In almost all animal groups, the mode of speciation is geographic speciation. It is essential that a population which is a prospective new species is geographically isolated from the parental species. Geographical isolation, according to Ernst Mayr, is almost invariably necessary for speciation to occur. Geographic speciation, as we pointed out earlier, in turn can be typed into two categories:
Speciation by populations of parent species which occupy quite separate territories is the most common mode of species formation among animals. Allopatric speciation occurs in large continental areas, and perhaps, also in the vast continental slopes and ocean floors. A once continuous series of population may be divided into two parts hy a new barrier. The barrier may be a newly formed desert or river or mountain range. The population on other side of new barrier becomes geographically isolated.