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共生关系是两个或多个物种之间的相互作用，其中一个物种生活在另一个物种中，或生活在另一个物种上。

A symbiotic relationship is an interaction between two or more species in which one species lives in or on another species.

共生关系主要有三种类型：寄生关系、偏利共生关系和互利共生关系。

There are three main types of symbiotic relationships: parasitism, commensalism, and mutualism.

第一个和第三个可能是生物群落结构的关键因素；也就是说，所有的生物种群都生活在一起，并可能在一个特定的区域内相互作用。

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.

寄生是一种捕食者与被捕食者之间的关系，寄生者以其共生伙伴——宿主为代价获取食物。

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.

寄生虫的一个例子是绦虫，它生活在一个更大的动物的肠道内，从它的宿主那里吸收营养。

An example of a parasite is a tapeworm that lives inside the intestines of a larger animal and absorbs nutrients from its host.

自然选择是让最能够适应它们居住条件的那些个体或群体存活下来的过程，这一过程有利于最有能力找到宿主并以宿主为食的寄生虫。

Natural selection the process that results in the survival of individuals or groups best adjusted to the conditions under which they live favors the parasites that are best able to find and feed on hosts.

同时，还要选择宿主的防御能力。

At the same time, defensive abilities of hosts are also selected for.

例如，植物产生对真菌和细菌寄生虫有毒的化学物质，以及对食肉动物有毒的化学物质（有时它们是相同的化学物质）。

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).

在脊椎动物中，免疫系统对体内寄生虫提供多重防御。

In vertebrates, the immune system provides a multiple defense against internal parasites.

有时，在宿主和寄生生物的关系中观察自然选择的影响实际上是可能的。

At times, it is actually possible to watch the effects of natural selection in host-parasite relationships.

例如，在20世纪40年代，澳大利亚被数亿只欧洲兔子占领。

For example, Australia during the 1940’s was overrun by hundreds of millions of European rabbits.

这些兔子摧毁了澳大利亚的大片土地，并威胁到羊和牛的产业。

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

在1950年，粘液瘤病毒，一种影响兔子的寄生虫，被故意引入澳大利亚来控制兔子的数量。

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

这种病毒通过蚊子迅速传播，使兔子数量锐减。

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

然而，这种病毒对幸存下来的兔子后代的致命性较低，多年来造成的伤害也越来越小。

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

显然，兔子种群中的基因型（生物体的遗传组成）被选择为更能抵抗寄生虫的基因型。

Apparently, genotypes (the genetic make-up of an organism) in the rabbit population were selected that were better able to resist the parasite.

与此同时，最致命的病毒株随着宿主的死亡而消失，因为自然选择倾向于感染宿主但不会杀死它们的病毒株。

Meanwhile, the deadliest strains of the virus perished with their hosts as natural selection favored strains that could infect hosts but not kill them.

因此，自然选择稳定了这种宿主和寄生生物的关系。

Thus, natural selection stabilized this host-parasite relationship.

与寄生不同的是，在偏利共生关系中，一方受益而另一方不受明显影响。

In contrast to parasitism, in commensalism, one partner benefits without significantly affecting the other.

绝对偏利共生的案例可能很少，因为其中一方不太可能完全不受影响。

Few cases of absolute commensalism probably exist, because it is unlikely that one of the partners will be completely unaffected.

偏利共生关系有时涉及一个物种获取被另一个物种不经意暴露的食物。

Commensal associations sometimes involve one species’ obtaining food that is inadvertently exposed by another.

例如，几种鸟类以吃草的牛驱赶出的昆虫为食。

For instance, several kinds of birds feed on insects flushed out of the grass by grazing cattle.

很难想象这会对牛产生怎样的影响，但这种关系可能会在某种未知的地方帮助或阻碍它们。

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.

第三种共生类型，互利共生关系，对双方都有好处。

The third type of symbiosis, mutualism, benefits both partners in the relationship.

豆科植物和它们的固氮细菌，以及开花植物和它们的传粉者之间的相互作用，都是互利共生的例子。

Legume plants and their nitrogen-fixing bacteria, and the interactions between flowering plants and their pollinators, are examples of mutualistic association.

在第一种情况下，植物为细菌提供碳水化合物和其他有机化合物，细菌有酶作为催化剂，最终向土壤中添加氮，丰富土壤。

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.

在第二种情况下，传粉者(昆虫、鸟类)从开花植物中获取食物，植物的花粉分布和种子传播比只靠风传播要有效得多。

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.

互利共生关系的另一个例子是生长在中美洲和南美洲的牛角相思树。

Another example of mutualism would be the bull’s horn acacia tree, which grows in Central and South America.

这棵树为牛角刺槐蚁提供了一个生存的地方。

The tree provides a place to live for ants of the genus Pseudomyrmex.

蚂蚁生活在巨大的中空的刺里，吃树分泌的糖。

The ants live in large, hollow thorns and eat sugar secreted by the tree.

蚂蚁也吃小叶尖端的黄色结构；这些结构富含蛋白质，除了吸引蚂蚁外，似乎对树没有任何作用。

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.

蚂蚁通过攻击几乎任何接触到树东西而使宿主树受益。

The ants benefit the host tree by attacking virtually anything that touches it.

它们会叮其他昆虫和大型食草动物（只吃植物的动物），甚至会剪掉生长在树附近的植物。

They sting other insects and large herbivores (animals that eat only plants) and even clip surrounding vegetation that grows near the tree.

当蚂蚁被移除后，树木通常会死亡，可能是因为食草动物对它们的伤害太大，导致它们无法与周围的植物竞争光照和生长空间。

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.

物种在共生关系中的复杂相互作用突出了群落的一个重要特点：它们的结构依赖于生物之间多样的联系网络。

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.

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

The word “derives” in the passage is closest in meaning to

According to paragraph 2, which of the following is true of the action of natural selection on hosts and parasites?

The word “devastated” in the passage is closest in meaning to

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

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

The word “inadvertently” in the passage is closest in meaning to

According to paragraph 5, the relationship between legumes and bacteria benefits the soil by

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.

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?

The word “highlights” in the passage is closest in meaning to

What is the main purpose of this passage?

Look at the four squares  that indicate where the following sentence could be added to the passage. This massive population began a century earlier as a mere twelve pairs of imported rabbits that reproduced quickly and developed into a major problem.  Where would the sentence best fit? Click on a square  to add the sentence to the passage.

Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.

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 the process that results in the survival of individuals or groups best adjusted to the conditions under which they live 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.