Allopatric Speciation - Background
Allopatric speciation occurs when a population is separated geographically, restricting the gene flow between the subpopulations. Genetic differences between the two populations accumulate as genetic drift and differential selection pressures affect the subpopulations. These genetic differences lead to the development of postzygotic isolation mechanisms which may prevent successful interbreeding between the two populations if they were to rejoin.
Given enough time, the populations may be different enough that if they were to come into secondary contact with each other, they would remain two genetically distinct populations. Two forces can keep these populations from rejoining upon secondary contact: mating preferences (a prezygotic isolation mechanism), and hybrid inferiority (a postzygotic isolation mechanism). If postzygotic isolation mechanisms are strong, prezygotic isolations may then be reinforced, which serve to maintain the divergence between the two subpopulations.
Genetic drift occurs more rapidly in small populations. Therefore, subpopulations of small size accumulate genetic differences faster accelerating the speciation process. However, small subpopulations are also more susceptible to extinction since deleterious mutations may also be amplified by genetic drift.