Question

When the Grand Canyon formed many small mammal population were divided. Some members of the population were on one side and others are on the other, therefore, they were no able to mate with one another. Over time this separation and inability to mate with one another led to the emergence of new species. This type of speciation is called

Answer 1

Allopatric speciation is your answer. 

Answer 2

This type of speciation is called Allopatric Speciation

In an allopatric speciation, a certain species gets divided into two groups and each group live in isolation of other group. This might happen due to any physical barrier such as a mountain range, waterway etc. which makes interbreeding between the two groups impossible. Each species evolve differently as per their habitat requirement and their physical and genetic characteristics