CONVERGENT evolution—the arrival, independently, by different species at the same answer to a question posed by nature—is a topic of great interest to biologists. One aspect of the phenomenon which has not yet been much looked at, however, is its underlying genetics. In particular, an issue not previously addressed is how often such changes arise from similar mutations in the two convergent lines, and how often they have different genetic causes that happen to have similar effects on the organisms’ forms and functions. That has now been rectified by an examination of two creatures which, though only distantly related, share an unusual feeding habit, an unusual anatomical feature and an unusual name: panda.
趋同进化——生物学家最感兴趣的主题——不同物种对大自然抛出的问题分别给出了相同的答案。然而，关于趋同进化现象中还没有的深入了解的一面是在基因方面。两条趋同线中相似基因突变带来的变化多久会出现一次，这些变化拥有的不同遗传原因（这些原因对机体的形式和功能效果类似）多久出现一次，这些问题之前都没有得到解决。如今对两种生物的检验给出了答案。虽然这两种生物是远亲，但他们都有独特的饮食习惯、解剖特征和名字：熊猫
The giant panda is a black and white bear. The red panda (pictured above) is related to weasels, raccoons and skunks. Their habitats—mountainous areas of southern China and its neighbours—overlap, but their last common ancestor lived 43m years ago. They do, though, share a limited kinship, for both are members (along with dogs, cats, hyenas, mongooses, seals and so on) of the mammalian order Carnivora. Which is curious, because both are vegetarian.
大熊猫是一种黑白相间的熊。小熊猫（见以上图片）是黄鼠狼、浣熊、臭鼬的亲戚。它们的栖息地在中国南方山区及邻国山区，但这两个物种最后一个共同祖先生活在4300万年前。它们确实是亲属关系，两者都是食肉目哺乳动物的一员，这一目还有犬科、猫科、鬣狗、猫鼬、海豹等。大熊猫和小熊猫都是食草动物，这令人感到好奇。
More intriguing still, both subsist almost entirely on bamboo (some etymologists think their mutual name is derived from the Nepali phrase nigalya ponya, meaning “bamboo-eater”). And most curiously of all, both have a sixth digit on their forepaws—a kind of ancillary thumb derived from one of the bones of the wrist that helps them hold bamboo stalks for consumption. These common features led Hu Yibo of the Institute of Zoology, in Beijing, and his colleagues, to wonder if the two vegetarian carnivores also shared genetic modifications that might explain those features. And, as Dr Hu observes in a paper just published in the Proceedings of the National Academy of Sciences, they do.
更奇怪的是，两者都完全依赖竹子，有些词源学家认为它们共同的名字来源于尼泊尔语中的词组 nigalya ponya，意思是“食竹者”。最让人好奇的是，两个物种的前爪都有从手腕处的骨头进化而来的六个脚趾，有助于吃竹子时抓住竹竿。这些共性吸引着北京动物研究所的胡义博和他的同事。他们想要知道这两个食草动物是否有着相同的基因突变，而这些突变可能是这些共性的原因。胡义博士在《国家科学院学报》上发表的论文称，确实如此。
To search for such genetic commonalities, Dr Hu and his colleagues compared the DNA of the two pandas with that of four other members of the Carnivora: polar bears (close relatives of giant pandas), ferrets (close relatives of red pandas), and tigers and dogs (close relatives of neither). If pandas share features of their DNA with each other, but not with the four comparison species, he reasoned, then it is likely that those features encode recent adaptations common to the two of them.
为了研究这些基因共性，胡义博士和同事将这两个物种的基因和其他四种食肉目动物的基因进行比较：北极熊（大熊猫的近亲）、雪豹（小熊猫的近亲）、老虎和狗（和大小熊猫关系较远）。胡义博士解释道，如果大小熊猫的DNA特点彼此相同，但与其他四个物种的不同，那么，这些共同特性有可能会是后来在它们身上出现的。
Altogether, the team identified 70 genes (out of the 20,000 or so that mammals have) which sport bits of DNA that seemed to be shared. They also found ten genes which have been disabled by crippling mutations in both of the pandas, but not in the other four species. Not all of the shared genetic features obviously tied in with the shared peculiarities of pandas, but some did. These fell into three categories: genes affecting anatomy, genes affecting appetite and genes affecting the digestion and metabolism of nutrients.
