Answer:
Allopatric speciation
Explanation:
Speciation is the gradual process by which two different species are formed from a single species due to evolution.
Allopatric speciation will occur when the speciation is due to geographical isolation of a species to the extend it prevents or interferes with the gene flow. Therefore, the populations can not mate with each other and they evolve separately.
This is what happened to the fox species when they were separated by a mountain range.