|
Over the last 150 years, large stretches of salmonhabitat have been eliminated by human activity: mining, livestock grazing,timber harvesting, and agriculture as well as recreational and urbandevelopment. The numerical effect is obvious: there are fewer salmon indegraded regions than in pristine ones; however, habitat loss also has thepotential to reduce genetic diversity. This is most evident in cases where itresults in the extinction of entire salmon populations. Indeed, most analystsbelieve that some kind of environmental degradation underlies the demise of many extinctsalmon populations. Although some rivers have been recolonized, the uniquegenes of the original populations have been lost.
Large-scale disturbances in one locale also have the potential to alter thegenetic structure of populations in neighboring areas, even if those areas havepristine habitats. Why? Although the homing instinct of salmon to their natal stream is strong, afraction of the fish returning from the sea (rarely more than 15 percent) strayand spawn in nearby streams. Low levels of straying are crucial, since theprocess provides a source of novel genes and a mechanism by which a locationcan be repopulated should the fish there disappear. 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 become diluted. Straying rates remain relatively low when environmental conditions arestable, but can increase dramatically when streams suffer severe disturbance.The 1980 volcanic eruption of Mount Saint Helens, for example, sent mud anddebris into several tributaries of the Columbia River. For the next couple ofyears, steelhead trout (a species included among the salmonids) returning fromthe sea to spawn were forced to find alternative streams. As a consequence,their rates of straying, initially 16 percent, rose to more than 40 percentoverall.
Although no one has quantified changes in the rate of straying as a result ofthe disturbances caused by humans, there is no reason to suspect that theeffect would be qualitatively different than what was seen in the aftermathof the Mount Saint Helens eruption. Such a dramatic increase in straying fromdamaged areas to more pristine streams results in substantial gene flow, whichcan in turn lower the overall fitness of subsequent generations.
It can be inferredfrom the passage that the occasional failure of some salmonto return to their natal streams in order to spawn provides a mechanismby which
A. pristine streams that are near polluted streamsbecome polluted themselves B. the particularadaptations of a pollutedstreams salmon population can be preserved without dilution C. the number of salmon in pristine habitats decreasesrelative to the number in polluted streams D. an environmentally degradedstream could be recolonized by new salmon populations should the stream recover E. the extinction of the salmonpopulations that spawn in pollutedstreams is accelerated
答案选D，我看懂了。但我选的是B，为什么错了呢？ 原文对应部分说"high rates of straying can be problematic because misdirect fish may interbreed with the existing stock to such a degree that any local adaption that are present become diluted"，也就是说high rates of straying导致了local adaption become diluted。那么low rates of straying就应该是without dilution啊。正好选B啊。
| 
京公网安备11010202008513号 京ICP证101109号 京ICP备12012021号
发表于 2018-1-30 21:07:16
收藏
收藏
楼主
