How does the geographic insolation affect speciation? provide an example

Hola!

Geographic Isolation prevents the cross breeding between the populations that have been separated by the isolation.

Without the separation the two growing population would be brought closer together and members of the two populations will cross breed.

♦ Example:
There are two populations of squirrels on either side of the grand canyon.
↑↑↑
These populations are considered two different species. 

We can conclude that these 2 populations were once one population.
Isolation allowed the two populations to move apart and speciation occurred.

hope it helps!