Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

Basically, parapatric speciation forms an intermediate link between allopatric speciation and sympatric speciation (Gavrilets 2000; Pennisi 2000a). Parapatric speciation could occur, for example, in not very mobile species whose members form mutually neighboring local populations within their range of occurrence. Crossing occurs between the organisms within these populations, while the gene flow between populations is much less and tends to be mediated by isolated migrants. The fact that the individual populations are constantly in contact means that there is a continuous gene flow between their members, even during speciation. This, of course, complicates their phenotype differentiation and the formation of post-zygotic reproductive isolation barriers. However, if the gene flow is sufficiently limited, for example because the ranges of occurrence of the two populations come into contact only at a certain restricted site, the individual populations can nonetheless adapt to the local conditions of their environments. Crosses born as a consequence of penetration of migrants into the territory of a foreign population exhibit a combination of the traits of the two populations and consequently have suboptimal phenotype and lower fitness in both environments. They are thus gradually eliminated from the population, again reducing the gene flow between the two populations.
The effectiveness of parapatric speciation, similar to the effectiveness of gradual sympatric speciation, is sometimes doubted. Mathematical models indicate that even very small gene flow, for example, exchange of a single individual per generation between the two populations, is mostly enough to prevent genetic differentiation of a new species. However, in actual fact, these conclusions are valid only for differentiation occurring through the action of genetic drift. If more effective processes participate in the differentiation of the two populations, such as evolutionary drives or selection, the intensity of the gene flow would have to be much greater to prevent differentiation.