托福TPO42閱讀真題解析：Passage 2

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Geographic Isolation of Species

 paragraph 1
Biologist Ernst Mayr defined a species as “an actually or potentially interbreeding population that does not interbreed with other such populations when there is opportunity to do so.” A key event in the origin of many species is the separation of a population with its gene pool (all of the genes in a population at any one time) from other populations of the same species, thereby preventing population interbreeding. With its gene pool isolated, a separate population can follow its own evolutionary course. In the formation of many species, the initial isolation of a population seems to have been a geographic barrier. This mode of evolving new species is called allopatric speciation.

1. The word “key” in the passage is closest in meaning to 【詞匯題】
A. early B. crucial C. noticeable D. frequent
答案：B

解析：key, 關(guān)鍵的。
Early, 早期的。Crucial, 重要的，決定性的。Noticeable, 顯而易見的，值得注意的。Frequent, 頻繁的。所以B選項(xiàng)正確。

2. The word “initial” in the passage is closest in meaning to 【詞匯題】
A. best B. usual C. first D. actual
答案：C

解析：initial, 最初的。
Best, 最好的。Usual, 通常的。First, 第一，首先。Actual, 真正的。所以C選項(xiàng)正確。

3. According to paragraph 1. allopatric speciation is possible when 【事實(shí)信息題】
A. a population contains all the different genes present in a species at a particular time
B. a population becomes isolated due to the presence of a geographic barrier
C. genetic mixing begins to occur in previously separate populations of a species
D. a species is successful in crossing a geographic barrier
答案：B

解析：根據(jù)allopatric speciation定位至原文the initial isolation of a population seems to have been a geographic barrier. This mode of evolving new species is called allopatric speciation.其中have been a geographic barrier對(duì)應(yīng)B選項(xiàng)中的the presence of a geographic barrier. 所以B選項(xiàng)正確。

paragraph 1&2
Biologist Ernst Mayr defined a species as “an actually or potentially interbreeding population that does not interbreed with other such populations when there is opportunity to do so.”A key event in the origin of many species is the separation of a population with its gene pool (all of the genes in a population at any one time) from other populations of the same species, thereby preventing population interbreeding With its gene pool isolated, a separate population can follow its own evolutionary course. In the formation of many species, the initial isolation of a population seems to have been a geographic barrier This mode of evolving new species is called allopatric speciation.
Many factors can isolate a population geographically. A mountain range may emerge and gradually split a population of organisms that can inhabit only lowland lakes, certain fish populations might become isolated in this way. Similarity, a creeping glacier may gradually divide a population, or a land bridge such as the Isthmus of Panama may form and separate the marine life in the ocean waters on either side.

4. How is paragraph 2 related to paragraph 1? 【組織結(jié)構(gòu)題】
A. Paragraph 2 points out a number of ways in which the phenomenon of geographic isolation mentioned in paragraph 1 can occur
B. Paragraph 2 identifies discoveries that led to the conclusion presented in paragraph 1 that geographic isolation has played a rote in the origin of many species
C. Paragraph 2 provides evidence supporting the statement in paragraph 1 that a population can follow its own evolutionary course once its gene pool becomes isolated
D. Paragraph 2 explains why the term “allopatric” was adopted to describe the method of speciation described in paragraph 1
答案：A

解析：根據(jù)第二段第一句話Many factors can isolate a population geographically.可知接下來的整段內(nèi)容都將圍繞能引起異域種化的因素或途徑。而后列舉的mountain range, creeping glacier, land bridge都是進(jìn)一步的對(duì)其進(jìn)行解釋說明。所以A選項(xiàng)正確。

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paragraph 3
How formidable must a geographic barrier be to keep populations apart? It depends on the ability of the organisms to move across barriers. Birds and coyotes can easily cross mountains and rivers. The passage of wind-blown tree pollen is also not hindered by such barriers, and the seeds of many plants may be earned back and forth on animals In contrast, small rodents may find a deep canyon or a wide river an effective barrier. For example, the Grand Canyon, in the southwestern United Slates, separate the range of the while-tailed antelope squirrel from that of the closely related Harris’ antelope squirrel. Smaller, with a shorter tail that is white underneath, the white-tailed antelope squirrel inhabits deserts north of the canyon and west of the Colorado River in southern California Hams' antelope squirrel has a more limited range in deserts south of the Grand Canyon.

