Allopatric (Greek: 'other country') describes a species or populations that are geographically separated by physical barriers (mountains, oceans, rivers). Allopatric speciation is the most common mode of speciation — populations separated by geographic barriers evolve independently and may eventually become reproductively isolated. Sympatric species occupy the same geographic area; sibling species are morphologically similar but reproductively isolated; biospecies are defined by reproductive isolation.