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Over the last 150 years, large stretches一大片 of salmon habitat have been eliminated by human activity:mining, livestock grazing, timber harvesting, and agriculture as well as recreational and urban devel- opment. The numerical effect is obvious: Q2there are fewer salmon in degraded regions than in pris- tine ones一个后果; however, habitat loss also has the potential to reduce genetic diversity第二个后果.解释第二个后果 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.---第一段,提出现象:许多habitat被人类活动给减少了,本栖息地的salmon数量和gene多样性被减少了
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崭新的,清新的 habitats---第二段中心句:邻近地区也受影响. Why?后面就在解释这个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迷失 and spawn in nearby streams. 低迷失率和高迷失率的不同:Low levels of straying are crucial, since the process provides a source of novel genes and a mechanismQ3 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. 举例说明:内容不重要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量化 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 Q1aftermath of the Mount Saint Helens eruption. Such a dra- 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 generations.
Q1:
The author mentions the "aftermath of the Mount Saint Helens eruption" most likely in order to
B indicate the extent to which the disturbance of salmon habitat by human activity in one stream might affect the genetic structure of salmon populations elsewhereC provide a standard of comparison against which the impact of human activity on the gene flow among salmon populations should be measured
B,C之间徘徊了很久,最终选错。---其实就是对effect有个直观的解释。这句话的意思是:尽管没有人可以量化人类行为引起的对于salmon迷失率的变化,但是没有理由怀疑(人类行为)带来的后果和MSH爆发带来的后果确实有本质的上的不同。这里强调的是一个类比。
思路错了---看见MS前文出现过,本能地往前顺着找回第二段,MS eruption出现在例子中,明确例子的作用---解释前一句话---看见disturbance---选错
再次犯低级错误:问的是function,联系的应该是前后句。不需要关心这个details的内容。
往后看就可以了,看见一个such(其实就是解释MS的内容),紧接着一个result in---其后就是选项的改写。---这么一想,其实B选项跨段了。
Q2 教训:没发现可以被绝对proved的选项,思考是不是定位出了问题?
According to the passage, human activity has had which of the following effects on salmon populations?
A An increase in the size of salmon populations in some previously polluted rivers 是decrease
B A decline in the number of salmon in some rivers
C A decrease in the number straying salmon in some rivers 跨段
D A decrease in the gene flow between salmon populations that spawn in polluted streams and populations that spawn in pristine streams 跨段
E A decline in the vulnerability of some salmon populations to the effects of naturally occurring habitat destruction 在第三段,也与原文意思相反
这题其实定位在第一段。特别容易读了后面的忘了前面的。
Q3 It can be inferred from the passage that the occasional failure偶尔性错误---低迷失率 of some salmon to return to their natal streams in order to spawn provides a mechanism by which
A pristine streams that are near polluted streams become polluted themselves
B the particular adaptations of a polluted stream's salmon population can be preserved without dilution---高迷失率相关,且无证据说明低迷失率就会preserved
C the number of salmon in pristine habitats decreases relative to the number in polluted streams
D an environmentally degraded stream could be recolonized by new salmon populations should the stream recover
E the extinction of the salmon populations that spawn in polluted streams is accelerated
难句子翻译: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.低迷失率是关键的,因为这个过程提供了一个新奇的基因来源和机制:这个地方(if) should the fish there disappear虚拟语气 可以(通过[by which]这个process)be repopulated.
我庆幸有一个英专的死党
搞定,吃饭。
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