5. In paragraph 3, the author contrasts a variety of organisms to illustrate which of the following points? 【事實(shí)信息題】
A. Geographic barriers are less likely to keep apart populations of plants than populations of animals.
B. Geographic barriers are more likely to keep apart populations of large organisms than populations of small organisms
C. Some members of a species are able to cross geographic barriers, while other members of the same species are not.
D. The effectiveness of geographic barriers in keeping organisms apart depends on an organism’s ability to move across barriers.
答案：D

解析：根據(jù)organisms定位至原文How formidable must a geographic barrier be to keep populations apart? It depends on the ability of the organisms to move across barriers.其中How formidable must a geographic barrier be to keep populations apart對(duì)應(yīng)D選項(xiàng)中的The effectiveness of geographic barriers in keeping organisms apart. 所以D選項(xiàng)正確。

6. Paragraph 3 supports the idea that white-tailed antelope squirrels and Harris' antelope squirrels have which of the following in common? 【事實(shí)信息題】
A. They are the two smallest rodents now found in the southwestern United States.
B. They have white coloring underneath their tails
C. They cannot cross the Grand Canyon
D. They cannot survive in desert conditions
答案：C

解析：根據(jù)white-tailed antelope squirrels定位至原文For example, the Grand Canyon, in the southwestern United States, separate the range of the while-tailed antelope squirrel from that of the closely related Harris’ antelope squirrel.可知，科羅拉多大峽谷將白尾巴羚羊松鼠和哈里斯羚羊松鼠隔絕開了，所以C選項(xiàng)正確，這兩種羚羊松鼠的共同特點(diǎn)就是無法跨越科羅拉多大峽谷。

paragraph 4
Geographic isolation creates opportunities for new species to develop, but it does not necessarily lead to new species because speciation occurs only when the gene pool undergoes enough changes to establish reproductive barriers between the isolated population and its parent population. The likelihood of allopatric speciation increases when a population is small as well as isolated, making it more likely than a large population to have its gene pool changed substantially. For example, in less than two million years, small populations of stray animals and plants from the South American mainland that managed to colonize the Galapagos Islands gave rise to all the species that now inhabit the islands.

7.The word “undergoes” in the passage is closest in meaning to 【詞匯題】
A.experiences B.allows C.prevents D.causes
答案：A

解析：undergoes, 經(jīng)歷，忍受。
Experiences, 經(jīng)歷。Allows, 允許。Prevents, 阻止。Cause, 引起。所以A選項(xiàng)正確。

8.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 【句子簡(jiǎn)化題】
A. Geographic isolation is sometimes but not always the reason for the creation of reproductive barriers between a parent population and the populations descended from it
B. Genetic changes in a geographical isolated population do not necessarily make the population look different enough from its parent population to be considered a new species
C. Geographical isolation allows the separated populations to evolve independently of each other and so can lead to the formation of new species
D. Geographic isolation can lead to new species only if the gene pool of the isolated population changes enough to prevent it from reproducing with the parent population
答案：D

解析：注意句子中的轉(zhuǎn)折和因果關(guān)系，句子主干出現(xiàn)在轉(zhuǎn)折后面，且要注意only后面的內(nèi)容。句子大意為，只有當(dāng)物種被隔絕的基因庫(kù)改變的足夠多來防止它們和親本種群繁殖時(shí)，地理隔絕就會(huì)產(chǎn)生新的物種。所以D選項(xiàng)正確。