研究小组识别了70种有相似之处的基因（哺乳动物的基因有20000多种）。他们还发现，两种熊猫体内剧烈的基因突变导致了10种基因残缺，但另外四个物种没有这种情况。并不是两种熊猫身上所有的基因特性都存在这种情况，只有部分如此。这些基因分为三类：骨骼基因、饮食基因和消化及新陈代谢基因。
The anatomy-related adaptations were those that seemed to control the development of the pandas’ second thumbs. Two genes in particular, DYNC2H1 and pericentrin, have mutations that cause identical changes in each type of panda in the proteins encoded by these genes. In mice, mutations in these genes are known to encourage extra digits to grow, so it is not unreasonable to suspect that they are also the cause of this in pandas.
骨骼基因似乎控制着熊猫第二拇指的发育。尤其是 DYNC2H1 和中心粒周围蛋白，这两种基因导致两种熊猫在加密蛋白质上发生了明显变化。老鼠的基因突变就促进了另外一个脚趾的生长，因此，对熊猫的这一假设是有理可循的。
The appetite-modifying change involved one of the disabled genes. When working, this gene encodes a protein which forms part of the tongue’s taste buds for umami, a savoury “meatiness” separate from the other four tastes of sweetness, sourness, bitterness and saltiness. Umami perception is stimulated by glutamic acid, one of the 20 amino acids that make up proteins—but one that is more characteristic of animal proteins than plant ones. Sensitivity to umami is valuable to a carnivore but unnecessary (and possibly harmful) to a herbivore.
饮食基因中包括一种残缺基因。这种基因在发挥作用时会产生一种蛋白质，使之成为味蕾的一部分，产生“鲜味”，与甜、酸、苦和咸等味道不同。鲜味味觉的刺激源于谷氨酸，即构成蛋白质的20中氨基酸中的一种，而谷氨酸更具有动物蛋白的特色，植物蛋白特色较少。鲜味感知对食肉动物很有价值，对食草动物则不仅没有意义，还可能造成伤害。
The other genetic convergences Dr Hu recorded were related to the digestion and metabolism of substances scarce in, or absent from, bamboo. He and his colleagues noted parallel changes in the genes for three digestive enzymes whose job is to liberate two particular amino acids, lysine and arginine, from proteins. Both lysine and arginine are abundant in meat, but in short supply in bamboo. The team also noted parallel changes in four genes that encode proteins involved in the metabolism of two vitamins, A and B12, and of arachidonic acid, a lipid essential for bodily function. All of these are scarce or non-existent in bamboo as well. Exactly how the genetic changes seen alter the effectiveness of the proteins involved remains to be determined. But the prediction would be that they enhance the availability of the nutrients in question.
胡义博士记录的其他基因趋同与竹子中稀缺的物质消化和代谢有关。他及其同事发现了三种消化酶基因的平行变化。这些酶的作用是从蛋白质中分离赖氨酸和精氨酸这两种谷氨酸。肉类中的赖氨酸和精氨酸很丰富，在竹子里却很稀缺。研究人员还注意到四种基因发生的平行变化。这些基因对负责代谢维生素A和维生素B12的蛋白质，以及新陈代谢花生四希酸的蛋白质产生，其中花生四希酸是人体机能必须的脂质。所有这些基因在竹子中都很稀缺甚至不存在。这些基因变化是怎样改变有关蛋白质效果的这一问题还没有定论。不过，有人预测这些基因有助于获得缺失的营养。
The upshot is that the two pandas do indeed seem to have similar genetics underlying their similar peculiarities. Such similarities are, admittedly, easier to find than different genetic routes to the same outcome would be. But Dr Hu has established that, at least in the case of pandas, natural selection has often taken the same paths to arrive at the same outcomes.
结论是两种熊猫的相似基因似乎确实决定了类似性征。不可否认，与不同基因路径得到相同结果相比，这种相似性更加容易发现。然而，胡义博士认为，自然选择通常采取相同模式，最后得到同样的结果，至少在熊猫案例中如此。
编译：刘恬
审校：周小靖
编辑：翻吧君
来源：经济学人
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