Last summer, researchers at MIT and Harvard announced a shocking possibility: Humans and chimps may have shared some lovin', well after they had evolved into two distinct species from a common ancestor. And in the past few years, rumors have spread of improprieties between Homo erectus and Neanderthals, also considered separate species. These suggestions refer not to isolated cases of pervy humanoids and a few willing shaggy primates, but to the full-scale mixing of entities that biologists have long considered, by definition, unmixable. They also hint at the growing rumble among biologists who study the vast diversity of organisms that populate the planet. How to define the word "species" lies at the core of the debate.
"Species is just so ingrained in the popular culture," says Brent Mishler, a plant biologist at the University of California, Berkeley, and director of Berkeley's Jepson Herbarium. "Everybody's heard of the biological species concept, the one that says organisms that can interbreed are part of one species, and if they can't, they're not. The average educated person, if you ask them what a species is, they'll be able to tell you that."
For most biologists, there is no such consensus. By one count in a recent publication, they have populated the planet with at least 22 different definitions of the word while we lay folks weren't looking. The revolution in genetics and molecular biology over the past decade has sharpened the focus on new ways to make distinctions between groups of organisms.
But in a world where conservation groups count species to develop policies to protect them from extinction, how they are classified makes a big difference. "The theoretical debate influences exactly which thing gets recognized as a species," says Kevin de Queiroz, a research zoologist at the Smithsonian Institution in Washington, D.C. "That has a direct effect on what gets protected."
What shall we count?
How many species are there, anyway? That's a slippery question. Apart from the issue of definitions, many unique types of organisms have yet to be discovered. That's especially true in remote parts of the world. But even here in California, botanists describe about a dozen novel species of plants each year. "The basic job of naming the [plant] species of the state is still not done," says Mishler.
The nexus of this effort is the Jepson Herbarium, the archive of every known plant in the state. "The actual flora of California is documented better in here than outdoors," Mishler notes.
If the word "herbarium" makes you think of fecund plant beds and the lush decadence of a greenhouse, you are mistaken. Pass under the giant ferns reaching over the Jepson Herbarium's entrance and through the glass doors, and inside, the closest thing to living foliage you would see on this winter Friday afternoon is the white-on-green fern pattern on Mishler's faded button-down shirt. Rather, it is like a library, pristine and ordered, with most of the collection hidden away in long rows of two-meter-high metal cabinets, all a bland beige.
The drawers are filled with more than two million specimens, dried and mounted on large paper sheets. Some are holotypes, the proto-ancestor of its name, the single find that spawned human understanding of a species. Most are stored as single specimens, but one sheet is covered with semi-ordered rows of yellow-petaled flowers like peeling wallpaper in an old Victorian house.
On each cardboard sheet is a white sticker with descriptions of the species' key traits, scribbled by the botanist who first brought it to light. The Grindelia hirsutula, for example, has been glued down since 1862, when William Brewer, working with the California Geological Survey, gathered the samples that founded the Jepson collection. Botanists who pull a specimen for study leave their own notes on new stickers. "It's like a calling card," Mishler explains, a piece of each species' personal history.
But it's not just the quiet, the order, and the history here that feel soothing. It's also that policymakers don't often fight over plants, save for the occasional endangered one in the path of a bulldozer. Rather, conservationists direct most of their attention to creatures, preferably fuzzy or feathered ones. That makes the decisions more charged.
Who says sex is so important?
The word "species" has roots in ancient Latin. Its original meaning, a "kind" or a "sort," is as simple as it is unscientific. Indeed, early taxonomists believed God created each living thing as a distinct entity. "There were botanists who insisted that we ought to have 100 families of plants, because it was just a nice number," says Mishler.
In the century before Charles Darwin, Swedish botanist Carolus Linnaeus invented a way to classify these entities into a ranking system, covering the broadest to the slightest differences between groups of organisms. Most everyone in high school biology memorizes the seven ranks, often along with a helpful mnemonic: Kingdom, Phylum, Class, Order, Family, Genus, Species. "King Philip came over for great spaghetti!"
