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tpo-42 - Reading - Passage1 < 上一篇 下一篇 >
Question 1 of 14
1. The word“key” in the passage is closest in meaning to

   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.ln 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 low land 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 lsthmus of Panama may form and separate the marine life in the ocean waters on either side.

  How formidable must a geographic barrier be to keep populations apart?lt depends on the ability of the organisms to move across blown tree pollen is also not hindered by such barriers, and the seeds of many plants may be earned back and forth on animals.ln contrast,,small rodents may find a deep canyon or a wide river an effective barrier.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.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.

  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 lslands gave rise to all the species that now inhabit the islands.

  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.0rganisms can also be carried to islands by other organisms, such as sea birds that travel long distances with seeds clinging to their feathers.

  The species on the Galapagos lslands today,,most of which occur nowhere else, descend-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.lsolated 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.

生物学家厄恩斯特迈尔将那些有机会和别的物种杂交却没有杂交的物种定义为“实际或潜在的杂交群体”。许多物种起源的一个关键事件是:基因库的一个种群和同一基因库的另一种群分离(在任何一个时间一个种群的所有基因) , 修防止种群繁殖 。 和基因库分离, 一个种群可以遵循它自己的进化过程。在许多物种的形成中, - 个种群的初始隔离似乎是地理上的障碍。这种模式进化的新物种被称为异域种化。    

许多因素可以在地理上隔离忠犬。一个山脉可能会出现,并逐渐分製一个可以栖息在低地湖泊的生物种群, 某些鱼类群可能会在这种方式中被隔离 。 同样地, 缓慢行进的冰川可能会逐渐将种群分隔开, 或者像巴拿马海山実这样路桥的建成分离了两边海洋水域的海洋生物。    

一个地理上的障碍是如何强大的,从而使种群分隔?这取决于生物能跨越障碍的能力。鸟和狼可以轻易越过山川 。 风吹树花粉的通道也不受这种障碍的约束, 许多植物的种子可以在动物身上来回地来回移动。相反的是,小老鼠可能会找到一 个深山実谷或一条宽河成为有效屏障。 例如, 在美国西南部的大山実谷, 将尾羚羊松鼠以及与它密切相关的哈里斯羚羊松鼠分隔开来。白尾羚羊松鼠体型较小,尾巴更短(白色在下面) ,栖息于大山実谷北部、 南加利福尼亚科罗拉多河的西部的沙漠。哈里斯羚羊松鼠的(栖息)范围比较有在大山実谷南部的沙漠 。    

地理隔离创造了新物种发展的机会,但它不一定会导致新物种,因为只有当基因产生足够的变化, 使得分隔种群与其起源种群之间产生繁殖障碍之。这个时候新的物种才会发生。当种群数量小且被分隔的时候,异域的可能性就增加了,它比一 个大数量的种群更容易使基因库发生重大变化。例如,在不到二百万年,来自南美大陆的小种群流浪动物和植物设法成功在加拉帕戈斯群岛大量繁殖,这促使了所有岛山与生存的物种的产生 。    

当海洋岛山与离得足够远, 远到可以使种群分离, 但又足够近,使得偶尔的分散得以发生,他们是进化有效的户外实验室 。 加拉帕戈斯群岛是世界上最大进化展示 。 每个岛山与都诞生于海底火山, 并且逐渐被生物体覆盖,这些生物体乘着洋流和风从其他岛山与和大陆过来 。 生物也可以被其他生物携帯到岛山与上, 比如说, 在长距离的旅行中, 种子会附着在他们的羽毛上的。    

在今天的加拉帕戈斯群岛的物种, 其中大多数出现在其他地方,从南美大陆生飘过来、飞过来或被风吹过来的生物 。 比如, 加拉帕戈斯群岛共有十三种密切相关的加拉帕戈斯雀类鸟。这些鸟有许多相似之处, 但他们的口畏养习惯和的嘴型不同,这是与他们吃什么有关。积累的证据表明所有的雀是从同一个小种群的祖先鸟进化过来的, 这种鸟曾经生活在岛山与之一。 从大陆迁移到岛上完全分离后, 起源的种群的基因库可能已经发生了显着的变化然后成为了新的种群 。 后来, 新物种的一些个体可能被风暴吹到邻近的岛山与 。 孤立在这二岛, 第二个起源物种可能进化成第二个新物种, 可后可能移居到它起源物种移居的岛山与 。 今天每个加拉帕戈斯岛有多种雀, 一些岛上有多达十几种 。


正确答案: B

key:关键的,重要的。对应B选项的crucial (重要的) 。文中说的是物种起源的key event,结合上下文BC更合适一点, B选项和 Key的意思最接近,所以选B

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