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  3. Despite the basic biological, chemical, and physical similarities found in all living things, a diversity of life exists not onl

Despite the basic biological, chemical, and physical similarities found in all living things, a diversity of life exists not onl

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问题     Despite the basic biological, chemical, and physical similarities found in all living things, a diversity of life exists not only among and between species but also within every natural population. The phenomenon of diversity has had a long history of study because so many of the variations that exist in nature are visible to the eye.
    【F1】It has been suggested that sexual reproduction became the dominant type of reproduction among organisms because of its inherent advantage of variability, which is the mechanism that enables a species to adjust to changing conditions. New variations are potentially present in genetic differences, but how preponderant a variation becomes in a gene pool depends upon the number of offspring the mutants or variants produce(differential reproduction).【F2】It is possible for a genetic novelty(new variation)to spread in time to all members of a population, especially if the novelty enhances the population’s chances for survival in the environment in which it exists. Thus, when a species is introduced into a new habitat, it either adapts to the change by natural selection or by some other evolutionary mechanism or else it eventually dies off. Because each new habitat means new adaptations, habitat changes have been responsible for the millions of different kinds of species and for the heterogeneity within each species.
    The total number of animal and plant species is estimated at between 2,000,000 and 4,500,000; authoritative estimates of the number of extinct species range from 15,000,000 up to 16,000,000,000.【F3】Although the use of classification as a means of producing some kind of order out of this staggering number of different types of organisms appears as early as the book of Genesis—with references to cattle, beasts, fowl, creeping things, trees, etc. —the first scientific attempt at classification is attributed to the Greek philosopher Aristotle, who tried to establish a system that would indicate the relationship of all things to each other. He arranged everything along a scale, or "ladder of nature", with nonliving things at the bottom; plants were placed below animals, and man was at the top.【F4】Other schemes that have been used for grouping species include large anatomical similarities, such as wings or fins, which indicate a natural relationship, and also similarities in reproductive structures.
    【F5】At the present time taxonomy is based on two major assumptions; one is that similar body construction can be used as a criterion for a classification grouping; the other is that, in addition to structural similarities, evolutionary and molecular relationships between organisms can be used as a means for determining classification.
【F3】
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答案虽然早在《物种起源》那本书中就使用分类方法把为数众多的不同种类的生物排列出了一个次序，书中提到的生物包括：家养牲畜、野兽、禽类、爬行动物、树木等。不过，真正从科学上首次尝试对物种分类还应归于希腊哲学家亚里士多德。他曾试图建立一个体系，来表明世界万物彼此之间的关系。
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