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题库解析   >   主旨题题型   >  
    本题由chris提供
1054次学习

Over the last 150 years, large stretches of salmon habitat have been eliminated by human activity: mining, livestock grazing,

timber harvesting, and agriculture as well as recreational and urban development. The numerical effect is obvious: there are fewer salmon in degraded regions than in pristine ones; however, habitat loss also has the potential to reduce genetic diversity. This is most evident in cases where it results in the extinction of entire salmon populations. Indeed, most analysts believe that some kind of environmental degradation underlies the demise of many extinct salmon populations. Although some rivers have been recolonized, the unique genes of the original populations have been lost.

Large-scale disturbances in one locale also have the potential to alter the genetic structure of populations in neighboring areas, even if those areas have pristine habitats. Why? Although the homing instinct of salmon to their natal stream is strong, a fraction of the fish returning from the sea (rarely more than 15 percent) stray and spawn in nearby streams. Low levels of straying are crucial, since the process provides a source of novel genes and a mechanism by which a location can be repopulated should the fish there disappear. Yet high rates of straying can be problematic because misdirected fish may interbreed with the existing stock to such a degree that any local adaptations that are present become diluted. Straying rates remain relatively low when environmental conditions are stable, but can increase dramatically when streams suffer severe disturbance. The 1980 volcanic eruption of Mount Saint Helens, for example, sent mud and debris into several tributaries of the Columbia River. For the next couple of years, steelhead trout (a species included among the salmonids) returning from the sea to spawn were forced to find alternative streams. As a consequence, their rates of straying, initially 16 percent, rose to more than 40 percent overall.

Although no one has quantified changes in the rate of straying as a result of the disturbances caused by humans, there is no reason to suspect that the effect would be qualitatively different than what was seen in the aftermath of the Mount Saint Helens eruption. Such a dramatic increase in straying from damaged areas to more pristine streams results in substantial gene flow, which can in turn lower the overall fitness of subsequent generations.

The primary purpose of the passage is to



    A. argue against a conventional explanation for the extinction of certain salmon populations and suggest an alternative

    B. correct a common misunderstanding about the behavior of salmon in response to environmental degradation caused by human activity

    C. compare the effects of human activity on salmon populations with the effects of natural disturbances on salmon populations

    D. differentiate the particular effects of various human activities on salmon habitats

    E. describe how environmental degradation can cause changes in salmon populations that extend beyond a numerical reduction


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答案:
E
正确答案为
(E)。这篇文章属于现象解释型文章,与典型的解释型文章不同的是文章给出了不止一个现象,但只是具体解释了其中一个现象:具体而言其实就是人类活动导致鲑鱼栖息地退化对鲑鱼三个影响,第一是数量效应,第二是基因多样性降低,第三是基因结构的改变,后两者其实可以归入一个大影响。但是文章主体部分也就是第二段着重解释邻近栖息地其他鲑鱼种群基因结构改变的原因。 (A) 态度相反:解释型文章的态度偏正或无,而A项的开头动词是argue against,表负态度 (B) 无中生有:文章没有纠正人们对鲑鱼行为的误解,只进行了描述 (C) 无关选项:主旨题选项中以Compare开头的项表示手段而非目的 (D) 无中生有:文章开头确实提到了不同的人类活动,但是根据文章信息,这些不同的活动导致的结果是一致的:归于栖息地的退化或者减少

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