4.18 考研英语外刊阅读模拟试题【难度9.5】

Section II Reading Comprehension 

Part A 

Directions: Read the following four texts. Answer the questions below each text by choosing A, B, C or D. Mark your answers on the ANSWER SHEET. 

Text 4

Humans have never been immune from the pressures of natural selection. Throughout the history of the species, circumstances have arisen to give individuals with certain beneficial mutations an advantage over their peers, allowing their valuable genetic variants to spread among a population. Still, the emergence and rapid spread of such variants—known as strong directional selection—was thought to have been a rare occurrence in human evolution. A new study published in Nature, however, reveals it has played a much more influential role.

The dynamics of evolutionary biology have historically been hard to extract from ancient DNA. Small sample sizes are partly to blame, with general conclusions about human adaptation hard to draw from a handful of (often poorly preserved) fragments. Another complication is that sustained genetic variation does not emerge exclusively as a result of natural selection. Genetic mutations can also propagate by chance or through mixing between populations. Sorting the genetic wheat from the chaff many thousands of years after the fact is a challenging task.

To overcome these limitations, Ali Akbari and David Reich, a pair of geneticists from Harvard University, developed a new statistical toolkit to use on collections of ancient DNA samples. The researchers hoped it could make sufficiently detailed comparisons between pairs of samples in a collection to distinguish signatures of directional selection from those of non-adaptive mutations.

The researchers then applied their tools to the genetic data of 15,836 ancient individuals of West Eurasian ancestry as well as 6,438 modern humans. The dataset spanned 18,000 years and included new data for 10,016 ancient genomes, representing a doubling of the global pool of ancient DNA data and a 14-fold increase in sample size compared with previous studies searching for natural selection.

The results of the analysis surprised even the authors. Until now, only 21 incidences of directional selection have been found in humans in the past 11,000 years. The new paper, however, identifies 479 gene variants as highly likely to have emerged as a result of natural selection over the same period. That suggests directional selection was pervasive across West Eurasia in recent millennia. “It’s an analytical masterpiece,” says Pontus Skoglund, a geneticist who was not involved in the study.

Some of the most intriguing findings emerged when researchers focused on variations arising in individual genes. One genetic signature that helps the body identify pathogens is also associated with an increased susceptibility to an autoimmune digestive disorder. Their analysis suggests that this signature, which is found in roughly one in five of their modern-day samples, was almost absent until around 4,000 years ago. Its emergence may not, as had been previously thought, have been driven by the spread of agriculture, which happened long before.

Another hypothesis they suggest may be wrong concerns genes associated with a higher risk of developing a severe genetic lung disease, which some have speculated emerged because of the resistance to cholera it granted carriers. Researchers found no indication of selection for these genes during the period that cholera is thought to have been endemic, weakening the case for such a connection.

Co-ordinated shifts across collections of genes can also influence more complex traits. The researchers found that genes associated with type 2 diabetes were selected against, for example, as were those linked to psychiatric conditions such as schizophrenia and bipolar disorder.

Strong directional selection, pervasive though it may be, does not operate at a constant rate. The researchers found that such instances appear to have dramatically intensified for West Eurasians during the Holocene epoch, beginning roughly 11,700 years ago, perhaps because of changing lifestyles following the rise of agriculture.

36. According to Paragraph 2, identifying the role of natural selection in ancient DNA is difficult partly because ________.[A] genetic changes could also be driven by random, non-adaptive mechanisms[B] poorly preserved DNA fragments always yield misleading conclusions[C] the ancient human populations were completely isolated from each other[D] sustained genetic variation relies exclusively on rare beneficial mutations

