2. 外语
  3. Sexual Reproduction Birds do it. Bees do it. But dandelions don’t. The prodigious spread of these winsome weeds underscores a

Sexual Reproduction Birds do it. Bees do it. But dandelions don’t. The prodigious spread of these winsome weeds underscores a

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问题                            Sexual Reproduction
   Birds do it. Bees do it. But dandelions don’t. The prodigious spread of these winsome weeds underscores a little-appreciated biological fact. Contrary to human experience, sex is not essential to reproduction. Asexual organisms can often churn out multiple generations of clones, gaining a distinct edge in the evolutionary numbers game. And therein lies the puzzle: If sex is such an inefficient way to reproduce, why is it so widespread?
   Sex almost certainly originated nearly 3. 5 billion years ago as a mechanism for repairing the DNA of bacteria. Because ancient earth was such a violent place, the genes of these unicellular organisms would have been frequently damaged by intense heat and ultraviolet radiation. " Conjugation" — the intricate process in which one bacterium infuses genetic material into another — provided an ingenious, if cumbersome, solution to this problem, although bacteria continued to rely on asexual reproduction to increase their numbers.
   Animal sex, however, is a more recent invention. Biologist Lynn Margulis of the University of Massachusetts at Amherst believes the evolutionary roots of egg and sperm cells can be traced back to a group of organisms known as rotests that first appeared some 1. 5 billion years ago.(Modern examples include protozoa, giant kelp and malaria parasite. ) During periods of starvation, Margulis conjectures, one rotest was driven to devour another. Sometimes this cannibalistic meal was incompletely digested, and the nuclei of prey and predator fused. By joining forces, the fused cells were better able to survive adversity, and because they survived, their penchant for union was passed on to their distant descendants.
   From this vantage point, human sexuality seems little more than a wondrous accident, born of a kind of original sin among protozoa. Most population biologists, however, believe sex was maintained over evolutionary time because it somehow enhanced survival. The mixing and matching of parental genes, they argue, provide organisms with a novel mechanism for generating genetically different offspring, thereby increasing the odds that their progeny could exploit new niches in a changing environment and, by virtue of their diversity, have a better chance of surviving the assaults of bacteria and other tiny germs that rapidly evolve tricks for eluding their hosts’ defenses.
The purpose of the passage is to______.
选项 A、justify the necessity of sexual reproduction
B、explain the origin of sexual reproduction
C、trace the history of sexual reproduction
D、demonstrate the differences between sexual and asexual reproduction
答案A
解析 篇章主旨题型，答案是A。本题考查全文主旨。文章除了开头，主干部分基本未提及无性繁殖，可见无性繁殖只是引子，不是作者的论证重心，由此排除D选项“阐释有性繁衍和无性繁衍之间的差异”。其余三个选项总结的内容原文均有涉及，需细读原文考查作者的论证逻辑。从第一段最后一句“If sex is such an inefficient way to reproduce，why is it so widespread?”(如果有性繁殖是低效的，为何会如此普遍?)可初步推定，作者的论证重点是有性繁殖存在的必要性。再考查随后三段论证，尽管三段文字依次谈到了生物、动物以及人类的有性繁殖，并在段落开头提及了三者的起源，但在具体论证过程中，突出的是有性繁殖发生的环境、原因以及生物进化带来的结果，可见“必要性”才是作者的论述主旨，应选A。本题核心：应同时关注两方面内容，一是文章的开头和结尾，二是具体段落的论证重点，以得出正确结论。
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