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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. 1) 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). 2)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. 3)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. 4)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. 5)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.
列车车门验票由列车员负责。
海螵蛸具有的功效是（ ）。
分析酒剂时用萃取法除杂的步骤是（）
三角肌位于肩部皮下，属（）状肌，其近侧端分别起自（）、（）和（）。
凡是商标均可以转让。
19世纪，对开辟人类生活新时代起决定作用的自然科学成就是（）
邻里关系要讲究礼让友好地和睦相处，因此我们要形成一种互敬、互信、互助、互让、互谅的新型邻里关系。
关于劳动法律体系的说法，错误的是（ ）。
《无地自容》是（）演唱的