Allopatric speciation occurs due to the geographical isolation between individuals of the same species. The geographical isolation can be due to canyon development, mountain formation, migration, etc.
The geographical isolation gives different environmental condition to the separated population of same species to live which allow these two population to evolve differently and evolve in different species.
So when two population becomes different species and not able to interbreed with each other when give opportunity then the allopatric speciation between the two populations becomes complete.
