Speciation is especially likely to occur in small populations that have become separated from the main populations, e.g., on islands, or in small lakes. One famous example is Hawaiian flies. Many of the small islands have endemic fly species. Theoretically, flies have migrated to the smaller islands when the islands had recently emerged by volanic action. Separated from the main population, the small fly poulation diverges genetically, adapting to the new island until it is a separate species from the original parent population. This example also illustrates the founder's effect.
Darwin found similar patterns of species on the Galapagos and Canary islands. It is an example of how patterns can illuminate processes (eg, allopatric speciation) in evolution.
The isolation can occur when: