TPO17 托福阅读Passage3 Symbiotic Relationships文本+题目原文+答案解析

Symbiotic Relationships

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 1940s 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.

TPO17托福阅读Passage3 Symbiotic Relationships 题目

Question 1 of 14: Which of the following statements about commensalism can be inferred from paragraph 1?

A. It excludes interactions between more than two species.

B. It makes it less likely for species within a community to survive.

C. Its significance to the organization of biological communities is small.

D. Its role in the structure of biological populations is a disruptive one

Question 2 of 14: The word “derives” in the passage is closest in meaning to

A. digests

B. obtains

C. controls

D. discovers

Question 3 of 14: According to paragraph 2 which of the following is true of the action of natural selection on hosts and parasites?

A. Hosts benefit more from natural selection than parasites do.

B. Both aggression in predators and defensive capacities in hosts are favored for species survival

C. The ability to make toxic chemicals enables a parasite to find and isolate its host

D. Larger size equips a parasite to prey on smaller host organisms.

Question 4 of 14: The word “devastated” in the passage is closest in meaning to

A. influenced

B. infected

C. strengthened

D. destroyed

Question 5 of 14: Which of the following can be concluded from the discussion in paragraph 3 about the Australian rabbit population?

A. Human intervention may alter the host, the parasite, and the relationship between them

B. The risks of introducing outside organisms into a biological community are not worth the benefits.

C. Humans should not interfere in host-parasite relationships.

D. Organisms that survive a parasitic attack do so in spite of the natural selection process.

Question 6 of 14: According to paragraph 3, all of the following characterize the way natural selection stabilized the Australian rabbit population EXCEPT:

A. The most toxic viruses died with their hosts.

B. The surviving rabbits were increasingly immune to the virus.

C. The decline of the mosquito population caused the spread of the virus to decline.

D. Rabbits with specific genetic make-ups were favored

Question 7 of 14: The word “inadvertently” in the passage is closest in meaning to

A. indefensibly

B. substantially

C. unintentionally

D. partially

Question 8 of 14: According to paragraph 5, the relationship between legumes and bacteria benefits the soil by

A. adding enriching carbohydrates

B. speeding the decay of organic matter

C. destroying enzymes that pollute it

D. contributing nitrogen to it

Question 9 of 14:  Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information

A. The relationship between flowering plants and pollinators provides pollinators with food and flowers with efficient reproduction

B. In some cases birds obtain food from the seeds that are dispersed in the wind.

C. The wind not only helps the flowers distribute their seeds but enables birds to find more food.

D. Animals and insects are more effective in distributing pollen and seeds than the wind

Question 10 of 14: According to paragraph 5, which of the following is NOT true of the relationship between the bull’s horn acacia tree and the Pseudomyrmex ants?

A. Ants defend the host trees against the predatory actions of insects and animals.

B. The acacia trees are a valuable source of nutrition for the ants.

C. The ants enable the acacia tree to produce its own chemical defenses.

D. The ants protect the acacia from having to compete with surrounding vegetation

Question 11 of 14: The word “highlights” in the passage is closest in meaning to

A. defines

B. emphasizes

C. reflects

D. suggests

Question 12 of 14: What is the main purpose of this passage?

A. To explain the concept of symbiosis by expanded descriptions of its principal types

B. To make a comparison between human relationships and symbiotic interactions in the natural world

C. To demonstrate the unforeseen benefits of natural processes that at first seem wholly destructive

D. To argue that parasitism is a problem that can be solved by scientific intervention

Question 13 of 14: Look at the four squares ■ that indicate where the following sentence could be added to the passage. Where would the sentence best fit?

This massive population began a century earlier as a mere twelve pairs of imported rabbits that reproduced quickly and developed into a major problem.

A. The rabbits destroyed huge expanses of Australia and threatened the sheep and cattle industries.

B.  In 1950, myxoma virus, a parasite that affects rabbits, was deliberately introduced into Australia to control the rabbit population. 

C. Spread rapidly by mosquitoes, the virus devastated the rabbit population. 

D. The virus was less deadly to the offspring of surviving rabbits, however, and it caused less and less harm over the years.

Question 13 of 14: Symbiotic relationships involve the interaction of two or more organisms acting as partner

Answer Choices

A. Parasitic relationships involve the interplay of aggression by the parasite and resistance and adaptation by the host.

B. Mutualism ordinarily involves an interaction between two members of the same species

C. Mutualism is unique among symbiotic relationships in that it benefits both partners involved in the relationship.

D. Parasitic damage to Australian rabbits was never reversed because the rabbits were unable to adapt to the parasites attacks.