Linnaeus's classification grouped organisms based on similar morphology, or appearance. This boils down to the notion that if it looks like a duck and walks like a duck, especially when scrutinized with a handheld magnifying glass, well, it's probably a duck. In the context of today's high-powered techniques this may sound imprecise, but overall, biologists agree that it worked very well and still does.
But with his theory of evolution, Darwin gave a new explanation of where species come from, and how the diversity we see can evolve from a common ancestor. This new vision, says Mishler, proclaimed that "a genus isn't a plan that God had—it's [an evolutionary] lineage." Although the Linnaean naming system remained unchanged, taxonomists gradually came to see ranks as largely a convention for grouping organisms that we humans use to represent an organism's evolutionary history. Most ranks, particularly the higher ones, were considered somewhat arbitrary.
And so things stood for another century, until Harvard ornithologist Ernst Mayr brought together evolution and the theory of genetics, devised by the 18th-century monk Gregor Mendel. Mayr formulated a genetic explanation for how different species evolve from a single common ancestor. And he cemented the definition that most of us now take for granted: If two populations' individuals don't reproduce in nature, they are distinct species. This idea is now known as the biological species concept.
The problem is that some species do breed with others, especially if they became distinct relatively recently. "Despite how beautiful that picture is, it's nice in theory but it just doesn't work," says Mishler. "Lineages diverge from their relatives until such point as it's convenient to call them species. But there's no magic moment when they go, 'Oh, I'm a species now.'"
And for the many species of plants that reproduce asexually, the biological species concept simply doesn't apply. Evolutionary biologists consider Mayr one of the greatest 20th-century minds in the field, but some blame him for attaching an importance to the word species that it should not carry.
The problem runs even deeper, straight down to the core of the Linnaean system of naming. Ranks are "just hold-overs, like the English metric system," says Mishler, from a time when they had an almost religious significance. He and a growing faction of biologists advocate tossing the Linnaean system altogether. Instead, they wish to classify species based on how closely they are related—or to put it another way, how long ago they diverged.
This approach, which involves determining differences in visible traits, behavior, or genes to determine evolutionary closeness, is called phylogenetics. The idea, phylogeneticists say, is to place each organism in relation to all others on a "tree of life." Mapping relationships in this way is like using a coordinate system with longitude and latitude, says David Baum, a botanist at the University of Wisconsin. "We'll be able to better handle diversity if the system is relationship-based," he explains.
Species concepts galore
Some scientists insist the biological species concept needs no overhaul. Loren Rieseberg, a plant geneticist at the University of British Columbia, examined studies that looked at variation. For the most part, he found, the groups that tended to have morphological differences between them were the ones that didn't interbreed. "There would have been an even better correspondence if [the phylogenetic species concept] hadn't spread," he notes. While there are exceptions, Rieseberg says, "The bottom line is, there are good species [using the biological species concept] and taxonomists are doing fairly well in discovering them."
What's more, the alternative may be causing trouble. The phylogenetic species concept looks for differences—any differences, some charge—whether they be single mutations in one gene, or major behavioral changes such as a breeding preference. "But if you take that criterion to its endpoint, we're all distinct," says Keith Crandall, a conservation biologist at Brigham Young University in Provo, Utah, who works on crayfish.
Taxonomists debating the status of species often get thrown into categories themselves. Consider two similar populations of birds that now live on two different islands. The lumpers might call them one species, while the splitters might divide them into two. Phylogeneticists tend to be splitters. And that could result in researchers identifying many more species.
Others argue that both approaches are misguided. "I think it's better to define species as a kind of an effect that you can see in nature," says James Mallet, a zoologist at University College London, who works on butterflies. "Phylogeny is something that you can infer about species, but it's not what species are. Similarly, reproductive isolation is what causes species, often, but it's not what species are." The problem, he says, is that biologists are mixing up two concepts that should remain separate.
