gwd1-12有严重不同看法

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, 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 Mount Saint

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.

Q12:

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

1. provide an example of the process that allows the repopulation of rivers whose indigenous salmon population has become extinct


2. 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


3. provide a standard of comparison against which the impact of human activity on the gene flow among salmon populations should be measured


4. show how salmons’ homing instinct can be impaired as a result of severe environmental degradation of their natal streams


5. show why straying rates in salmon populations remain generally low except when spawning streams suffer severe environmental disturbance

这到题大家争论的焦点集中在b,c 但我认为这两个都有问题

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

文中最后一段开头已经说了Although no one has quantified

首先文中并没提什么标准 ，只是说人为破坏和大自然破坏没有qualitatively 的区别，更加没讲should be measured, 如果有人选c，那么请问 到底应该怎样measure?

我选d， show how salmons’ homing instinct can be impaired as a result of severe environmental degradation of their natal streams，

注意文中there is no

如果选D：show how salmons’ homing instinct can be impaired as a result of severe environmental degradation of their natal streams，就需要看看是否“其本能被影响”。我认为，文中没有提到“回游的本能受到影响”。

至于B和C，有很多争论了，我选C。

GWD-1-Q9:

The primary purpose of the passage is to

1. argue against a conventional explanation for the extinction of certain salmon populations and suggest an alternative


2. correct a common misunderstanding about the behavior of salmon in response to environmental degradation caused by human activity


3. compare the effects of human activity on salmon populations with the effects of natural disturbances on salmon populations


4. differentiate the particular effects of various human activities on salmon habitats


5. describe how environmental degradation can cause changes in salmon populations that extend beyond a numerical reduction

如楼上所说，文章最后是在对比自然的影响和人的影响。那为什么9提不选C 呢？谢谢了

顶一下

同意斑竹的说法。

GWD-1-Q9:

对比自然的影响和人的影响只是文章的部分内容，并不是文章的PURPOSE。

正因为是在文章最后才说得，是细节内容，所以不选C。

应该选E。

回楼主！我认为：的确没有人可以衡量人类破坏的影，所以我们要找一个参照物阿！这时the Mount Saint Helens eruption.首当其冲 作了这个参照物！所以一定是bc两项里选！个人倾向b项！再看看讨论！