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  3. Sexual Reproduction Birds do it. Bees do it. But d

Sexual Reproduction Birds do it. Bees do it. But d

游客2023-12-26  33
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. [br] According to biologist Lynn Margulis, ______.
选项 A、rotests have both egg and sperm cells
B、rotests devoured each other because of starvation
C、rotests devoured each other for the survival of its species
D、protozoans, like their distant ancestors, devour each other
答案 B
解析 观点主旨题型，答案是B。本题考查生物学家Lynn Margulis的具体观点。原文给出的信息大意如下：精子和卵子产生的源头可追溯到rotests。该类生物因为饥饿而相互吞噬，由于吞噬不完全，吞噬者和被吞噬者的细胞核发生了聚合。两者的合力使得聚合后的细胞在恶劣环境中更容易存活，而这种相互结合的倾向也随着其存活被其继承下来。逐一对比选择项可见：A选项与原文不符，尽管原文出现了roots一词，但并非指rotests本身具有精子和卵子细胞，而是指这种生物在相互吞噬过程中产生了细胞核聚合，导致后来同类生物之间精子和卵子细胞存在结合的倾向。C选项与原文不符，根据原文逻辑，“物种存续”是“互相吞噬”的结果而非原因。D选项与原文不符，尽管根据modern examples一词可判定protozoa和rotests之间存在共性，但原文protozoa一词出现在作者探讨rotests的吞噬现象之前，可见其共性并非吞噬本能，而只是从物种分类上可大致划归一类。本题核心：完整理解段落逻辑，而非孤立依据单独字句断章取义。
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