The failure to pin down the term “species” continues to vex many evolutionary biologists today. Physicists have the atom. Molecular biologists have DNA. Some evolutionary biologists worry that failing to define their fundamental unit of study with the same precision leaves them open to criticism that they are doing something less than hard science. In Colorado, for instance, state and federal fisheries experts were recently spending hundreds of thousands of dollars a year over five years to restore populations of a threatened subspecies of trout, the greenback cutthroat. Then in 2007, genetic analysis suggested that most of the fish being protected belong to a much more common subspecies, the Colorado River cutthroat trout. Even conservationists could not tell the difference just by looking.
If biologists cannot define it and sometimes do not know it when they see it, is the idea of species real? In many cases, especially among insects, separate species can appear identical except for minute differences in their genitalia. Because they can have such a direct influence on reproductive success, genitalia evolve more quickly and in more bizarre ways than any other animal trait. And since they may determine whether two individuals can interbreed, genitalia often provide a reliable guide to species identity. “Pull out the genitalia,” says Maxi Polihronakis, a beetle taxonomist at the Santa Barbara Museum of Natural History, “and often everything becomes clear.”
Or not. One difficulty with using any morphological trait is that there are not enough experts in the world with a working knowledge of the differences that distinguish closely related species, whether it is the pattern of bristles on the genitalia of Anopheles mosquitoes, say, or the dorsal prickles in the pufferfish genus Lagocephalus. And even that expertise does not guarantee that the morphological differences will yield absolute answers. The question is always where to draw the line: Are the differences just a matter of normal variation among individuals within a species? Or do they suggest that individuals or varieties belong to separate species?
Genetic analysis might sound like the perfect tool for resolving these messy complications. The term “DNA bar coding” suggests that the process is as straightforward as using a laser scanner to separate chicken noodle soup from beef barley in the supermarket checkout line. And it is, in fact, quick and cheap. A gene sequencing machine followed by analysis can produce bar coding results on a batch of specimens in several hours at $10 apiece. But bar coding is seldom conclusive when it comes to designating a new species.
Bar coding typically involves sequencing a few short segments of animal DNA from the mitochondria, the mini-organs that produce energy within every cell. Mitochondrial DNA has a fast mutation rate and hence is a quick-and-dirty indicator of a possible species difference. But since this DNA is inherited only from the maternal line, it does not go through the normal genetic process of division and recombination. That means traits are not steadily diluted to the point of insignificance. If two species have mixed in the past, the genetic evidence of that indiscretion may linger like an archaeological record for 10,000 years or more. That persistence can give the misleading impression that these species still interbreed today. Bar coding suggests, for instance, that savanna elephants and forest elephants belong to the same species, Loxodonta africana. DNA from the cell nucleus, which includes both maternal and paternal lines, tells a different story: The two types of elephants are in fact separate species, leading recently to a proposed relisting of the forest elephant as Loxodonta cyclotis.
Even so, DNA bar coding is turning taxonomy on its head, suggesting that valid species can exist in the absence of any morphological difference whatsoever. In Costa Rica’s Area de Conservacion Guanacaste, a group of researchers have collected some 450,000 caterpillars over three decades and reared them in captivity. Among other things, they were interested in parasitoid insects whose reproductive strategy is to find a caterpillar and lay an egg on or in it. The egg produces a larva that develops by devouring the caterpillar’s innards, eventually bursting out, Alien-style, to become an adult fly or wasp. Recently the researchers used bar coding to take a closer look at 16 species of parasitoid flies known to scientists for more than a century. Hidden within each of the 16 was evidence of four or five cryptic species that looked identical even to experts but that were nonetheless separated from one another by an average genetic distance of about 4 percent. (By comparison, humans and chimpanzees differ genetically by about 2 percent.)
The scientists then went back and looked at the caterpillars from which the flies were reared. It turned out that the genetically different individuals were ecologically and behaviorally different, too. Researchers had assumed that the original 16 species were all generalists parasitizing any caterpillar that happened to be in the wrong place at the wrong time. But at least 64 of the 73 new species were actually specialists, each focusing its deadly attentions on just one or two caterpillar species.
That distinction is important in understanding how an ecosystem works, according to University of Pennsylvania conservation biologist Dan Janzen, a leader of the Guanacaste research team. It is also the sort of evidence that biologists, Janzen included, have traditionally missed. “To me a species is a very real thing,” he says. But separating species based on “how they look to a six-foot-tall diurnal mammal” may not have much relevance to the creatures themselves. DNA bar coding alerts scientists that they need to figure out “what is actually there, rather than what we perceive as humans.” The key difference between species may be a matter of scent, seasonal timing, vocalization, auditory targeting of a particular prey, or some other trait. Such invisible distinctions may leave no trace in a museum specimen drawer, but they can make a life-or-death, sex-or-solitude difference in the wild, and not just for the species themselves.
For instance, bar coding studies in malaria zones around the world are splitting Anopheles mosquitoes into multiple cryptic species, all of them identical to human eyes. Why should we care? Because some of those species cause disease, while others are harmless. A detailed picture of invisible differences helps public health workers target limited funds more effectively. The result is that children now live who, just a year or two ago, probably would have died.
On the other hand, the proliferation of new species also complicates life for conservationists, not least because it opens the door to environmental skeptics. An editorial in The Economist not long ago suggested that the scientific currency is “being subtly debauched by over-eager taxonomists.” The magazine wondered if organisms were simply being “rebranded” to help conservation. Some biologists share that concern, particularly about certain primates that have recently been split off into separate species. They fear that unwarranted “species inflation” could jeopardize the credibility of their work. Taxonomists could become “like expert witnesses,” says Kent Redford of the Wildlife Conservation Society Institute. “You know, ‘You tell me if you want them to be separate species, and I’ll tell you what philosophy of species designation I’m going to use to give you the answer you want.’”
The discovery of valid new species divisions can also present conservationists with major new headaches. Individual species that had seemed relatively healthy can suddenly look endangered when split up into multiple separate ones. Protected areas that once seemed adequate may not include what turns out to be essential habitat. But when genetic, morphological, and behavioral differences all point to a new species, says David Brown, the geneticist whose study argued for dividing giraffes into six species, that is not rebranding. It is science.
Brown says his research team did not know what to expect when it began its study. In zoos, the different giraffe types now being proposed as separate species did the one thing that has traditionally defined animals as a single species: They bred together and produced what seemed like viable offspring. As a result, past taxonomists had categorized the variants as subspecies at best, meaning that although they bred together, they were morphologically or geographically distinct. It seemed likely that they would interbreed in the wild, too. The genetic evidence in Brown’s study showed otherwise. Even neighboring giraffe types almost never interbreed. Some populations that look identical to us turn out to have been going their separate ways for up to 1.5 million years.
One possible explanation for these divisions has to do with climate. Masai giraffes, living just south of the equator, give birth during the dry season from December to March, meaning their offspring are ready to wean just as the wet season arrives and produces new foliage to browse on. North of the equator, where reticulated giraffes live, the dry season starts in July. A hybrid of the two species with a blended reproductive cycle might do fine in a zoo. But in the wild, predators kill 50 to 70 percent of young giraffes in the first year of life. Being born in the right season, so there is plenty of browse to support fast growth, can be critical to a young giraffe’s survival. The species difference that seems like an imperceptible nuance to us is anything but to them.