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GWD1-Q9 to Q12:讨论

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楼主
发表于 2008-1-21 23:59:00 | 只看该作者

GWD1-Q9 to Q12:讨论


    


    

     
Over the last 150 years, large stretches of salmon habitat have been
eliminated by human activity: mining, livestock grazing(
畜牧), timber(树木)
        
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
(淳朴原始) 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
(出生) 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
(支流) 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.


    

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                      


    

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



大家解释一下,为什么D正确,why not B?


沙发
发表于 2008-1-22 00:30:00 | 只看该作者

Low levels of 。。。。。。。。。。mechanism by which a location can be repopulated should the fish there disappear

这就是D对应的原文,B对应的是Yet high rates of straying 。。。。。。。。。。。。interbreed with the existing stock to such a degree that any local adaptations that are present become diluted

问题问的是occasional failure of some salmon to return ,应该找low level,就是D

板凳
发表于 2009-4-15 11:36:00 | 只看该作者
楼上说的对,new对应原文的“novel”,recolonized对应“repopulated”
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