tpo31-1 阅读Word版

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 absences 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 glacier 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. The formation of the isthmus had important

consequences for global patterns of ocean water flow. ■ 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 adaption. 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.

1. The word “promotes” in the passage is closest in meaning to A. describes B. encourages C. delays D. Requires

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

A. The division of a population into subspecies.

B. The reuniting of separated populations after they have become distinct species.

C. The movement of a population to a new homeland. D. The absence of gene flow between subpopulations.

3. Why does the author provide the information that “the subpopulations of most species

are separated from each other by some measurable distance”?

A. To indicate how scientists are able to determine whether subpopulations of a species are allopatric.

B. To define what it means for a group of animals or plants to be a subpopulation.

C. To suggest that allopatric speciation is not the only route to subpopulation.

D. To help explain why allopatric speciation is a common way for new species to come About.

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

A. Become more significant B. Occur randomly

C. Gradually increase in number D. Cause changes

5. 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?

A. To make the point that some kind of physical barrier separates the subpopulations of most species.

B. To support the claim that the condition of allopatry can sometimes arise in a short time.

C. To provide an example of a situation in which gene flow among the subpopulations of a species happens at a slow rate.

D. To explain why insects living along opposite shores of the Mississippi River are very different from each other.

6. According to paragraph 3, separation of subpopulations by glaciers resulted in

speciation in those groups of plants and animals that

A. were reproductively isolated even after the glaciers disappeared

B. had adjusted to the old conditions caused by the glaciers

C. were able to survive being separated from their parent population

D. had experienced some genetic divergences from their parent population.

7. The word “colossal” in the passage is closet in meaning to

A. consistent B. gradual C. enormous D. Effective

8. According to paragraph 4, which of the following is true of the geologic movements that

brought about the Isthmus of Panama?

A. The movements brought populations of certain fishes and marine organisms into

contact with one another for the first time.

B. The movements transferred populations of fishes and other marine animals between the Pacific and Atlantic Oceans.

C. The movements created conditions that allowed water to flow more freely between the Pacific and Atlantic Oceans.

E. The movements created conditions for the formation of new species of fishes and other marine animals.

9. The word “sequence” in the passage is closet in

meaning to A. quality B. order C. function D. Number

10. According to paragraph 5, by comparing the enzymes from two related groups of

fishes on opposite sides of the isthmus, Graves found evidence that

A. there were slight genetic divergences between the two groups

B. the Atlantic group of fishes were descended from the Pacific group of fishes

C. the temperature of water on either side of the isthmus had changed

D. genetic changes in the Atlantic group of fishes were more rapid and frequent than in the Pacific group of fishes

11. 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

A. each of the Pacific enzymes works better in cooler waters

B. the Enzymes of the Atlantic fish populations had not changed since the formation of the Isthmus of Panama

C. gel electrophoresis showed that the changes benefited both the Atlantic and the Pacific fish populations

D. the differences between the enzymes disappeared when the two fish populations

were experimentally switched to other side of the isthmus

12. Which of the sentence 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.

A. Graves’s study provides evidence that isthmus fishes are in the process of becoming geographically isolated.

B. Graves’s study of mutating isthmus fishes yields results that differ from results of other

studies involving allopatric speciation.

C. Graves’s study of isolated populations of isthmus fishes provides some evidence that

allopatric speciation might be beginning

D. Grave’s study indicates that when isolated, populations of isthmus fished register

neutral or adaptive mutations.

13. Look at the four squares [■] that indicate where the following sentence can 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? ■ A ■ B ■ C ■ D

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 passages or are

minor ideas in the passage. This question is worth 2 points.

Allopatric speciation takes place when physically separated populations of a

single species gradually diverge genetically to the point of becoming unable to interbreed ● ● ●

Answer Choices

A. Allopatric speciation is common because the gene flow between subpopulations is

generally limited and the barriers that completely separate subpopulations can arise in a variety of ways.

B. During past ice ages, some, but not all, subpopulations separated by glaciers evolved into distinct species.

C. Speciation does not need to take place through allopatry because subpopulations will

form distinct species whenever there are adaptive advantages or not

D. Physical barriers from glaciers and the movement of tectonic plates form so slowly

that the subpopulations on either side of the barriers usually do not form distinct species.

