1、tpo 31文本Speciation in Geographically Isolated Populations1 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 su
2、bpopulations 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 diffe
3、rences 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 a
4、llopatry (“allo-” means “different”, and “patria” means “homeland”).Para.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
5、. 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 ri
6、ver 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.Para.3 Geographic isolation can also proceed slowly, over great spans of time. We find evidence of such extende
7、d 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 separate
8、d 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 cou
9、ld still interbreed for them, reproductive isolation was not completed, and so speciation had not occurred.Para.4 Allopatric speciation can also be brought by the imperceptibly slow but colossal movements of the tectonic plates that make up Earths surface. About 5 million years ago such geologic mov
10、ements created the land bridge between North America and South America that we call the Isthmus of Panama 巴拿马地峡. 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
11、 set the stage for allopatric speciation among populations of fishes and other marine species.Para.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 m
12、uscles 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
13、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. Para.6 Graves drew two conclusions from these observations. First, at least some of the observed di
14、fferences 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 st
15、udy 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 me
16、aning toO DescribesO EncouragesO DelaysO Requires2.According to paragraph 1, allopatric speciation involves which of the following?O The division of a population into subspeciesO The reuniting of separated populations after they have become distinct speciesO The movement of a population to a new hom
17、elandO The absence of gene flow between subpopulations3.Why does the author provide the information that “subpopulations of most species are separated from each other by some measurable distance?O To indicate how scientists are able to determine whether subpopulations of a species are allopatricO To
18、 define what it means for a group of animals or plants to be a subpopulationO To suggest that allopatric speciation is not the only route to speciationO To help explain why allopatric speciation is a common way for new species to come about4.The word “accumulate” in the passage is closest in meaning
19、 toO Become more significantO Occur randomlyO Gradually increase in numberO Cause changes5.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?O To make the point that some kind
20、 of physical barrier separates the subpopulations of most speciesO To support the claim that the condition of allopatry can sometimes arise in a short timeO To provide an example of a situation in which gene flow among the populations a species happens at a slow rateO To explain why insect living al
21、ong opposite shores of the Mississippi River are very different from each other6.According to paragraph 3, separation of subpopulations by glaciers resulted in speciation in those groups of plants and animals thatO Were reproductively isolated even after the glaciers disappearedO Had adjusted to the
22、 old conditions caused by the glaciersO Were able to survive being separated from their parent populationO Had experienced some genetic divergences from their parent population7.The word “colossal” in the passage is closest in meaning toO ConsistentO GradualO EnormousO Effective8.According to paragr
23、aph 4, which of the following is true of the geologic movements that brought about the Isthmus of Panama?O The movements brought populations of certain fishes and marine organisms into contact with one another for the first time.O The movements transferred populations of fishes and other marine anim
24、als between the Pacific and Atlantic Oceans.O The movement created conditions that allowed water to flow more freely between the Pacific and Atlantic Oceans.O The movement created conditions for the formation of new species of fishes and other marine animals.9.The word “sequence” in the passage is c
25、losest in meaning toO QualityO OrderO FunctionO Number10.According to paragraph 5, by comparing the enzymes from two related groups of fishes on opposite sides of the isthmus, Graves found evidence thatO There were slight genetic divergences between the two groupsO The Atlantic group of fishes were
26、descended from the Pacific group of fishesO The temperature of water on either side of the isthmus had changedO Genetic changes in the Atlantic group of fishes were more rapid an frequent than in the Pacific group of fishes11.It can be inferred from paragraph 5 and 6 that the reason Graved concluded
27、 that some of the differences between the Pacific and Atlantic enzymes were not random was thatO Each of the Pacific enzymes works better in cooler watersO The enzymes of the Atlantic fish populations had not changed since the formation of the Isthmus of PanamaO Gel electrophoresis showed that the c
28、hanges benefited both the Atlantic and the Pacific fish populationsO The differences between the enzymes disappeared when the two fish populations were experimentally switched to other side of the isthmus12.Which of the sentence below best expresses the essential information in the highlighted sente
29、nce in the passage? Incorrect choices change the meaning in important ways or leave out essential information.O Gravess study provides evidence that isthmus fishes are in the process of becoming geographically isolated.O Gravess study of mutating isthmus fishes yields results that differ from result
30、s of other studies involving allopatric speciation.O Gravess study of isolated populations of isthmus fishes provides some evidence that allopatric speciation might be beginning.O Gravess study indicates that when isolated, populations of isthmus fishes register neutral or adaptive mutations. 13.Loo
31、k 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?14.Prose Summary Allopatric speciation takes place when physically separat
32、ed populations of a single species gradually diverge genetically to the point of becoming unable to interbreed.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 the
