BUTTERFLY EFFECT: Genetic bar codes and ecological data revealed that the neotropical skipper butterfly Astraptes fulgerator is actually a complex of 10 species. Whether bar coding by itself can define species is controversial.
Bending to Bar Codes Is a one-gene method to define species truly effective?
By Sara Beardsley
By the close of the 20th century, taxonomy had reached a crossroads. Funds were declining and academic interest dwindling, even as biologists and conservationists raced to identify and quantify species. "During my long engagement in the tropics, I've been confronted with the frustration that all biologists feel, with no knowledge of the life systems around them," explains evolutionary biologist Paul D. N. Hebert, who holds the Canada Research Chair in Molecular Biodiversity. So, in 2003, Hebert proposed a new ID system, sidestepping the cumbersome taxonomic legwork: "tag" species according to a segment of a mitochondrial gene. These so-called DNA bar codes instantly won public favor, heralding a day when researchers could run simple DNA tests in the field, perhaps even with a Star Trek-ian "tricorder" device. But since its advent, scores of taxonomists have decried the shortcut, claiming it will undermine the purposely elaborate systems developed to ensure ID precision and accuracy.
Hebert's scheme focuses on a sequence fragment from the cytochrome c oxidase I (COI) gene, which he claims is unique to separate taxa. He and his colleagues demonstrated proof of principle last year, when DNA bar codes correctly predicted independent species in groups of previously undistinguished birds and butterflies. In a February meeting in London, the Consortium for the Barcode of Life announced plans to bar-code all birds and fish within the next five years, as well as identify all flowering plants in Costa Rica. The initiatives are stepping-stones on the way to their much grander goal: a gene tag for every living thing and a catalogue of the earth's biodiversity (only about one tenth of the world's species are formally known).
But like anything, warns entomologist Quentin D. Wheeler of the Natural History Museum in London, bar coding "can be used for good or evil." A standardized species marker is exciting, provided it corresponds to formal descriptions and classifications. To Wheeler and other critics, it is bar coders' more ambitious goal--applying the COI system backward to create "provisional" new-species definitions--that threatens to hamper taxonomic progress.
The problem, naysayers argue, is oversimplification. "Nature is messy," points out entomologist Daniel Rubinoff of the University of Hawaii at Manoa. Multiple species definitions exist today because nobody knows what qualifies as speciation; the very "science" of taxonomy involves analyzing hundreds of characters to make these distinctions--which is why the one-character data sets used by bar coders "are like returning to the Dark Ages," Rubinoff says. Biologist Brent D. Mishler of the University of California at Berkeley concurs, calling bar coding for species identification "extremely wrongheaded and damaging to the fabric of systematics," which currently relies on extensive morphological, ecological and genetic data to frame species in an evolutionary context.
Critics also raise eyebrows over bar coding's accuracy. Hebert pegs the error rate at 2 percent, small enough to validate the approach for animals. But so far only a few proofs of principle have emerged, and the tests have been easy. "Close sister species are usually the most important to identify correctly," says biologist Felix Sperling of the University of Alberta. And those are the ones scientists may have the greatest trouble resolving through COI. Recently split taxa or cases of hybridization, where independent species have produced offspring, pose particular challenges because sequences may not have evolved to reflect those events yet.
Hebert argues that the system is meant to augment current taxonomy by "heaping life into piles" that can later be revised. But with a price tag between $1 billion and $2 billion, critics worry that the initiative will only divert funds and leave "real" taxonomy to clean up the mess. "In the age of cyber-infrastructure, digital tools, and IT," Wheeler and others write in a paper in press for the journal Systematic Biology, "most of the weights that have held taxonomy back are gone. [But] now ... it is in danger of being tossed out like rubbish for the latest parlor trick."
Experts also point out that bar codes cannot be integrated with the other major systematics enterprise--the Tree of Life, a peer-reviewed cladogram linking all known phylogenetic relationships. (Bar codes provide too little evidence to justify formal species designations on it.) At best, Hebert's database will e xist alongside the tree, superimposing untested "leaves." Hebert claims that despite the obstacles "we're finally tapping into an automated, digital information stream," but others note that cut-and-dry codes seem to overlook the essential meaning of species: ever changing end points in the hands of evolution.
据Sciam网5月9日报道，随着二十世纪的结束，生物分类学也走到了十字路口。尽管生物学家和自然资源保护学家争相寻找和鉴别新的物种，但是全世界投入到生物分类学的资金仍然日益减少，分类学的学术价值也在不断下降。
加拿大生物多样化研究带头人、生物学家Paul·Hebert说道，他在热带雨林生活了很长一段时间，遇到过许多和其他生物学家一样的问题，热带雨林中的生物种类实在太多了，到目前为止人们对它们还是知之甚少。所以Hebert于2003年提出一种新的生物物种标识体系，借此改变现有分类学烦杂的分类方法。这种分类方法通过分析生物自身遗传的线粒体DNA中的某一部分 ，就可以准确的判断出这种生物是属于什么种类。这种分类方法很快受到了生物学家们的欢迎与认可，因为利用简单的分析仪器，生物学家就可以在热带雨林中进行实地生物分类。但是这种分类方法的使用也遭到了许多分类学者的诋毁，他们认为这种分类方法严重破坏了现有精细的分类体系，使得分类的准确度大大降低。
Hebert的基因检测分类方法主要通过分析细胞色素C氧化酶中的一段基因序列碎片来测定生物的种类，这段序列碎片可以准确标识出不同生物种类间的差别。他和同事于2004年证实了此项分类方法的正确性。他们运用此方法准确地发现了新的鸟类和蝴蝶物种。之前这些物种都被混杂在其它已有的物种中，没有被生物学家鉴别出来。 生物科学家们于2005年2月份在伦敦讨论后宣布，在未来的五年里，生物学家将对所有的鸟类、鱼类和有花植物进行基因分类。此项举动是生物分类学领域通往新的辉煌的一块基石。目前世界上只有大约十分之一的物种为人们所认识和分类，生物分类学的最终目标是把地球上存在的所有物种进行系统而准确的分类。
