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阅读理解(51) Senescence  

2011-12-14 08:08:01|  分类: 阅读理解 |  标签: |举报 |字号 订阅

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阅读理解(51) Senescence - 佳中 - 嘉华相恋

   阅读理解(52)           

                                Senescence

                                                                      佳中命题


 

 

 

                                                                           Text 4

       Much has been discussed about the reasons why organisms age. Some have claimed that senescence

is a mechanism for culling the aged from the population to prevent overcrowding. Others have believed aging

to be the unavoidable outcome of tissue metabolism. Modern evolutionary theory offers a rational explanation

for senescence — natural selection of individuals. 

      One should be skeptical of the claim that organisms deteriorate with age as an inevitable outcome of living.

A valid theory of senescence must explain why different species have different senescence rate. For example,

 why salmon usually deteriorate rapidly and die after spawning, at an age of two or three, while trees and

tortoises may live for hundreds of years without degenerating.

      Senescence, in and of itself, can never help the individual’s Darwinian fitness. However, if senescence is

to be compatible with natural selection, there must be some concomitant benefit associated with it that

outweighs its disadvantage. Since Darwinian fitness is measured by total reproduction, the advantage must

be that senescence is inevitably tied up with reproductive effort.

      Consider that all organisms have a finite probability of dying of natural causes within any given time

period. This puts a premium on reproducing sooner rather than later. In other words, an organism’s Darwinian

fitness tends to be enhanced by not deferring reproduction till a later time, for that organism might be killed or

injured in the interim. The degree of risk an organism is subject to determines how much pressure there is to

reproduce soon.

      An insect, for example, may have a ten percent chance of being killed each day; it is not surprising that

insects have high reproductive rates, and high rates of senescence. Tortoises, on the other hand, have

heavy shells which presumably provide efficient protection against predation and injury; because their

mortality risk is low, they are able to rely on lives and can defer reproduction until times are most favorable.

Their senescence rates are low.

       If an organism is able to have more offspring as it ages, it experiences an evolutionary pressure

opposing that caused by mortality. In such cases, low senescence rates are favored to take advantage of

the increased reproductive rate at future times. This theory would predict that female fish which produce

more eggs as they grow older and larger would age at a slower rate than male fish whose sperm production

does not rise with size.

      This line of reasoning suggests that when organisms have little or no chance of reproducing, they tend

to die early as a result of physical degenerative changes called senescence.

                                                                                                                        (425 words)

 

36. The passage is primarily about

      [A] the validity of Darwin’s theory on natural selection.

      [B] the effect of senescence on Darwinian fitness.

      [C] different theories about why organisms deteriorate with age.

      [D] an illustration of varying rates of senescence in different species.

37. We learn from the passage that senescence is

      [A] an inevitable process of physical deterioration of an organism.

      [B] totally determined by Darwinian fitness of an organism.

      [C] closely associated with the reproduction of an organism.

      [D] always compatible with the theory of natural selection.

38. Which of the following statements is true according to the fourth and fifth paragraphs?

      [A] High reproductive rate and high senescence rate are frequently linked in a species.

      [B] An organism which reproduces later tends to have a high senescence rate.

      [C] Organisms with high reproductive rates usually have a smaller reproductive pressure.

      [D] The less danger for an organism’s dying, the greater pressure for its early reproduction.

39. The example of fish is cited in the passage to illustrate that

     [A] some organisms can take advantage of low senescence rates.

     [B] organisms which produce more offspring will deteriorate faster.

     [C] some organisms have more mortality pressures than evolutionary pressure.

     [D] male organisms usually have lower senescence rates than female organisms.

40. According to the author, organisms deteriorate

     [A] simply because they are worn out from old age.

     [B] as a result of the principle of Darwinian fitness.

     [C] when they have fulfilled their reproductive function.

     [D] in random fashion, not governed by any consistent principle.

 

 答案解析

 

36.  【答案】 D

       【考点】 主旨大意。     针对全文命题。

      【解析】   从篇章分析,第一段,作者列举关于生物为什么会衰老的不同说法;第二段,作者提出自己的观

       点,认为关于衰老的理论必须能解释为什么不同的生物有不同的衰老率;第三段至第七段,作者围绕为什么

       不同的生物有不同的衰老率展开讨论。文章的主题句是:A valid theory of senescence must explain why

       different species have different senescence rate. 由此可以断定D项是本题的正确答案。

              C项的意思是提出生物为什么衰老的各种理论,这仅是第一段的内容,不是全文论述的中心。

 

37. 【答案】 C

      【考点】 具体细节。     针对第三段命题。

      【解析】   作者在解释为什么不同的生物会有不同的衰老率时强调的一个重要观点是:生物的衰老与其生殖

       能力密切相关(senescence is inevitably tied up with reproductive effect)。如昆虫的生殖率高,其衰老

       率也高;乌龟的生殖率低,其衰老率也低, 因此C项是本题的正确答案。

              A项的意思与第二段第1句所述不符,文中说是对这种说法表示怀疑(skeptical of the claim );B项的

       意思与第三段第2句所述不符,文中说生物的达尔文适者生存能力决定于其总的生殖能力,而不是生物衰老

       决定于其达尔文适者生存的能力。

 

38.  【答案】 A

       【考点】 细节判断。     针对第四段、第五段命题。

      【解析】   作者在第四段末提出:生物受到生存危险的程度决定于其需要早繁殖的压力。从作者在第五段举

        昆虫和乌龟的例子可以看出生物繁殖早晚与受生存危险的程度有关。生存危险大的生物繁殖得早,其繁殖

        率高,衰老率也高;而生存危险大小的生物繁殖得晚,其繁殖率低,衰老率也低。A项的意思是“高繁殖率

        与高衰老率在某一生物中通常是相关的”,与文中所述相符,因此是本题的正确答案。B、C、和D项所述

        都与文中所述相脖。

       

39.  【答案】 A

       【考点】 举例目的。     针对第六段命题。

       【解析】   一般来说,生殖率高的生物衰老得快,但作者在第六段中指出,有些生物如鱼在grow older时

        仍能have more offsprings ,其衰老速度缓慢(age at a much slower rate )。这是因为这些生物承受的

       是进化的压力而不是死亡的威胁,在这种情况下,低衰老率对于利用不断增长的繁殖率更为有利(In such

       cases, low senescence rates are favored to take advantage of the increased reproductive rate at future

       times)。可见作者举鱼的例子是为了说明有些生物能利用低衰老率增加繁殖,A项为本题的正确答案。

 

40.  【答案】 C

       【考点】 概念含义。     针对最后一段命题。

      【解析】   作者在阐明衰老与生殖的关系后,在最后一段给衰老下定义为:When organisms have little or

       no chance of reproducing, they tend to die early as a result of physical degenerative changes called

       senescence. 也就是说,当生物完成其生殖后,衰老就开始,因此C项是本题的正确答案。

 

 

 

 

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