现代达尔文
Modern Darwins
达尔文逝世的两周前写了一篇短文，谈到英国密德兰地区的池塘里有颗小蛤夹在一只水生甲虫的腿上。那是他最后发表的文章，送甲虫给他的是一位年轻的鞋匠兼业余的博物学家，名叫华特‧德劳布里吉‧克里克。这名鞋匠后来结了婚，育有一子哈利。哈利也育有一子，取名叫弗朗西斯。达尔文对演化所做的一切推论，几乎都因为弗朗西斯‧克里克以及美国青年詹姆士‧华生在1953年的一项共同发现而成功地获得了证明。
Just two weeks before he died, Charles Darwin wrote a short paper about a tiny clam found clamped to the leg of a water beetle in a pond in the English Midlands. It was his last publication. The man who sent him the beetle was a young shoemaker and amateur naturalist named Walter Drawbridge Crick. The shoemaker eventually married and had a son named Harry, who himself had a son named Francis. In 1953, Francis Crick, together with a young American named James Watson, would make a discovery that has led inexorably to the triumphant vindication of almost everything Darwin deduced about evolution.
那个证明不是来自化石、现存生物的标本或器官解剖，而是一本书。华生和克里克发现，每种生物的细胞里都带有和自己生命有关的化学码，那是以所有生物的共同语言所写成的：DNA简单的四字母码。达尔文写道：「所有曾经生活在地球上的有机体都是源自于某种原始型态。」坦白讲，达尔文只是臆测。现代的科学家想了解演化的故事与机制时无须臆测，他们可以参考遗传信息。
The vindication came not from fossils, or from specimens of living creatures, or from dissection of their organs. It came from a book. What Watson and Crick found was that every organism carries a chemical code for its own creation inside its cells, a text written in a language common to all life: the simple, four-letter code of DNA. "All the organic beings which have ever lived on this earth have descended from some one primordial form," wrote Darwin. He was, frankly, guessing. To understand the story of evolution—both its narrative and its mechanism—modern Darwins don't have to guess. They consult genetic scripture.
以加拉巴哥群岛上知名的雀为例，达尔文发现牠们的喙形状各异，有的宽而深、有的细长，还有的短小。他推测（不过时间上有点落后），尽管有这些差异，但加拉巴哥群岛上所有的雀都是近亲。达尔文在《小猎犬号航海记》里面这么写道：「看到这一小群密切相关的鸟有这样多元的结构差异，我们可能会真的以为，有人从这个群岛上为数稀少的鸟类中抓走其中一种，然后为了不同目的加以修改。」
这段话也是达尔文一时的臆测。不过，如今科学家分析牠们彼此近似的基因密码，确定了加拉巴哥群岛的雀的确是源自于同一个始祖物种（血缘上最接近这种鸟的现存生物是暗色草雀）。
Consider, for instance, the famous finches of the Galápagos. Darwin could see that their beaks were variously shaped—some broad and deep, others elongated, still others small and short. He surmised (somewhat belatedly) that in spite of these differences, all the Galápagos finches were close cousins. "Seeing this gradation and diversity of structure in one small, intimately related group of birds," he wrote in The Voyage of the Beagle, "one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends."
This, too, was inspired guesswork. But by analyzing the close similarity of their genetic codes, scientists today can confirm that the Galápagos finches did indeed descend from a single ancestral species (a bird whose closest living relative is the dull-colored grassquit).
DNA不仅确认了演化的事实，也显示它如何从根本重新塑造生物。最近，美国哈佛大学的阿哈特‧阿布札诺夫与哈佛医学院的克里夫‧塔宾找到决定某些鸟喙形状的基因。基因是DNA字母的序列，细胞启动基因时就会产生特殊的蛋白质。阿布札诺夫和塔宾发现，雀胚胎里的颚在发育时，蛋白质BMP4的基因一经启动（科学家用的字眼是「表现」），就会让鸟喙变得比较深、比较宽。这个基因在大嘴地雀身上表现得最为明显，牠们是用强而有力的喙撬开大型的种子与坚果。在其它的雀身上，有一种基因会表现名叫携钙蛋白的蛋白质，让鸟喙变得又细又长。这种基因在大仙人掌地雀身上最为活跃，牠们会利用细细长长的喙探查仙人掌果实里面的种子。
在美国佛罗里达州墨西哥湾岸外的另一群岛上，东南白足鼠的毛色比美国本土的鼠来得淡。这让牠们在白沙上更容易隐身：猫头鹰、鹰、鹭会捕食伪装效果较差的鼠，使得其它的老鼠得以繁衍。哈佛大学的贺琵‧胡克斯特拉和同事追踪发现，毛色差异是源自于单一基因里单一字母的改变，它会减少毛皮中色素的产生。这种突变从将近6000年前这些沙滩岛形成时就出现了。
DNA not only confirms the reality of evolution, it also shows, at the most basic level, how it reshapes living things. Recently, Arhat Abzhanov of Harvard University and Cliff Tabin of Harvard Medical School pinned down the very genes responsible for some of those beak shapes. Genes are sequences of DNA letters that when activated by the cell make a particular protein. Abzhanov and Tabin found that when the gene for a protein called BMP4 is activated (scientists use the word "expressed") in the growing jaw of a finch embryo, it makes the beak deeper and wider. This gene is most strongly expressed in the large ground finch (Geospiza magnirostris), which uses its robust beak to crack open large seeds and nuts. In other finches, a gene expresses a protein called calmodulin, which makes a beak long and thin. This gene is most active in the large cactus finch G. conirostris, which uses its elongated beak to probe for seeds in cactus fruit. |
发表于 2009-1-27 10:56
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