A gene pool undergoes allopatric speciation when geographical distance, or a physical barrier such as a stretch of ocean, permits variation to grow such that gene flow across the population is permanently interrupted. The radiation of Darwin's finches in the Galapagos Islands is the classic example of allopatric speciation.
The Galapagos Islands, off the west coast of South America, are home to 13 species of finches, often referred to as Darwin's finches. These species derive from an initial colonization event, in which the ancestral mainland finch arrived, and its descendants spread throughout the chain. Isolation, both from the mainland and from one island to the next, allowed variation to accumulate among the finches until they became separate species.