【OG24-532题】The primary purpose of the passage is to_mofa留学圈mofaGRE在线题库系统_GRE在线搜题_GRE题库解析_GRE真题机经_GRE课程_GRE培训机构

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A small number of the forest species of lepidoptera (moths and butterflies, which exist as caterpillars during most of their life cycle) exhibit regularly recurring patterns of population growth and decline—such fluctuations in population are known as population cycles. Although many different variables influence population levels, a regular pattern such as a population cycle seems to imply a dominant, driving force. Identification of that driving force, however, has proved surprisingly elusive despite considerable research. The common approach of studying causes of population cycles by measuring the mortality caused by different agents, such as predatory birds or parasites, has been unproductive in the case of lepidoptera. Moreover, population ecologists’ attempts to alter cycles by changing the caterpillars’ habitat and by reducing caterpillar populations have not succeeded. In short, the evidence implies that these insect populations, if not self-regulating, may at least be regulated by an agent more intimately connected with the insect than are predatory birds or parasites.

Recent work suggests that this agent may be a virus. For many years, viral disease had been reported in declining populations of caterpillars, but population ecologists had usually considered viral disease to have contributed to the decline once it was underway rather than to have initiated it. The recent work has been made possible by new techniques of molecular biology that allow viral DNA to be detected at low concentrations in the environment. Nuclear polyhedrosis viruses are hypothesized to be the driving force behind population cycles in lepidoptera in part because the viruses themselves follow an infectious cycle in which, if protected from direct sunlight, they may remain virulent for many years in the environment, embedded in durable crystals of polyhedrin protein. Once ingested by a caterpillar, the crystals dissolve, releasing the virus to infect the insect’s cells. Late in the course of the infection, millions of new virus particles are formed and enclosed in polyhedrin crystals. These crystals reenter the environment after the insect dies and decomposes, thus becoming available to infect other caterpillars.

One of the attractions of this hypothesis is its broad applicability. Remarkably, despite significant differences in habitat and behavior, many species of lepidoptera have population cycles of similar length, between eight and 11 years. Nuclear polyhedrosis viral infection is one factor these disparate species share.

The primary purpose of the passage is to

describe the development of new techniques that may help to determine the driving force behind population cycles in lepidoptera present evidence that refutes a particular theory about the driving force behind population cycles in lepidoptera present a hypothesis about the driving force behind population cycles in lepidoptera describe the fluctuating patterns of population cycles in lepidoptera question the idea that a single driving force is behind population cycles in lepidoptera

正确答案： C

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文章大意逻辑
文章开篇指出鳞翅目（飞蛾和蝴蝶）森林物种中的一小部分在生命周期大部分时间以毛毛虫形态存在，且展现出规律性的种群数量增长和下降模式（种群周期）。尽管有很多变量影响种群数量，但这种规律模式似乎意味着存在主导驱动力，然而确定该驱动力很困难。接着提到通过测量捕食性鸟类或寄生虫等因素导致的死亡率来研究种群周期原因的常见方法在鳞翅目案例中未取得成果，改变毛毛虫栖息地和减少其数量的尝试也未成功。然后表明证据暗示这些昆虫种群若不是自我调节，至少是由与昆虫关系更密切的因素调节，而非捕食性鸟类或寄生虫。最近研究表明这个因素可能是病毒，过去种群生态学家认为病毒疾病是在种群数量下降过程中起作用而非引发下降，而新技术使检测低浓度病毒DNA成为可能，核多角体病毒被假设为鳞翅目种群周期背后的驱动力，因为它们自身遵循感染周期。 文章类型及逻辑简图
文章类型为提出问题给出假设。逻辑简图： Lepidoptera have population cycles. Identifying driving force of cycles is elusive. Common approaches unproductive. Alter cycle attempts failed. Recent work suggests virus may be the driving force. New techniques enable virus detection. Polyhedrosis viruses hypothesized as driving force. 做题方法及问题类型
这是一道主旨题，做题方法是通过梳理文章整体内容，把握文章围绕鳞翅目种群周期驱动力展开的论述脉络来选择答案。 选项分析
A选项：文章重点不是描述有助于确定驱动力的新技术的发展，新技术只是最近研究的一个工具，故A错误。 B选项：文章没有反驳关于驱动力的特定理论，只是指出过去方法未成功，提出新假设，故B错误。 C选项：文章主要就是提出了关于鳞翅目种群周期背后驱动力（可能是病毒）的假设，该项正确。 D选项：描述种群周期波动模式只是开头引入内容，不是文章主要目的，故D错误。 E选项：文章不是质疑单一驱动力的观点，而是试图找出这个驱动力，故E错误。

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