GWD-1-10

libbytt

  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, 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.

      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?  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.  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 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, 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.

      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 laceType>MountlaceType> laceName>SaintlaceName>

Helens eruption.  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.

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

              

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

answer

不好意思，这篇文章好长，麻烦大家了：）划线部分是我认为的与此题相对应的部分。这句话是什么意思？是倒装吗？有点看不懂。。。答案是怎样与它对应的？乍一看似乎对应，但有点不理解是什么意思。

magic_cube

我也有同样的问题。

定位应该没有错 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.

D：an environmentally degraded stream could be recolonized by new salmon populations should the stream recover

但是句子读不太懂，这两个地方的should究竟是什么用法？如果是倒装的话，正常的语序应该是什么？

请NN指点一下。谢谢！！

sophia_liwei

我也有同样的问题！求助！！！

ayang1017

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.  较低程度上鱼的离群至关重要，因为这个过程提供了新的基因来源和一种机制，通过这种机制，那些鱼群消失的地方能够得以重新繁荣起来。从意思上来讲（语法上略有区别），should=if，

10．一些s鱼回到出生地产卵的偶尔的失败提供了一种机制通过什么，定位39偶尔说明离群少，在一个可能消失的地方重新繁荣，d一个已遭破坏的环境可被重新改善通过新的s种群的重新覆盖。同意变换

imharper

以下是引用magic_cube在2005-10-7 11:37:00的发言：

我也有同样的问题。

定位应该没有错 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.

D：an environmentally degraded stream could be recolonized by new salmon populations should the stream recover

但是句子读不太懂，这两个地方的should究竟是什么用法？如果是倒装的话，正常的语序应该是什么？

请NN指点一下。谢谢！！

黄色部分定位不对!

SHOULD THE FISH THERE DISAPPEAR的SHOULD是IF的意思,与下面的SHOULD...RECOVER是不对应的.应该是定位a mechanism by (其中MECHANISM是抽象名词,定位首选它或者其同义词)

zhuminmin