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.
Paragraph 1: 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.
1. Which of the following statements about commensalism can be inferred from paragraph 1?
○It excludes interactions between more than two species.
○It makes it less likely for species within a community to survive.
○Its significance to the organization of biological communities is small.
○Its role in the structure of biological populations is a disruptive one.
Paragraph 2: 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.
2. The word derives in the passage is closest in meaning to
○Digests
○Obtains
○Controls
○Discovers
3. According to paragraph 2. which of the following is true of the action of natural selection on hosts and parasites?
○Hosts benefit more from natural selection than parasites do.
○B(yǎng)oth aggression in predators and defensive capacities in hosts are favored for species survival.
○The ability to make toxic chemicals enables a parasite to find and isolate its host.
○Larger size equips a parasite to prey on smaller host organisms.
Paragraph 3: 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.
4. The word devastated in the passage is closest in meaning to
○ Influenced
○ Infected
○ strengthened
○ destroyed
5. Which of the following can be concluded from the discussion in paragraph 3 about the Australian rabbit population?
○Human intervention may alter the host, the parasite. and the relationship between them.
○The risks of introducing outside organisms into a biological community are not worth the benefits.
○Humans should not interfere in host-parasite relationships.
○Organisms that survive a parasitic attack do so in spite of the natural selection process.
6. According to paragraph 3, all of the following characterize the way natural selection stabilized the Australian rabbit population EXCEPT:
○The most toxic viruses died with their hosts.
○The surviving rabbits were increasingly immune to the virus.
○The decline of the mosquito population caused the spread of the virus to decline.
○Rabbits with specific genetic make-ups were favored.
Paragraph 4: 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.
7. The word inadvertently in the passage is closest in meaning to
○Indefensibly
○Substantially
○Unintentionally
○Partially
Paragraph 5: 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.
8. According to paragraph 5, the relationship between legumes and bacteria benefits the soil by
○adding enriching carbohydrates
○speeding the decay of organic matter
○destroying enzymes that pollute it
○contributing nitrogen to it
9. 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.
○The relationship between flowering plants and pollinators provides pollinators with food and flowers with efficient reproduction.
○In some cases birds obtain food from the seeds that are dispersed in the wind.
○The wind not only helps the flowers distribute their seeds but enables birds to find more food.
○Animals and insects are more effective in distributing pollen and seeds than the wind.
10. 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?
○Ants defend the host trees against the predatory actions of insects and animals.
○The acacia trees are a valuable source of nutrition for the ants.
○The ants enable the acacia tree to produce its own chemical defenses.
○The ants protect the acacia from having to compete with surrounding vegetation.
Paragraph 6: 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.
11. The word highlights in the passage is closest in meaning to
○Defines
○Emphasizes
○Reflects
○Suggests
12. What is the main purpose of this passage?
○To explain the concept of symbiosis by expanded descriptions of its principal types
○To make a comparison between human relationships and symbiotic interactions in the natural world
○To demonstrate the unforeseen benefits of natural processes that at first seem wholly destructive
○To argue that parasitism is a problem that can be solved by scientific intervention
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.
13. 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?
14. 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 involve the interaction of two or more organisms acting as partners.
●
●
●
Answer Choices
○Parasitic relationships involve the interplay of aggression by the parasite and resistance and adaptation by the host.
○Mutualism ordinarily involves an interaction between two members of the same species.
○Mutualism is unique among symbiotic relationships in that it r benefits both partners involved in the relationship.
○Parasitic damage to Australian rabbits was never reversed because the rabbits were unable to adapt to the parasites' attacks.
○The rarity of commensal relationships stems from the difficulty of finding relationships that benefit one species without affecting the other.
○The structure of biological communities depends on the types of relationships that exist among the species within.
