劍橋雅思10Test4Passage3閱讀原文翻譯 When evolution runs backwards

劍橋雅思10Test4Passage3閱讀原文翻譯 When evolution runs backwards

劍橋雅思10 Test4 Passage3閱讀原文翻譯

第1段

The description of any animal as an ‘evolutionary throwback’ is controversial. For the better part of a century, most biologists have been reluctant to use those words, mindful of a principle of evolution that says ‘evolution cannot run backwards’. But as more and more examples come to light and modern genetics enters the scene, that principle is having to be rewritten. Not only are evolutionary throwbacks possible, they sometimes play an important role in the forward march of evolution.

將任何動物描述為“進化倒退”是有爭議的。該世紀的大部分時間里，大多數(shù)生物學(xué)家一直不愿意使用這些詞匯。他們謹記著進化的一條基礎(chǔ)原則，“進化不能后退”。但隨著越來越多的例子出現(xiàn)，以及現(xiàn)代基因科學(xué)的誕生，這條規(guī)則不得不被改寫。進化的倒退不僅是可能的，它們還在向前的進化中起著重要作用。

第2段

The technical term for an evolutionary throwback is an ‘a(chǎn)tavism’, from the Latin atavus, meaning forefather. The word has ugly connotations thanks largely to Cesare Lombroso, a 19th-century Italian medic who argued that criminals were born not made and could be identified by certain physical features that were throwbacks to a primitive, sub-human state.

用于描述進化后退的技術(shù)詞匯是“返祖現(xiàn)象”，它來自于拉丁語atavus，意思是祖先。該詞具有一定的丑陋含義。這在很大程度上要歸功于19世紀意大利醫(yī)務(wù)人員Cesare Lombroso。他認為犯罪分子是天生的，而不是后期造成的。并且可以根據(jù)特定的身體特征來進行識別。而這些特征可以追溯到原始的亞人類狀態(tài)。

第3段

While Lombroso was measuring criminals, a Belgian palaeontologist called Louis Dollo was studying fossil records and coming to the opposite conclusion. In 1890 he proposed that evolution was irreversible: that ‘a(chǎn)n organism is unable to return, even partially, to a previous stage already realised in the ranks of its ancestors’. Early 20th-century biologists came to a similar conclusion, though they qualified it in terms of probability, stating that there is no reason why evolution cannot run backwards—it is just very unlikely. And so the idea of irreversibility in evolution stuck and came to be known as ‘Dollo’s law’.

Lombroso在描述罪犯時，一名叫做Louis Dollo的比利時古生物學(xué)家正在研究化石記錄，并得出相反的結(jié)論。1890年，他提出進化是不可逆的，“一種生物無法回到其祖先在進化序列中已經(jīng)完成的任何先前階段，即使是部分回歸也不可能”。二十世紀早期的生物學(xué)家得出了相似的結(jié)論，盡管他們在概率上對其進行了限定，認為并沒有任何理由支持進化無法后退，它僅僅是不太可能。因此，這種進化不可逆的觀念扎根下來，并被稱為“Dollo定律”。

第4段

If Dollo’s law is right, atavisms should occur only very rarely, if at all. Yet almost since the idea took root, exceptions have been cropping up. In 1919, for example, a humpback whale with a pair of leg-like appendages over a metre long, complete with a full set of limb bones, was caught off Vancouver Island in Canada. Explorer Roy Chapman Andrews argued at the time that the whale must be a throwback to a land-living ancestor. ‘I can see no other explanation,’ he wrote in 1921.

如果Dollo定律是正確的，那么返祖現(xiàn)象哪怕確實存在，也應(yīng)該十分少見。然而，幾乎從這一概念扎根以來，例外就一直出現(xiàn)。例如，1919年，加拿大溫哥華島上捕獲了一頭座頭鯨。它有一雙類似于腿的、一米多長的附屬物，并擁有完整的四肢骨骼。探險家Roy Chapman Andrews那時認為這頭鯨魚一定退化成了其生活在陸地上的祖先狀態(tài)。他于1921年寫到，“我找不到任何其他解釋”。

第5段

Since then, so many other examples have been discovered that it no longer makes sense to say that evolution is as good as irreversible. And this poses a puzzle: how can characteristics that disappeared millions of years ago suddenly reappear? In 1994, Rudolf Raff and colleagues at Indiana University in the USA decided to use genetics to put a number on the probability of evolution going into reverse. They reasoned that while some evolutionary changes involve the loss of genes and are therefore irreversible, others may be the result of genes being switched off. If these silent genes are somehow switched back on, they argued, long-lost traits could reappear.

從那時起，發(fā)現(xiàn)了如此之多的其他例子，以至于堅持進化不可逆這一理論不再有什么意義。這就提出了一個難題：那些幾百萬年前消失的特征怎么會再次突然出現(xiàn)？1994年，美國印第安納大學(xué)Rudolf Raff和他的同事決定利用基因?qū)W量化進化倒退的可能性。他們考慮到，雖然一些進化變化涉及到基因的缺失，并因此是不可逆的。但其他的則可能是基因被關(guān)閉的結(jié)果。他們認為，如果這些沉默的基因以某種方式再次被打開，消失了很長時間的特征就會再次出現(xiàn)。

第6段

Raff’s team went on to calculate the likelihood of it happening. Silent genes accumulate random mutations, they reasoned, eventually rendering them useless. So how long can a gene survive in a species if it is no longer used? The team calculated that there is a good chance of silent genes surviving for up to 6 million years in at least a few individuals in a population, and that some might survive as long as 10 million years. In other words, throwbacks are possible, but only to the relatively recent evolutionary past.

