Gene flow

In population genetics, gene flow or gene migration is the transfer of alleles or genes from one population to another. In other definition, is the propagation of genes from one breeding population to another by migration, possibly leading to changes in allele frequency. The overall effect of gene flow is that it hinders the genetic divergence between populations and increases the genetic variation within populations. When gene flow between populations is reduced or absent, the two populations may go to diverge to the point that members of a population can not be able to interbreed successfully with members of the other.

Isolating barriers
Gene flow between biological species can be largely or entirely prevented by isolating mechanisms or barriers to gene flow. These barriers are most of the time physical like impassable mountain ranges, canyons, oceans or other large bodies of water, extensive deserts and so on.