阅读 怪鱼 求解惑

bruce0816

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 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. 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.

对应问题
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
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
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
真是TM的崩溃读的，B说high level是diluted那低level preserved without dilution不也挺好，D说的stream recover原文写的是fish disappear，怎么就同意替换了？这为啥是同意替换啊？

理解不能，希望高人能手把手的授业解惑别跳步，这篇破阅读烦死了

adamzjw

a mechanism很明显是指这个句子：
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.
这个mechanism可以让一个鱼已经消失的地区重新出现鱼。

那很明显是和D同个意思
an environmentally degraded stream could be recolonized by new salmon populations should the stream recover
一个环境退化了的溪流被salmon鱼重新占领（如果这个地方还能恢复的话）

B中的adaptations of a polluted stream，在文章就没有被讨论过。
更何况，adaptation是一种基因特质，不被稀释就是意味着基因的稳定。
而low level of straying provides lots of novel genes，这明显是相反的。

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这题一开始我一直看不懂should the fish there disappear这个成分，原来是倒装省略句。

the process provides (a source of novel genes) and (a mechanism (by which a location can be repopulated) [should the fish there disappear]

用倒裝句表示非真實條件
如果條件副詞子句的謂語動詞中含有were, had, should, 可以省略連接詞if，把were, had, should,放在主語前構成倒裝。如果條件副詞子句是否定形式，倒裝時只能把not放在主語的後面，不能用縮寫形式而把not一起放到主語之前。
Should there be a flood, what should we do?
萬一發生了水災，我們該怎麼辦呢？