The Wallace Line is the biogeographic boundary named after Alfred Russel Wallace that separates Asian and Australasian animal distributions.
The boundary runs through the Indonesian region, passing between islands such as Bali and Lombok and between Borneo and Sulawesi. Although some of these islands are close together, their animal communities differ sharply. Asian-associated species include monkeys, tigers, and woodpeckers, while Australasian-associated fauna include marsupials and cockatoos.
Wallace recognized this pattern during his nineteenth-century travels in the Malay Archipelago. Deep water channels remained between some islands even when sea levels were lower during ice ages, preventing many land animals from crossing. As a result, nearby islands could have separate evolutionary histories.
The line is not an absolute barrier for every species, and biogeographers use related boundaries to describe more gradual transition zones. The Weber Line and Lydekker Line are nearby concepts, but they mark different interpretations of the division between Asian and Australasian biotas.