附注：根据历年考研英语真题阅读题源外刊等，摘选最新文章，模拟仿真出题。

参考答案见以下。

Quick look: ABCDB

36.【正确答案】A【解析】题型：因果细节题定位： 第二段第三、四句“Another complication is that sustained genetic variation does not emerge exclusively as a result of natural selection. Genetic mutations can also propagate by chance or through mixing between populations.”（另一个复杂因素是，持续的遗传变异并不完全是自然选择的结果。基因突变也可能通过偶然或种群间的混合来传播。）分析： 原文指出，除了样本小之外，从古代DNA中提取进化动态很困难的另一个原因是：基因变异不全是自然选择导致的，也可能是因为“偶然（by chance）”或“混合（mixing）”等非适应性因素。选项A“基因变异也可能是由随机的、非适应性的机制驱动的”精准概括了这一复杂因素。干扰项：[B] 保存不佳的片段总是得出误导性结论，“总是（always）”过于绝对；[C] 古代人类种群彼此完全隔离，原文明确提到了“种群间的混合（mixing between populations）”；[D] 持续的基因变异完全依赖于罕见的有益突变，与原文“并不完全是自然选择的结果（does not emerge exclusively...）”相悖。

37.【正确答案】B【解析】题型：例证推断题定位： 第五段第三、四句“The new paper, however, identifies 479 gene variants as highly likely to have emerged as a result of natural selection... That suggests directional selection was pervasive across West Eurasia in recent millennia.”（然而，新论文确定了在同一时期极有可能是自然选择结果的479个基因变异。这表明定向选择在近几千年的西欧亚大陆是普遍存在的。）分析： 作者列举“发现了479个基因变异（对比以前只发现了21个）”这一数据，紧接着用“That suggests...”引出了该数据的结论：定向选择在最近几千年是“普遍存在的（pervasive）”。选项B“定向选择在近代史中是一种广泛的进化力量”是对原文结论的同义替换（widespread 对应 pervasive）。干扰项：[A] 古代DNA数据的全球库被严重误解，数据翻倍是第四段的内容，并不是列举479个变异的目的；[C] 人类适应主要发生在西欧亚大陆之外，原文探讨的正是西欧亚大陆（across West Eurasia），而非其外部；[D] 人类的进化率保持着完美的恒定，第九段明确指出“定向选择并不以恒定的速度运作（does not operate at a constant rate）”。

38.【正确答案】C【解析】题型：事实细节题定位： 第六段最后两句“...was almost absent until around 4,000 years ago. Its emergence may not... have been driven by the spread of agriculture, which happened long before.”（……直到大约4000年前几乎是不存在的。它的出现可能并不像以前认为的那样，是由农业的传播驱动的，农业的传播发生在这之前很久。）分析： 原文指出，这种与自身免疫性疾病（immune disorder）相关的基因特征，在4000年前几乎不存在，说明它是相对较晚才出现的。选项C“某些免疫疾病变异在人类历史上出现得相对较晚”完全契合原文意思（4000年前在18000年的跨度中相对较晚，且原文强调之前 almost absent）。干扰项：[A] 农业的传播直接引发了自身免疫性疾病的抬头，与原文“may not... have been driven by the spread of agriculture”事实相反；[B] 对霍乱的基因抵抗力在西欧亚大陆高度流行，第七段明确表示研究“削弱了这种联系的理由（weakening the case for such a connection）”；[D] 协同的基因转变故意增加了遗传性肺病的风险，无中生有。

39.【正确答案】D【解析】题型：句意推断题定位： 第八段第二句“The researchers found that genes associated with type 2 diabetes were selected against, for example, as were those linked to psychiatric conditions...”分析： 考查核心生物学短语“selected against”。在进化生物学和自然选择的语境中，被“selected against（逆向选择/淘汰）”意味着这些基因在进化过程中处于劣势，从而被自然选择逐渐清除或抑制。选项D“它们在进化上处于劣势并逐渐被抑制”完美解释了该短语的学术内涵。干扰项：[A] 它们被自然选择积极偏爱和保留，这与“selected against”的意思完全相反（那是 selected for）；[B] 它们在孤立的现代种群中引发了协同转变，原文未提“孤立种群”；[C] 它们随着早期农业的传播而迅速发展，这是第九段讨论进化速度加快时的背景，而非这些特定疾病基因的特征。