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14. Parasitic relationships…
The rarity of…
Mutualism is unique…
共生關(guān)系是兩種或更多物種之間的一種交互作用，其中一個(gè)物種要么在另一個(gè)物種中生存要么依賴另外一個(gè)物種生存。共生關(guān)系共有三種類型：寄生、共棲和互利共生。其中第一種和第三種是一個(gè)生物群落結(jié)構(gòu)的關(guān)鍵要素。所謂生物群落，指的是在某個(gè)特定區(qū)域內(nèi)，所有生物體共同生存并且潛在地相互影響。
寄生現(xiàn)象是一種捕食式的關(guān)系，其中，寄生物通過(guò)削弱其寄主而獲得自身所需食物。寄生物的形體往往小于寄主。絳蟲(chóng)是寄生的例子之一，它生存在較大型動(dòng)物的腸道中，并吸收寄主體內(nèi)的營(yíng)養(yǎng)。自然選擇青睞那些尋找寄主并且寄生能力強(qiáng)的寄生蟲(chóng)，同時(shí)，防御能力強(qiáng)的寄主也被選擇出來(lái)。比如說(shuō)，有些植物會(huì)產(chǎn)生對(duì)真菌和細(xì)菌寄生物有毒的化學(xué)物質(zhì)，也會(huì)產(chǎn)生那些對(duì)捕食動(dòng)物有毒的化學(xué)物質(zhì)（有時(shí)這些化學(xué)物質(zhì)是一樣的）。而對(duì)于脊椎動(dòng)物來(lái)說(shuō)，其身體的免疫系統(tǒng)可以對(duì)體內(nèi)的寄生物進(jìn)行多層防御。
有時(shí)候，寄生關(guān)系的自然選擇效應(yīng)也可能在現(xiàn)實(shí)中被觀察到。比如說(shuō)，二十世紀(jì)四十年代，億萬(wàn)只歐洲兔在澳大利亞泛濫成災(zāi)。兔子肆虐了廣袤的土地，并給牛羊業(yè)帶來(lái)了極大的威脅。1950年，為了控制兔災(zāi)，澳大利亞特意引進(jìn)了一種名為粘液瘤病毒的可影響兔子的寄生蟲(chóng)。通過(guò)蚊子的快速傳播，兔子數(shù)量急劇減少。然而，這種寄生蟲(chóng)對(duì)于生存下來(lái)的兔群的后代就沒(méi)有那么致命了，而且這種傷害逐年減小。顯然，該兔群的遺傳性狀（生物體的基因結(jié)構(gòu)）經(jīng)過(guò)了自然選擇，已經(jīng)具備了更好地抵抗粘液瘤病毒的能力。同時(shí)，由于自然選擇更傾向于那些能夠感染寄主但不致其死的品系，這種病毒最致命的品系也逐漸地衰敗了。這樣，自然選擇便使得寄主-寄生蟲(chóng)的關(guān)系趨于穩(wěn)定。
與寄生關(guān)系相反，在共生關(guān)系之中，一方受益，也不會(huì)給另一方帶來(lái)嚴(yán)重影響。然而在現(xiàn)實(shí)中，純粹的共生關(guān)系幾乎不存在，因?yàn)楹茈y有一方會(huì)完全不受影響。共生關(guān)系有時(shí)候表現(xiàn)為這樣一種方式，一個(gè)物種尋覓食物會(huì)經(jīng)由另外一個(gè)物種不經(jīng)意地暴露出來(lái)。比如說(shuō)，有一些以昆蟲(chóng)為食的鳥(niǎo)類會(huì)被放牧中的牛群趕出草地。很難說(shuō)這個(gè)對(duì)牛群會(huì)帶來(lái)什么影響，但這樣的關(guān)系也許正以一種我們尚未認(rèn)知到的方式在幫助或阻礙著它們。
第三種關(guān)系，互利共生，是指共生雙方能夠互利互惠。其中典型的例子有豆科植物和固氮細(xì)菌，以及開(kāi)花植物和授粉生物。在第一個(gè)例子中，植物可以為細(xì)菌提供碳水化合物以及其他一些化合物，而這些細(xì)菌則能產(chǎn)生一種起催化作用的酶，它最終增加土壤中的氮元素來(lái)豐富土壤。在第二個(gè)例子中，授粉生物（昆蟲(chóng)、鳥(niǎo)類等）從開(kāi)花植物中獲取食物，而植物則可以通過(guò)它們來(lái)傳遞花粉和種子，這比僅僅依靠風(fēng)來(lái)傳遞要高效得多。還有一個(gè)互利共生的例子是生長(zhǎng)在美國(guó)中南部的牛角金合歡樹(shù)。這種樹(shù)為一種偽蟻屬的螞蟻提供了棲居地。這些螞蟻?zhàn)≡诖蟮闹锌盏那G棘叢中，汲取金合歡樹(shù)分泌出來(lái)的糖分。它們還吃樹(shù)葉末端的黃色組織：這個(gè)部分富含蛋白質(zhì)，但是除了吸引螞蟻，似乎對(duì)樹(shù)本身沒(méi)有任何功能。而這些螞蟻們則可以幫助它們的寄主攻擊外界幾乎所有的威脅。它們會(huì)叮咬昆蟲(chóng)和食草動(dòng)物（只以植物為食的動(dòng)物），甚至可以削減生長(zhǎng)在樹(shù)周圍的其他植物。一旦螞蟻被清除掉，這種樹(shù)就難以存活，很可能是因?yàn)樗鼈儽皇巢輨?dòng)物損害而無(wú)力與周圍的其他植物爭(zhēng)奪陽(yáng)光和生長(zhǎng)空間。
共生關(guān)系中物種間錯(cuò)綜復(fù)雜的相互影響揭示了群落中很重要的一點(diǎn)，即結(jié)構(gòu)的建立依賴于生物間千變?nèi)f化的聯(lián)系網(wǎng)絡(luò)。
