Very important factor is the impenetrability and permanence of the geographical barrier. Gene flow must be blocked long enough to give the founding population long enough to develope intrinsic reproductive barriers- prezygotic and or post zygotic. Sympatric speciation evolved reproductive isolation- can occur in a single generation in some species of organisms. Autopolyploidy results in the doubling of the number of chromosomes. The tetraploid cannot interbreed with the 2 N types to yield a viable 3N therefore the 4 N is reproductively isolated.
Wheat is allopolyploid.
A subpopulation begins to exploit resources not used by the parent population. Example includes the cichlids in Lake Victoria which is less than a million years old yet there are some 200 species of cichlids in the lake. A balanced polymorphism coupled with assortative mating could result in sympatric speciation in animals. Dimorphic bills in birds, mating between individuals of the same bill type could result in eventual speciation. The reproductive barriers may evolve without being favored directly by natural selection.