Allopatric speciation occurs when a physical barrier arises between members of a population. The two new populations become geographically isolated in different environments by, for example, a mountain, river or the sea.Finches in the Galapagos islands are birds that have formed different colonies on the different islands and have become geographically isolated by the sea. Random mutations and natural selection have meant that the finch populations are unable to breed with each other.In addition their beaks have become adapted to the different food sources on the different islands. This is an example of adaptive radiation.