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When scientists concluded in 2007 that the giraffe—long regarded as a single species, Giraffa camelopardalis—should in fact be classified as six or more species, the news worried schoolchildren and conservationists alike. The finding, based largely on genetic evidence, suggested that these graceful, long-necked animals were in trouble. Lumped together as a single species, the giraffe seemed relatively healthy, with a population of up to 110,000 individuals scattered across sub-Saharan Africa. But split apart into at least six separate groups, some of the most beloved animals on earth suddenly looked “hyper-endangered,” as one researcher put it. The study was unsettling on a more basic level, too. The idea that an animal so well known and so big—the giraffe is the tallest animal on earth—could have so many cryptic species hiding beneath its familiar dappled flesh seemed to call into question the notion of species itself.

The concept of species is among the most familiar scientific ideas in our lives. We celebrate (or bemoan) the human species, get excited about the discovery of new species, obsess over the fate of endangered ones, and shout at one another about the book called On the Origin of Species. The word derives from the Latin specere, “to look at” or “to behold.” What we behold, in the conventional view of natural history, is a comforting and lovely sense of order. In a drawer at a museum, the butterflies, dragonflies, beetles, and other insects stand discretely apart, like jewels, each neatly labeled on its mounting pin.

The real world, by contrast, can seem like a seething mess, with one species smudging uncertainly into another. “Fuzzy species are common,” says Rutgers University geneticist Jody Hey. Taxonomists, the scientists who specialize in classification, frequently disagree about how to determine where one species ends and another begins. Ask the big question—“So what is a species, anyway?”—and you discover there is no universally accepted definition. Instead, some 20-odd concepts and interpretations vie for eminence.


The notion of biological species dates back at least to Carolus Linnaeus, the Swedish botanist who invented the system of classification in 1735. A compulsive organizer, he divided life on earth into distinct entities with fixed forms given to them by God. Even at the time, other naturalists saw shades of gray, with one species often separated from another only by barely perceptible nuances.

In the mid-19th century Charles Darwin made these nuances the basis for his theory of evolution by natural selection. He saw that the normal variations among individuals within a species tended to become more significant among separate populations of the same species, and even more so among separate varieties, as each moved down its own evolutionary path. The natural world was a continuum, he concluded, with isolated populations perpetually in the process of becoming species in their own right. The evolutionary perspective meant acknowledging the species designation as “arbitrarily given, for the sake of convenience, to a set of individuals closely resembling each other,” Darwin wrote.

Scientists have been arguing ever since about where to draw the line. For years taxonomists tended to follow the “biological species” concept. As articulated in the 1940s by ornithologist and evolutionary thinker Ernst Mayr, it defines a species as a population of organisms that interbreed and live in reproductive isolation—that is, they do not normally breed with similar populations.

But nothing in nature is as simple as this sounds. Among its other drawbacks, this definition excludes the vast majority of organisms on earth. Bacteria, for instance, do not interbreed at all; they reproduce asexually—and also swap genes in ways that can blur the distinction between species. Even some sexual species would not qualify, according to Richard Mayden, an ichthyologist and evolutionary theorist at Saint Louis University. For instance, certain fish species produce no males, but the females must have sex to trigger parthenogenic development of unfertilized eggs; the female therefore “mates” with males of other species. It’s not exactly virgin birth, but since the males don’t fertilize the eggs, it isn’t interbreeding, either. Strict followers of the biological species concept might also have to classify some dog breeds as separate species, Hey suggests, because a Chihuahua cannot jump high enough to make puppies with a mastiff.

A more general problem, according to critics of the biological species concept, is that it does not really help scientists figure out where one species ends and another begins. Determining whether different populations of a species are interbreeding is difficult at best, especially for scientists looking at specimens in a museum. Figuring out the sex lives of fossil species is nearly impossible.

Influenced by his work with fossils, George Gaylord Simpson, one of the great paleontologists of the 20th century, proposed his “evolutionary species” concept in 1951. It defines a species as a lineage—individuals descended from a common ancestor—that maintains a distinct identity and follows a common evolutionary path through time. The addition of a time line suited scientists limited to working with fossils, often of extinct species. Simpson’s concept was also broad enough to accommodate asexual and parthenogenic species. But in terms of providing an obvious criterion for recognizing a species, it was not much of an improvement.

Looking at species is like looking at clouds. On a sunny day they can seem like distinct entities, with sharp boundaries separating them. On other days they pile up together in dense banks.

Tantalized by these efforts, a small army of evolutionary thinkers fanned out, beginning in the 1960s, on a quest for what Hey calls “the big one,” the magic formula that would somehow address all the murky complications of the natural world and “lay the species problem to rest.” What they ended up with was more like an alphabet soup: The “phenetic” concept defines species mainly according to observable differences in physiological traits. The “genetic” concept puts more emphasis on DNA. The “ecological” concept focuses on ecological niches or adaptive zones. And the “phylogenetic” concept combines descent with “diagnosable” differences in physical traits. But all these concepts suffer from arbitrary cutoff points, arguable assumptions, and cases in which they flat out do not work. One formidable critic, until his death at 101 in 2005, was Mayr, who clung to the biological species concept. He derided rivals as “armchair taxonomists” and asserted that some of them had “never personally analyzed any species populations or studied species in nature.”