prep2012一篇RC求助

sandefia

原文和GWD一篇类似，讲三文鱼的基因多样化下降的
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。

The author's argument that increased straying can “lower the overall fitness of subsequent generations” (see highlighted text) is based on which of the following assumptions?
A. A disturbance of salmonid spawning streams caused by human activity could increase the straying rate of affected salmonid populations as much as the aftermath of the Mount Saint Helens eruption did.
B. In the streams in which the straying salmonid spawn, these straying salmonid would amount to no more than 40 percent of the total spawning population.
C. Salmonid in some streams benefit from particular local adaptations.
D. Nonenvironmental factors have no effect on salmonid straying rates.
E. At least some of the streams in which straying salmonid would spawn are pristine, affected by neither natural nor artificial disturbances.
这题假设还是想不太通阿。首先我看到这个问题，定位就错了，定在了第三段。然后想假设是不是和第三段第一第二句有关，就是人类对环境影响足够大，所以就导致后代适应性变差，就错选了A

正确答案是C。
是怎么理解的呢？？？

sandefia

顶!!!!!!!!!!!!!!!!!!!!!1111

baoxuhui

供参考( 我也做错了，参考了下答案以及网上的各种意见）；这道题，解题时用到两个句子：
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.  高比例的straying 是有问题的，因为鱼的杂交会使local adaptation 稀释。
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。
straying 从被毁坏的区域到未污染的地方会导致大量的gene flow，可能会导致降低后代总体的fitness.

Explanation:
straying would lower the fitness of subsequent generations.
High rate of straying is problematic and may leads to interbreed, which diluted the local adaptation.

OA: C;
If the local adaptation is not beneficial, then diluted local adaption, is not problematic. ( Negate the assumption would undermine the argument.)
So C is the result.

q635058726

这题我也错选了A  怎么也没想到highlight题要跨越文段去找答案啊  唉........

ritsusong

baoxuhui 发表于 2015-11-30 18:15
供参考( 我也做错了，参考了下答案以及网上的各种意见）；这道题，解题时用到两个句子：
Yet high rates of ...

还是不懂为什么diluted adaption is not problematic？not beneficial的话，then diluted，所以是problematic啊。