Some species on Earth may not be what they seem – 02/21/2024 – Science

Naturalists have tried for centuries to catalog every species on Earth, and this effort remains one of the great unfinished works in science. So far, researchers have named about 2.3 million species, but there are millions — perhaps even billions — yet to be discovered.

As if this search wasn’t already difficult enough, there is no consensus on what a species is. A 2021 survey, for example, found that biologists used 16 different approaches to categorize species. Two randomly chosen scientists were overwhelmingly likely to use different approaches.

“Everyone uses the term, but no one knows what it is,” said Michal Grabowski, a biologist at the University of Lodz in Poland.

Debating species is more than an academic pastime. In the current extinction crisis, scientists urgently need to take inventory of the world’s biological diversity. But even some of Earth’s best-known species may not be what they seem.

Take the giraffe, for example.

In 1758, Swedish taxonomist Carl Linnaeus described a single species of giraffe: Giraffe camelopardalis. Although the species has declined in recent decades, 117,000 giraffes survive across Africa, prompting an international conservation group to classify the species as vulnerable rather than endangered.

But some biologists argue that giraffes are in great danger because what appears to be one species is actually four. Genetic studies have found that giraffe DNA falls into four distinct groups: the northern giraffe, the reticulated giraffe, the Masai giraffe, and the southern giraffe.

The northern giraffe, which lives in pockets ranging from Niger to Ethiopia, has suffered catastrophic losses due to civil wars, poaching and destruction of its wild habitat. If the northern giraffe were considered a separate species, it would be one of the most endangered large mammals in the world, according to Stephanie Fennessy, executive director of the Giraffe Conservation Foundation, a nongovernmental conservation organization.

For Linnaeus, species were divinely created life forms, each with its own traits. A century later, Charles Darwin recognized that living species had evolved, like young branches sprouting from the tree of life. This understanding made it more difficult to say exactly when a new group became a species of its own, rather than just a subspecies of an old one.

In the 1940s, Ernst Mayr tried to solve this problem with a new definition of species based on how animals reproduce. If two animals could not breed with each other, the German ornithologist argued, then they were separate species.

The biological species concept, as it became known, had an enormous influence on later generations of researchers.

In recent years, Christophe Dufresnes, a herpetologist at Nanjing Forestry University in China, has used this concept to classify different frog species in Europe.

Some groups of frogs interbred a lot, while others had no hybrids. By analyzing their DNA, Dufresnes discovered that groups with a recent ancestor – that is, those most closely related – easily produced hybrids. He estimates that it takes about 6 million years of divergent evolution for two groups of frogs to become incapable of interbreeding — in other words, to become two distinct species.

“That’s really cool,” Dufresnes said. “Now we know what the limit is to consider them species or not.”

Dufresnes’ method for finding new species requires a lot of field work. Other researchers have sought more efficient ways to identify species. One popular method is to sequence the DNA of organisms and observe differences in their genetic code.

This search can reveal many surprises, such as giraffes in Africa. Grabowski’s team discovered even greater diversity among European crustaceans, a group of aquatic creatures that includes lobsters, shrimp and crabs. Researchers have shown that animals that look identical to each other and appear to belong to a single species may actually be dozens of new species.

As scientists gather more genetic data, new questions are emerging about what appear, at first glance, to be clearly separate species.

You don’t need to be a breast zoologist to understand that polar bears and brown bears are different. Just take a look at their white and brown fur.

The difference in their colors is the result of their ecological adaptations. White polar bears camouflage themselves in their arctic habitats, where they hunt seals and other prey. Brown bears have adapted to life on land further south. The differences are so distinct that paleontologists can distinguish fossils of the two species that date back hundreds of thousands of years.

And yet, the DNA inside these ancient bones is revealing a surprising story of interbreeding between polar bears and brown bears. After the two lineages split about half a million years ago, they exchanged DNA for thousands of years. They then became more distinct, but about 120,000 years ago they underwent another extraordinary gene exchange.

Between 25,000 and 10,000 years ago, bears interbred in various parts of their range. The exchanges left a significant mark on today’s bears: about 10% of brown bears’ DNA comes from polar bears.

Beth Shapiro, a paleogeneticist at the University of California, Santa Cruz, said the interbreeding likely occurred when climate change forced polar bears down from the Arctic and into brown bear territory.

But the DNA swap didn’t mix the bears into a single species. Some of the traits that benefit polar bears in their own environment may become a burden for grizzly bears, and vice versa.

“Clearly they require separate strategies for conservation management,” Shapiro said. “To me, it makes sense to consider them separate species.”

Uncertainties about what defines a species have left taxonomists with countless conflicts. Separate groups of ornithologists have created their own lists of all bird species on Earth, for example, and these lists often conflict.

Even a common species like the barn owl — found on every continent except Antarctica, as well as remote islands — is cause for disagreement.

The conservation group BirdLife International recognizes barn owls as a species, Tyto alba, which lives all over the world. But another influential inventory, called the “Clements Checklist of Birds of the World,” separates barn owls living on an Indian Ocean island chain as their own species, Tyto deroepstorffi. Another recognizes barn owls in Australia and New Guinea as Tyto delicatula. And a room divides Tyto alba into four species, each covering its own extensive area of ​​the planet.

Some ornithologists are trying to resolve these conflicts with a low-tech approach: voting.

In 2021, the International Union of Ornithologists formed a working group to replace the four main bird lists with a single catalogue. Nine experts are going through the lists and voting on more than 11,000 potential species.

“Discussions can get very heated,” said Leslie Christidis, president of the group. Some experts tend to group species together, while others separate them. “We’re just trying to negotiate a peaceful system.”