E. Graves’s study of fish populations separated by the

Isthmus of Panama may well

provide a picture of the beginning stages of speciation. F. Graves’s study of physically separated fish populations show that there must be large

differences between the environments of the isolated populations if allopatric speciation is to take place.

参考译文：

进化生物学家认为物种形成（也就是新物种的产生）经常是某种物理障碍（地理隔离）

的出现把一个单一物种群分为隔离的亚种群。亚种群的身体隔离促进了新物种的产生，因为

一旦亚种群中的成员无法与其他亚种群中的成员交配，那么亚种群之间就没有变种基因的交

换。缺乏了基因交流，亚种群之间的遗传差异就开始变大。最终这些亚种群的基因差异变得

十分明显，以至于它们无法进行种族内的交配，即使它们之间的物理障碍已经消除。这时，

这些亚种族就演化成了独立的物种。这样的物种形成的方式被称作异域性（“allo-”意思是 “不同的”，“patria”意为“故乡”）

异域性物种形成可能是最主要的物种形成方式。这没什么惊讶的，因为异域现象太常见

了。一般而言，大多数的物种的亚种族都是被遥远距离所隔开了。所以即使在正常条件下，

这些亚物种间的基因流动倒更像是一条断断续续的细流，而不是一条不停流淌的小溪。并且，

障碍也会迅速出现来截断这条细流。例如，19 世纪的一次大地震改变了密西西比河（美国

中部的一条大河）的流向。这个变化使得当地的昆虫分离开来，它们现在居住于河的两岸，

彼此的基因流动已经完全被截断了。

地理隔离也能缓慢地进行，会跨越很长的时间段。我们在化

石记录中找到了这种长期证

据，从中我们可以瞥见先前延续环境的破碎。比如说，在过去的冰河世纪，经过北美和欧洲

的冰川最终把种群的部分彼此切断，当冰川消退后，这些分离的动植物种群又会连到一起。

一些起源于同一母群的群体变得不再那么容易兼容——它们演化成了分开的物种。然而，

在另一些群体里，遗传差异并没有那么明显，后代们还是可以种族内互相交配的——繁殖

隔离并没有彻底，所以物种形成就没有发生。

异域性物种的形成也可能是由构成地球表面的地壳构造板块运动所产生的，这个运动虽

略微缓慢但却异常剧烈。大约五百万年前这种地质运动形成了连接北美和南美的大陆桥，也

就是我们今天所说的巴拿马地峡。巴拿马地峡的形成对全球洋流的模式产生了重大的影响。

之前，大陆间的这个缺口可以让水自由流动，而现在巴拿马地峡在大西洋和太平洋之间形成

了一道障碍。这个地域分割为鱼和其他海洋物种的异域性物种形成创造了条件。

20 世纪80 年代，约翰?格雷夫斯研究了两个紧密相关的鱼种，一种来自地峡的大西洋

一侧，另一种来自太平洋那一侧。他比较了每个鱼种肌肉里的四种酶。格雷夫斯发现在低温

条件下，位于太平洋一侧的鱼种里的全部四类酶比大西洋一侧的四种酶功能更好。这个发现

很重要，因为太平洋的海水的水温通常要比地峡另一侧大西洋的海水低2 到3 度。凝胶电

泳的分析显示，四对中的两对酶的氨基酸序列略有不同。这也很重要，因为酶的氨基酸序列 是由基因决定的。

根据这些观测，格雷夫斯得出了两个结论。第一，在这些观测到的大西洋和太平洋鱼种

体内酶的差异中，至少有一些不是偶然的，它们是适应进化

的结果。第二，似乎地峡两侧原

本密切相关的鱼种开始有了基因分化。因为格雷夫斯关于地峡鱼类地域隔离的种群研究为中

性或适应性突变的逐渐累积过程的开始提供了一些线索。这里的差异可能会给正在进行的异 域性物种的形成提供证据。

参考答案：

1. B 2. D 3. D 4. C 5. B 6. A 7. C 8. D 9. B 10. A 11. A 12. C 13. B 14. ABE

本文来源：https://www.bwwdw.com/article/v1ud.html