托福

　　在以往的托福阅读考试中有关于自然选择与生物灭绝的考试文章。针对这道托福考题，前程百利小编为大家普及一下相关背景知识，这样有助于考生在面对这类题目时方便作答。

　　托福阅读真题：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。最后说是一些偶然，包括进化等等造就了生存下来的早期生物。

　　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。

　　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” “as if by some murderous pestilence.”