An Evolving Question: What Is a Species?
Naturalists have been trying for centuries to catalog all of the species on Earth. So far, researchers have named about 2.3 million species, but there are millions — perhaps billions — left to be discovered.
As if that is not hard enough, biologists cannot agree on what a species is. A 2021 survey found that practicing biologists used 16 different approaches to categorizing species.
“Everyone uses the term, but no one knows what it is,” said Michal Grabowski, a biologist at the University of Lodz in Poland.
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 may not be what they seem. Consider 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 across Africa, prompting an international conservation group to designate the species as vulnerable, rather than endangered.
But some biologists argue that giraffes are in great peril, because what looks like one species is actually four. Genetic studies have found that giraffe DNA falls into four distinct clusters: Northern giraffe, reticulated giraffe, Masai giraffe and Southern giraffe.
The Northern giraffe, which lives in pockets from Niger to Ethiopia, has suffered catastrophic losses from civil wars, poaching and the destruction of its habitat. If it were considered a separate species, it would be “one of the most threatened large mammals in the world,” said Stephanie Fennessy, executive director of the Giraffe Conservation Foundation.
In the 1800s, Charles Darwin recognized that living species had evolved, making it harder to say exactly when a new group became a species of its own, instead of just a subspecies of an old one.
In the 1940s, Ernst Mayr, a German ornithologist, argued that if two animals could not breed with each other, then they were separate species. The biological species concept, as it came to be known, had a huge 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 species of frogs in Europe. He estimates that it takes about six million years of diverging evolution for two groups of frogs to become unable to interbreed — and thus become two distinct species.
One popular method to identify species is to sequence DNA from organisms and observe the differences in genetic code. This can yield surprises, as shown by the giraffes in Africa.
Dr. Grabowski’s team has studied European crustaceans. They found that animals that look identical and appear to belong to a single species may actually be dozens of new species.
For example, a species of common freshwater shrimp, 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 of Gammarus fossarum might in fact be 32 species — or as many as 152.
“For us, it’s mind-blowing,” Dr. Grabowski said.
Even a common species like the barn owl is a source of disagreement. The conservation group BirdLife International recognizes barn owls as a species, Tyto alba, that lives across the world. But another influential inventory, called the Clements Checklist of Birds of the World, carves off the barn owls that live on an Indian Ocean island chain 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 broad part of the planet.
It can take decades for a new species of plant to be formally named in a scientific publication after it is first discovered. Thomas Wells, a botanist at the University of Oxford, said the sluggish pace is unacceptable, at a time when three out of four undescribed species of plants are threatened with extinction.
Dr. Wells and his colleagues are taking photographs of plants both in the wild and in museums and using computer programs to spot samples that seem to cluster together because they have similar shapes. The researchers are also rapidly sequencing DNA from the samples to see if they cluster together genetically.
If they get clear clusters from approaches such as these, they call the plants a new species. The methods may make it possible for Dr. Wells’s team to describe more than 100 new species of plants each year.
“We don’t really have the luxury of agonizing over, ‘Is this a species, or is this a subspecies?’ ” he said.