Reading Comprehension

[Reading Comprehension] 试题详情

    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 author mentions the "aftermath of the Mount Saint Helens eruption" (the highlighted text) most likely in order to

Options:

provide an example of the process that allows the repopulation of rivers whose indigenous salmon population has become extinct
indicate the extent to which the disturbance of salmon habitat by human activity in one stream might affect the genetic structure of salmon populations elsewhere
provide a standard of comparison against which the impact of human activity on the gene flow among salmon populations should be measured
show how salmons' homing instinct can be impaired as a result of severe environmental degradation of their natal streams
show why straying rates in salmon populations remain generally low except when spawning streams suffer severe environmental disturbance

Correct:

C

Reading Comprehension

3评分
255浏览

[点此查看答疑][GWD]Reading Comprehension

错选了E。我觉得火山爆发的例子完全就是用来举例支持环境突变会引发straying rate上升的。哪里看出是为了建立对比人类活动的标准呢??? 整体脉络理解的是:人类活动不仅造成S鱼数量下降,而且会导致S鱼物种基因出问题。 重大环境变化就会triger一系列活动导致S鱼基因变化,比如SH火山爆发的例子。人类活动也会导致类似重大环境变化,所以也会威胁S鱼的物种基因
0评分
187浏览

[点此查看答疑][GWD]Reading Comprehension

错选D 感觉这个例子出现的前面一直在说environmental degradatio的破坏所以选了D
0评分
123浏览

[点此查看答疑][GWD]Reading Comprehension

老师您好,这题我不知读黄线前面还是后面那句,剩下ABC,下一步不知如何辨析,谢谢老师
0评分
122浏览

[点此查看答疑][GWD]Reading Comprehension

B我看成第一段首句了。说了habitat被人类毁了,然后就导致了数量少了等等,请问老师为什么不选B。谢谢!
-1评分
100浏览

[点此查看答疑][GWD]Reading Comprehension

我选的E 正确答案是C 整篇读得都不叫晕 在这了问目的 也没明确出来时应该读前边 还是读后面 觉得such 是接着说的 就读了后面 读完感觉没有特别沾边的 就选了E
3评分
255浏览

[点此查看答疑][GWD]Reading Comprehension

错选了E。我觉得火山爆发的例子完全就是用来举例支持环境突变会引发straying rate上升的。哪里看出是为了建立对比人类活动的标准呢??? 整体脉络理解的是:人类活动不仅造成S鱼数量下降,而且会导致S鱼物种基因出问题。 重大环境变化就会triger一系列活动导致S鱼基因变化,比如SH火山爆发的例子。人类活动也会导致类似重大环境变化,所以也会威胁S鱼的物种基因
0评分
187浏览

[点此查看答疑][GWD]Reading Comprehension

错选D 感觉这个例子出现的前面一直在说environmental degradatio的破坏所以选了D
0评分
123浏览

[点此查看答疑][GWD]Reading Comprehension

老师您好,这题我不知读黄线前面还是后面那句,剩下ABC,下一步不知如何辨析,谢谢老师
0评分
122浏览

[点此查看答疑][GWD]Reading Comprehension

B我看成第一段首句了。说了habitat被人类毁了,然后就导致了数量少了等等,请问老师为什么不选B。谢谢!
-1评分
100浏览

[点此查看答疑][GWD]Reading Comprehension

我选的E 正确答案是C 整篇读得都不叫晕 在这了问目的 也没明确出来时应该读前边 还是读后面 觉得such 是接着说的 就读了后面 读完感觉没有特别沾边的 就选了E

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