在2014年6月15日的托福阅读考试中有这样一道题：自然选择所带来的相关灭绝。针对这道托福考题，出国留学网(www.liuxue86.com)小编来为大家普及一下关于自然选择的背景知识，这样有助于考生在面对这类题目时方便作答。小编在此提醒大家：本文讨论的是自然选择这个常考话题，该话题历年来都是常考话题。
托福阅读真题再现：
版本一：
2，自然选择的限制，进化并非好处，很大可能导致灭绝。分述前者进化会对后代造成限制，器官间的相互作用。最后说是一些列的偶然，包括进化等等造就了生存下来的早期生物
版本二：
natural selection不是总是靠谱的因为据记录地球上百分之九十九点几的出现过的进化分支(不知道中文怎么说原文也不记得了可能是evolutionary line?)都灭绝了 这好像是达尔文说的 大概说进化好多时候不是按natural selection或者进化不到perfection.原因一是基因结构的形成互相影响(interact)，不会轻易因为 natural selection改变。还说早期的改变会影响后来的进化，举例说脊椎动物和另一种估计是节肢什么的动物分别进化了internal和external的骨骼，导致后来脊椎动物很多长得很大个，节肢动物顶多有只大个的螃蟹噗哈哈哈
还有一个原因好像是自然环境因素，这个记不清了似乎说自然环境变了动物fit in environment也变反正是说可能比起由于natural selection产生的变化多??还有什么面对环境变化幸存的动物后代可能失去一些基因??
版本三：
题目好像是EXTINCTION 讲的是生物进化的知识 第一段亮观点进化不一定都是物种的选优(原词是perfection)达尔文的进化论虽然对滴但是也有99.9%的evolutionary line(原词好像是这个) 灭绝了 主题就是自然选择不一定都是好事 可能灭绝一些
往下继续是一些比较零散的信息： 有一个词汇题resistance 正确选项是opposition 那个词附近是这么个事情 这段的主题是interation 各个结构和器官之间不可能相互independent 一个器官或者组织不足以应对自然界的变化 所以器官之间的resistance会在某种程度上决定这个物种的去留
再往下：提到了另一个因素 就是Chance 就是一个物种能不能留下也得靠人品 有一个词汇题indiscriminate答案应该是random 那个附近说了这么个事情 就是一些大灾难用的是catastrophe会是的一些就算是比较NB的物种也完蛋 就是一视同仁不管是谁 所以用了indiscriminate这个词
补充resistance的那一段 还提到了genotype基因型 好像是这个意思 在进化过程中优秀的基因会保留 不行的直接完蛋
解析：
本文讨论的是自然选择这个常考话题，该话题历年来都是常考话题。与以往不同的是，本文侧重的角度是自然选择所带来的相关灭绝，而且没有像之前的话题那样结合某种物种来讨论，同学们在解题中的关键是，阅读完英文后，能对应的理解所描述表达的概念，对同学们抽象内容理解能力也是一定的挑战。
托福参考阅读：
Natural selection acts solely through the preservation of variations in some way advantageous, which consequently endure. Owing to the high geometrical rate of increase of all organic beings, each area is already fully stocked with inhabitants; and it follows from this, that as the favoured forms increase in number, so, generally, will the less favoured decrease and become rare. Rarity, as geology tells us, is the precursor to extinction. We can see that any form which is represented by few individuals will run a good chance of utter extinction, during great fluctuations in the nature of the seasons, or from a temporary increase in the number of its enemies. But we may go further than this; for, as new forms are produced, unless we admit that specific forms can go on indefinitely increasing in number, many old forms must become extinct. That the number of specific forms has not indefinitely increased, geology plainly tells us; and we shall presently attempt to show why it is that the number of species throughout the world has not become immeasurably great.
We have seen that the species which are most numerous in individuals have the best chance of producing favourable variations within any given period. We have evidence of this, in the facts stated in the second chapter showing that it is the common and diffused or dominant species which offer the greatest number of recorded varieties. Hence, rare species will be less quickly modified or improved within any given period; they will consequently be beaten in the race for life by the modified and improved descendants of the commoner species.
From these several considerations I think it inevitably follows, that as new species in the course of time are formed through natural selection, others will become rarer and rarer, and finally extinct. The forms which stand in closest competition with those undergoing modification and improvement will naturally suffer most. And we have seen in the chapter on the Struggle for Existence that it is the most closely-allied forms,—varieties of the same species, and species of the same genus or of related genera,—which, from having nearly the same structure, constitution, and habits, generally come into the severest competition with each other; consequently, each new variety or species, during the progress of its formation, will generally press hardest on its nearest kindred, and tend to exterminate them. We see the same process of extermination amongst our domesticated productions, through the selection of improved forms by man. Many curious instances could be given showing how quickly new breeds of cattle, sheep, and other animals, and varieties of flowers, take the place of older and inferior kinds. In Yorkshire, it is historically known that the ancient black cattle were displaced by the long-horns, and that these “were swept away by the shorthorns” (I quote the words of an agricultural writer) “as if by some murderous pestilence.”
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