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. 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 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.
【Paragraph 1】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”).
1.According to paragraph 1, allopatric speciation involves which of the following?
○The division of a population into subspecies
○The reuniting of separated populations after they have become distinct species
○The movement of a population to a new homeland
○The absence of gene flow between subpopulations
【Paragraph 2】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.
2.Why does the author provide the information that “the subpopulations of most species are separated from each other by some measurable distance”?
○To indicate how scientists are able to determine whether subpopulations of a species are allopathic.
○To define what it means for a group of animals or plants to be a subpopulation.
○To suggest that allopathic speciation is not the only route to speciation.
○To help explain why allopatric speciation is a common way for new species to come about.
3. 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?
○To make the point that some kind of physical barrier separates the subpopulations of most species
○To support the claim that the condition of allopatry can sometimes arise in a short time
○To provide an example of a situation in which gene flow among the subpopulations of a species happens at a slow rate
○To explain insects living along opposite shores of the Mississippi River are very different from each other
【Paragraph 3】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.
4.According to paragraph 3, separation of subpopulations by glaciers resulted in speciation in those groups of plants and animals that
○were reproductively isolated even after the glaciers disappeared
○had adjusted to the old conditions caused by the glaciers
○were able to survive being separated from their parent population
○had experienced some genetic divergences from their parent population
【Paragraph 4】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.
5.According to paragraph 4, which of the following is true of the geologic movements that brought about the Isthmus of Panama?
○The movement brought populations of certain fishes and marine organisms into contact with one another for the first time.
○The movement transferred populations of fishes and other marine animals between the Pacific and Atlantic Oceans.
○The movement created conditions that allowed water to flow more freely between the Pacific and Atlantic Oceans.
○The movement created conditions for the formation of new species of fished and other marine animals.
【Paragraph 5】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.
6.According to paragraph 5, by comparing the enzymes from two related groups of fishes on opposite sides of the Isthmus, Graves found evidence that
○there were slight genetic divergences between the two groups
○the Atlantic group of fishes were descended from the Pacific group of fishes
○the temperature of water on either side of the Isthmus had changed
○genetic changes in the Atlantic group of fishes were more rapid and frequent than in the Pacific group of fishes
【Paragraph 6】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.
7.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
○each of the Pacific enzymes works better in cooler waters
○the Enzymes of the Atlantic fish populations had not changed since the formation of the Isthmus of Panama
○gel electrophoresis showed that the changes benefited both the Atlantic and the Pacific fish populations
○the differences between the enzymes disappeared when the two fish populations were experimentally switched to other side of the isthmus
8. 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.
○Graves’s study provides evidence that isthmus fishes are in the process of becoming geographically isolated.
○Graves’s study of mutating isthmus fishes yields results that differ from results of other studies involving allopatric speciation.
○Graves’s study of isolated populations of isthmus fishes provides some evidence that allopatric speciation might be beginning.
○Graves’s study indicates that when isolated, populations of isthmus fishes register neutral or adaptive mutations.
【Paragraph 4】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.■
9. 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?
10.【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.
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Answer Choices
○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.
○During past ice ages, some, but not all, subpopulations separated by glaciers evolved into distinct species.
○Speciation does not need to take place through allopatry because subpopulations will form distinct species whenever there are adaptive advantage to not.
○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.
○Graves’s study of fish populations separately by the Isthmus of Panama may well provide a picture of the beginning stages of speciation.
○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.
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