E. The rarity of commensal relationships stems from the difficulty of finding relationships that benefit one species without affecting the other

F. The structure of biological communities depends on the types of relationships that exist among the species within

TPO17托福阅读Passage3 Symbiotic Relationships 真题解析

Question 1 of 14

正确答案：C

解析：对应到第一段倒数第二句和最后一句前半部分：共生关系共有三种类型：寄生、共栖和互利共生。其中第一种和第三种是一个生物群落结构的关键要素。言下之意第二种不是关键要素，对应C选项。

Question 2 of 14

正确答案：B

解析：derive“源于，获得”，所以正确答案是obtain。原文提到Parasitism是一种捕食与被捕食的关系，寄生者从寄主那儿_____食物。这里获得食物显然是最恰当的，控制食物和消化食物都不合文意，发现食物表现的是一个从没有到有的过程，而那些食物本来就是存在的，所以不是被发现。

Question 3 of 14

正确答案：B

解析：对应到第二段第四句、第五句：自然选择青睐那些善于寻找寄主并且寄生能力强的寄生虫，同时，防御能力强的寄主也被选择出来。选B。A与原文相反；原文提到是植物分泌毒素，不是寄生虫分泌，C错误；D错在原文说寄生虫一般比寄主小。

Question 4 of 14

正确答案：D

解析：devastated毁灭，所以D destroyed正确。原句的前一句说一种病毒被引入以控制兔子的种群数量，接着说由于蚊子的携带导致病毒的扩张，使得兔子的种群_____了，既然前面都说了引入这种病毒是为了使兔子减少，答案当然是destroyed。strengthened意思相反了；infected“传染”只是在说一个过程，没提到控制兔子种群数量的结果，influenced程度太浅，而且也没说怎么影响。

Question 5 of 14

正确答案：A

解析：原文提到：兔子曾在澳大利亚肆掠横行，后来人为干涉引入一种寄生病毒，兔子数量大大减少。说明人为干涉可以影响寄主和寄生虫，并改变两者之间的关系。选Ａ。原文没有将引入物种的优缺点进行比较，Ｂ错误；也没有讲作者的态度，Ｃ错误；Ｄ与原文相反，第三段最后一句是肯定了自然选择的作用的。

Question 6 of 14

正确答案：C

解析：EXCEPT题，A的most toxic viruses做关键词定位至第三段倒数第二句的deadliest strains of the virus，所以A正确，不选；B的surviving rabbits做关键词定位至倒数第四句，正确，不选；C的mosquito population做关键词定位至第五句，跟选项说的完全无关，所以C错误，可选；D的genetic make-up做关键词定位至倒数第三句，所以D正确，不选。

Question 7 of 14

正确答案：C

解析：inadvertently“不注意地”，所以正确答案是C unintentionally。原文提到一个物种寻觅食物会经由另外一个物种_____地暴露出来。紧接着下句给出了例子帮忙理解这个单词。例子说有一些以昆虫为食的鸟类会被放牧中的牛群赶出草地，也就是这些鸟无意间被发现了，所以答案是C，A“不能防御地”B“确实地”D“部分地”都不合文意。

Question 8 of 14

正确答案：D

解析：以legume，bacteria和soil做关键词定位至第三句，提到植物给细菌提供有机物，而细菌中的酶可以作为催化剂向土壤中添加氮，所以答案是D。

Question 9 of 14

正确答案：A

解析：原句的结构是传花粉者可以从植物那获得食物，而植物能使花粉和种子更有效地传播。即共生关系给传花粉者和植物分别带来的好处。对应A选项。B混淆概念，原文只是说传花粉者帮助传播种子，而不吃种子；C同样混淆概念，wind在原句只是次要信息，而且也没说它能帮助鸟类找到更多食物；D只是说植物从中得到的好处，没提到pollinator的好处，缺失重要信息，错误。

Question 10 of 14

正确答案：C

解析：EXCEPT题，A的predatory actions和insects and animals做关键词定位至第五段倒数第二句，所以A正确，不选；B的nutrition 做关键词定位至倒数第四句的protein rich，正确，不选；C的chemical defenses在原文没有对应点，原文没有讲到蚂蚁可以使植物生成化学防御，这种植物是靠蚂蚁叮咬别的昆虫和食草动物而防御自己，C错误，可选；D的surrounding vegetation 做关键词定位至最后一句，所以D正确，不选。

Question 11 of 14

正确答案：B

解析：highlight“强调”，所以B的emphasize正确。

Question 12 of 14

正确答案：A

解析：提问的是整个文章的目的是什么，所以应该关注各段的中心句。文章第一段提到共生关系有多种类型，接着后面几段分别说了几个类别的共生关系，最后一段是总结，所以答案是A，分类描述了概念。B错误，不是比较自然和人；C和D都和文章内容无关。

Question 13 of 14

正确答案：A

解析：插入句里的this massive population强调了这种群种数量之多，这里的this说明前面也提到过这一现象，对应A选项前一句的hundreds of millions of European rabbits。另外插入句里还提到a major problem，而A选项后面一句就解释了问题具体是什么，所以联系紧密。

Question 14 of 14

正确答案：ACE

解析：A选项对应原文第二段第四句、第五句，正确；C选项对应第五段第一句，正确；E选项对应第四段第二句，正确。

B选项与第一段第一句中的between two or more species不符，错误。D选项与第三段第六句内容相反，病毒对存活兔子的后代来说没那么致命了，造成的伤害逐年减少。F选项与最后一段内容相反，原文说的是a web of diverse connections among organisms，而不是among the species within。

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