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.
