托福 新托福 31 - Speciation in Geographically Isolated Populations
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9.Look at the four squares [█] that indicate where the following sentence can be added to the passage. Where would the sentence best fit?
The formation of the isthmus had important consequences for global patterns of ocean water flow.
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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 if 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.

进化论生物学家认为物种的形成,即形成一个新的物种,通常由某一障碍将一个族群分割成亚族群开始的。亚族群间的物理隔离可以促进物种形成是因为隔离之后一个亚族群里的个体无法与其他亚族群个体交配,因此该族群里突变的基因将无法与其他亚族群交流。之后亚族群间的基因差异将逐渐累积。最终即便物理隔离不复存在,亚族群间的基因差异过大而无法杂交。此时亚族群就进化为不同的物种。这种物种形成的轨迹称为异域分布。 异域分布可能是主要的物种形成路线。这不足奇怪,因为异域分布很常见。一般,绝大部分的物种的亚族群都由一定程度的距离隔离。所以就算在正常情况下,亚族群间基因交流更多可能像是间歇的细流而不是稳定的流动。另外,障碍可以迅速形成并切断这种细流。例如,十九世纪一场巨大的地震改变了密西西比河,美国中部的一条大河,的河道。这一改变将两岸的昆虫隔离开来,完全隔断了他们之间的基因交流。 地理隔离也可能进程缓慢,跨越很长时间。在化石记录中发现过这类事件。原本连续的环境突然终止。例如在以往的冰河时期,冰河从北美和欧洲向下扩张,逐渐切断了族群间的联系。冰河消退后分隔的动植物种群又重新接触。一些来自同一物种的后代就不能再相互交配----它们已形成新的物种。然而其它亚族群间基因差异没有如此之大,其后代仍可相互交配----对他们而言,繁殖隔离并未完成,因而新物种没有形成。 异域分布导致的物种形成也可能源自察觉不到但巨大的板块运动,他们造就了地球表面。约500万年前,这种地质运动产生了连接北美洲和南美洲的陆地通道,我们称为巴拿马地峡。地峡的形成对全球洋流的形式产生重大影响。之前两大陆之间的间隙可使洋流畅通流动,现在地峡将大西洋和太平洋隔开。这种分割为海洋生物和鱼类提供了异域分布导致物种形成的舞台。 在二十世纪80年代,John Graves研究了两种极相关的鱼群,一个族群来自地峡的大西洋一侧,另一个来自地峡太平洋一侧。他对比了族群个体肌肉中的四种酶。这很重要因为太平洋的海水一般比大西洋的低2-3度。胶体电泳分析表明四对酶的氨基酸系列有微小差别。其重要性在于酶的氨基酸系列是由基因决定的。 Graves得出两条结论。首先至少太平洋族群和大西洋族群酶的一些差别不是随机的,而是进化适应的结果。第二,地峡两侧关系紧密的鱼群的基因正在开始变异。Graves的关于地峡分割的鱼群的研究得以让我们窥见那些适应性变化的积累过程的开始。这里的变异可能是异地分布物种形成过程的证据。  

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  • wx_6697
    觉得B C 意思一样,不知道选哪个
  • wx_5576
    这道题C为什么对,E为什么不对?
  • wx_5576
    B为什么不能选啊?
  • wx_6697
    TPO30 passage 2 Q5我选的 D,不明白为啥不对?
  • wx_6697
    鑫哥,TPO6passage3Q5 答案是给错了吗?好多人都选A
  • wx_6697
    这题也很容易选错选成了D
  • wx_6697
    这道题A为什么错了
  • 芊儿
    为什么这道题不选c??a中的variety不是应该对应文中的differentiating 吗??求解!
  • wx_1000
    这道题不选E是因为太细节了吗
  • 王金阁
    这个题为什么不选C啊。。。
  • 芊儿
    这道题的D选项不是和文中的better able to reproduce in open settings相对应么??
  • 风荨火
    有大佬解释一下这个为啥选D嘛?
  • 以沫
    请问这个D 在哪里提现?为什么D错?
