Wallace’s Line

Wallace's Line, as drawn by Alfred Wallace in 1863

Wallace’s Line, as drawn by Alfred Wallace in 1863

Alfred Russel Wallace, 1823-1913, deserves better than history has given him. Wallace was a naturalist, explorer, geographer, anthropologist and biologist. He is best known today for independently conceiving the theory of evolution through natural selection; after all, his paper on the natural selection was jointly published with some of Charles Darwin’s writings in 1858. That paper, and the risk that Wallace would publish first, prompted Darwin to publish his own ideas in On the Origin of Species.

Wallace did extensive fieldwork, first in the Amazon River basin and then in the Malay Archipelago, where he identified the faunal divide now termed the Wallace Line, which separates the Indonesian archipelago into two distinct parts: a western portion in which the animals are largely of Asian origin, and an eastern portion where the fauna reflect Australasia.

He was considered the 19th century’s leading expert on the geographical distribution of animal species and is sometimes called the “father of biogeography.” Wallace was one of the leading evolutionary thinkers of the 19th century and made many other contributions to the development of evolutionary theory besides being co-discoverer of natural selection. These included the concept of warning coloration in animals, and the Wallace effect, a hypothesis on how natural selection could contribute to speciation by encouraging the development of barriers against hybridization.

The development of such radically different flora and fauna on different sides of such a narrow physical boundary was a compelling argument in support of evolution. Natural barriers isolate populations, and promote speciation. Later science has only refined Wallace’s recognition of the physical barrier.

Wallace could not know of the two refinements later science has brought to his observation and deductions. Today we know that during the last glacial maximum, the much lower sea levels caused the islands of either side of Wallace’s Line to be part of continuous land masses. It’s the same effect that created Beringia, linking Russia and Alaska. Effectively, there were no water barriers to migration of flora and fauna. The channel between Borneo and Sulawesi (Celebes), and along the Lombok Strait between Bali and Lombok, was too deep for the lower sea level to bridge. Wallace could had no way to know of the consequences of the Ice Ages in the 1860s.

There’s an even more recently discovered chunk of science that explains Wallace’s Line. The opposing sides sit on different continental plates, which have – in geologic terms, at least – only recently drifted this close together. And the deep channels dividing the Asian and Australian plates are the result of that geologically recent collision. Modern geology, specifically plate tectonics, completely supports both evolution and Wallace’s line.

Which WC thinks is very cool.