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  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.
According to Paragraph 1, asexual reproduction is advantageous for its______.
选项 A、large offspring numbers
B、creation of offspring without a mate
C、creation of genetically identical offspring
D、creation of genetically different offspring
答案A
解析 段落主旨题型，答案是A。本题考查点“无性繁衍的优点”与段落的主旨密切相关。本段为全文引段，旨在点明主题。作者采用的具体策略是反面对照法，对照的两方分别为有性和无性繁衍，意在通过彰显无性繁衍的优点来引发读者思考有性繁衍存在的必要性。段中涉及无性繁衍的叙述有两处，一是the prodigious spread of these winsome weeds，二是“Asexual organisms can often churn out multiple generations of clones，gaining a distinct edge in the evolutionary numbers game．”。从prodigious spread(疯狂的传播)、chum out multiple generations(大量制造子子孙孙)和numbers game(数字游戏)等描述词可见作者意在突出其繁衍数量多这一优点，故选A。本题核心：B、C选择项的内容(无须交配；与后代基因相同)同样属于无性繁殖的特征，尽管在原文中亦有所提及，但甄选正确答案有赖于把握段落的论证逻辑，找准表意重心，明确作者在此处提及无性繁殖的用意。
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