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  3. Natural selection describes the biological process in which the differences of individuals within a population influences their

Natural selection describes the biological process in which the differences of individuals within a population influences their

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问题     Natural selection describes the biological process in which the differences of individuals within a population influences their abilities to survive and reproduce in an environment. The differences in individuals is a result of their genetic inheritance from their parents. In a population, any characteristic which blocks reproduction success tends to decrease generation by generation. In time, the ill-adapted die out. On the other hand,the individuals who do survive and reproduce will tend to produce offspring which are better adapted to the environment. Natural selection tends to promote adaptations that will increase the organism’s ability to survive in an environment.
    Natural selection can serve to stabilize a population if the new traits, called mutations, are eliminated when they appear because they are not as well-adapted to the environment. The opposite effect is obtained when a new trait is introduced which allows individuals to adapt better over time, the species will change as this mutation becomes more widespread in the population. In human beings, increased brain size helped individuals to adapt better, and so brain size increased gradually in the species. Changes in the overall genetic makeup of a population are normal when there are environmental changes, especially sever environmental disruptions. Specialized adaptations to specific environments can lead, over time, to the development of subpopulation of individuals, ones who are better adapted to particular soil conditions food sources and so on. Given enough time, these subpopulations may develop into separate species, such as zebras and horses, living in distinct environments and not interbreeding.  
The topic of this passage would be______.
选项 A、why natural selection leads to population increase over generations
B、how natural selection maintains and changes the makeup of a population
C、why natural selection created the large brain size in human beings
D、how natural selection promotes individuals who care for their offspring
答案B
解析 题目询问文章的主题。从第一段可知自然淘汰是“减少无法适应环境的个体”，第二段中则描述自然，淘汰是“去除与环境无法融合的特征，吸取可使个体安定的特征”，另外还提及：“为了能充分适应环境进而演化出各种种类”，因为文章的主旨是B。“自然淘汰如何改变和维持一个种群的构造。”
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