So is the major battle between the biological species concept and the phylogenetic species concept? "Well, you could say that," Mallet says. Citing a well-known book in the field, he adds a third, called the genotypic cluster. "Biologists are embracing the idea that species are genotypic clusters, and we just have to decide the level of clustering that we want to call a species."
After a string of conversations with taxonomists describing their varied theories, the difference seems subtle to say the least—a fact that is not lost on Mallet. "I regard mine as acceptable to everyone," he says. "My interpretation of it is, I'm right and everyone else is wrong. Unfortunately, that's everyone else's interpretation of it too."
From concept to critter
According to the Endangered Species Act, a group of organisms must be a species or a subspecies to be listed for protection. Another level of protection, referred to as a distinct population segment, or DPS, is available only to vertebrates.
But the DPS is used sparingly, explains Brad Shaffer, a zoologist at the University of California, Davis. "Some people, and I happen to be one of them, believes [the DPS classification] gives the act some flexibility," he says. "If you lack that, it's a roll of the dice"—either you're in, as a species, or you're not. The act also contains what's known as a "hybrid clause." Organisms whose populations exchange genes are not eligible for protection, because they are not unique entities. "With subspecies this is almost by definition a problem," Shaffer says. "If they didn't hybridize, you'd call them species."
Regardless of where they stand in the debate, most biologists agree that ultimately, determining whether a bunch of organisms makes a species is a subjective call. "It's just a tough thing, lining up those crawfish from Arkansas and figuring out who's who," says Crandall. "You can have whatever theory you want, but when you get out into the field, all hell breaks loose."
That disorder, in turn, fuels endless disputes over conservation policy that often have an overtly politicized bent. In recent years, disagreements (and often lawsuits) have erupted over the species or subspecies status of animals, including the polar bear, the red tree squirrel in Arizona, a beach bird called the snowy plover, the California sea otter, West Coast salmon, a cave spider called the Karst Meshweaver in Texas, the Idaho springsnail, and a long list of others.
Another example is the California tiger salamander. Shaffer has long studied the creature, a little guy about 8 to 12 centimeters long, with a shiny black coat and yellow dots and stripes. Biologists generally divided the species into three different subspecies, found in Santa Barbara, Sonoma, and in the Central Coast region. But gene studies on the Santa Barbara population suggested it might be different from the other two. The data were perfectly clear, he says, but only for one gene.
Still, Shaffer recalls, he got a lot of pressure from environmentalists looking to ensure the salamander's protection. "A lot of people said, let's call it a species," he says. Evidence from one gene was not enough, he says—although further studies showed other genetic differences, and now he's convinced. "The only way scientists can make their policy relevant is to turn off their political opinions," Shaffer says.
But the process is stymied because there are no good standards for sampling the data or for analyzing it. Determining whether a population is a species means studying several individuals at many locations. "What a lot of people mean by population is a genetically circumscribed unit," says Crandall. "What other people mean is, 'Where I parked my car to sample.'"
More and more, such studies rely on genetics. "There's a tendency to think genetic information is a silver bullet," says Sylvia Fallon, Conservation Genetics Fellow at the Natural Resources Defense Council in Washington, D.C. But researchers and agencies have no agreed-upon system—such as a cut-off for the percent of divergence—on which to base decisions, particularly below the species level. And because the type and amount of genetic data used can vary so widely, she says, "That type of data is open to manipulation."
In a study two years ago, Crandall and his colleagues examined published studies of researchers weighing data on species decisions that would impact conservation policy. Most of the time, they found, researchers claimed to use a set of criteria. In cold, hard fact, however, they didn't. "We all love our species, and we all want them conserved," Crandall says. "We're happy to bend our rules to get whatever we want saved."
