GWD1-Q9 to Q12: Over the last 150 years, large stretches of salmon habitat have been eliminated by human activity: Line mining, livestock grazing, timber (5) 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- (10) tine ones; however,(转折词后强调重点) 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 (15) populations. Indeed, most analysts believe that some kind of environmental degradation underlies the demise of many extinct salmon populations. (20) Although some rivers have been recolonized, the unique genes of the original populations have been lost.(让步略读,重点是让步后的转折句) 这一段总结human activity使—〉salmon habitat减少,而habitat的减少—〉也会造成genetic diversity的减少。所以环境的恶化是一些salmon物种消失。human activity—〉salmon habitat—〉genetic diversity的减少)
Large-scale disturbances in (25) one locale also have the potential to alter the genetic structure of populations in neighboring areas, even if those areas have pristine habitats. Why? (明显标志下文开始解释) Although the (30) 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 (35) streams. (让步略读,重点是让步后的转折句) Low levels of straying are crucial, since the process provides a source of novel genes and a mechanism by which a location can be (40) repopulated should the fish there disappear. Yet (转折词后强调重点) high rates of straying can be problematic because misdirected fish may interbreed with the existing stock (45) to such a degree that any local adaptations that are present become diluted. Straying rates remain relatively low when environmental conditions are (50) stable, but can increase dramati- cally when streams suffer severe disturbance. The 1980 volcanic eruption of Mount Saint Helens, for example(举例通常可以略读,只提取例子中的关键词,举例的关键看前后的总结句,通常会是for example前面的一句话,因为例子都是用来说明总结句的,只要明白总结句的意思就行了), sent mud and debris (55) into several tributaries of the Columbia River. For the next couple of years, steelhead trout (a species included among the salmonids) returning from the (60) 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 (65) overall. (这一段概括,即使是在好的环境下, Large-scale disturbances也会造成genetic structure的变化。会造成high rates of straying会造成genetic问题,而severe disturbance造成rates of straying增加 Large-scale disturbances—〉high rates of straying –〉genetic structure的变化 ) Although no one has quantified changes in the rate of straying as a result of the disturbances caused by humans, there is no (70) reason to suspect that the effect would be qualitatively different than what was seen in the aftermath of the Mount Saint Helens eruption. (让步略读,重点是让步后的转折句) Such a dra- (75) 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.
总结increase in straying from damaged areas to more pristine streams results in substantial gene flow, -------------------------------------------------------------------------------- GWD1-Q9: The primary purpose of the passage is to (Main idea ) - argue against a conventional explanation for the extinction of certain salmon populations and suggest an alternative
- correct a common misunderstanding about the behavior of salmon in response to environmental degradation caused by human activity
- compare the effects of human activity on salmon populations with the effects of natural disturbances on salmon populations
- differentiate the particular effects of various human activities on salmon habitats
- describe how environmental degradation can cause changes in salmon populations that extend beyond a numerical reduction
结论解释性文章主旨题找概括全面的,包括信息多且严谨的选项 Indeed, most analysts believe that some kind of environmental degradation underlies the demise of many extinct salmon populations
-------------------------------------------------------------------------------- GWD1-Q10: 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 (Inference ) 找到问题中的关键词回文章定位 - pristine streams that are near polluted streams become polluted themselves
- the particular adaptations of a polluted stream’s salmon population can be preserved without dilution
- the number of salmon in pristine habitats decreases relative to the number in polluted streams
- an environmentally degraded stream
could be recolonized by new salmon populations should the stream recover - the extinction of the salmon populations that spawn in polluted streams is accelerated
D这两句话怎么理解翻译? since the process provides a source of novel genes and a mechanism by which a location can be repopulated should the fish there disappear should the fish there disappear=if the fish there should disappear -------------------------------------------------------------------------------- GWD1-Q11: According to the passage, human activity 定位在第一段has had which of the following effects on salmon populations? (supporting idea )找到问题中的关键词回文章定位 - An increase in the size of salmon populations in some previously polluted rivers
- A decline in the number of salmon in some rivers
- A decrease in the number straying salmon in some rivers
- A decrease in the gene flow between salmon populations that spawn in polluted streams and populations that spawn in pristine streams
- A decline in the vulnerability of some salmon populations to the effects of naturally occurring habitat destruction
The numerical effect is obvious: there are fewer salmon in degraded regions than in pristine ones;
-------------------------------------------------------------------------------- GWD1-Q12: The author mentions the “aftermath of the Mount Saint Helens eruption” (lines 73-74) most likely in order to 举例作用题
- provide an example of the process that allows the repopulation of rivers whose indigenous salmon population has become extinct
- 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
- provide a standard of comparison against which the impact of human activity on the gene flow among salmon populations should be measured
- show how salmons’ homing instinct can be impaired as a result of severe environmental degradation of their natal streams
- show why straying rates in salmon populations remain generally low except when spawning streams suffer severe environmental disturbance
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.
[此贴子已经被作者于2007-9-13 13:52:24编辑过] |