Prep2012_Pack1_RC(题目版)(6)

C. offer evidence held to undermine a common view of why Wegener's theory was not

accepted in its day

D. give an example of a modern scientist who believes that Wegener’s theory was rejected

because it failed to adequately explain the mechanical basis of continental drift

E. support Gould's rationale for why Wegener's theory was rejected by most geologists in

the early twentieth century

PREP2012-Pack1-RC-007-05 VRC000416-05 Medium

The author of the passage discusses the “essential presumptions” (see highlighted text) of The Origin of Continents and Oceans most in order to

A. indicate features of Wegener's theory that caused it to be doubted in its day

B. show why Wegener's theory is now regarded as prescient

C. indicate differences between plate tectonics and the theory of continental drift

D. cite features of the theory of continental drift for which no evidence was available in

Wegener’s day

E. point out aspects of Wegener's theory that were accepted well before the advent of plate

tectonics

PREP2012-Pack1-RC-008

Over the last 150 years, large stretches of salmon habitat have been eliminated by human activity: mining, livestock grazing, timber 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 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 beenrecolonized, 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 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 repopulates 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 of the Columbia River. For the next couple of years, steelhead trout (a species included among the salmon ) 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. PREP2012-Pack1-RC-008-01 VRC000460-01 Medium The primary purpose of the passage is to

A.

B. argue against a conventional explanation for the extinction of certain salmon populations and suggest an alternative correct a common misunderstanding about the behavior of salmon in response to

environmental degradation caused by human activity

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

disturbances on salmon populations

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

E. describe how environmental degradation can cause changes in salmon populations that

extend beyond a numerical reduction

PREP2012-Pack1-RC-008-02 VRC000460-02 Hard

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 streams 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 PREP2012-Pack1-RC-008-03 VRC000460-03 Medium

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

A.

B.

C.

D.

E. An increase in the size of salmon populations in some previously polluted rivers A in the number of salmon in some rivers A decrease in the number of 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

PREP2012-Pack1-RC-008-04 VRC000460-05 Hard

The author's argument that increased straying can “lower the overall fitness of subsequent generations” (see highlighted text) is based on which of the following assumptions?

A. A disturbance of salmonid spawning streams caused …… 此处隐藏：6042字，全部文档内容请下载后查看。喜欢就下载吧 ……