9.According to paragraph 4, why does the size of a population affect the likelihood of allopatric speciation? 【事實(shí)信息題】
A. Because smaller populations are more likely than larger ones to become geographically isolated
B. Because the gene pool of a small isolated population is more likely to undergo substantial change than is the gene pool of a larger population
C. Because a isolated population can become a new species with substantially less change to its gene pool than would be required by a larger population
D. Because smaller populations are more likely to be made up of stray animals or plants than larger populations are
答案：B

解析：根據(jù)likelihood定位至原文The likelihood of allopatric speciation increases when a population is small as well as isolated, making it more likely than a large population to have its gene pool changed substantially.注意句中的比較關(guān)系，并且其中have its gene pool changed substantially對(duì)應(yīng)B選項(xiàng)中的undergo substantial change. 所以B選項(xiàng)正確。

10.The word “managed” in the passage is closest in meaning to 【詞匯題】
A.were able B.were forced C.arrived D.expanded
答案：A

解析：managed,設(shè)法完成。
Were able, 能夠。Were forced, 被迫。Arrived, 到達(dá)。Expanded, 擴(kuò)大。所以A選項(xiàng)正確。

paragraph 5
When oceanic islands are far enough apart to permit populations to evolve in isolation, but close enough to allow occasional dispersions to occur, they are effectively outdoor laboratories of evolution. The Galapagos island chain is one of the world s greatest showcases of evolution Each island was born from underwater volcanoes and was gradually covered by organisms derived from strays that rode the ocean currents and winds from other islands and continents. Organisms can also be carried to islands by other organisms, such as sea birds that travel long distances with seeds clinging to their feathers.

11.Paragraph 5 supports the idea that the Galapagos island chain was able to become “one of the world's greatest showcases of evolution” primarily because of 【事實(shí)信息題】
A. the richness of the volcanic soil of each of the islands in the chain
B. the distance of the individual islands from each other and from the mainland
C. the relativity long time it took for the islands to become covered by organisms
D. the outdoor laboratories that scientists have built on the islands to study evolution
答案：B

解析：根據(jù)“one of the world's greatest showcases of evolution”定位至原文前一句話When oceanic islands are far enough apart to permit populations to evolve in isolation, but close enough to allow occasional dispersions to occur, they are effectively outdoor laboratories of evolution.可知，島嶼之間的距離和陸地的距離最完美的詮釋了進(jìn)化。所以B選項(xiàng)正確。

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paragraph 6
The species on the Galapagos Islands today, most of which occur nowhere else, descended from organisms that floated, flew, or were blown over the sea from the South American mainland For instance, the Galapagos island chain has a total of thirteen species of closely related birds called Galapagos finches These birds have many similarities but differ in their feeding habits and their beak type, which is correlated with what they eat Accumulated evidence indicates that all thirteen finch species evolved from a single small population of ancestral birds that colonized one of the islands. Completely isolated on the island after migrating from the mainland, the founder population may have undergone significant changes in its gene pool and become a new species. Later, a few individuals of this new species may have been blown by storms to a neighboring island. Isolated on this second island, the second founder population could have evolved into a second new species, which could later recolonize the island from which its founding population emigrated. Today each Galapagos island has multiple species of finches, with as many as ten on some islands.

12.According to paragraph 6. what is true about the thirteen species of Galapagos finches?【事實(shí)信息題】
A. All thirteen species are now found on most of the Galapagos Islands
B. All thirteen species are descended from the same population of ancestral birds
C. All thirteen species evolved on the island that was originally colonized by finches from the mainland.
D. All thirteen species occur only in small, completely isolated populations.
答案：B

解析：根據(jù)thirteen species定位至原文Accumulated evidence indicates that all thirteen finch species evolved from a single small population of ancestral birds that colonized one of the islands.其中a single small population of ancestral birds對(duì)應(yīng)B選項(xiàng)中的the same population of ancestral birds.所以B選項(xiàng)正確。

