One basis that is more evident on this model is that the separation of population would be isolated at the edge of a species range of barrier that divides the whole and the other basis is the pattern of geographic variation of modern species.
This is one model of allopatric speciation which represents the vital evolutionary process. Ring species give clear indication for its possibility but the evidence comes from the general existence and degree of geographic variation in all species and theoretical chance that reproductive isolation could evolve as part of the genetic divergence in an isolated allopatric population.
In case of two populations that have been move away in allopatry and consequently re-encounter each other in another geographical location, the gene flows become supplementary part as selection might strengthen any isolation between populations. On the other hand gene flow will perform to equalize thegene frequencies.