In 2016, scientists published a newspaper with a bold claim: that the giraffe, first described as a species by the Swedish biologist Carl Linnaeus in 1758, is actually always four kinds. Unlike Linneaus, the researchers had access to modern genetic tools, which showed 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 said. said at the time.
The news sent shockwaves through the giraffe conservation community, which suddenly had four species to protect instead of one. But from the start, there was disagreement about this new classification, and even today the International Union for Conservation of Nature – an organization that oversees the list of endangered species – lists the giraffe as a single species, Giraffe camelopardaliswith nine subspecies.
This and similar fuss highlights the “species problem”: a fundamental uncertainty about the way we analyze organisms that continues to vex biologists around the world.
Arguments often revolve around decades-old definitions. In 1942, biologist Ernst Mayr coined 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 developed definitions based on shared ancestry (the phylogenetic species concept), physical characteristics (the morphological species concept), or shared ecology (the ecological species concept), with species diverging as they occupy different environmental niches. In total, there are at least 16 species definitions, and possibly as many as 32, circulating among scientists today.
However, no single definition seems to exist without exception. There are species in which individuals look very different, as well as “cryptic species” that appear identical but are genetically distinct. Hybridization is also common, resulting in animals such as the liger (a lion-tiger hybrid) and the beefalo (a cross between domestic cattle and the American bison). There is even evidence that humans once interbred with two other ancient hominins usually considered separate species, the Neanderthals and the Denisovans, suggesting that they may not have been so different from us after all.
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“Some of the rules we put in place don’t work, and sometimes it gets quite messy,” Jordan Caseya marine molecular ecologist at the University of Texas at Austin Marine Science Institute, told Live Science. “People naturally 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 unnecessarily bend things into different species.”
But establishing the definition of a species is not just an academic exercise — much of the world’s conservation policy is structured around species as the de facto unit of protection. Ultimately, it also raises more existential questions. After all, if there are four species of giraffe, does it really matter if one becomes extinct?
To answer these questions, groups are now coming together to create guidelines for how species should be named and ordered on the tree of life, and how to handle disputes when they arise. Creating a working list of agreed-upon rules is indeed crucial, even if it is not perfect, biologists say.
‘It’s getting pretty messy’
The concept of a species is an old concept. In 343 B.C. for example, Aristotle wrote “History of Animals,” in which he described differences between individual animals and between groups.
But it was not until the mid-18th century that the concept of taxonomy — the formal classification of living things — became truly popular and was turned into an official discipline by Linnaeus. Taxonomy flourished for a while as scientists around the world began naming new species, but as the field and related species developed, 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, apparently well-studied animals, adaptations are quite common, and iconic animals such as 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 different as mammals, birds, fish, plants and bacteria. Still others argue whether such an exercise is even useful, noting that scientists have pressed on for centuries without consensus and will continue to have to do so as the world’s creatures are lost at a staggering rate.
“We are losing things before we even put a name to them, and that is why we absolutely must continue to push to achieve our conservation goals,” Terry Goslineran evolutionary biologist and taxonomist at the California Academy of Sciences who has discovered thousands of species in his decades-long career, told Live Science. “But in some cases we also need to put aside the question of what a species is to move forward in meaningful ways.”
Today’s scientists are tackling the species problem in different ways. Some are attempting to reconcile existing definitions with modern methods, such as by renaming Mayr’s biological species concept as the genetic species conceptwhich still suggests an inability to reproduce, but specifically links the mechanism to genetic incompatibility.
There are many concepts in science that have no clear meaning, yet we manage just fine in that space of uncertainty.
Yuichi Amitani, Aizu University
Others continue to develop new ideas. Jeannette Whitton, an evolutionary biologist at the University of British Columbia, developed the retrospective reproductive community concept with others. Rather than a strict definition, this concept encourages scientists to embrace uncertainty and recognize that speciation is an ongoing process — that organisms we observe today were shaped by past forces.
Adopting this holistic view, which encompasses facets 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 arrive at the final language, in part because of how challenging it was to reconcile their own conflicting ideas.
Still others have argued for putting the species issue aside, noting that the question itself could be a distraction. Yuichi Amitani, an associate professor of biology at Aizu University in Japan, noted in 2022 that scientists’ fears that a lack of consensus would lead to communication breakdowns and make it impossible to compare research have not materialized.
“There are a lot of concepts in science that don’t have a uniform meaning, and we’re still doing just fine in that space of uncertainty,” he told Live Science, adding that there seems to be something about the idea of a species “that elicits such a strong emotional response.”
Confrontation with ‘taxonomy anarchy’
In many ways, conservation is the issue where these emotions boil over, and fierce debates take place in the scientific literature. In 2017, Leslie Christidisa taxonomist at Southern Cross University in Australia, argued in a newspaper that the continued explosion of newly described species in biology – what he called “taxonomic anarchy” – made it difficult for conservationists to distribute resources or raise support.
Christidis told LiveScience that this idea was indeed controversial, prompting more than 180 scientists to sign an agreement public reprimand. But Christidis insists he never meant to suggest that taxonomy has no place in conservation. Instead, he said, he advocated one uniform framework for naming new species and resolving disputes.
As scientists develop increasingly sophisticated tools that combine taxonomy with genomics, tagging studies, modeling, and even machine learning, it is clear that the optimal solution is unlikely to be a one-size-fits-all definition.
It is not even true that research into new species inevitably leads to more species. When Tomas nearbyan evolutionary biologist at Yale University, studies the evolutionary history of fish. He often finds that individual species, including several popular sport fishare in fact the same.
“We have to let science lead us where it leads us, and that doesn’t always have to lead to more species,” Near told Live Science.
Working groups are now trying to establish new guidelines. The Catalog of lifefor example, develops rules for naming within each kingdom of life, while other groups carve out even smaller pieces of the puzzle. World Register of Marine Species keeps track of marine species, while the Cat Specialist Group reevaluates the taxonomy of the world’s felines.
Christidis leads an effort to merge three existing lists of bird species and hopes to release a report later this year. After a controversial 2016 paper doubled the number of bird species based on a new definition, the field was clearly due for a reckoning, he said. Fortunately, the group’s efforts show that “it is often possible to reach consensus — if not universal agreement — once all the evidence has been presented,” he said. From there it is easier to assess which species need protection the most.
“As scientists, we all want to protect our biodiversity,” Christidis said, “and I think starting from that shared foundation has helped tremendously.”