When several individuals of the same species or of several different species depend on the same limited resource, a situation may arise that is referred to as competition. (给出竞赛的界说)The existence of competition has been long known to naturalists; its effects were described by Darwin in considerable detail. Competition among individuals of the same species (intraspecies competition), one of the major mechanisms of natural selection, is the concern of evolutionary biology. Competition among the individuals of different species (interspecies competition) is a major concern of ecology. It is one of the factors controlling the size of competing populations, and extreme cases it may lead to the extinction of one of the competing species. This was described by Darwin for indigenous New Zealand species of animals and plants, which died out when competing species from Europe were introduced.(种群内部竞赛和种群外部竞赛各自的界说)
No serious competition exists when the major needed resource is in superabundant supply, as in most cases of the coexistence of herbivores (plant eaters). (主题句：必要资源足够的情况下不存在剧烈竞赛)Furthermore, most species do not depend entirely on a single resource, if the major resource for a species becomes scarce, the species can usually shift to alternative resources. If more than one species is competing for a scarce resource, the competing species usually switch to different alternative resources. (Furthermore作为逻辑词提示考生能够先略过这以后边的句子内容，大约了解后边是关于资源竞赛的一个现实阐明就好，if引起对上一句的进一步阐明)Competition is usually most severe among close relatives with similar demands on the environment. But it may also occur among totally unrelated forms that compete for the same resource, such asseed-eating rodents and ants. (近亲之间的竞赛往往最剧烈，这一点也是主题之下的一个阐明)The effects of such competition are graphically demonstrated when all the animals or all the plants in an ecosystem come into competition, as happened 2 million years ago at the end of Pliocene, when North and South America became joined by the Isthmus of Panama. North and South American species migrating across the Isthmus now came into competition with each other. The result was the extermination of a large fraction of the South American mammals, which were apparently unable to withstand the competition from invading North American species—although added predation was also an important factor.(as引起比如，在扫读时能够略过先不看)
阶段剖析：阶段宗旨是必要资源足够的情况下不存在剧烈竞赛。然后阐明近亲之间因为环境资源需求类似存在剧烈竞赛、不相关但需求一致资源物种之间存在剧烈竞赛、一个生态系统中的一切动植物都参加到竞赛中来时竞赛剧烈。阶段中的并排逻辑词 furthermore；举例逻辑词 such as 、as；条件逻辑词If 等都能够协助考生串联内容。
To what extent competition determines the composition of a community and the density of particular species has been the source of considerable controversy. (主题句：竞赛在多大程度上决议特定群落组成和特定物种密度引起争议)The problem is that competition ordinarily cannot be observed directly but must be inferred from the spread or increase of one species and the concurrent reduction or disappearance of another species. (进一步解说争议的问题详细是什么)The Russian biologist G. F. Gause performed numerous two-species experiments in the laboratory, in which one of the species became extinct when only a single kind of resource was available. On the basis of these experiments and of field observations, the so-called law of competitive exclusion was formulated, according to which no two species can occupy the same niche. (俄罗斯生物学家的试验和定论：竞赛灭绝法则是建立的)Numerous seeming exceptions to this law have since been found, but they can usually be explained as cases in which the two species, even though competing for a major joint resource, did not really occupy exactly the same niche.(阐明试验定论例外情况的原因)
Competition among species is of considerable evolutionary importance. (物种间的竞赛机制对进化过程很重要)The physical structure of species competing for resources in the same ecological niche tends to gradually evolve in ways that allow them to occupy different niches. Competing species also tend to change their ranges so that their territories no longer overlap. (两种进化的方向：依靠的资源改变和生活区域)The evolutionary effect of competition on species has been referred to as “species selection”; however, this description is potentially misleading. Only the individuals of a species are subject to the pressures of natural selection. The effect on the well-being and existence of a species is just the result of the effects of selection on all the individuals of the species. Thus species selection is actually a result of individual selection.
阶段剖析：本段中的并排逻辑词also、转机逻辑词 however与定论逻辑词Thus给出了阶段的明晰结构：竞赛对进化的2种影响成果+“species selection”概念的评论(其间包含了一个thus引导的定论)
Competition may occur for any needed resource. (任何有必要的资源都可能引发竞赛)In the case of animals it is usually food; in the case of forest plants it may be light; in the case of substrate inhabitants it may be space, as in many shallow-water bottom-dwelling marine organisms. (几个举例)Indeed, it may be for any of the factors, physical as well as biotic, that are essential for organisms. (表示断定的逻辑词Indeed)Competition is usually the more severe the denser the population. Together with predation, it is the most important density-dependent factor in regulating population growth.