第12题，看了所有的讨论，又重复看了N遍原文、题干和选项，最终，我认为是B是正确的，C不可能是正确答案.
Over the last 150 years, large
stretches of salmon habitat have
been eliminated by human activity:
Line mining, livestock grazing, timber 第一段：以人类活动使salmon habitat消失开头，列举人类活动对salmon population的影响：
(5) harvesting, and agriculture as well 1、数量减少， 2、(人类活动造成habitat loss，而hatbitat loss会）减少基因多样性。
as recreational and urban devel-
opment. The numerical effect is (注意，第2出现在however之后，是作者 强调和将说明的重点）之后均是顺接关系。
obvious: there are fewer salmon
in degraded regions than in pris-
(10) tine 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
(15) populations. Indeed, most
analysts believe that some kind
of environmental degradation
underlies the demise of many
extinct salmon populations.
(20) Although some rivers have 第一段最后一句，再次强调：虽然一些河流可以再度有鱼，但原来鱼群的独有的基因却已丢失
been recolonized, the unique 表明：鱼数量的减少并非无可挽回（can be recolonized）,而独有基因的丢失却无可挽回（have
genes of the original populations been lost）。注意：Although...,...句式的强调重心。
have been lost. 可见，文章讨论的重点将是人类活动对鱼群影响的第二条——对基因方面的影响。
Large-scale disturbances in 第二段提出更深一层的研究：一个地区被大规模扰乱（不仅会使当地的鱼群基因丢失）还会
(25) one locale also have the potential 改变邻近地区鱼群的基因结构。（继续说对基因的影响）并解释原因。
to alter the genetic structure of
populations in neighboring areas,
even if those areas have pristine
habitats. Why? Although the
(30) 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
(35) streams. Low levels of straying 说明low level of straying很至关重要(会影响邻近区域的鱼群)，
are crucial, since the process 它提供：为邻近区域的鱼群提供新的基因组合的来源
provides a source of novel 还提供：如果原来的鱼群消失，一个区域的河流可被repopulate的机制。
genes and a mechanism
by which a location can be
(40) repopulated should the fish
there disappear.
Yet high rates
of straying can be problematic
because misdirected fish may
interbreed with the existing stock
(45) to such a degree that any local 注意：这里的Yet不仅起到转折和对比的作用（low与high比），还提出了新问题：
adaptations that are present high rates of straying将引发问题——misdirected fish会与邻近区域的鱼群大量杂交——
become diluted. 造成any local adaptations that are present become diluted.
Straying
rates remain relatively low when
environmental conditions are
(50) stable, but can increase dramati- 当河流被严重扰乱时，鱼的迷路就会激增——出现high rates of straying。
cally when streams suffer severe 例如：1980年的Mount Saint Helens火山爆发使哥伦比亚河的许多支流受污。这使rates of
disturbance. The 1980 volcanic straying从16％激增到40％，从大海回来的鱼群也被迫寻找其它河流产卵。（与
eruption of Mount Saint Helens, 邻近区域的鱼群大量杂交)
for example, sent mud and debris
(55) into several tributaries of the
Columbia River. For the next
couple of years, steelhead trout
(a species included among the
salmonids) returning from the
(60) 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
(65) overall.
Although no one has quantified 尽管还没有人量化（由人类干扰导致的）rate of straying的变化，但没有理由怀疑：
changes in the rate of straying 结果会与Mout Saint Helens火山爆发的后果有质的不同。
as a result of the disturbances （也就是说，结果不会与Mout Saint Helens火山爆发的后果有质的不同）
caused by humans, there is no
(70) reason to suspect that the effect
would be qualitatively different
than what was seen in the
aftermath of the laceType>MountlaceType> laceName>SaintlaceName>
Helens eruption. Such a dra- 这种迷路的增加导致substantial gene flow, 进而降低后代的整体适应性。
(75) matic 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 这里的后代就是大量straying的鱼与existing stock杂交后的鱼，这些后代
generations. 丢失了straying鱼原有的local基因特性（any local adaptations that are present become diluted），
从而比straying鱼具有lower overall fitness.
注意：straying鱼和邻近区域的existing stock的基因都发生了变化，共同形成了subsequent generations.
Q12:
The author mentions the “aftermath of the Mount Saint Helens eruption” (lines 73-74) most likely in order to
- 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
题干、选项与原文的对应，我已用相同的颜色进行标注，以帮助理解。
解12题的思路可由文中推理链得出：
Severe disturbance of salmon habitat by human activity in one stream→ high rates of straying from damaged areas to more pristine streams→ misdirected fish interbreed with the existing stock to such a degree that any local adaptation that are present become diluted→ substantial gene flow which can in turn lower the overall fitness of subsequent generations.
注意: 相应词汇的对应。
“aftermath of the Mount Saint Helens eruption”—— not be qualitatively different，所以是Severe disturbance of salmon habitat by human activity in one stream的一个具有代表性的实例。
注意体会原文中的such的含义：先说明以aftermath of the Mount Saint Helens eruption为代表的sever disturbance会引起dramatic increase in straying from damaged areas to more pristine streams. 再进而（在最后一句话中）总结性地重申了上面的推理链。
B为什么是正确答案应该很清楚了！！！
京公网安备11010202008513号 京ICP证101109号 京ICP备12012021号
发表于 2005-11-21 19:10:00
