【OG24-535题】According to the passage, before the discovery of new techniques for detecting viral DNA, population ecologists believed that viral diseases_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.

According to the passage, before the discovery of new techniques for detecting viral DNA, population ecologists believed that viral diseases

were not widely prevalent among insect populations generally affected only the caterpillar life stage of lepidoptera were the driving force behind lepidoptera population cycles attacked already declining caterpillar populations infected birds and parasites that prey on various species of lepidoptera

正确答案： D

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文章大意逻辑
文章围绕鳞翅目昆虫（飞蛾和蝴蝶）的种群数量周期展开。指出虽然许多因素影响其种群水平，但种群周期背后的主导驱动力难以确定。过去研究方法在该问题上成效不佳。近期研究表明病毒可能是驱动力，此前生态学家认为病毒是在种群数量下降过程中起作用而非引发下降，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背后驱动力，并介绍了其感染过程。 文章类型及逻辑简图
文章类型为新旧观点对比。逻辑简图： Old view: Many variables influence lepidoptera population levels, but identifying the driving force of population cycles is elusive. Traditional research methods are unproductive. Population ecologists thought viral diseases contributed to decline once it was underway. New view: A virus may be the driving force. New techniques in molecular biology make it possible to detect viral DNA at low concentrations. Nuclear polyhedrosis viruses are hypothesized to be the driving force and their infectious cycle is described. 做题方法及问题类型
这是一道细节题，通过定位“before the discovery of new techniques for detecting viral DNA, population ecologists believed”相关内容来解题。 选项分析
A选项：文中未提及病毒在昆虫种群中普遍不流行，无中生有，排除。 B选项：文中没有说病毒只影响鳞翅目昆虫的毛毛虫阶段，无依据，排除。 C选项：这是新观点，不是之前生态学家的观点，排除。 D选项：根据“but population ecologists had usually considered viral disease to have contributed to the decline once it was underway rather than to have initiated it”可知，之前生态学家认为病毒攻击的是已经在下降的毛毛虫种群，该项正确。 E选项：文中未提及病毒感染捕食鳞翅目昆虫的鸟类和寄生虫，排除。

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