This occurs when two species hybridize and a small number of the offspring manage to mate with the general population. This occurs in plants more so than in animals. This type of speciation is produced when a physical barrier separated a species into two separate areas and does not allow any further contact. Mountain building, glacial movement, river boundary movement, etc. are examples of geographical situations that can divide up a single species into two distance areas. When the species is split, microevolution will cause changes in the species to make them different in phenotype.
Small populations have a much better chance to develop into a new species than larger populations.
Adaptive radiation is the evolution of many diversely adapted species from a common ancestor. Geographical isolation lends itself to this type of new species development. This is a type of speciation that develops within the range of the parent population. This type of speciation does not include geographical isolation. It can occur rapidly if a genetic change results in a barrier between the mutants and the parent population.