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[机经题库答案] 托福阅读生物类真题Constraints on Natural Selection原文+题目汇总

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发表于 2017-5-15 15:50 |显示全部楼层
类别:生物类
真题140615CN-P2
Title:Constraints on Natural Selection
Natural selection is the process  in which organisms with certain traits  survive and reproduce while organisms that are less able to adapt to their environment die off. As Darwin pointed out,  natural selection does not necessarily produce evolutionary progress,  much less perfection. The limits  to the effectiveness of natural selection are most clearly revealed by the universality of extinction. More than 99.9  percent of all evolutionary lines  that once existed  on Earth have become  extinct. Mass extinctions remind us forcefully that evolution is not a steady  approach to an ever-higher perfection but an unpredictable process  in which the best-adapted organisms may be suddenly exterminated y a catastrophe and  their place taken  by lineages that  prior to the catastrophe seemed  to be without distinction or prospects.    
There are numerous constraints, or limits, on the power  of natural selection to bring about change. First, the genetic  variation needed to perfect a characteristic may not be forthcoming.  Second, during evolution, the adoption of one among several possible  solutions to a new environmental opportunity may greatly  restrict the possibilities for subsequent evolution. For instance, when a  selective advantage for a skeleton  developed among the ancestors of the vertebrates and the arthropods, the  ancestors of the arthropods had the prerequisites for developing an  external skeleton, and those of the vertebrates had the prerequisites for acquiring  an internal skeleton. The entire subsequent history  of these two large groups  of organisms was affected by the two different paths  taken by their remote  ancestors. The vertebrates were able to develop such huge creatures as  dinosaurs, elephants, and whales. A large crab is the largest type that  the arthropods were able to achieve.   
Another constraint on natural selection is  developmental interaction. The different components  of an individual organism—its structures and organs—are not independent of one another, and none of them responds to selection  without interacting with the others. The whole developmental machinery is a single interacting system. Organisms are compromises among  competing demands. How far a particular structure or organ can respond  to the forces of selection depends, to a considerable extent, on the resistance offered  by other structures and organs, as well  as components of the genotype (the totality of an individual’s genes).  
The structure of the genotype itself imposes  limits on the power of natural selection.  The classical  metaphor of the genotype was that of a beaded string on which the genes were  lined up like pearls in a necklace. According to this view,  each gene was more or less independent of the others. Not much is left of this previously accepted image. It is now  known that there  are different functional classes of genes,  some charged to produce material, others to regulate it, and still  others that are apparently not functioning at all. There are single coding  genes, moderately repetitive DNA, highly repetitive DNA, and many other kinds  of DNA. Discovering exactly how they all interact with one another is still a rather poorly  understood area of genetics.
A further constraint on natural selection is the capacity for nongenetic modification. The more plastic  the organism’s body  characteristics are (owing to developmental flexibility), the more this reduces the force  of adverse selection pressures. Plants, and particularly  microorganisms, have a far greater  capacity for individual modification than do animals. Natural  selection is involved even in this  phenomenon, since the capacity for nongenetic  adaptation is under strict genetic control. When  a population shifts  to a new specialized environment, genes will be selected during  the following generations that reinforce and may eventually largely  replace the capacity for nongenetic adaptation.
Finally, which organisms  survive and reproduce in a population is partly the result of chance, and this also limits the power of natural selection. Chance operates at every level  of the process of  reproduction, from the transmission of parental chromosomes to the survival  of the newly formed individual.  Furthermore, potentially favorable gene  combinations are often destroyed by indiscriminate  environmental forces such as storms, floods, earthquakes, or volcanic eruptions, without natural selection  being given the opportunity to favor these  genotypes. Yet over time, in the survival of those few individuals that become the  ancestors of subsequent generations,  relative fitness always  plays a major  role.
  
Paragraph 1  
Natural selection is the process  in which organisms with certain traits  survive and reproduce while organisms that are less able to adapt to their environment die off. As Darwin pointed out,  natural selection does not necessarily produce evolutionary progress,  much less perfection. The limits  to the effectiveness of natural selection are most clearly revealed by the universality of extinction. More than  99.9 percent of all evolutionary lines that once existed on Earth have become  extinct. Mass extinctions remind us forcefully that evolution is not a steady  approach to an ever-higher perfection but an unpredictable process  in which the best-adapted organisms may be suddenly exterminated y a catastrophe and  their place taken  by lineages that  prior to the catastrophe seemed  to be without distinction or prospects.  
  
