2. 外语
  3. This section measures your ability to understand academic passages in English. The Reading section of TOEFL iBT is divided i

This section measures your ability to understand academic passages in English. The Reading section of TOEFL iBT is divided i

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问题     This section measures your ability to understand academic passages in English.
    The Reading section of TOEFL iBT is divided into 2 or 3 separately timed parts.
    Most questions in the Reading section are worth 1 point, but the last question for each passage is worth more than 1 point. The directions for the last question include the point value of the question.
    Some passages will include a word or phrase that is underlined. You can see its definition or an explanation in the Glossary box.
    Within each part in the real test, you can go on to the next question by clicking the Next icon. You may skip questions and go back to them later. If you want to go back to previous questions, click the Back icon. You may click the Review icon at any time and the review screen will show you which questions you have answered and which you have not. From the review screen, you can go directly to any question you have already seen in the Reading section.
    You will now begin the Reading section. There is 1 passage for this part of the section, and you will have 20 minutes to read the passage and answer the questions.
Natural Selection
    First coined by Darwin, the term natural selection refers to the process at the core of evolutionary theory. Simply put, Darwin believed that every living organism had to struggle to survive and reproduce. By random chance, some individuals in a species would have characteristics that made them better adapted to this struggle than their contemporaries. Such individuals would be more likely to reproduce and pass their characteristics on to their offspring. Over many generations, a series of small differences in characteristics would accumulate, until eventually the creatures that were produced would be so different from their original ancestors that they would constitute a new species.
(A) Although Darwin did not know how variations between individual members of the same species were able to arise in the first place or how such traits could then be passed on from species to species, we now know that this process occurs through genes.
(B) Genes determine what characteristics an organism will possess, from its size to its skin color to its gender.
(C) Sometimes these genes become damaged or altered--a process known as mutation.
(D) Most mutations are either too small to have any effect, or have detrimental effects on the organism. A few, however, can be beneficial to helping an organism survive. The organisms with these beneficial mutations are the ones that .survive and reproduce, passing their particular genetic mutation on to future generations.
    Originally, Darwin thought that two particular types of traits were naturally selected for evolution. The first type included any and all traits that helped an organism survive, either by giving it easier access to food, better protection from predators, or superior adaptability to shifting environmental conditions. Several examples of the natural selection of such traits have been observed since Darwin first proposed his theory. The most often cited example is the coloration of moths near industrial centers. Some of them have white wings and the others have black wings. Under normal conditions, the lighter-colored moths predominate since white moths are less visible against the light-colored bark of the trees in the forests where they live, which means they can more easily avoid predators. Near industrial centers, however, darker-colored moths predominate, as the pollution from the factory has darkened the bark of the nearby trees. Moths landing on these trees are much more difficult to see if they have black wings. This trait has now become most common in moth populations living in areas where pollutants have changed the color of the tree bark.
    The second type of naturally selected traits envisioned by Darwin involved those characteristics of an organism that made it easier for that organism to find a mate. This form of natural selection, often called sexual selection, was thought to explain the existence of highly developed traits in certain animals that seemed to play no particular role in their survival, and that in some cases seemed to make it even harder for them to survive. A prime example of such a trait is the peacock’s elaborate, colorful tail feathers. A long, unwieldy tail that can expand into a display of bright colors that draws the attention of any predator in the vicinity seems less likely than that of other kinds of birds to help the bird survive. However, researchers discovered that peahens invariably chose to mate with those peacocks whose tail feathers produced the most artistic displays. Thus, although the peacock’s tail feathers have no adaptive function in terms of coping with survival pressure, the gene that controls feather color is the one that is most likely to be passed on because of its role in the mating process.
    Many scientists now believe that a third type of naturally selected characteristics exists, namely those that may be harmful to an individual organism but beneficial to that organism’s species. This helps to explain the persistence of certain traits that were viewed under traditional evolutionary theory as not assisting in adaptation. For instance, in many animal species, including humans, a small subset of the population is homosexual. Under the old view of evolution, this could not be accounted for since homosexual individuals do not normally reproduce, meaning the gene that codes for it should have long been phased out of the gene pool. However, recent studies among wild sheep populations have found that the prevalence of homosexuality in the sheep population increases in times of severe food shortages, causing the sheep to form stronger bonds with sheep of the same sex than they otherwise would. As a result, instead of fighting over a particular food source, these sheep share the food they find with each other, increasing the species’ chance of surviving hard times. Though still controversial, the idea that traits may be naturally selected because of their benefits to the species instead of their benefits to the individual is rapidly gaining acceptance in the scientific community because of the wide array of characteristics that it can explain, including the human tendency towards altruism
Look at the four squaresin the passage that indicate where the following sentence can be added to the passage. All of the information about an organism’s physiology is contained in its DNA, which is a strand of organic chemicals composed of sequences of nucleotides called genes. Where would this sentence best fit?
选项 A、(Ａ)
B、(Ｂ)
C、(Ｃ)
D、(Ｄ)
答案B
解析 句子插入题 待插入的句子是对基因句子的定义。gene(基因)这个词第一次出现A处后面一句的句末。B处后面的句子提到“基因决定着有机体所具有的特征，包括身材、肤色、性别等”(Genes determine what characteristics an organism will possess…)。这一部分是对基因的具体说明，因此对基因的定义应该出现在这之前。
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