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托福TPO阅读17- Symbiotic Relationships(3)【雷哥托福名师精评版】

本帖最后由 小托君 于 2018-01-29编辑

TPO 17-3 Symbiotic Relationships(自然科学--物种)




A symbiotic relationship is an interaction between two or more species in which one species lives in or on another species. There are three main types of symbiotic relationships: parasitism, commensalism, and mutualism. The first and the third can be key factors in the structure of a biological community; that is, all the populations of organisms living together and potentially interacting in a particular area.

Parasitism is a kind of predator-prey relationship in which one organism, the parasite, derives its food at the expense of its symbiotic associate, the host. Parasites are usually smaller than their hosts. An example of a parasite is a tapeworm that lives inside the intestines of a larger animal and absorbs nutrients from its host. Natural selection favors the parasites that are best able to find and feed on hosts. At the same time, defensive abilities of hosts are also selected for. As an example, plants make chemicals toxic to fungal and bacterial parasites, along with ones toxic to predatory animals (sometimes they are the same chemicals). In vertebrates, the immune system provides a multiple defense against internal parasites.

At times, it is actually possible to watch the effects of natural selection in host-parasite relationships. For example, Australia during the 1940 s was overrun by hundreds of millions of European rabbits. The rabbits destroyed huge expanses of Australia and threatened the sheep and cattle industries. In 1950, myxoma virus, a parasite that affects rabbits, was deliberately introduced into Australia to control the rabbit population. Spread rapidly by mosquitoes, the virus devastated the rabbit population. The virus was less deadly to the offspring of surviving rabbits, however, and it caused less and less harm over the years. Apparently, genotypes (the genetic make-up of an organism) in the rabbit population were selected that were better able to resist the parasite. Meanwhile, the deadliest strains of the virus perished with their hosts as natural selection favored strains that could infect hosts but not kill them. Thus, natural selection stabilized this host-parasite relationship.

In contrast to parasitism, in commensalism, one partner benefits without significantly affecting the other. Few cases of absolute commensalism probably exist, because it is unlikely that one of the partners will be completely unaffected. Commensal associations sometimes involve one species' obtaining food that is inadvertently exposed by another. For instance, several kinds of birds feed on insects flushed out of the grass by grazing cattle. It is difficult to imagine how this could affect the cattle, but the relationship may help or hinder them in some way not yet recognized.

The third type of symbiosis, mutualism, benefits both partners in the relationship Legume plants and their nitrogen-fixing bacteria, and the interactions between flowering plants and their pollinators, are examples of mutualistic association. In the first case, the plants provide the bacteria with carbohydrates and other organic compounds, and the bacteria have enzymes that act as catalysts that eventually add nitrogen to the soil, enriching it. In the second case, pollinators (insects, birds) obtain food from the flowering plant, and the plant has its pollen distributed and seeds dispersed much more efficiently than they would be if they were carried by the wind only. Another example of mutualism would be the bull's horn acacia tree, which grows in Central and South America. The tree provides a place to live for ants of the genus Pseudomyrmex. The ants live in large, hollow thorns and eat sugar secreted by the tree. The ants also eat yellow structures at the tip of leaflets: these are protein rich and seem to have no   function for the tree except to attract ants. The ants benefit the host tree by attacking virtually anything that touches it. They sting other insects and large herbivores (animals that eat only plants) and even clip surrounding vegetation that grows near the tree. When the ants are removed, the trees usually die, probably because herbivores damage them so much that they are unable to compete with surrounding vegetation for light and growing space.

The complex interplay of species in symbiotic relationships highlights an important point about communities: Their structure depends on a web of diverse connections among organisms.






1. 推断题



2. 词汇题(动名搭配)



3. 事实信息题

解析:根据natural selection定位到第4句,说自然选择更喜欢那些最能够找到寄主的寄生虫,下句说同时也会选择寄主的防御能力,也就是说寄生者和寄主都要有能力要能生存,选B


4. 词汇题(词根+动名搭配)



5. 推断题(结论关注首尾句。但是整段都没有提到人类。答案是什么鬼)

解析:问题问的是结论,而Australian rabbit是一个例子,例子是为了说明论点,首句说自然选择对寄生关系的影响能够被观测,而最后一句说自然选择稳定了寄生关系,所以可以推断人类的打扰可能会改变寄生关系。B原文没有这个比较。C原文没说人类不应该干扰。D原文说的自然选择有影响。


6. 事实信息否定题

解析:A根据most toxic(与deadliest最致命的同义) virus定位到倒数第2句,对。B根据surviving rabbits定位到倒数第4句,对。C根据mosquito定位到第5句,内容不同,选CD根据genetic make-up定位到倒数第3句,对。


7. 词汇题(后文有解释)



8. 事实信息题

解析:根据legume, bacteria定位到第1-2句,说植物像细菌提供碳水化合物和其他有机物,细菌的酶作为催化剂给土壤添加氮,所以选D

9. 句子简化题



10. 事实信息否定题



11. 词汇题



12. 段落主旨题



13. 句子插入题

解析:插入句提到this massive population,说明前文应该有有关大量数量的内容;最后又提到这发展成了一个主要problem,所以后文应该要说到具体的问题A前说明有大量的兔子,A后说兔子毁掉了大片区域,所以选A


14. 段落小结题


Bsame species 没说,不选




Famong species 与最后一段不同,最后一段说的是among organism,不选。



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