Such a debate still rages over the Prebles' meadow jumping mouse, a bushy brown critter from Colorado with a tail half its length, currently described as a subspecies. In 1998, the mouse was listed as endangered. Two years ago, a genetic study conducted by a researcher from the Denver Museum of Nature and Science claimed the mouse was not a subspecies, because it seemed to exchange genes with other subspecies.
But Crandall, who advised the U.S. Fish and Wildlife Service on this issue and other policy questions, says the study was fraught with bad science. Problems ranged from where the investigators found their mice to the genes they studied. After the agency commissioned a new study, says Crandall, "They did find genetic differences, which was all they wanted." But to his mind, the second study was just as flawed, though for different reasons. "The government is literally spending millions of dollars building tunnels [so the mice can cross safely under roads] without spending the money to do the proper study."
From critter to diversity
Such cases are usually resolved by individual experts, who rarely butt heads over the biological versus the phylogenetic species concepts. But on the population level, things could get messier still. "If you're the Nature Conservancy and you do a study looking for species hotspots, that number is really biased depending on your species concept," says Crandall.
According to a paper published by Mallet and two colleagues in 2004, the number of listed primate species has doubled since the 1980s. A few genuinely new primates surfaced, but most arose because zoologists recategorized subspecies as species. Counts could differ by a full order of magnitude depending on the taxonomy used, the study concluded.
Other researchers, including Crandall, have also found such "taxonomic inflation." In his review of 89 studies assessing thousands of species, the number of species of plants, animals, fungi, and lichen multiplied like rabbits under the phylogenetic species concept as opposed to the biological species concept. Some animals, such as birds of paradise and parakeets, shot up threefold or more.
Why should we care? Well, first, there's the cost. More than a decade ago, the Fish and Wildlife Service estimated that saving a single species on the endangered list would cost about $2.76 million. Double the species means double the listings—and double the money. Additionally, some researchers say this kind of confusion could lead people to mistrust the science behind conservation work.
Phylogeneticists, however, insist that public trust, as well as scientific accuracy, is precisely the point. "We've probably undercounted, and the number does need to be increased," says Mishler. "If we have a group that we now call a species but it's actually heterogeneous, the conservation community needs to know about that."
Some taxonomists and experts in the conservation community are starting to promote the idea that simply counting species may not be the best way to measure diversity at all. A conservation plan that saves ten very closely related organisms—even if they are all distinct species—is not as good as one that saves five more diverse ones. Consensus on the definition so far seems a distant fantasy.
"Folks have been debating that for centuries," says Crandall. "Turns out, we're not even close to resolving it."
ABOUT THE WRITER
Alla Katsnelson
B.A. (College Scholar), linguistics, psychology, and neurobiology, Cornell
University
D.Phil., physiology, Oxford University
First job: The Scientist (Philadelphia)
My interest in science began with language. Growing up bilingual, I
wondered why I experienced things differently in my two languages and what
this said about the software and the hardware that made up "me." In a
college lab class, I became fascinated with physiology as we poked snail
cells with electrodes and watched the arcs of ion channels that regulate
their simple nervous systems. So I went to graduate school to study
cellular aspects of brain development. But many dead rats later, I began
to suspect that I’d much prefer writing about science to doing it. Doing
research on mashed up cells and on whole humans taught me that scientific
method is as much capricious art as ordered process. I believe this is
science’s biggest untold story, and it's the one I most want to tell.
ABOUT THE ILLUSTRATOR
Valerie L. Layne
B.S., botany, and M.A., biology, San Jose State University
I’ve always been curious about what makes ecosystems work, and I’ve had
the good fortune to be able to explore this while working as a biologist
in many parts of California and in Alaska. Drawing has always been
secondary to "real work," but it’s starting to take a much larger role in
my life. Now, I’d like to take what I’ve learned working over the last
several years and combine it with art. I have long thought about creating
a field guide to wetland vegetation, and about depicting habitat values
I’ve observed on the agricultural landscapes of the Central Valley. I am
very excited to be in this program and to explore my dreams!