The riches of the natural world are not spread evenly across the globe. Some places, such as the tropical Andes in South America, are simply stacked with unique species of plants and animals, many found no place else on Earth. So-called biodiversity “hotspots” are thought to cover just 2.3 percent of the planet’s surface, mostly in the tropics, yet they account for half of all known plant species and 77 percent of land vertebrates.
Until recently, the predominant explanation for the embarrassment of biological riches concentrated in places like the Amazon Basin was that such places must be engines of biodiversity, with new species evolving at a faster rate than other parts of the world. But now, new research on bird evolution may turn that assumption on its head, instead supporting the idea that areas with fewer species actually tend to produce new species at a faster clip than those with the most dizzying arrays of flora and fauna.
The researchers behind the new study, published today in the journal Science, say these biodiversity “coldspots” are generally found in harsher environments featuring frigid, dry and unstable conditions. Though the researchers found that these locations with few bird species tend to produce new ones at high rates, they fail to accumulate a large number of species because the harsh, unstable conditions frequently drive the new life forms to extinction.
The more well-known hotspots, by contrast, have accumulated their large numbers of species by being balmy, hospitable and relatively stable. Indeed, the researchers found that the myriad bird species that call the Amazon home tend to be older in evolutionary terms.
“Our results suggest that these hotspots for biodiversity are not hotspots for speciation or diversification,” says Elizabeth Derryberry, an evolutionary biologist at the University of Tennessee and one of the paper’s senior authors. “New species do form in places like the Amazon, just not as frequently as in more extreme environments, like the dry puna grasslands in the Andes.”
These findings are outgrowths of what began as a massive genetic study of nearly 1,300 species of birds from a mainly South American group known as the suboscines. This group of mostly small, passerine birds is spread across several continents but their greatest diversity is found in South America, where they make up a third of all known bird species. North American bird aficionados may be familiar with suboscines by way of flycatchers, while some notable South American representatives include the woodcreepers, antbirds, manakins and cotingas.
The researchers’ main goal was to create an accurate, detailed evolutionary tree, or phylogeny, for this large group of birds by sequencing the DNA of every single species in the group. When they finished, the team hoped to analyze the assembled trove of evolutionary data to determine how some places came to have more species than others.
Over eight years, an international collaboration managed to wrangle 1,940 samples representing 1,287 of the 1,306 suboscine bird species. Even though the collection is missing 19 species, the final tally is still good enough to comprise more than 10 percent of Earth’s roughly 10,000 known bird species.
“Creating a robust phylogeny showing how all these birds are related to each other is a monumental task,” says Ben Winger, an evolutionary biologist focusing on birds at the University of Michigan’s Museum of Zoology who was not involved in the research. “Many of these birds have tiny ranges in really remote places, and the samples that are in museum collections are scattered in drawers and freezers all over the world.”
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What’s more, each of the DNA samples used to establish the evolutionary relationships between the collected species was sequenced across more than 2,000 gene locations. Looking at so many locations on the genomes of so many suboscine birds creates a phylogeny that is both huge in scale and highly accurate, writes Jason Weir, an evolutionary biologist at the University of Toronto who was not involved in the new paper, in an email.
Tracking down and sequencing the DNA of all the samples at the heart of this phylogeny took the project’s 21 collaborators six years. They started by scouring museum collections around the world, but they soon realized that they would also need to spend many months in the field collecting the more than 100 species that couldn’t be found in museums. Once the team had the samples in hand, they sent them off to a lab in Florida for genetic sequencing. In 2018, the researchers assembled their data into a phylogenetic tree that showed how the nearly 1,300 species of birds were related to one another. Finally, the team set about analyzing what amounted to nearly four terabytes of genetic code to look for patterns showing where and when new species arose.
The analysis revealed that the best predictor of whether an area would produce new species at a high rate was how many species lived there, rather than environmental factors like climate or geographic features like mountains. The team was surprised to find that the relationship between species richness and the rate at which new species arose was inverse. Species-rich areas tended to produce new species more slowly across the more than 1,000 types of suboscine birds.
“What seems to be happening is that these places like the Amazon have higher species richness despite having low rates of speciation because the environment there is more stable and favorable to the species’ survival over time,” says Gustavo Bravo, an evolutionary biologist at Harvard’s Museum of Comparative Zoology and one of the lead authors of the paper. “So, the number of species in the Amazon has kept growing because the species tend to stay alive longer.”
On the other hand, Bravo says places with low levels of species diversity and high rates of speciation may have so few species because their extreme, changeable environments keep killing off the newcomers.
“The authors of this paper have pointed out an evolutionary pattern that flips some common assumptions, and I think it’s likely to hold for other animal groups besides birds,” says Rebecca Kimball, an evolutionary biologist at the University of Florida who was not involved in the research. “This shows the power of our ability to collect and analyze data from nearly every species in major groups to start asking these kinds of big questions about evolutionary patterns.”
Bravo says the team behind this study hopes to follow it up by seeing how the patterns they identified hold up when more biological factors are included, such as a species’ behavior, ecology or body size and shape. “One explanation for our results in this paper is that maybe harsh environments generate new species more frequently because there’s less competition and more available opportunities for new species,” he says “This next layer of data can help explain that.”
On a more practical level, Kimball says the study’s findings add new urgency for conserving ecosystems that may look barren, but may actually be nature’s hothouses for the evolution of new species. She says the study also offers a sobering framing for the accelerating loss of species and habitat in the world’s hotspots of biodiversity. “Humans are driving extinction rates up around the world, and this study suggests that the places with the most biodiversity may be slowest to recover because of their low speciation rates,” she says.
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