In 2016, scientists published a letter with a bold claim: that the giraffe, first described as a species by the Swedish biologist Carl Linnaeus in 1758, may actually have been four types throughout time. Unlike Linneaus, researchers had access to modern genetic tools, which revealed that giraffes fall into distinct groups based on differences in their DNA, some of which are “greater than the differences between brown bears and polar bears “, the authors. said at the time.
The news sent ripples through the giraffe conservation community, which suddenly had 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 – an organization that oversees the list of threatened and endangered species – lists the giraffe as a single species, Giraffe camelopardaliswith nine subspecies.
The dust and others like it highlight the “species problem,” a fundamental uncertainty about how we analyze organisms, and it continues to trouble biologists around the world.
Arguments often hinge on decades-old definitions. In 1942, biologist Ernst Mayr coined what is perhaps the most enduring: the biological species concept, which labels two organisms as different species if they cannot reproduce and produce fertile offspring. Researchers have since established definitions based on shared ancestry (phylogenetic species concept), physical features (morphological species concept), or shared ecology (ecological species concept), where species change as they occupy different nodes in the environment. theirs. In total, there are at least 16 species definitions, and potentially as many as 32, circulating among scientists today.
However, no definition seems to be without exception. There are species in which individuals look very different from each other, as well as “cryptic species” that look identical but are genetically distinct. Hybridization is also common, leading to animals such as the liger (a lion-tiger hybrid) and the beefalo (a cross between domestic cattle and the 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 might not be so different from us after all.
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“Some of the rules we’ve put in place don’t work, and sometimes it gets pretty messy.” Jordan Casey, a marine molecular ecologist at the University of Texas at Austin Marine Science Institute, told Live Science. “Humans inherently want to put order in things, and even I have to make a lot of decisions about whether I’m just seeing diversity among individuals or trying to turn things unnecessarily into different species.”
But determining the definition of a species 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 raises even more existential questions. If there are four species of giraffe, after all, does it really matter if one goes extinct?
To answer these questions, groups are now meeting to establish guidelines for how species should be named and ranked on the tree of life, and how to handle disagreements when they arise. Indeed, creating a working list of agreed-upon rules is essential, even if it’s not perfect, biologists say.
“It gets pretty messy”
The concept of a species is an ancient concept. In 343 BC, for example, Aristotle wrote The History of Animals, in which he described the differences between individual animals as well as between groups.
But it wasn’t until the mid-1700s that the concept of taxonomy—the formal classification of living things—really took off and was turned into an official discipline by Linnaeus. Taxonomy flourished for a time as scientists around the globe began naming new species, but as the field and others progressed, conflicts inevitably arose.
Scientists have officially described about 2 million species, and others are constantly being added or reclassified based on new evidence. Even for large, seemingly well-studied animals, adjustments are quite common, and iconic animals such as the giraffe, African elephant and orca have come under scrutiny.
The problem is that scientists cannot agree on a universal definition that can classify organisms as diverse and varied as mammals, birds, fish, plants and bacteria. Still others argue whether such an exercise is even useful, noting that scientists have continued in the absence of consensus for centuries and will have to do so again as the world’s creatures disappear at an alarming rate.
“We’re losing things before we even have a name on them, and so we have to keep pushing in order 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 meaningful ways.”
Today’s scientists are tackling the species problem in different ways. Some are trying to reconcile existing definitions with modern methods, such as renaming Mayr’s concept of biological species as The concept of genetic specieswhich still suggests an inability to reproduce, but links the mechanism specifically to genetic incompatibility.
There are many concepts in science that lack a unified understanding, and we still do well in that 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, coded the retrospective concept of the reproductive community. Rather than adopting a strict definition, this concept encourages scientists to embrace uncertainty and acknowledge that speciation is an ongoing process—that the organisms we observe today have been shaped by past forces.
Taking this holistic view, which includes aspects of several existing definitions, means that scientists can still make predictions or explain natural phenomena even in the absence of a clear definition. Whitton told Live Science that it took her and a colleague seven years to settle on the final language, in part because of how challenging it was to reconcile their conflicting ideas.
Still others have argued for setting aside the species problem, noting that the question itself can be a distraction. Yuichi Amitani, a senior associate professor of biology at Aizu University in Japan, noted in 2022 that scientists’ fears that a lack of consensus would lead to a breakdown in communication and make it impossible to compare research have not happened
“There are many concepts in science that lack a unified understanding, and we still do well in that space of uncertainty,” he told Live Science, adding that there seems to be something about the idea of a species “that teases such a strong. emotional response”.
Confronting “taxonomic anarchy”
In many ways, conservation is where those emotions boil over, with fierce debates playing out in the scientific literature. in 2017, Leslie Christidisa taxonomist at Southern Cross University in Australia, argued in a letter that the continuing explosion of newly described species from biology—what he called “taxonomic anarchy”—was making it challenging for conservationists to direct resources or rally support.
Christidis told Live Science that the idea was indeed controversial, prompting more than 180 scientists to sign a public reprimand. But Christidis insists he never meant to suggest that taxonomy has no place in conservation. Instead, he said, he was advocating for one unified framework for naming new species and managing disputes.
Indeed, as scientists develop more sophisticated tools that combine taxonomy with genomics, tagging studies, modeling and even machine learning, it is clear that the optimal solution is likely not a single definition.
Nor is it true that the search for new species leads inevitably more species. When Thomas Nearan evolutionary biologist at Yale University, investigates the evolutionary histories of fish, he often finds that particular species, including some popular sport fishin fact they are the same.
“We have to let science take us where it will, and that’s not always necessarily for more species,” Near told Live Science.
Working groups are now trying to create new guidelines. of Catalog of Life, for example, is developing naming rules within each kingdom of life, while other groups are carving out even smaller pieces of the puzzle. of World Register of Marine Species is the tracking of marine species, while Cat Specialist Group is reevaluating the taxonomy of the world’s felids.
Kristidis is in charge an attempt to merge three existing lists of bird species and hopes to publish a report later this year. After a controversial 2016 article doubled the number of bird species based on a new definition, the field was clearly in need of an account, he said. Fortunately, the group’s efforts are revealing that “it is often possible to reach consensus — if not universal agreement — after all the evidence has been presented,” he said. From there, it is easier to judge which species are most in need of protection.
“As scientists, we all want to protect our biodiversity,” Christidis said, “and I think starting from that common ground has helped tremendously.”