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英['lɪs(ə)n] 美['lɪsn]
vi. 听,倾听;听从,听信
n. 听,倾听

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草莓小菇凉:说的非常好,十分有道理,棒棒棒!

06-08 15:44:55

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TPO-31-Speciation in Geographically Isolated Populations

Evolutionary biologists believe that speciation, the formation of a new species, often begins when some kind of physical barrier arises and divides a population of a single species into separate subpopulations. Physical separation between subpopulations promotes the formation of new species because once the members of one subpopulation can no longer mate with members of another subpopulation, they cannot exchange variant genes that arise in one of the subpopulations. In the absence of gene flow between the subpopulations, genetic differences between the groups begin to accumulate. Eventually the subpopulations become so genetically distinct that they cannot interbreed even if the physical barriers between them were removed. At this point the subpopulations have evolved into distinct species. This route to speciation is known as allopatry (“allo-” means “different”, and “patria” means “homeland”).


Allopatric speciation may be the main speciation route. This should not be surprising, since allopatry is pretty common. In general, the subpopulations of most species are separated from each other by some measurable distance. So even under normal situations the gene flow among the subpopulations is more of an intermittent trickle than a steady stream. In addition, barriers can rapidly arise and shut off the trickle. For example, in the 1800s a monstrous earthquake changed the course of the Mississippi River, a large river flowing in the central part of the United States of America. The change separated populations of insects now living along opposite shores, completely cutting off gene flow between them.

Geographic isolation can also proceed slowly, over great spans of time. We find evidence of such extended events in the fossil record, which affords glimpse into the breakup of formerly continuous environments. For example, during past ice ages, glaciers advanced down through North America and Europe and gradually cut off parts of populations from one another. When the glaciers retreated, the separated populations of plants and animals came into contact again. Some groups that had descended from the same parent population were no longer reproductively compatible – they had evolved into separate species. In other groups, however, genetic divergences had not proceeded so far, and the descendants could still interbreed – for them, reproductive isolation was not completed, and so speciation had not occurred.

Allopatric speciation can also be brought by the imperceptibly slow but colossal movements of the tectonic plates that make up Earth’s surface. About 5 million years ago such geologic movements created the land bridge between North America and South America that we call the Isthmus of Panama . While previously the gap between the continents had allowed a free flow of water, now the isthmus presented a barrier that divided the Atlantic Ocean from the Pacific Ocean. This division set the stage for allopatric speciation among populations of fishes and other marine species.

In the 1980s, John Graves studied two populations of closely related fishes, one population from the Atlantic side of isthmus, the other from the Pacific side. He compared four enzymes found in the muscles of each population. Graves found that all four Pacific enzymes function better at lower temperatures than the four Atlantic versions of the same enzymes. This is significant because Pacific seawater is typically 2 to 3 degrees cooler than seawater on the Atlantic side of isthmus. Analysis by gel electrophoresis revealed slight differences in amino acid sequence of the enzymes of two of the four pairs. This is significant because the amino acid sequence of an enzyme is determined by genes.

Graves drew two conclusions from these observations. First, at least some of the observed differences between the enzymes of the Atlantic and Pacific fish populations were not random but were the result of evolutionary adaptation. Second, it appears that closely related populations of fishes on both sides of the isthmus are starting to genetically diverge from each other. Because Graves’ study of geographically isolated populations of isthmus fishes offers a glimpse of the beginning of a process of gradual accumulation of mutations that are neutral or adaptive, divergences here might be evidence of allopatric speciation in process.

进化生物学家认为,物种形成,新物种的形成,往往始于某种物质屏障的出现,并将一个物种的种群分为不同的亚群。亚群之间的物理分离促进了新物种的形成,因为一旦一个亚群的成员不能再与另一个亚群的成员交配,它们就不能交换其中一个亚群中出现的变异基因。在亚群之间没有基因流的情况下,群之间的遗传差异开始积累。最终,这些亚种群在遗传学上是非常独特的,即使它们之间的物理障碍被消除,它们也不能杂交。在这一点上,亚种已经演变成不同的物种。这种物种形成的途径被称为异种(“allo-”是指“不同的”,“patria”是指“家乡”)。

