GWD1-Q12

DeliciaAn

刚才分析了一下，想跟前辈们share一下。取了第二段和第三段

第2段：Large-scale disturbancesin one locale also have the potential to alter the genetic structure of populationsin 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) strayand spawn in nearby streams.  Low levelsof straying are crucial, since the process provides a source of novel genes anda mechanism by which a location can be repopulated should the fish theredisappear.  Yet high rates of strayingcan be problematic because misdirected fish may interbreed with the existingstock to such a degree that any local adaptations that are present becomediluted. Straying rates remain relatively low when environmental conditions arestable, but can increase dramatically when streams suffer severe disturbance.  （这之前是两组对比，low level的stray: crutial+ environmental conditions stable; high rates的stray: problematic+ stream suffer disturbance) The 1980 volcanic eruption of Mount SaintHelens, for example(这里就举了例子来说明前面的东西，前面的核心的内容就是disturbance alter the genetic structure), sent mud and debris into several tributaries of the ColumbiaRiver.  For the next couple of years,steelhead trout (a species included among the salmonids) returning from the seato spawn were forced to find alternative streams.  As a consequence, their rates of straying,initially 16 percent, rose to more than 40 percent overall(这是afterman of the MH eruption 发生的事情）.

第3段：Although no one has quantified changes inthe rate of straying as a result of the disturbances caused by humans, there isno reason to suspect that the effect would be qualitatively different than whatwas seen in the aftermathof the Mount Saint Helens eruption. (没有理由怀疑这种效果和MSH eruption 的后果有质的差别） Such a dramatic increase in straying from damaged areas to more pristinestreams results in substantial gene flow, which can in turn lower the overallfitness of subsequent generations.

Mount SH是作为例子在第二段有所体现的
Q12:

The author mentions the“aftermath of the Mount Saint Helens eruption” (lines 73-74) most likely inorder to

A.   provide an example of the process that allows therepopulation of rivers whose indigenous salmon population has become extinct

B.    indicate the extent to which the disturbance of salmonhabitat by human activity in one stream might affect the genetic structure ofsalmon populations elsewhere

C.   provide a standard of comparison against which the impactof human activity on the gene flow among salmon populations should be measured

第三段的第一句话不是想和后面的MSH做对比的，而是为了承接上文（第二段）的内容，上文说了stray->high level->sever disturbance. 第三段开头说“尽管没有人数出来到底是有多少disturbances是由人导致的，但是没有理由怀疑和MSH火山爆发带来的后果有质的区别）。上面提到的standard是一方面，这里说提供数量比较是另一方面。因为上文虽然提到了spray 在MSH火山爆发后的比例是改变了的（16%－>40%)，但是问题问的是author mentions 这个most likely in order to...,是为了说明我们应该衡量一下由于人的活动，干扰了salmon gene flow,还是说我们想用例子来说明这二者的本质后果是一样的？所以我是支持B选项的。

D.   show how salmons’ homing instinct can be impaired as aresult of severe environmental degradation of their natal streams

E.    show why straying rates in salmon populations remaingenerally low except when spawning streams suffer severe environmentaldisturbance

myselfhjl

DeliciaAn 发表于 2015-6-11 14:03
刚才分析了一下，想跟前辈们share一下。取了第二段和第三段
第2段：Large-scale disturbancesin one locale ...

我支持B选项。B中extent也可以理解为费数量上的程度。而C中standard measured文中不是这个意思啊。