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[GMAT阅读题讨论] PREP2012阅读第8篇第3题

icetong123 发表于 2015-1-9 11:43:07 | 显示全部楼层 |阅读模式
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 repopulates 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 salmon ) 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
A. An increase in the size of salmon populations in some previously polluted rivers
B. A in the number of salmon in some rivers
C. A decrease in the number of 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




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