40.【正确答案】B【解析】题型：主旨大意题定位： 全文逻辑结构及第一段核心论点。分析： 文章第一段开宗明义：过去认为强定向选择（自然选择的一种）在人类进化中很罕见（a rare occurrence），但新研究表明它扮演了“具影响力的多得多的角色（a much more influential role）”。后续所有段落（统计工具的开发、479个变异的发现、对具体疾病基因旧假设的推翻）都在用证据支撑这一核心观点：自然选择在近代人类进化中非常普遍且强大。选项B“自然选择在人类进化中被低估的作用”最精准地概括了全文主旨。干扰项：[A] 人类基因突变的不可预测性，虽然提到了随机突变，但这只是第二段的背景介绍，不是全文核心；[C] 追踪现代自身免疫性疾病的隐藏起源，这只是第六段的一个具体案例，以偏概全；[D] 新统计工具如何解码古代DNA片段，统计工具只是第三段提到的研究手段，并非文章探讨的最终科学主题。

【词汇注释】

【参考译文】

人类从未对自然选择的压力免疫。在整个人类物种的历史中，出现的各种环境赋予了带有某些有益突变的个体超越同伴的优势，使得他们宝贵的基因变异得以在种群中传播。尽管如此，这种变异的出现和快速传播——被称为强定向选择——曾被认为是人类进化中的罕见现象。然而，发表在《自然》杂志上的一项新研究表明，它发挥了比以前认为的影响大得多的作用。

历史上，进化生物学的动态很难从古代DNA中提取出来。小样本量是部分原因，仅凭少数（通常保存不佳的）片段很难得出关于人类适应性的普遍结论。另一个复杂因素是，持续的基因变异并不完全是自然选择的结果。基因突变也可能通过偶然或种群间的混合来传播。在事发几千年后还要对基因进行“去粗取精”，是一项艰巨的任务。

为了克服这些局限性，哈佛大学的两位遗传学家阿里·阿克巴里和大卫·赖希开发了一种新的统计工具包，用于处理古代DNA样本集。研究人员希望它能在样本集的样本对之间进行足够详细的比较，从而区分出定向选择的特征与非适应性突变的特征。

然后，研究人员将他们的工具应用于15836名具有西欧亚血统的古代个体以及6438名现代人类的基因数据。该数据集跨越了18000年，包括了10016个古代基因组的新数据，这意味着全球古代DNA数据池翻了一番，且样本量比以前寻找自然选择的研究增加了14倍。

分析结果甚至让作者也感到惊讶。到目前为止，在过去的11000年里，人类只发现了21次定向选择的事件。然而，这篇新论文确定了在同一时期内极有可能是自然选择结果的479个基因变异。这表明定向选择在近几千年的西欧亚大陆是普遍存在的。“这是一项分析上的杰作，”未参与该研究的遗传学家蓬图斯·斯科格伦德说。

当研究人员将注意力集中在单个基因中产生的变异时，出现了一些最有趣的发现。一种帮助身体识别病原体的基因特征，也与对某种自身免疫性消化系统疾病的易感性增加有关。他们的分析表明，这种在今天大约五分之一的样本中都能找到的特征，直到大约4000年前几乎是不存在的。它的出现可能并不像以前认为的那样，是由农业的传播驱动的，农业的传播发生在这之前很久。

他们提出可能错误的另一个假设，涉及与患严重遗传性肺病风险较高相关的基因，一些人推测这些基因的出现是因为它赋予了携带者对霍乱的抵抗力。研究人员在霍乱被认为在西欧亚大陆流行期间，没有发现针对这些基因进行自然选择的迹象，这削弱了这种联系的理由。

整个基因群的协同转变也会影响更复杂的性状。研究人员发现，例如与2型糖尿病相关的基因在进化中被淘汰（逆向选择）了，那些与精神分裂症和躁郁症等精神疾病相关的基因也是如此。

强定向选择尽管可能很普遍，但并不以恒定的速度运作。研究人员发现，在大约11700年前开始的全新世期间，由于农业兴起后生活方式的改变，西欧亚人的此类选择实例似乎急剧加强。

附注：

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