What exactly is a species?

Naturalists have been trying to catalog all of Earth's species for centuries, and the effort remains one of the great unfinished jobs in science. So far, researchers have mentioned that about 2.3 million speciesbut there are millions – maybe even billions – has yet to be discovered.

As if this search isn't difficult enough, biologists can't agree on what a species is. A 2021 questionnaire found that practicing biologists used 16 different approaches to categorize species. Any two of the randomly selected scientists had an overwhelming chance of using different ones.

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

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

Take the giraffe.

In 1758, the Swedish taxonomist Carl Linnaeus described a single species of giraffe: Giraffa camelopardalis. Although the species has declined in recent decades, 117,000 giraffes still survive in Africa, prompting an international conservation group to assign the species is more vulnerable than endangered.

But some conservation biologists say giraffes are in grave danger because what looks like one species is actually four. Genetic studies have done that found it that giraffe DNA falls into four different clusters: the northern giraffe, the reticulated giraffe, the Masai giraffe and the southern giraffe.

The northern giraffe, which lives in areas from Niger to Ethiopia, has suffered catastrophic losses due to civil wars, poaching and the 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,” says Stephanie Fennessy, executive director of the Giraffe Conservation Foundation, a nongovernmental conservation organization.

For Linnaeus, species were life forms created by God, each with its own distinctive properties. A century later, Charles Darwin recognized that living species had evolved, like young branches sprouting from the tree of life. That realization made it harder to say exactly when a new group became its own species, rather than just a subspecies of an old one.

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

The biological species concept, as it came to be 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 several species of frogs in Europe.

Some groups of frogs interbred extensively, while others had no hybrids at all. By analyzing their DNA, Dr. Dufresnes that groups with a recent ancestor—that is, those who were more closely related—easily produced hybrids. He estimates that it takes about six million years of divergent evolution for two groups of frogs to become unable to reproduce with each other – in other words, to become two different species.

“This is really cool,” said Dr. Dufresne. “Now we know what the threshold is for considering them as a species or not.”

Dr.'s method Dufresne for finding new species requires a lot of work in the field. Other researchers have looked for more efficient ways to identify species. A popular method is to sequence the DNA of organisms and observe the differences in their genetic code.

This search can yield many surprises, as evidenced by the giraffes in Africa. The team of Dr. Grabowski has discovered even more dramatic diversity lurking among European crustaceans, a group of aquatic animals that also includes lobsters, shrimp and crabs. The researchers have shown that animals that look identical and appear to belong to one species can actually be dozens of new species.

For example, a species of common freshwater shrimp called Gammarus fossarum split 25 million years ago into separate lineages that are still alive today. Depending on how researchers classify their DNA differences, the single species Gammarus fossarum may actually consist of 32 species – or as many as 152.

“To us it's mind-boggling,” said Dr. Grabowski.

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

You don't have to be a mammalogist to understand that polar bears and brown bears are different. Just one look at their white and brown coats is enough.

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

And yet the DNA in those ancient bones is revealing an amazing history of the cross 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 clearer, but about 120,000 years ago they again underwent an extraordinary exchange of genes.

Between 25,000 and 10,000 years ago, bears interbred in different parts of their range. The exchanges have left a significant impression on bears today: About 10 percent of the DNA in brown bears comes from polar bears.

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

But the exchange of DNA hasn't blurred the bears into one species. Some traits that benefit polar bears in their native environment can become a burden for brown bears, and vice versa.

“They clearly require separate conservation management strategies,” said Dr. Shapiro. “It makes sense to me to think of them as different species.”

The uncertainties about what constitutes a species have left taxonomists with numerous conflicts. For example, separate groups of ornithologists have created their own lists of all the bird species on Earth, and those lists often conflict.

Even a species as common as the barn owl – found on every continent as well as on remote islands – is a source of contention.

The conservation group BirdLife International recognizes barn owls as a species, Tyto alba, that lives around the world. But another influential inventory, called the Clements Checklist of Birds of the World, labels the barn owls that live on an island chain in the Indian Ocean as their own species, Tyto deroepstorffi. Yet another recognizes the barn owls in Australia and New Guinea as Tyto delicatula. And a fourth splits Tyto alba into four species, each covering its own wide swath of the planet.

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

In 2021, the International Ornithologists Association formed a workgroup to replace the four leading bird checklists with a single catalogue. Nine experts work their way through the lists and vote on more than 11,000 potential species.

“The discussions can get very heated,” said Leslie Christidis, president of the group. Some experts tend to lump species together, while others split them up. “We are just trying to negotiate a peaceful system.”

Thomas Wells, a botanist at the University of Oxford, is concerned that debates about the nature of species are slowing down the discovery of new ones. Taxonomy is traditionally a slow process, especially for plants. It may take decades that a new plant species is formally named in a scientific publication after it is first discovered. That slow pace is unacceptable, he said, when three of the four undescribed plant species already are threatened with extinction.

Dr. Wells and his colleagues are developing a new method to speed up the process. They take photos of plants in the wild and in museum collections and use computer programs to spot specimens that appear to come together because they have similar shapes. They also quickly analyze the DNA from the samples to see if they clump together genetically.

If they get clear clusters through such approaches, they call the plants a new species. The method – which Dr. Wells calls a “rough and easy” triage in our age of extinction – could make it possible for his team to describe more than 100 new plant species every year.

“We don't really have the luxury of worrying about, 'Is this a species or is this a subspecies?'” he said. “We need to make decisions quickly and as accurately as possible, based on the evidence we have available.”