  • 芊儿
    第六题 的C选择为什么不对,感觉A是明显驳斥啊...
  • wx_6697
    鑫哥,这道题D是从哪里看出来的
  • wx_6697
    这题选的A,根据是Joly’s calculations clearly supported those geologists who insisted on an age for Earth far in excess of a few million years.想问鑫哥为啥不选A
  • wx_6697
    这题我选的是C依据是into a new habitat outside of its natural range, it may adapt to the new environment and leave its enemies behind.C为啥错了呢?鑫哥
  • wx_8861
    F选项的weather-related destruction在哪里体现了呢?原文最后一段的开头Among the costs里的costs是不是打错了?应该是coast?
  • wx_6697
    求问这道题B为啥不选,原文依据:viable seeds of pioneer species can be found in large numbers on some forest floors.
  • 与托福的斗争史
    与托福的斗争史 去解答 去解答
    这题为什么选C?
  • 小雨淅沥哗啦的下
    小雨淅沥哗啦的下 去解答 去解答
    B哪里错了
  • 小雨淅沥哗啦的下
    小雨淅沥哗啦的下 去解答 去解答
    B为啥不对
  • 李浩然
    B选项错误,是因为残缺么?
  • wx_100
    请问在做题的时候如何排除c呢。看了答案,感觉是该选a的,但是当时做题脑子一热,就特别钟爱c,也没看其他选项。。求敲醒。。
  • wx xxxxx
    请问鑫哥,这段开头有写As one pesticide replaces another为什么不是对应a new pesticide is developed?
  • wx_7695
    鑫哥,从哪里看出来这个masks 不是use呀,原文说了wear呀
  • haiyuqiao
    @鑫哥,这题the damage will continue 不应该对应前面的 the target species evolves resistance to it,然后As one pesticide replaces another,不应该是结束了time cycle 吗
  • wx_2065
    鑫哥,想知道E错在哪里?
  • wx_7695
    鑫哥,B选项 cannot extended to earlier geological periods. 原文说的意思是后来的进化无法估计吧
  • wx_2163
    B为什么不选
  • wx_7780
    鑫哥,这个哪里看不use了。BD是修饰错,C是无中生有,怎么能直接选出A?
  • 100
    看到第一句话,以为是中心句就选了A... 为什么不能选A呢
  • 100
    为什么选b?
  • gu33
    请问下 这里选D的原因是 因为 evolutionary approach 对应着 原文的 Rates of evolution 嘛? 这里我选了C。。不是很懂 插入句和 D的关系 求解答
  • 我是啦啦啦
    我是啦啦啦 去解答 去解答
    这个题A哪里错了?是因为主语不对吗?这个C比A多一步推理啊
  • haiyuqiao
    鑫哥,D选项里的19世纪出现了很多假设,原文中并没有提到啊
  • wx_7060
    为什么选a 呢。我觉得a是细节。F哪错了?
  • wx_1105
    我想问一下,这道题为什么不能选A呢?
  • wx_8122
    D为什么不选
  • wx_1655
    f选项哪里说了
  • chaulaw
    鑫哥,原文是below经济损害水平,D是一触发经济损害就用,这也对吗?
  • chaulaw
    interclan婚姻是对的吗?不是只在自己的family结婚扩大家族吗?
  • wx_6697
    鑫哥,这道题答案是不是错了,好多人选D 我也选的D求解答
  • wx_6697
    这道题应该是一道易错题,每个选项的都有,然而我选的A错了,求解
  • wx_6697
    鑫哥,这题的C是怎么得到的?B也没有找到啊?难道不是应该选B
  • wx_6697
    鑫哥,还有这个,好多人选A答案是不是错了
  • wx_6697
    求问D是从哪里得出来的,我选的B呀哎呀
  • wx_4185
    it is difficult to say how far they were intended to be portraits rather than generalized images 这句话怎么理解呢
  • 此楠楠
    请问下这个插入题怎么选的呢?
  • 此楠楠
    求鑫哥讲解下A选项。。。 Even though in error, Joly’s calculations clearly supported those geologists who insisted on an age for Earth far in excess of a few million years.