GWD1-Q11

triangel8866

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.

GWD1-Q11:

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

A     [url=]An increase in the size of salmon populations in some previously polluted rivers[/url]

B     [url=]A decline in the number of salmon in some rivers[/url]

C     [url=]A decrease in the number straying salmon in some rivers[/url]

D      [url=]A decrease in the gene flow between salmon populations that spawn in polluted streams and populations that spawn in pristine streams[/url]

E      [url=]A decline in the vulnerability of some salmon populations to the effects of naturally occurring habitat destruction[/url]

答案B，可是D有什么不对么？

翻了以前的讨论，但是没有第11题，谢谢各位！！

hanlei84921

原文关于Gene Flow这么说的Such a dra-matic increase in straying from damaged areas to more pristine streams results in substantial gene flow, ... 意思是什么情况下导致了Gene Flow

而D是说derease in the Gene Flow between A and B，有两处明显错误

1）Gene Flow被between/and限定，不符合原文

2）原文并未提及Gene Flow的decrease

cherry_rui

那gene和number有什么关系呢？

zhangmile

我选的E，我觉得从文章最后一句lower the overall fitness of subsequent generations可以得出E阿。

E为什么不对呢？

lovesiqi4665

我也选了d，唉怎么阅读做起来这么不上手啊

clairesslc

我觉得D肯定是错的吧~宁可是increase也不是decrease啊~the rate of spraying increased so more salmon spawn in pollute stream instead of pristine stream and the gene flow should be increased~不知道对不对

clairesslc

Bis correct because more salmon find alternative stream so that the amount ofsalmon in some rivers declined naturally.