子机构
400-600-1123
logo GMAT备考资讯 答疑社区 GMAT公开课 提分课程

城市电话

申友-中心

免费咨询

咨询热线:

地址:
登录
注册

我的备考方案

获取方案

题库解析   >   细节题题型   >  
    本题由chris提供
1041次学习

Over the last 150 years, large stretches of salmon habitat have been eliminated by human activity: mining, livestock grazing,

timber harvesting, and agriculture as well as recreational and urban development. The numerical effect is obvious: there are fewer salmon in degraded regions than in pristine 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 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.

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.

Although no one has quantified 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 aftermath of the Mount Saint Helens eruption. Such a dramatic 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.

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

    B. A decline in the number of salmon in some rivers

    C. A decrease in the number straying salmon in some rivers

    D. A decrease in the gene flow between salmon populations that spawn in polluted streams and populations that spawn in pristine streams

    E. A decline in the vulnerability of some salmon populations to the effects of naturally occurring habitat destruction


登录申友留学,查看答案及解析
答案:
B
正确答案为
(B)。根据题干关键词human activity, effects on salmon populations,定位回原文第一段。第一段第二句说: the numerical effect is obvious: there are fewer salmon in degraded regions than in pristine ones.即受到人类活动影响的退化区域鲑鱼数量减少了。 (A):与原文相反。一些鲑鱼种群应该数量下降而不是上升。 (C):与原文相反:迷失率应该上升而不是下降 (D):与原文相反:基因漂流上升而不是下降 (E):与原文相反:受到栖息地被破坏影响的鲑鱼种群的适应性降低,也就是脆弱性上升。

参与讨论

提交
吉祥物小蜜蜂
联系申友 全国免费咨询热线:400-600-1123

友情链接: 申友雅思 | 申友留学 | 申友GMAT官方微博 | 申友GMAT官方知乎机构账号

Copyright © 2020 All Right Reserved 申友教育 版权所有 沪ICP备17005516号-4 免责声明