Constraints on Natural Selection

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 by 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.   

 

O Evolution is an unpredictable process because in mass extinctions highly-evolved organisms are exterminated.  

 

O 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.  

 

O 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.   

 

O 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?   

 

O Most evolutionary lines that once existed on Earth have become extinct.  

 

O Evolutionary lines usually weaken as they increase in age. 

 

O The history of evolution shows that many evolutionary lines can become extinct at the same time.   

 

O So far, less than one percent of evolutionary lines have achieved such perfect adaptation to their environment that they will never become extinct. 

 

 

 

 

 

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. In paragraph 2, why does the author discuss the ancestors of vertebrates and arthropods?   

 

O To explain how a single feature can cause the extinction of entire groups of organisms   

 

O To identify some factors that determine how large an organism can become   

 

O To illustrate the point that earlier developments influence the possibilities for future development  

 

O To emphasize the role of the environment in the development of organisms 

 

 

 

4. Which of the following can be inferred from paragraph 2 about arthropods?   

 

O There are fewer of them than there are of vertebrates.   

 

O Their ancestors had a selective advantage over the ancestors of vertebrates.   

 

O Their ancestors once possessed internal skeletons.   

 

O Their body features prevent them from becoming large organisms.

 

 

 

 

 

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). 

 

 

 

5. According to paragraph 3, why must organisms compromise between competing demands?   

 

O A particular organ or structure may be unable to respond to selection pressures due to the needs of other parts of the organism.  

 

O An organism’s ability to respond to the forces of selection depends on the demands of other organisms within its environment.   

 

O An organism’s environment and its genotype try at the same time to influence its ability to respond to natural selection.   

 

O Different elements of the environment call for adaptations that are often incompatible with one another. 

 

 

 

 

 

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. 

 

 

 

6. Paragraph 4 supports all of the statements about genes EXCEPT:   

 

O Our understanding of the extent to which genes act independently has changed over time.   

 

O Genes are classified into groups on the basis of their function.   

 

O Some genes seem to have no function.   

 

O Studies to identify ways that genes interact have been largely successful.

 

 

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. 

 

 

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. 

 

 

 

7. What point does paragraph 6 make about the individuals that survive a particular natural disaster?   

 

O They were the small number of organisms that happened to be well-adapted to survive that kind of natural disaster.   

 

O Their descendants will likely be shaped by natural selection to evolve genes that will make them fit to survive the next natural disaster.   

 

O Their survival may have been due to chance, but fitness also contributes to their survival over time.  

 

O They will probably have comparatively weak offspring because the natural disaster eliminated the individuals that had more favorable gene combinations. 

 

 

 

8. According to paragraph 6, environmental forces limit the power of natural selection in which of the following ways? 

 

O They change the reproductive process in significant ways.   

 

O They destroy potentially favorable gene combinations before they can be selected.  

 

O They interfere with the transmission of chromosomes from parent to offspring.   

 

O They weaken the ability of individuals to maintain high fitness levels. 

 

 

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. 

 

 

 

9. 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. 

 

 

 

 

 

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 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. 

 

 

 

O 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. 

 

O A single adaptation to an organism’s environment may determine the way in which the organism’s subsequent ancestors are able to evolve.

 

O 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. 

 

O The development of vertebrates and arthropods from a single ancestor demonstrates the power of the environment to limit natural selection. 

 

O 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.

 

O Both environmental and genetic chance may prevent even the fittest organisms from surviving and reproducing. 

 

 

 

 

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