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Speciation in Geographically Isolated Populations

进化生物学家认为，物种形成，即一个新物种的形成，通常始于某种物理屏障的出现，并将一个单一物种的种群划分为不同的亚种群。

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.

亚种群间的物理分离促进了新物种的形成，因为一旦一个亚种群的成员不能与另一个亚种群的成员交配，他们就不能交换在一个亚种群中出现的变异基因。

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.

在亚群体之间缺乏基因流动的情况下，群体之间的遗传差异开始积累。

In the absence of gene flow between the subpopulations, genetic differences between the groups begin to accumulate.

最终，这些亚种群在基因上变得如此不同，以至于即使消除了它们之间的物理障碍，它们也无法杂交。

Eventually the subpopulations become so genetically distinct that they cannot interbreed even if the physical barriers between them were removed.

在这一点上，亚种群已经进化成不同的物种。

At this point the subpopulations have evolved into distinct species.

这条通往物种形成的道路被称为“异域性”（“allo-”意为“不同的”，“patria”意为“家园”）。

This route to speciation is known as allopatry (“allo-” means “different”, and “patria” means “homeland”).

异域物种形成可能是物种形成的主要途径。

Allopatric speciation may be the main speciation route.

这应该不足为奇，因为异域性相当普遍。

This should not be surprising, since allopatry is pretty common.

一般来说，大多数物种的亚种群之间都有一定的距离。

In general, the subpopulations of most species are separated from each other by some measurable distance.

因此，即使在正常情况下，亚种群之间的基因流动也更像是断断续续的细流，而不是稳定的细流。

So even under normal situations the gene flow among the subpopulations is more of an intermittent trickle than a steady stream.

此外，障碍会迅速出现，将细流拒之门外。

In addition, barriers can rapidly arise and shut off the trickle.

例如，在19世纪，一场巨大的地震改变了密西西比河的流向，这是一条流经美国中部的大河。

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.

这种变化使生活在两岸的昆虫种群分离，完全切断了它们之间的基因流动。

The change separated populations of insects now living along opposite shore, completely cutting off gene flow between them.

地理上的隔离也可能在很长一段时间内进展缓慢。

Geographic isolation also can proceed slowly, over great spans of time.

我们在化石记录中找到了这种扩展事件的证据，这让我们得以一窥之前连续环境崩溃的情况。

We find evidence of such extended events in the fossil record, which affords glimpses into the breakup of formerly continuous environments.

例如，在过去的冰河时代，冰川向下穿过北美和欧洲，逐渐切断了部分种群的联系。

For example, during past ice ages, glaciers advanced down through North America and Europe and gradually cut off parts of populations from one another.

当冰川消退时，分离的动植物种群再次接触。

When the glaciers retreated, the separated populations of plants and animals came into contact again.

一些由同一亲本群体进化而来的群体在繁殖上不再相容——他们进化成了不同的物种。

Some groups that had descended from the same parent population were no longer reproductively compatible— they had evolved into separate species.

然而，在其他群体中，遗传差异目前并没有继续，后代仍然可以杂交——对他们来说，生殖隔离还没有完成，因此还没有出现物种形成。

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.

构成地球表面的板块缓慢但巨大的运动也可能带来异域物种形成。

Allopatric speciation can also be brought by the imperceptibly slow but colossal movements of the tectonic plates that make up Earth’s surface.

大约500万年前，这样的地质运动在北美和南美之间架起了一座大陆桥，我们称之为巴拿马地峡。

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.

以前，大陆之间的鸿沟允许水的自由流动，而现在地峡形成了一道屏障，把大西洋和太平洋分隔开来。

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.

这种分割为鱼类和其他海洋物种的异域物种形成奠定了基础。

This division set the stage for allopatric speciation among populations of fishes and other marine species.

上世纪80年代，约翰·格雷夫斯（John Graves）研究了两种亲缘关系密切的鱼类，一种来自地峡的大西洋一侧，另一种来自太平洋一侧。

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.

他比较了每个群体肌肉中的四种酶。

He compared four enzymes found in the muscles of each population.

格雷夫斯发现，这四种太平洋酶在较低的温度下比四种大西洋酶的功能更好。

Graves found that all four Pacific enzymes function better at lower temperatures than the four Atlantic versions of the same enzymes.

这一点意义重大，因为太平洋的海水通常比地峡大西洋一侧的海水低2至3度。

This is significant because Pacific seawater is typically 2 to 3 degrees cooler than seawater on the Atlantic side of isthmus.

经凝胶电泳分析，发现四对中有两对的氨基酸序列略有差异。

Analysis by gel electrophoresis revealed slight differences in amino acid sequence of the enzymes of two of the four pairs.

这很重要，因为酶的氨基酸序列是由基因决定的。

This is significant because the amino acid sequence of an enzyme is determined by genes.

格雷夫斯从这些观察中得出了两个结论。

Graves drew two conclusions from these observations.

首先，至少一些观察到的大西洋和太平洋鱼类种群中酶的差异不是随机的，而是进化适应的结果。

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.

其次，似乎地峡两侧的紧密相关的鱼类种群开始在基因上彼此分离。

Second, it appears that closely related populations of fishes on both sides of the isthmus are starting to genetically diverge from each other.

由于格雷夫斯对地理上隔离地峡的鱼类种群研究，使我们得以一窥突变积累过程的开端，这些突变是中性的或适应性的，这里的分歧可能是在这一过程中异域物种形成的证据。

Because Graves’s 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.

The word “promotes” in the passage is closest in meaning to

According to paragraph 1, allopatric speciation involves which of the following?

Why does the author provide the information that “the subpopulations of most species are separated from each other by some measurable distance”?

The word “accumulate” in the passage is closest in meaning to?

In paragraph 2, why does the author mention that some insect populations were separated from each other by a change in the course of Mississippi River caused by an earthquake?

According to paragraph 3, separation of subpopulations by glaciers resulted in speciation in those groups of plants and animals that

The word “colossal” in the passage is closest in meaning to

According to paragraph 4, which of the following is true of the geologic movements that brought about the Isthmus of Panama?

The word “sequence” in the passage is closest in meaning to

According to paragraph 5, by comparing the enzymes from two related groups of fishes on opposite sides of the Isthmus, Graves found evidence that

It can be inferred from paragraph 5 and 6 that the reason Graves concluded that some of the differences between the Pacific and Atlantic enzymes were not random was that

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.

Look at the four squares  that indicate where the following sentence could be added to the passage. The formation of the isthmus had important consequences for global patterns of ocean water flow.  Where would the sentence best fit? Click on a square  to add the sentence to the passage.

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.

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 shore, completely cutting off gene flow between them. Geographic isolation also can proceed slowly, over great spans of time. We find evidence of such extended events in the fossil record, which affords glimpses 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’s 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.