A form of allopatric speciation in which a founder population becomes established beyond the existing range of the main population and remains isolated because of difficult terrain or other factors. Such a founder population may involve just a few individuals or even a single fertilized female, and so has a much smaller gene pool compared with the main population. Also, it may be subject to different selection pressures. Together these factors cause potentially rapid evolutionary divergence, provided there is little or no interbreeding with the main population, so that in time it becomes a new species. Compare dichopatric speciation.