What exactly is a seed?

Naturalists have been trying to catalog every species on Earth for centuries, but this effort remains one of science’s great unfinished tasks. To date, researchers have named about 2.3 million species, but there are still millions, perhaps billions, yet to be discovered.

As if this quest wasn’t difficult enough, biologists can’t agree on what a species is. A 2021 study found that practicing biologists use 16 different approaches to classify species. Her two randomly selected scientists were significantly more likely to use different ones.

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

Debates over species are more than academic entertainment. The current extinction crisis requires scientists to urgently assess the world’s biodiversity. But some of the most well-known species on Earth may not be what they seem.

Take the giraffe.

In 1758, Swedish taxonomist Karl Linnaeus described a species of giraffe, the giraffe (Giraffa camlopardalis). Although the species has declined in recent decades, there are still 117,000 giraffes across Africa, prompting international conservation groups to list the species as endangered rather than endangered. I am.

But some conservation biologists argue that giraffes are in great danger because what appears to be one species is actually four species. Genetic research has revealed that giraffe DNA falls into four distinct clusters: giraffe, reticulated giraffe, Masai giraffe, and southern giraffe.

The northern giraffe, which ranges from Niger to Ethiopia, has been devastated by civil war, poaching and destruction of its wild habitat. If the northern giraffe were considered a separate species, it would make it “one of the most endangered large mammals in the world,” said Stephanie, executive director of the Giraffe Conservation Foundation, a non-governmental conservation group. Fennessy said.

For Linnaeus, species are forms of life created by God, each with unique characteristics. A century later, Charles Darwin recognized that species evolved like young branches sprouting from the tree of life. This realization has made it difficult to say exactly when a new group becomes its own species, rather than just a subspecies of an old group.

In the 1940s, German ornithologist Ernst Mayer tried to solve this problem with a new definition of species based on how animals reproduce. If two animals cannot reproduce with each other, Meyer argued, they are different species.

The concept of biological species, as it came to be known, greatly influenced subsequent generations of researchers.

Recently, Christophe Dufresne, a herpetologist at China’s Nanjing Forestry University, used this concept to classify different species of European frogs.

Some groups of frogs interbred frequently, while others did not interbreed at all. By analyzing their DNA, Dr. Dufrens discovered that groups with recent ancestry, or more closely related groups, easily produce hybrids. He estimates that about 6 million years of divergent evolution is required before the two groups of frogs can no longer interbreed, meaning they become two different species.

“This is really great,” Dr. Dufresne said. “Now we know what the criteria are for whether or not they are considered a species.”

Dr. Dufresne’s method for discovering new species requires a lot of effort in the field. Other researchers were looking for more efficient ways to identify species. One common method is to sequence DNA from organisms and observe differences in their genetic codes.

As seen with African giraffes, this study could yield many surprises. Dr. Grabowski’s team discovered even more dramatic diversity hidden within European crustaceans, a group of aquatic organisms 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.

For example, a common freshwater shrimp species called Gammarus fossarum split into separate lineages 25 million years ago that still exist today. His single species of Gammarus phossarum could actually be 32 species, or as many as 152 species, depending on how researchers classify the DNA differences.

“To us, that’s surprising,” Dr. Grabowski said.

As scientists collect more genetic data, new questions are arising about what on the surface appears to be a distinctly different species.

You don’t have to be a mammalogist to understand that polar bears and brown bears are different. One look at the white and brown fur is enough.

Differences in color are the result of ecological adaptation. White polar bears blend into their arctic habitat and hunt seals and other prey. Brown bears have adapted to life on land further south. This difference is so clear that a paleontologist can go back hundreds of thousands of years to distinguish fossils of the two species.

But the DNA contained in these ancient bones is revealing a remarkable history of interbreeding between polar and brown bears. After the two lineages diverged about 500,000 years ago, they exchanged DNA for thousands of years. Later, they became more distinct, but about 120,000 years ago, another surprising genetic exchange took place.

Between 25,000 and 10,000 years ago, bears interbred in several parts of their range. This exchange has a huge impact on today’s bears. About 10 percent of a brown bear’s DNA comes from polar bears.

Beth Shapiro, a paleogeneticist at the University of California, Santa Cruz, said interbreeding most likely occurred when polar bears moved down from the Arctic to brown bear habitat due to climate change.

However, the DNA exchange did not identify bears as one species. Some of the traits that are advantageous to polar bears in their own environment may be burdensome to brown bears, and vice versa.

“They clearly require a separate strategy for conservation management,” Dr. Shapiro said. “To me, it makes sense to think of them as separate species.”

Uncertainty about what constitutes a species has led taxonomists to countless contradictions. For example, different groups of ornithologists create their own lists of all the bird species on Earth, but those lists often conflict.

Even a common species like the barn owl (found on all continents as well as remote islands) is a source of disagreement.

The conservation organization BirdLife International recognizes the barn owl as a species called Tito alba, which occurs worldwide. But another influential inventory called Clements’ Checklist of the Birds of the World has the barn owl, native to the Indian Ocean archipelago, carved out as its own species, Tito deloepstorphi. Still other researchers recognize the Australian and New Guinea barn owl as Tyto delicatula. And his fourth species, Tito, he divides his Alba into four species, each covering its own wide area on Earth.

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

In 2021, the International Union of Ornithologists established a working group to replace the four major bird checklists with a single catalog. Nine experts have reviewed the list and voted on more than 11,000 potential species.

“The debate can be very heated,” said Leslie Christidis, the group’s chair. Some experts tend to lump the species together, while others divide them. “We’re just trying to negotiate a peaceful regime.”

Thomas Wells, a botanist at the University of Oxford, worries that debates over the nature of species are slowing down the process of discovering new species. Taxonomy is traditionally a time-consuming process, especially for plants. After a new plant species is first discovered, it can take decades for it to be officially named in a scientific publication. He said the slow pace was unacceptable when three out of four undescribed plant species were already at risk of extinction.

Dr. Wells and his colleagues are developing new methods to speed up the process. They take photos of plants, both in the wild and in museum collections, and use a computer program to find samples that appear to cluster together because they are similar in shape. We also rapidly sequence DNA from samples to see if genetic clusters exist.

If such an approach yields a distinct cluster, they call the plant a new species. This method, which Dr. Wells calls “roughly ready” triage in the era of extinction, could allow his team to describe more than 100 new plant species each year.

“We don’t have the luxury of wondering, ‘Is this a species or a subspecies?'” he says. “We need to make decisions as quickly and accurately as possible based on the evidence we have.”