The eight biogeographic realms
Biogeographic realms, also called ecozones, divide Earth's land surface by the broad evolutionary history of its plants and animals rather than by climate alone. The widely used scheme recognizes eight: the Nearctic (most of North America), the Neotropical (Central and South America), the Palearctic (Europe, North Africa, and northern Asia), the Afrotropical (sub-Saharan Africa), the Indomalayan (South and Southeast Asia), the Australasian (Australia, New Guinea, and nearby islands), the Oceanian (the Pacific islands), and the Antarctic.
Each realm contains characteristic lineages that evolved in relative isolation, which is why marsupials dominate Australasia and placental mammals dominate the others.
Why the boundaries fall where they do
Realm boundaries trace deep barriers to dispersal: oceans, mountain ranges, and deserts that separated faunas long enough for distinct evolution. The Wallace Line between the Indomalayan and Australasian realms is the classic example, marking an abrupt faunal change across a narrow strait. The realm concept descends from Alfred Russel Wallace's nineteenth-century work and remains foundational to conservation planning and the study of how continental drift shaped life.