Small, isolated populations are commonly around the edge of a large ancestral population. Because such peripheral isolates are small, they may diverge by founder effect and by drift. Because of inbreeding, genetic rearrangements may be more likely to become fixed in small populations. Whether by such special processes or simply by selection for marginal conditions, allopatric speciation involving peripheral isolates may be commonplace. Allopatric speciation involving peripheral isolates provides the theoretical basis for the hypothesis of punctuated equilibrium , discussed in Chapter 20, p. 560.
Allopatric speciation is the least controversial mode of speciation.
It undoubtedly happens and is undoubtedly important. Most other modes of speciation remain controversial. Parapatric speciation presumes divergence across the range of a species, caused by selection operating differently across the range while gene flow is low enough that panmixis does not occur. The result of these of conditions is a transition, which may be broad or narrow, across which gene frequencies change from those which prevail on one side to those which prevail on the other. The transition across which allele frequencies change may be broad or narrow.