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.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.
No serious competition exists when the major needed resource is in superabundant supply,as in most cases of the coexistence of herbivores(plant eaters).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.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 as seed-eating rodents and ants.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.
To what extent competition determines the composition of a community and the density of particular species has been the source of considerable controversy.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.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.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.
Competition among species is of considerable evolutionary importance.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.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.
Competition may occur for any needed resource.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.Indeed,it may be for any of the factors,physical as well as biotic,that are essential for organisms.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.
1.The phrase"mechanisms of natural selection"in the passage(paragraph 1)is closest inmeaning to
A.types of natural selection.
B.dangers of natural selection.
C.problems natural selection solves.
D.ways natural selection works.
2.According to paragraph 1,what is one effect of competition among individuals ofdifferent species?
A.It results in the eventual elimination of the resource for which they are competing.
B.It leads to competition among individuals of the same species.
C.It encourages new species to immigrate to an area.
D.It controls the number of individuals in the competing populations.
2.通过competition among individuals of differentspecies定位到红色标注句子，但这句并没提到任何一个选项，继续往后看，下一句的主语是代词it，指代intraspecies competition，因此后面也是对物种间竞争的描述。而这一句提到了是控制population的因素，对应了选项D。
3.The word"indigenous"in the passage(paragraph 1)is closest in meaning to
4.In paragraph 1,why does the author mention what happened in New Zealand?
A.To indicate that Darwin understood the importance of competition.
B.To illustrate that competition can lead to the extinction of species.
C.To identify where the idea of competition among species first arose.
D.To argue against the idea that the process of selection is a natural occurrence.
5.According to paragraph 2,competition is not usually a significant factor among twocoexisting species when
A.one of the species has only recently moved into the territory of the other.
B.the species are closely related to each other.
C.the population of one species is much larger than that of the other.
D.both of the species are herbivores.
5.根据coexisting species定位到原文处，前文就是原因，因为资源供给充足。而coexistence ofherbivores是给出的例证，coexistence herbivores间没有激烈竞争是因为它们都吃草，而草资源是非常充足的。理想答案是当资源充足时，竞争不会成为重要的影响因素。但原文没有，唯一合适的选择就是D，也就是原文提到的例证。
6.The word"graphically"in the passage(paragraph 2)is closest in meaning to
7.In paragraph 2,why does the author talk about what happened as a result of Northand South America becoming joined at the Isthmus of Panama?
A.To make the point that predation can have as much effect on species survival ascompetition does.
B.To show how the ability to switch to an alternative resource can give a species acompetitive advantage.
C.To account for the current species composition of North and South America.
D.To provide an example of the serious effects of competition between unrelated species.
7.引出南北美洲物种竞争结果的主句是"The effects of such competition are graphicallydemonstrated…"由此推断，南北美洲动物竞争的结果就是为了说明竞争的Effects。因此选择D。A原文提到了，在最后的半句，但是前面的although说明了这不是主要观点。
8.Paragraph 3 supports the idea that Gause's experiments were important becausethey
A.provided a situation in which competition could be removed from the interaction betweentwo species.
B.showed that previous ideas about the extent to which competition determines thecomposition of a community were completely mistaken.
C.helped establish that competition will remove all but one species from any givenecological niche.
D.offered evidence that competition between species is minimal when there is anoverabundance of a single food source.
8.通过Gause's experiments定位，文章在描述完实验内容后，说on the basis of these experiment，那么说明后面就是实验得到的结论。也就是law of competitive exclusion.因此选择C。
9.Which of the sentences below best expresses the essential information inthehighlighted sentence in the passage(paragraph 3)?Incorrect choices change themeaningin important ways or leave out essential information.
A.Apparent exceptions to this law usually involves cases in which two species compete for thesame major resource but occupy slightly different niches.
B.Although it may appear that two species always have different niches,many exceptions showthat species compete with each other.
C.Cases in which two species not only compete for a shared resource but also occupy similarniches are considered exceptions to this law.
D.Cases in which the two species do not occupy that same niche yet still compete for thesame resource are believed to be exceptions to this law.
9.原文中句子的主干是seeming exceptions have been found，然后存在了转折逻辑之后说这种例外是可以被解释的。因此只有A符合。B，C，D都在一定程度上改变了原句的意思。
10.According to paragraph 4,how does competition affect evolution?
A.It results in the evolution of physical structures that allow the species to compete witheach other more effectively.
B.It results I the evolutionary extinction of all but one of the competing species.
C.It results in the competing species evolving in such a way that they no longer compete forthe same resources.
D.It results in the competing species evolving to become so much like each other thatcompetition between them eventually disappears.
10.第一句就提到了问题所问的内容，说competition具有evolutionary importance。后面应该就是具体的解释。往后看说同一小生态环境中的竞争物种tend to….那么tend to的内容就是答案。对应的就是C，说使得其领地不再重叠，也就是不再同一区域内进行资源竞争了。
11.According to paragraph 4,"species selection"is a misleading term because it
A.overemphasizes the role of selection pressure in species extinction.
B.suggests that selection pressures directly influence whole species.
C.does not make a distinction between species extinction and species evolution.
D.suggests that extinction always results whenever there is a competition.
11.通过species selection定位到标注处，后面出现了转折，说这个描写是misleading的。往后肯定就会解释原因。原文说natural selection其实是直接作用在individual上的，而不是species上。所以错误。对应了B选项，"species selection暗示了选择压力是直接作用于整个物种上的。"
12.The word"regulating"in the passage(paragraph 5)is closest in meaning to
13.Look at the four squares[■]that indicate where the following sentence could beadded to the passage.Where would the sentence best fit?That is,as the density of apopulation increases,competition has a greater impact and leads to greater mortality.
Competition may occur for any needed resource.■【A】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-waterbottom-dwelling marine organisms.■【B】Indeed,it may be forany of the factors,physical as well as biotic,that are essential for organisms.■【C】Competition isusually the more severe the denser the population.■【D】Togetherwith predation,itis the most important density-dependent factor in regulating populationgrowth.
14.Directions:An introductory sentence for a brief summary of the passage isprovided below.Complete the summary by selecting the THREE answer choices thatexpress the most important ideas in the passage.Some sentences do not belong inthe summary because they express ideas that are not presented in the passage or areminor ideas in the passage.This question is worth 2 points.
When necessary resources are limited,competition can occur among individuals of the samespecies or of different species.
A.Competition can eliminate a species,but since most species do not depend on a singleresource,competition is often reduced by switching to alternative resources.
B.Investigation of the ecological role of competition is difficult because ordinarily thecompetition cannot be observed directly and must be inferred from its presumed effects.
C.Competition between a pair of species tends to lessen over time because the species tendto evolve to occupy different ecological niches and ranges.
D.Competition between individual of the same species is usually for food whereas competitionbetween species is usually for habitat.
E.Experiments and field observation have established that competition between species isstrong enough to prevent two species from occupying the same ecological niche.
F.Competition is usually strongest when the density of the competing populations is the same.
14.A正确，对应原文第二段第二，三句。B原文并没有这样的意思，一直在说是在竞争resource，不选。C后半句在原文中提到了，但s是并没有与前半句的这样的因果关系，不选。D正确，对应原文第3段的on thebasis of那一句。E正确，对应原文第四段的第二句。
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