GMAT阅读经典高频拟真难题实例讲解：Over the last 150 years...
- 2018年04月23日18:01 来源：小站整理
- 参与（1） 阅读（2057）
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 devel- opment. The numerical effect is obvious: there are fewer salmon in degraded regions than in pris- tine 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 dra- matic 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.
1. The primary purpose of the passage is to
A argue against a conventional explanation for the extinction of certain salmon populations and suggest an alternative
B correct a common misunderstanding about the behavior of salmon in response to environmental degradation caused by human activity
C compare the effects of human activity on salmon populations with the effects of natural disturbances on salmon populations
D differentiate the particular effects of various human activities on salmon habitats
E describe how environmental degradation can cause changes in salmon populations that extend beyond a numerical reduction
The numerical effect is obvious: there are fewer salmon in degraded regions than in pristine ones; however, habitat lossalso has the potential to reduce genetic diversity.
第一部分说: 对 numerical effect的影响;
第二部分说: 对genetic diversity也有影响;
整体意思和答案E中的 “ extend beyond numerical” 意思一致.
(A) No conventional explanation is stated; 讲的不是 ”certain salmon“!
(B) No common misunderstanding; “environmental degradation“ is NOT“ caused by human activity”
(C) Compare is not the main idea.
(D) 重点不是” human activities“
(E) The passage is primarily discuss the environmental degradation (mainly)and human activities (in p1 only) are cause changes in salmon population.
2. 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
Although the homing instinct of salmon to their natal stream is strong, a fraction of the fishreturning 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.
so D ：should引导一个虚拟语气的条件句。
should the fish there disappear = if the fish there should disappear
3. 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
直接细节题：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。
4. 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
C将human activity和mountain eruption相比较，因此给了human activity对salmon的gene的破坏给了一个standard 因此为正确答案