【OG24-534题】Which of the following, if true, would most weaken the author's conclusion in highlight lines ?_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.

Which of the following, if true, would most weaken the author's conclusion in highlight lines ?

New research reveals that the number of species of birds and parasites that prey on lepidoptera has dropped significantly in recent years. New experiments in which the habitats of lepidoptera are altered in previously untried ways result in the shortening of lepidoptera population cycles. Recent experiments have revealed that the nuclear polyhedrosis virus is present in a number of predators and parasites of lepidoptera. Differences among the habitats of lepidoptera species make it difficult to assess the effects of weather on lepidoptera population cycles. Viral disease is typically observed in a large proportion of the lepidoptera population.

正确答案： B

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文章大意逻辑
文章开篇指出一小部分森林鳞翅目昆虫（飞蛾和蝴蝶）存在定期的种群数量增长和下降模式，即种群周期。尽管影响种群数量的变量众多，但这种规律模式暗示存在主导驱动因素，然而其识别却很困难。接着表明研究种群周期原因的常见方法（测量捕食性鸟类或寄生虫导致的死亡率）在鳞翅目昆虫上无效，生态学家改变栖息地和减少幼虫数量的尝试也未成功。随后提出近期研究暗示这种驱动因素可能是病毒，过去生态学家认为病毒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昆虫种群周期背后的驱动力，并介绍了该病毒的感染周期。 文章类型及逻辑简图
文章类型为问题解决方案型。逻辑简图： Problem: Some lepidoptera exhibit population cycles. Identifying the driving force is elusive. Common research approaches unproductive. Solution: Recent work suggests a virus may be the driving force. New techniques enable virus detection. Hypothesis about nuclear polyhedrosis viruses. 做题方法及问题类型
问题类型为削弱题。做题方法是定位到高亮句子（作者结论处），分析各个选项对该结论的削弱作用。 选项分析
A选项：捕食鳞翅目昆虫的鸟类和寄生虫数量显著下降。这可能表明是捕食者数量变化影响种群周期，而不是病毒，有一定削弱作用，但不如B直接。 B选项：以以前未尝试过的方式改变鳞翅目昆虫栖息地的新实验导致其种群周期缩短。说明是栖息地改变影响了种群周期，而不是病毒，直接削弱了作者认为病毒是驱动因素的结论，当选。 C选项：核多角体病毒存在于一些捕食者和寄生虫中。这与病毒是否是鳞翅目昆虫种群周期的驱动因素无关，无法削弱。 D选项：鳞翅目昆虫栖息地的差异使评估天气对种群周期的影响困难。与病毒是否是驱动因素没有关联，无法削弱。 E选项：在很大比例的鳞翅目昆虫种群中通常观察到病毒疾病。这其实在一定程度上支持了病毒与种群周期有关，而不是削弱。

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