Biogeography: Wallace and Wegener (1 of 2)

Alfred Russel Wallace

Today Alfred Russel Wallace (left) is a prisoner of scientific parentheses, as in, “the theory of evolution by natural selection proposed by Charles Darwin (and also by Alfred Russel Wallace).” Yet Wallace was a great naturalist in his own right, particularly in the way he used evolutionary theory to interpret the natural world. In one of his most important applications, he helped found the modern science of biogeography—the study of how species are scattered across the planet, and how they got that way.

Mammals of the Borneo forest

Patterns of Species’ Ranges
Wallace had already accepted evolution when he began his travels in 1848 through the Amazon and Southeast Asia. On his journeys, he sought to demonstrate that evolution did indeed take place, by showing how geography affected the ranges of species. He studied hundreds of thousands of animals and plants, carefully noting exactly where he had found them. The patterns he found were compelling evidence for evolution. He was struck, for example, by how rivers and mountain ranges marked the boundaries of many species’ ranges. The conventional explanation that species had been created with adaptations to their particular climate made no sense since he could find similar climatic regions with very different animals in them.

Wallace came to much the same conclusion that Darwin published in the Origin of Species: biogeography was simply a record of inheritance. As species colonized new habitats and their old ranges were divided by mountain ranges or other barriers, they took on the distributions they have today.

Left: Wallace’s 1876 book, The Geographic Distribution of Animals, has plates depicting the animal life of the biogeographic regions he identified. These are mammals typically found in the forests of Borneo.
Wallace's Oriental region
This map from Wallace’s 1876 book shows his Oriental biogeographic region, broken into four subregions (outlined in red). “Wallace’s Line” is indicated by the arrow. Click for an enlargement.

Wallace pushed the study of biogeography to grander scales than Darwin. As he traveled through Indonesia, for example, he was struck by the sharp distinction between the northwestern part of the archipelago and the southeastern, despite their similar climate and terrain. Sumatra and Java were ecologically more like the Asian mainland, while New Guinea was more like Australia. He traced a remarkably clear boundary that snaked among the islands, which later became known as “Wallace’s Line.” He later recognized six great biogeographical regions on Earth, and Wallace’s Line divided the Oriental and the Australian regions.

Read Wallace's 1858 manuscript on species.
  • Wallace (in 1848) image courtesy of The Alfred Russel Wallace Page. next page

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