1.        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.  
¡  Evolution is an unpredictable process because  in mass extinctions highly-evolved organisms  are exterminated.  
¡  Evolution does not progress steadily to  ever-higher levels of perfection because, as shown by mass extinctions,  lineages favored by evolution can  be suddenly replaced by those not favored  previously when circumstances change.  
¡  Catastrophes remind us that evolution is a  process in which the best-adapted organisms  are exterminated and their  place taken by lineages shown to be poorly  adapted.  
¡  When mass extinctions exterminate the  best-adapted organisms, less important lineages suddenly become better adapted and take their place.  
  
2.        According to paragraph 1, which of the following provides evidence that  natural selection does not always  lead to evolutionary progress?  
¡ Most evolutionary  lines that once existed on Earth have become  extinct.  
¡ Evolutionary lines  usually weaken as they increase in age.  
¡  The history of evolution shows that many  evolutionary lines can become extinct at the  same time.  
¡  So far, less than one percent of evolutionary  lines have achieved such perfect adaptation to their environment that they  will never become extinct.
  
Paragraph 1 is marked  with an arrow [→]
   
Paragraph 2  
There are numerous constraints, or limits, on the power  of natural selection to bring about change. First, the genetic  variation needed to perfect a characteristic may not be forthcoming.  Second, during evolution, the adoption of one among several possible  solutions to a new environmental opportunity may greatly  restrict the possibilities for subsequent evolution. For instance, when a  selective advantage for a skeleton  developed among the ancestors of the vertebrates and the arthropods, the  ancestors of the arthropods had the prerequisites for developing an  external skeleton, and those of the vertebrates had the prerequisites for acquiring  an internal skeleton. The entire subsequent history  of these two large groups  of organisms was affected by the two different paths  taken by their remote ancestors. The vertebrates were able to develop such huge creatures as  dinosaurs, elephants, and whales. A large crab is the largest type that  the arthropods were able to achieve.  
  
3.       The  word ”remote” in the passage is  closet in meaning to  
¡ corresponding  
¡ distant
¡ separate  
¡ direct  
  
4.       In paragraph 2, why does  the author discuss the ancestors of vertebrates and arthropods?  
¡ To  explain how a single  feature can cause the extinction of entire groups of organisms  
¡ To  identify some factors  that determine how large an organism can become  
¡  To illustrate the point  that earlier developments influence the possibilities for future development  
¡  To emphasize the role  of the environment in the development of organisms Paragraph 2 is marked with an arrow [→]  
  
5.       Which of the following  can be inferred from paragraph 2 about arthropods?  
¡ There are fewer of  them than there are of vertebrates.  
¡ Their ancestors had a  selective advantage over the ancestors of  vertebrates.  
¡ Their ancestors once  possessed internal skeletons.  
¡ Their  body features prevent them from becoming large organisms.  
Paragraph  2 is marked with an arrow [→]
   
Paragraph 3  
Another constraint on natural selection is developmental interaction.  The different components of an individual organism—its structures and organs—are not independent of one another, and none of them responds to selection  without   interacting with the others.  The whole developmental machinery is a single  interacting system. Organisms are compromises among  competing demands. How far a particular structure or organ can respond  to the forces of selection depends, to a considerable extent, on the resistance offered by other structures and organs, as well  as components of the genotype (the totality of an individual’s genes).  
  
6.       The  word ”resistance” in the passage is  closet in meaning to  
¡ opposition  
¡ pressure
¡ interaction  
¡ competition  
  
7.       According to paragraph 3, why must organisms compromise between competing demands?  
¡  Aparticular  organ or structure may be unable  to respond to selection pressures due to the needs of other parts of the organism.  
¡  An organism’s ability to respond to the forces of selection  depends on the demands of other  organisms within its environment.  
¡  An organism’s environment and its genotype  try at the same time to influence its ability to respond to natural selection.  
¡  Different elements of the environment call for adaptations that are often  incompatible with one another.
  
Paragraph 3 is marked  with an arrow [→]
  
Paragraph 4  
The structure of the genotype itself imposes limits on the power of  natural selection.  The classical  metaphor of the genotype was that of a beaded string on which the genes were  lined up like pearls in a necklace. According to this view,  each gene was more or less independent of the others. Not  much is left of this previously accepted image. It is now known that there are different functional classes of genes,  some charged to produce material, others to regulate it, and still  others that are apparently not functioning at all. There are single coding  genes, moderately repetitive DNA, highly repetitive DNA, and many other kinds  of DNA. Discovering exactly how they all interact with one another is still a rather poorly  understood area of genetics.  
  
