GWD-1-11和GWD-1-12

柿子头

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 pristine 质朴的

(10)    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成为基础 the demise死亡 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 漂泊迷路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 w:st="on">MountlaceType> laceName w:st="on">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.

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柿子头

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Q11:

According to the passage, human activity has had which of the following effects on salmon populations?

              

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

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Q12:

The author mentions the “aftermath of the Mount Saint Helens eruption” (lines 73-74) most likely in order to

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

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有哪位牛人能讲讲为什么？答案是11-B，12-C

rakesun

兄弟最好每次先到置顶的讨论贴里查一下，不然错过许多精彩讨论很可惜。第十二题至今没定论。请参考以下连接

http://forum.chasedream.com/dispbbs.asp?BoardID=25&ID=55875

柿子头

你附加的连接里只有争论不清的12题，没11题。而且CD其他地方也没有。

能不能帮我看看11题？

rakesun

11的答案就是这句话.

The numerical effect is

obvious:  there are fewer salmon

in degraded regions than in pristine 质朴的

(10)    ones;

无比

12题。。。B 和 C 。。。