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1. GWD-1-Q9 to Q12环境退化导致鲑鱼数量下降Large-scaledisturbances in one locale also have the potential to alter the geneticstructure of populations in neighboring areas, even if those areas have pristine habitats. Why? Although the hominginstinct of salmon to their natal stream is strong, a fraction of the fish returning fromthe sea (rarely more than 15 percent) stray and spawn in nearby streams. Low levels of straying are crucial, since the process provides asource of novel genes and a mechanism by which a location can be repopulatedshould the fish there disappear. Yet high rates of straying can be problematic because misdirected fish may interbreed with theexisting stock to such a degree that any local adaptations that are presentbecome diluted. Straying rates remain relatively low whenenvironmental conditions are stable,but can increase dramatically when streams suffer severe disturbance. The 1980 volcanic eruption of Mount Saint Helens, forexample, sent mud and debrisinto several tributariesof the Columbia River. For the next couple of years, steelhead trout (aspecies included among the salmonids) returning from the sea to spawn wereforced to find alternative streams. As aconsequence, their rates of straying, initially 16 percent, rose to more than40 percent overall.
It can beinferred from the passage that the occasional failure of some salmon to returnto their natal streams in order to spawn provides a mechanism by which
A. pristine streams that are near pollutedstreams become polluted themselves
B. the particular adaptations of a pollutedstream’s salmon population can be preserved without dilution
C. the number of salmon in pristine habitatsdecreases relative to the number in polluted streams
D. an environmentally degraded stream could berecolonized by new salmon populations should the stream recover
E. the extinction of the salmon populationsthat spawn in polluted streams is accelerated
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发表于 2012-8-13 10:58:43
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