8.       Paragraph 4 supports  all of the statements about genes EXCEPT:  
¡ Our understanding of the extent  to which genes  act independently has  changed over time.  
¡ Genes are classified  into groups on the basis of their function.  
¡ Some genes seem to  have no function.
¡ Studies  to identify ways that genes interact have been largely successful.  
Paragraph 4 is marked with an  arrow [→]  
  
Paragraph 5  
A further constraint on natural selection is the capacity for nongenetic modification. The more plastic  the organism’s body characteristics  are (owing to developmental flexibility), the more this reduces the force of  adverse selection pressures. Plants,  and particularly microorganisms, have a  far greater capacity for individual modification than do animals. Natural  selection is involved even in this phenomenon, since the capacity for nongenetic adaptation is under strict genetic control. When a population shifts to a new specialized environment, genes will  be selected during  the following generations that reinforce and may eventually largely  replace the capacity for nongenetic adaptation.  
  
9.       The  word ”adverse”  in the passage is closet in meaning  to  
¡ harmful  
¡ continuous  
¡ strong  
¡ excessive  
  
Paragraph 6  
Finally, which organisms  survive and reproduce in a population is partly the result of chance, and this also limits the power of natural selection. Chance operates at every level  of the process of  reproduction, from the transmission of parental chromosomes to the survival  of the newly formed individual.  Furthermore, potentially favorable gene  combinations are often destroyed by indiscriminate environmental forces such as  storms, floods, earthquakes, or volcanic eruptions,  without natural selection being given the opportunity to favor these genotypes. Yet over time, in the survival of those few  individuals that become the ancestors of subsequent generations, relative fitness always plays a major role.  
  
10.    What point does  paragraph 6 make about the  individuals that survive a particular natural disaster?  
¡  They were the small number of organisms that  happened to be well-adapted to survive that kind of natural disaster.  
¡  Their descendants will likely be shaped by natural selection to evolve genes that will make them fit to survive the next natural disaster.  
¡  Their survival may have been  due to chance, but fitness also  contributes to their  survival over time.  
¡  They will probably have comparatively weak  offspring because the natural disaster eliminated the individuals that had more favorable gene combinations.  
Paragraph 6 is marked  with an arrow [→]  
  
11.    According to paragraph 6, environmental forces  limit the power of natural selection in which  of the following ways?  
¡ They change the  reproductive process in significant ways.  
¡ They destroy  potentially favorable gene  combinations before they can be selected.  
¡ They interfere with  the transmission of chromosomes from parent to offspring.  
¡ They  weaken the ability of individuals to maintain high fitness levels.  
Paragraph 6 is marked with an  arrow [→]  
  
12.   The  word ”indiscriminate” in the passage  is closet in meaning to  
¡ dangerous  
¡ random  
¡  common
¡  powerful  
  
Paragraph 4  
The structure of the genotype itself imposes limits on the power of  natural selection. The classical  metaphor of the genotype was that of a beaded string on which the genes were lined  up like pearls in a necklace. ■According  to this view, each gene was more  or less independent of the others. ■Not  much is left of this previously accepted  image. ■It is now known that there are different functional classes of genes, some  charged to produce  material, others to regulate it, and  still others that are apparently not functioning at all. ■There are single  coding genes, moderately repetitive DNA, highly repetitive DNA, and many other kinds  of DNA. Discovering exactly how they all interact with one another is still a rather poorly  understood area of genetics.  
  
13.    Look at the four squares [■] that indicate where the following sentence could be added to the  passage.  
New models  of the genotype depict a much  more complicated relationship among genes.  
Where would the  sentence best fit? Click on a square [■] to add the sentence to the passage.  
  
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 answer choices 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.  
Drag your choices to the spaces  where they belong. To review  the passage, click  on View Text.
            
Genetics      and environmental factors often      prevent organisms from achieving      evolutionary perfection.
      
      
      
       Answer Choices  

¡   Darwin first challenged the  idea of evolutionary progress by observing that more than  99.9 percent of all evolutionary lines that once existed on Earth have become extinct.  
¡   A single adaptation to an organism’s  environment may determine the way in which the organism’s subsequent ancestors are able to evolve.  
¡   The structure of the  genotype itself restricts natural  selection, since genes must line up like pearls on a necklace and cannot be moved  out of their proper order.  
¡  The  development of vertebrates and  arthropods from a single ancestor demonstrates the  power of the environment to limit natural selection.
¡   A single structure or gene  in an organism cannot respond to the forces of natural selection without affecting the  functioning of other structures or genes.  
¡   Both environmental and  genetic chance may prevent even the fittest organisms from  surviving and reproducing.
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