With a new planetwide analysis of vertebrate life, an international team has used 21st century science to update an iconic 1876 map of Earth’s zoological regions.
By incorporating data on 21,037 species of mammals, birds and amphibians, Jean-Philippe Lessard, now at McGill University in Montreal, and his colleagues have revised a zoological map created by Alfred Russel Wallace, an oft-overlooked cofounder of the theory of evolution. Wallace’s map divided Earth’s landmasses into six major regions, each with its own distinctive blend of vertebrates.
Over the years scientists have redistricted Wallace’s wildlife precincts several times, mostly to fit the growing trove of information on what species live where. Lessard and his colleagues, however, use not just species distributions but family tree relationships. Incorporating degrees of kinship revives the evolutionary spirit of Wallace’s original map, Lessard and colleagues say online December 20 in Science.
The notion of regional mixes of animals intrigued 19th century thinkers as they struggled toward a theory of evolution by natural selection. Naturalists noted that similar habitats in far-flung places looked as if they could nourish the same fauna, but often didn’t.
A modern reader of Wallace’s and Darwin’s notes and letters on biogeography might say that the signature of evolutionary history lay right in front of them: Regional mixes reflect where animal lineages originated and how shifting continents and emerging barriers or bridges guided the descendants’ spread.
One of the most bizarre of these regional breaks occurs to the southeast of the Oriental suite of species in India, Indochina and the islands that trail off toward Australia. When Wallace visited Bali in 1856, he puzzled over how the neighboring island of Lombok has similar soil and climate but noticeably different creatures. Yet Lombok lies within sight of Bali.
Lombok marks the border where Australian zoology begins, Wallace eventually concluded. When he published his map of animal regions, he drew a boundary, now known as Wallace’s line, that separates Bali and Lombok and then wiggles northward through the Malay archipelago.
To reevaluate Wallace’s boundaries, Lessard and his colleagues used computers to divide the planet’s land into squares on a grid and compared how many species from the three vertebrate groups each square shared with other squares. Using massive family trees developed at the University of Copenhagen and other places, the researchers gave more weight to differences between squares if their species were only distantly related.
Analyzing just birds, researchers placed a new boundary largely along Wallace’s famous line. Adding mammals and amphibians, however, shifted the line.
The biggest change from older maps, Lessard says, stretches the old Palearctic realm of northern Eurasia into the Western Hemisphere.
The new evolutionary approach also highlights just how unusual Australian animals are, Lessard says. People think of kangaroos and other Australian mammals as odd, but it was the amphibians that launched the realm’s distinctness scores to extremes.
The new map is “really the closest we’ve got to mapping life on Earth while considering the way it evolved,” says Şerban Procheş of the University of KwaZulu-Natal’s Westville campus in South Africa. He and a colleague published their own version of zoological regions in March. Their map didn’t adjust for species relatedness, but Procheş muses that it may be easier for people to grasp because it offers examples of characteristic species.
These, of course, are animal species. Plants tend to be better than animals at dispersing across oceans, according to genetic analyses, says James Richardson of the Royal Botanic Garden Edinburgh. “For this reason floristic divisions may not be as clear as zoogeographic ones,” he says.