The whole population, of dogs, are together and all a happy population.
An earthquake occurred and seperated the population
The two populations continued to live, but they started to become different from each other.
Because of the earthquake, their populations grew, and they created different habits from each other
If they were to mix, then the two populations would not be able to breed because they would not be attracted to each other because of the different habits they created(as a population) (x= cannot mix)
Since the two populations have evolved so much over the time, they have become so different that they are now considered two different species.