[阅读] BeyondGMAT阅读讲义2.113和2.114鲑鱼消失与基因多样性减少

ellen.li问道：
RC 2.113 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
我选择了E选项，我认为该题细节对应原文中的 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的此句话，但对这句话中the fish there disappear 的there指代什么？词句话该如何翻译？

RC 2.114. The author mentions the “aftermath of the Mount Saint Helens eruption” most likely in
order to
A. provide an example of the process that allows the repopulation of rivers whose indigenous
salmon population has become extinct
B. indicate the extent to which the disturbance of salmon habitat by human activity in one stream
might affect the genetic structure of salmon populations elsewhere
C. provide a standard of comparison against which the impact of human activity on the gene flow
among salmon populations should be measured
D. show how salmons’ homing instinct can be impaired as a result of severe environmental
degradation of their natal streams
E. show why straying rates in salmon populations remain generally low except when spawning
streams suffer severe environmental disturbance
此题我选择了B，我认为作者此处的目的虽然是提供了测量标志的，但标志应该是为了体现影响程度的。所以我认为更侧重的应该是程度，就选了B。但答案为什么选C?

原文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

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