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题库 / Prep2012-RC-38
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 repopulates 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 salmon ) 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.
It can be inferred from the passage that the occasional failure of some salmon to return to their natal streams in order to spawn provides a mechanism by which
pristine streams that are near polluted streams become polluted themselves
the particular adaptations of a polluted streams salmon population can be preserved without dilution
the number of salmon in pristine habitats decreases relative to the number in polluted streams
an environmentally degraded stream could be recolonized by new salmon populations should the stream recover
the extinction of the salmon populations that spawn in polluted streams is accelerated
此讲解的内容由AI生成，还未经人工审阅，仅供参考。
正确答案是 D。
因为文章说，如果一个环境受到严重的破坏，那么一小部分的鲑鱼会在邻近的清澈的河流里产卵，这提供了一种重新建立这个河流的方法。因此，D 选项是正确的，环境恶化的河流可以通过新的鲑鱼群体重新被建立。
pristine 未触动过的
stray 走失
spawn 产卵
dilute 稀释
低水平的游离是至关重要的，因为这个过程提供了一个新基因的来源，也提供了一个机制，如果那里的鱼消失了，这个地方可以被重新填充。然而，高的游离率可能是有问题的，因为误入的鱼可能与现有种群杂交，以至于存在的任何当地适应性都被稀释。
should the stream recover=if the stream should recover虚拟语气
D选项 原文a mechanism by which a location can be repopulates should the fish there disappear对应选项an environmentally degraded stream could be recolonized by new salmon populations should the stream recover
pristine 原始的
since the process provides a source of novel genes and a mechanism by which a location can be repopulated should the fish there disappear
定位——since the process provides a source of novel genes and a mechanism by which a location can be repopulated should the fish there disappear.
occasional 偶尔的
题干关键词，occasional，定位：Low levels of straying are crucial, since the process provides a source of novel genes and a mechanism by which a location can be repopulates should the fish there disappear. 完美对应D
occasional偶尔的 不经常的
since the process provides a source of novel genes and a mechanism by which a location can be repopulated should the fish there disappear
这句话没读懂干扰做题，说的是should虚拟假设这些地方原有的鱼没了，还有新鱼来repopulate
注意题干occasional failure指的是low stray，要在low stray那两句找答案
直接细节题：Low levels of straying are crucial, since the process provides a source of novel genes and a mechanism by which a location can be repopulates should the fish there disappear.
Low levels of straying are crucial, since the process provides a source of novel genes and a mechanism by which a location can be repopulates should the fish there disappear.
细节题，“ since the process provides a source of novel genes and a mechanism by which a location can be repopulates should the fish there disappear ” 的rephrase
should the stream recover=if the stream should recover虚拟语气
Low levels of straying are crucial, since the process provides a source of novel genes and a mechanism by which a location can be repopulates should the fish there disappear.
occasional failure，对应low stray，根据原文since the process provides a source of novel genes and a mechanism by which a location can be repopulates should the fish there disappear，定位mechanism；二者结合选D
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.
细节题，“ since the process provides a source of novel genes and a mechanism by which a location can be repopulates should the fish there disappear ” 的rephrase
should the stream recover=if the stream should recover虚拟语气
…and a mechanism by which a location can be repopulated should the fish there disappear.