When the Panamanian land bridge was emplaced about 2.7 Ma, it triggered the Great American Biotic Interchange (GABI), a major mingling of land mammal faunas between North and South America. Four families of northern immigrants (Procyonidae, Felidae, Tayassuidae, and Camelidae) diversified at moderate rates, while four others, Canidae, Mustelidae, Cervidae, and especially Muridae, evolved explosively. As a consequence, half of living South American genera are descendants of northern immigrants. The other major consequence of the interchange was the conquest of tropical North America by immigrants from Amazonia, an episode that justifies the term Neotropical Realm.