evolution of two different species of African elephants.

The separated populations adapt to their own unique environments, becoming so genetically different from one another that members of separated populations cannot breed with members of the other.