In 2016, researchers published paper with the bold claim: that the giraffe, first described as a species by the Swedish biologist Carl Linné in 1758, may actually have been four kinds all the time. Unlike Linnaeus, scientists had access to modern genetic tools that revealed that giraffes fall into distinct clusters based on differences in their DNA, some of which are “larger than the differences between brown bears and polar bears,” the authors wrote. he said then.
The news sent a wave of the giraffe conservation community suddenly needing to protect four species instead of one. But from the beginning there was disagreement about this new classification, and even today the International Union for Conservation of Nature—the organization that oversees the list of threatened and endangered species—lists the giraffe as the only kind, Giraffe camelopardaliswith nine subspecies.
Dustup and others like it highlight the “species problem,” a fundamental uncertainty about how we analyze organisms that continues to vex biologists around the world.
Arguments are often based on definitions that are decades old. In 1942, biologist Ernst Mayr created what is perhaps the most enduring: the biological species concept, which designates two organisms as different species unless they can reproduce and produce fertile offspring. Researchers have since developed definitions based on shared ancestry (the phylogenetic species concept), physical traits (the morphological species concept), or shared ecology (the ecological species concept), where species diverge as they take over different niches in their environment. In total, there are at least 16 species definitions in circulation among scientists today, and potentially as many as 32.
However, no definition seems to be without exception. There are species in which individuals look very different from each other, as well as “mystery species” that look identical but are genetically different. Hybridization is also common, leading to animals such as the liger (a cross between a lion and a tiger) and a beefalo (a cross between a domestic cattle and an American bison). Evidence even suggests that humans once interbred with two other ancient hominins that are usually considered separate species, Neanderthals and Denisovans, suggesting that they weren’t so different from us after all.
Related: ‘More Neanderthal than human’: How your health may depend on the DNA of our long-lost ancestors
“Some of the rules we’ve set up don’t work and sometimes it’s quite confusing,” Jordan Casey, a marine molecular ecologist at the University of Texas at Austin Marine Science Institute, told Live Science. “People by nature want to put things in order, and even I have to make a lot of decisions about whether I’m just seeing diversity among individuals or trying to bend things unnecessarily into different kinds.”
But establishing a species definition is not just an academic exercise – many of the world’s conservation policies are structured around the species as the de facto unit of conservation. Ultimately, it also brings existential questions. If there are ultimately four species of giraffe, does it really matter if one goes extinct?
To answer these questions, groups are now meeting to set guidelines for how species should be named and ordered on the tree of life, and how to resolve disputes when they arise. Developing a working list of agreed-upon rules is indeed essential, even if it’s not perfect, biologists say.
“It’s pretty messy”
The concept of species is ancient. For example, in 343 BC, Aristotle wrote the “History of Animals” in which he described the differences between individual animals and between groups.
But it wasn’t until the mid-18th century that the concept of taxonomy – the formal classification of living things – really took off, and Linnaeus turned it into an official discipline. Taxonomy flourished for a time as scientists around the world began naming new species, but as the field and relatives progressed, conflicts inevitably arose.
Scientists have officially described around 2 million species, and more are constantly being added or reclassified based on new evidence. Even for large, seemingly well-studied animals, adaptations are fairly common, and iconic animals like the giraffe, African elephant, and killer whale have come under scrutiny.
The problem is that scientists cannot agree on a universal definition that can classify organisms as diverse and dissimilar as mammals, birds, fish, plants and bacteria. Still others argue whether such an exercise is even useful, noting that scientists have continued to lack consensus for centuries and will continue to do so as the world’s creatures are being lost at an astounding rate.
“We’re losing things before we even have a name on them, so we absolutely must push to advance our conservation goals,” Terry Goslin, an evolutionary biologist and taxonomist at the California Academy of Sciences who has discovered thousands of species during his decades-long career, told Live Science. “But in some cases we also have to put aside the question of what a species is in order to move forward in a meaningful way.”
Today’s scientists solve the species problem in different ways. Some try to reconcile existing definitions with modern methods, for example by renaming Mayr’s concept of biological species to the so-called genetic species conceptwhich still suggests an inability to reproduce, but links the mechanism specifically to genetic incompatibility.
There are many concepts in science that lack a single meaning, and we still do well in this space of uncertainty.
Yuichi Amitani, University of Aizu
Others continue to develop new ideas. Jeannette Whitton, an evolutionary biologist at the University of British Columbia, co-developed the retrospective concept of the reproductive community. Rather than adopting a rigid definition, the concept encourages scientists to embrace uncertainty and recognize that speciation is an ongoing process—that the organisms we observe today have been shaped by past forces.
This holistic view, which incorporates aspects of several existing definitions, means that scientists can still predict or explain natural phenomena even without a clear definition. Whitton told Live Science that it took her and her colleague seven years to settle on the final language, in part because of how challenging it was to reconcile their own conflicting ideas.
Still others have argued for setting aside the issue of species, noting that the question itself can be a distraction. Yuichi Amitani, a senior associate professor of biology at the University of Aizu in Japan, noted in 2022 that scientists’ fears that the lack of consensus would lead to communication breakdowns and make it impossible to compare research have not come to pass.
“There are many terms in science that lack a unified meaning, and we’re still managing well in that space of uncertainty,” he told Live Science, adding that it seems to be about the idea of ​​a species “that excites such a strong emotional response.”
Confrontation with “taxonomic anarchy”
In many ways, this is where these emotions boil over, with fierce debates in the scientific literature. in 2017 Leslie Christidistaxonomist at Southern Cross University in Australia, argued in paper that the continuing explosion of newly described species in biology—what he called the “anarchy of taxonomy”—challenged conservationists to manage resources or rally support.
Christidis told Live Science that the idea was indeed controversial, prompting more than 180 scientists to sign and public rebuke. But Christidis insists he never meant to suggest that taxonomy has no place in conservation. Instead, he said he was advocating a unified framework for naming new species and resolving disputes.
As scientists develop more sophisticated tools that combine taxonomy with genomics, marker studies, modeling and even machine learning, it is clear that a one-size-fits-all definition is unlikely to be the optimal solution.
Nor is it true that probing new species inevitably leads to more species. When Thomas Closeevolutionary biologist at Yale University, studies the evolutionary history of fish, often finds these separate species, including several popular sport fishthey are actually the same.
“We have to let science take us where it wants to go, and that’s not always necessarily for multiple species,” Near told Live Science.
Working groups are now trying to create new guidelines. The Catalog of life, for example, develops naming rules within each kingdom of life, while other groups carve out even smaller pieces of the puzzle. The World Register of Marine Species tracks marine species while A group of cat specialists reconsiders the taxonomy of the world’s felids.
Christidis leads effort to merge the three existing lists of bird species and hopes to issue a report later this year. After the controversial 2016 documentary doubled the number of bird species under the new definition, the field was clearly required to make a settlement, he said. Fortunately, the group’s efforts reveal that “it is often possible to reach consensus — if not universal agreement — once all the evidence is presented,” he said. From there, it is easier to make a judgment about which species are most in need of protection.
“As scientists, we all want to protect our biodiversity,” Christidis said, “and I think starting from this shared ground has helped tremendously.”