Allopatric speciation, also known as geographic speciation, is the phenomenon whereby biological populations are physically isolate by an extrinsic barrier and evolve intrinsic (genetic) reproductive isolation, such that if the barrier breaks down, individuals of the populations can no longer interbreed. Evolutionary biologists agree that allopatry is a common method by which new species arise. (The word is derived from the ancient Greek allos, "other" + Greek patrā, "fatherland".) By contrast, the frequency of other types of speciation, such as sympatric speciation, parapatric speciation, and heteropatric speciation, is debated.