13.Look at the four squares [■] that indicate where the following sentence could be added to the passage. 【句子插入題】
This process of speciation and colonization could have been repeated over and over again, gradually involving all the islands in the chain.
The species on the Galapagos Islands today, most of which occur nowhere else, descended from organisms that floated, flew, or were blown over the sea from the South American mainland For instance, the Galapagos island chain has a total of thirteen species of closely related birds called Galapagos finches These birds have many similarities but differ in their feeding habits and their beak type, which is correlated with what they eat Accumulated evidence indicates that all thirteen finch species evolved from a single small population of ancestral birds that colonized one of the islands.Completely isolated on the island after migrating from the mainland, the founder population may have undergone significant changes in its gene pool and become a new species. [■] Later, a few individuals of this new species may have been blown by storms to a neighboring island. [■]Isolated on this second island, the second founder population could have evolved into a second new species, which could later recolonize the island from which its founding population emigrated.[■]Today each Galapagos island has multiple species of finches, with as many as ten on some islands.[■]
答案：C

解析：根據(jù)B選項(xiàng)的這句話，Isolated on this second island, the second founder population could have evolved into a second new species, which could later recolonize the island from which its founding population emigrated.對(duì)應(yīng)待插入句子中的This process of speciation and colonization, 所以C選項(xiàng)正確。

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.
【概要小結(jié)題】
The geographic isolation of a population can result in the rise of a new species.
Answer Choices
A.Isolation can result when a geographic barrier forms and splits a population or when a few organisms somehow get carried across an existing geographic barrier and form a new population
B.Speciation is more likely when an isolated population is small because significant genetic changes are more likely to occur in a small population than in a large one
C.Because of the geographic isolation of the Galapagos Islands, the species that now inhabit them have gene pools that have not changed very much since the islands were first populated.
D.Fish populations are more easily isolated by geographic barriers than are populations of most other organisms because fish cannot move across areas where there is no water.
E.The Galapagos Islands are well situated for speciation because they provide opportunities for population isolation while also making occasional dispersions between islands possible.
F.Evidence indicates that the first organisms to reach the Galapagos Islands were probably a small population of finches that,in less than two million years of isolation,evolved into thirteen species.
答案：A, B, E

解析：全文都在講，當(dāng)形成地理障礙分隔了一個(gè)種群時(shí)，或者當(dāng)一些有機(jī)體跨越了地理障礙形成新的種群時(shí)，會(huì)產(chǎn)生隔離。所以A選項(xiàng)正確。
根據(jù)原文The likelihood of allopatric speciation increases when a population is small as well as isolated, making it more likely than a large population to have its gene pool changed substantially.所以B選項(xiàng)正確。
根據(jù)原文Completely isolated on the island after migrating from the mainland, the founder population may have undergone significant changes in its gene pool and become a new species.可知，在Galapagos島上的生物經(jīng)歷了很多的變化，并非C選項(xiàng)中的have not changed very much,所以C選項(xiàng)錯(cuò)誤。
根據(jù)原文A mountain range may emerge and gradually split a population of organisms that can inhabit only lowland lakes, certain fish populations might become isolated in this way.文中并沒有提及魚類容易被地理障礙隔離，也沒有提及是否因?yàn)闆]有水就不能跨越該區(qū)域等。所以D選項(xiàng)錯(cuò)誤。
根據(jù)原文When oceanic islands are far enough apart to permit populations to evolve in isolation, but close enough to allow occasional dispersions to occur, they are effectively outdoor laboratories of evolution.其中allow occasional dispersions to occur對(duì)應(yīng)E選項(xiàng)中的making occasional dispersions, 所以E選項(xiàng)正確。
根據(jù)原文Accumulated evidence indicates that all thirteen finch species evolved from a single small population of ancestral birds that colonized one of the islands.可知，原文中的ancestral birds 并非F選項(xiàng)中的finches, 所以F選項(xiàng)錯(cuò)誤。

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