Raff的團隊繼續(xù)計算這一現(xiàn)象發(fā)生的可能性。他們認為沉默的基因會積累隨機突變，并最終使得它們變得毫無用處。所以，如果一個基因不再被使用，那么它在一個物種中可以存在多長時間呢？該團隊計算，有很大概率，沉默的基因在種群中的一些個體里可以存在600萬年，一些甚至可能延續(xù)1000萬年那么久。換句話說，退化完全可能，但僅僅是退回相對較近的進化階段。

第7段

As a possible example, the team pointed to the mole salamanders of Mexico and California. Like most amphibians these begin life in a juvenile ‘tadpole’ state, then metamorphose into the adult form—except for one species, the axolotl, which famously lives its entire life as a juvenile. The simplest explanation for this is that the axolotl lineage alone lost the ability to metamorphose, while others retained it. From a detailed analysis of the salamanders’ family tree, however, it is clear that the other lineages evolved from an ancestor that itself had lost the ability to metamorphose. In other words, metamorphosis in mole salamanders is an atavism. The salamander example fits with Raff’s 10-million-year time frame.

該團隊列舉了墨西哥和加利福尼亞的鈍口螈為證。與大多數(shù)兩棲動物類似，它們以一種年幼的“蝌蚪”形態(tài)開始自己的一生，然后轉(zhuǎn)變?yōu)槌赡晷螒B(tài)。只有一個品種例外：蠑螈。它因一生都保持著年幼狀態(tài)而聞名。這一現(xiàn)象最簡單的解釋是，蠑螈這一單獨分支失去了變形的能力，而其他分支則保留了這一能力。然而，通過對蠑螈家族譜系的細致分析發(fā)現(xiàn)，其他分支顯然是從某一個已經(jīng)失去變形能力的祖先進化而來的。話句話說，鈍口螈的變形能力是一種返祖現(xiàn)象。鈍口螈的例子符合Raff一千萬年的時間框架。

第8段

More recently, however, examples have been reported that break the time limit, suggesting that silent genes may not be the whole story. In a paper published last year, biologist Gunter Wagner of Yale University reported some work on the evolutionary history of a group of South American lizards called Bachia. Many of these have minuscule limbs; some look more like snakes than lizards and a few have completely lost the toes on their hind limbs. Other species, however, sport up to four toes on their hind legs. The simplest explanation is that the toed lineages never lost their toes, but Wagner begs to differ. According to his analysis of the Bachia family tree, the toed species re-evolved toes from toeless ancestors and, what is more, digit loss and gain has occurred on more than one occasion over tens of millions of years.

然而，更近一些被報道的例子打破了時間限制，表明沉默的基因可能還不是故事的全部。在去年發(fā)表的一篇論文中，耶魯大學(xué)生物學(xué)家Gunter Wagner公布了一些自己對一種名為Bachia的南美洲蜥蜴的進化歷史所做的研究。它們許多都有微型四肢。一些看起來更像是蛇而不是蜥蜴。一些已經(jīng)完全失去了后肢的指頭。然而，其他物種的后肢上則出現(xiàn)了最多四個指頭。最簡單的解釋是，有指頭的分支從未喪失過這一特征。但Wagner持不同意見。根據(jù)他對Bachia家族譜系的研究，有指頭的物種從沒有指頭的祖先那里再次進化出了指頭。此外，這種腳趾的消失和再現(xiàn)在過去上千萬年里曾不只出現(xiàn)過一次。

第9段

So what’s going on? One possibility is that these traits are lost and then simply reappear, in much the same way that similar structures can independently arise in unrelated species, such as the dorsal fins of sharks and killer whales. Another more intriguing possibility is that the genetic information needed to make toes somehow survived for tens or perhaps hundreds of millions of years in the lizards and was reactivated. These atavistic traits provided an advantage and spread through the population, effectively reversing evolution.

那么，到底是怎么回事呢？一種可能是，這些特征先是消失，然后只是簡單的再次出現(xiàn)而已。就跟一些相似的結(jié)構(gòu)可能獨立出現(xiàn)在不相關(guān)的物種身上一樣，比如鯊魚和虎鯨的背鰭。另外一種更為有趣的可能是，需要用來制造指頭的基因信息在蜥蜴中存在了數(shù)千萬甚至上億年，然后被再次激活了。這些返祖體征提供了一項優(yōu)勢，并在族群中傳播開來，有效地逆轉(zhuǎn)了進化進程。

第10段

But if silent genes degrade within 6 to 10 million years, how can long-lost traits be reactivated over longer timescales? The answer may lie in the womb. Early embryos of many species develop ancestral features. Snake embryos, for example, sprout hind limb buds. Later in development, these features disappear thanks to developmental programs that say ‘lose the leg’. If for any reason this does not happen, the ancestral feature may not disappear, leading to an atavism.

但如果沉默的基因在600萬到1000萬年的時間里退化，那么已經(jīng)消失了很久的特征又是如何在更長的時間跨度里重現(xiàn)的呢？答案可能存在于子宮內(nèi)部。許多物種的早期胚胎都發(fā)展出祖先的特征。例如，蛇的胚胎會長出后腿突起物。在隨后的發(fā)育過程中，這些特征會由于發(fā)育程序的命令（丟掉這些腿）而消失。如果因為某種原因，這一機制沒有起作用，那么祖先的特征就不會消失，導(dǎo)致返祖現(xiàn)象。

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