 

异源物种可能是主要的物种形成途径。这应该不会令人感到惊讶,因为异种是相当普遍的。一般来说,大多数物种的亚种群之间相隔一定距离。所以即使在正常情况下,亚群之间的基因流动更像是一种间歇性的滴流,而不是稳定的流动。此外,障碍可能迅速出现,并关闭涓流。例如,在19世纪,一场巨大的地震改变了密西西比河的流程,密西西比河流经美利坚合众国中部。这种变化使现在居住在相对海岸的昆虫种群分离,完全切断了它们之间的基因流动。

 

地理隔离也可以在很长时间内缓慢进行。我们在化石记录中发现了这样的扩展事件的证据,这让我们看到了以前持续不断的环境的分裂。例如,在过去的冰河时代,冰川通过北美和欧洲向下推进,逐渐将部分人口相互切断。当冰川退缩时,分离的动植物种群再次接触。一些来自同一父母本族的群体不再具有生殖上的兼容性 - 他们已经演变成了不同的物种。然而在其他群体中,遗传分化还没有进行到底,后代仍然可以进行杂交 - 对于它们来说,生殖隔离没有完成,所以没有发生物种形成。

 

构成地球表面的大地构造板块也可以通过不可感知的缓慢而巨大的运动来引起异域物种形成。大约500万年前,这种地质运动创造了北美和南美之间的陆桥,我们称之为巴拿马地峡。虽然此前各大洲之间的差距已经让水自由流动,但现在峡谷提出了将大西洋与太平洋分开的屏障。这个部门为鱼类和其他海洋物种种群之间的异域物种形成奠定了基础。

 

在20世纪80年代,约翰·格雷夫斯研究了两个密切相关的鱼类种群,一个来自大西洋地区,另一个来自太平洋地区。他比较了每个人群肌肉中发现的四种酶。格雷夫斯发现,所有四种太平洋酶在四种大西洋版本的相同酶的较低温度下功能更好。这是非常重要的,因为太平洋海水通常比地峡大西洋一侧的海水低2至3度。通过凝胶电泳分析揭示了四对中的两对的酶的氨基酸序列的轻微差异。这是重要的,因为酶的氨基酸序列是由基因决定的。

 

格雷夫斯从这些意见中得出两个结论。首先,至少有一些观察到的大西洋和太平洋鱼类的酶的差异不是随机的,而是进化适应的结果。其次,地峡两侧的密切相关的鱼群似乎开始在遗传上相互分离。因为格雷夫斯对地理上分离的地峡鱼类群体的研究提供了中性或适应性突变渐进积累过程的开始,这里的分歧可能是过程中异源物种形成的证据。

中文翻译

The word "promotes" in the passage is closest in meaning to

  • Physical separation between subpopulations promotes the formation of new species because once the members of one subpopulation can no longer mate with members of another subpopulation A Describes B Encourages C Delays D Requires promotes
正确答案: B

网友解析

  • 用户头像

    十三个  发表于 2018-07-18 15:53:18

    题型分类:词汇题

    原文定位:Though these bees do no sting, rival colonies of some species fight fiercely over potential nesting sites. 大部分同学可能都熟悉rival作为名词词性的意思为“对手,竞争者”。那么从这个名词词性也不难猜测其形容词词性。再者,也可以从这句话的语境来推测,主要说的是这些物种会争夺地盘,那么rival就是形容这些物种之间的关系的,很明显,将这一层意思表达出来的只有B选项。rival: 竞争的。

    选项分析:



    A选项 确定的,已建立的;

    B选项 竞争的;正确

    C选项 附近的;

    D选项 不同的。

  • 用户头像

    kisenana  发表于 2017-12-06 14:03:34

    词汇题
    Physical separation between subpopulations promotes the formation of new species 亚群的身体区别促进产生新的物种,promote在句子意思中是促进的意思。选项A 描述 B 鼓励,促进 C 延迟 D 要求, 排除ACD,选B

题目讨论

  • 用户头像

    kisenana  发表于 12-06 14:04:00

    词汇题需要在句子意思,语境下选择

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