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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.
Paragraph 4 implies that human sexuality is the result of______.
选项 A、genetic mutation
B、gradual evolution
C、germ protection
D、bacterial transformation
答案B
解析 段落主旨题型，答案是B。本题考查人类性的来源，需准确把握段落的论证逻辑。第四段开头前两句提及了两种截然相反的观点，第一种认为人类性的出现可归于偶然事件，第二种认为人类性的出现是为了存续逐渐进化而来。从from this vantage point一词可知，第一种观点是对上段内容的总结，从however一词可见本段论证重心发生转变，之后整段的论述都围绕后一种观点展开。可见在A(基因突变)、B(逐渐进化)、C(胚芽保护)、D(细菌转变)四个选项中，B是正确答案。本题核心：重点关注段落开头以有效把握主旨。
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