That’s where mountains come in. There are mountains all over the world, in different climates with different evolutionary histories and different assemblages of species. But they share one thing in common: height. Scientists can compare the number of species at 1,000 meters to the number of species at 2,000 meters, at 2,500 meters, and so forth, on mountains around the world to test whether a particular elevational range is more conducive to speciation. McCain’s research has found that regardless of geographical location or evolutionary history, 45 percent of the world’s vertebrates are clustered around mid-elevational peaks of diversity, making them more common at elevations in the middle of mountains than anywhere else.
McCain thinks the percentage’s stability might be related to levels of precipitation on the mountains, which means climate, more so than the other three mechanisms, would be a major driver of diversity. But there’s still a lot of work to be done to understand just how big a role climate plays.
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Like Heaney, when McCain began studying mammalian diversity, in the 1990s, everyone believed the most diverse hubs were concentrated in the lowlands. Research on the Andes showed greater numbers of bird and bat species around the lowlands—yet fieldwork was rarely conducted in mountains. “People got a famous study stuck in their heads and forgot everything else, even their own data,” she says, referring to the Andes research. She’s taken the mistake to heart, resisting any overly simplistic rules about where diversity is found, even when it comes to mid-elevational peaks.
“I fight against the idea of people now trying to make the mid-elevational peak the only thing that’s there,” McCain says. She wants researchers to focus more on variability—the anomalies that can’t be explained in the standard model.
One example of such an anomaly came in a 2013 study, which found that harsh environments, like those in northern latitudes, contain more mammal and bird subspecies than other regions—that is, a greater variety within the species that live in harsh environments, even though the number of different species itself was lower. “It is surprising given that most indirect evidence suggests that the tropics, not the temperate regions of the world, are hotbeds for biological diversity,” the authors wrote. The researchers (who included McCain) reviewed 2,365 species of mammals for the study in hopes of understanding whether the tropical regions had greater diversity because the conditions helped produce more species (a phenomenon known as the “cradle of diversity” hypothesis), or simply because fewer species went extinct (the “museum of diversity” hypothesis).
“Any kind of change is a driver of evolution,” says study co-author Carlos Botero, assistant professor of biology at Washington University in St. Louis. “If the environment is really harsh and selection is really strong, that could be an engine for the generation of diversity. But even though the potential for the generation of species appears to be greater in temperate regions, when you subtract the cost from the benefits you see fewer species.”