What defines a species? Inside the fierce debate that's rocking biology to its core

In 2016, scientists published a paper with a bold claim: that the giraffe, first described as a species by Swedish biologist Carl Linnaeus in 1758, might actually have been four species all along. Unlike Linneaus, the researchers had access to modern genetic tools, which 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 said at the time. 

The news sent ripples through the giraffe conservation community, which suddenly needed to protect four species 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 listing of threatened and endangered species — lists the giraffe as a single species, Giraffa camelopardalis, with nine subspecies.

The dustup and others like it highlight the "species problem," a fundamental uncertainty over how we parse organisms, and it continues to rile biologists the world over.

Arguments often hinge on decades-old definitions. In 1942, biologist Ernst Mayr coined what is perhaps the most enduring one: the biological species concept, which labels two organisms as different species if they cannot reproduce and create fertile offspring. Researchers have since established definitions on the basis of shared ancestry (the phylogenetic species concept), physical features (the morphological species concept), or shared ecology (the ecological species concept), wherein species diverge as they take over different niches in their environment. In all, there are at least 16 species definitions, and potentially as many as 32, circulating among scientists today.

No definition seems to be without exception, however. There are species in which individuals look very different from one another, as well as "cryptic species" that appear identical but are genetically distinct. Hybridization is also common, leading to animals like the liger (a lion-tiger hybrid) and the beefalo (a cross between domestic cattle and the American bison). Evidence even suggests that humans once bred with two other ancient hominins that are usually considered separate species, the Neanderthals and the Denisovans, suggesting they might not have been so different from us after all.

Related: 'More Neanderthal than human': How your health may depend on DNA from our long-lost ancestors

"Some of the rules that we set don't work, and it gets quite messy sometimes," 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 on things, and even I have to make a lot of decisions about whether I'm just seeing diversity between individuals or trying to bend things needlessly into different species."