What is a geographic speciation model?
In order to geographic speciation to occur, two or more population of a species must be geografically sepparate; this can happen through migrations or natural disasters (volcano eruptions, island formations, deviations in the course of rivers, etc.).
Once sepparated, the distinct populations assume different geentic characteristics as they reproduce; this can happen due to adaptations to different environments ( Darwin's natural selection) or merely by chance ( genetic drift).
As generations go by, a time will come when the populations are so different genetically that they wont be able to crossbreed, thus characterizing two different species - therefore, a speciation mechanism.