In 2006, a team led by Ed Louis, Jr., a molecular biologist at Omaha’s Henry Doorly Zoo, identified fourteen new lemur species. The discovery was not made in some forgotten enclave of Madagascar, the only place where lemurs live in the wild, but rather in a laboratory. Last year, after reviewing the data, the International Union for the Conservation of Nature determined that all the new lemurs were at “high risk of extinction,” or worse.

In the last twenty years, the total number of lemurs known to Western science has expanded from thirty-three to a hundred and one. By comparing lemurs’ D.N.A. using sophisticated computer programs housed in laboratories in Europe and the United States, scientists are now able to differentiate between neighboring populations of only a few hundred individuals. According to a recent, controversial change in definition, each of these subdivided populations can be called its own species.

We used to have a more intuitive system for sorting out living things. When the taxonomist Carl Linnaeus first codified the modern scientific classification system in the eighteenth century, he believed each species represented the descendants of a first pair created by God. With his “Systema Naturae,” he sought to glorify God by cataloguing his designs. A century later, when Darwin upended the biblical view of creation with his theory of evolution by natural selection, he also threw taxonomy into chaos. He observed that the distinctions between species that make sense on a single day, in a particular place, often lose coherence when you take a global perspective or begin thinking in evolutionary time scales. If a type of bird has yellow feathers on one side of the continent, red feathers on the other side, and a mixture in between, is it one species? Two? Three? Likewise, if an animal has gradually evolved over millions of years from one species into another, where do you draw the line between the two? If you accepted evolution, Darwin concluded, then species would have to be treated as “artificial combinations made for convenience.”

In the early part of the twentieth century, several biologists independently converged on a possible solution to “the species problem,” as these issues have collectively become known. Species would be delineated based on whether they could mate and bear fertile offspring. The result was the Biological Species Concept, or B.S.C., which was most famously articulated in 1942 by Ernst Mayr, who wrote, “[Species are] groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups.”

It was a seductively simple definition that reassured biologists that species were real entities after all. The B.S.C. remained the dominant definition for many decades.

The B.S.C., of course, came with its own questions. For example, if two identical populations were separated by a geographical barrier, how could you assess the potential for interbreeding under natural conditions? And how could you apply the idea of interbreeding to asexual organisms like bacteria, which represent up to half of the biomass on earth?

In the eighties, D.N.A. sequencing and other techniques made it possible to create high-resolution evolutionary maps, known as phylogenies, that clearly showed that the ability to interbreed did not provide a good proxy for relatedness. Some distantly related taxa had retained the ability to interbreed while other, more closely related species had already lost it.

In 1983, Joel Cracraft, an ornithologist at the American Museum of Natural History, and his colleagues developed a new definition known as the Phylogenetic Species Concept. The P.S.C. takes the focus away from interbreeding. Instead, under this definition, if you can find an inherited trait that is common to one population and not found among its neighbors, then that population can be considered a species. It could be anything from the color of a wing feather to a tiny snippet of unique D.N.A. These sorts of characteristics can become fixed in small populations long before those populations lose the ability to interbreed with their neighbors. The P.S.C., as a result, delineates many more species than the B.S.C. It was under the P.S.C. that Louis, Jr., and his colleagues identified fourteen new species.

There are many purely scientific reasons to prefer the P.S.C. (it can be applied to asexual organisms, for example), but its appeal extends beyond the laboratory. Each year, governments and non-governmental institutions spend an estimated twenty-five billion dollars worldwide on protecting biodiversity, a term that is almost always quantified by counting species. Funding is frequently steered to those countries or regions with the most species, especially if they are endemic or charismatic. Endangered species have become especially potent tools for conservationists because they can generate funding and legal protection for entire ecosystems. P.S.C. species usually have a smaller number of individuals than the B.S.C. species they were derived from, and are therefore more likely to be classified as endangered.

Fifty years ago, during the heyday of the B.S.C., a group of influential environmentalists persuaded people all over the world that species preservation was of paramount importance. If scientists now change the meaning of the word “species” to take advantage of the laws and goodwill that have been built for their protection—if it even appears they are engaged in such legerdemain—the result could be a loss of credibility for both science and conservation.

Ian Tattersall, a lemur expert at the American Museum of Natural History, laments the “taxonomic inflation” that has occurred among lemurs and other groups as a result of work by people like Louis, Jr., and fears an impending backlash led by the traditional enemies of conservation. But he also feels conflicted over the harm that might be done by any deceleration of efforts to save endangered species that may come when the inflation gets corrected. Lemurs are spectacular animals, and the forests they depend on are being destroyed at a staggering rate. By some estimates, only ten per cent of the original forest cover on Madagascar remains. “If recognizing lots of species is actually going to help to conserve some of these environments, I can’t really be against that,” he said.

Anders Halverson is the author of “An Entirely Synthetic Fish: How Rainbow Trout Beguiled America and Overran the World.”

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Above: The ring-tailed lemurs of Madagascar. Photograph by Bas Czerwinski/AFP/Getty.