Small populations have a much better chance to develop into a new species than larger populations. These small populations usually occur on the edges of a larger population. These fringe populations are good candidates for speciation. Their gene pool differs from the parent population, genetic drift will continue to cause chance changes in the gene pool of small populations until a larger population is formed. Different selection pressures are at work on there peripheral 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. Sympatric Speciation . 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.