1、TPO-42Geographic Isolation of SpeciesBiologist Ernst Mayr defined a species as “an actually or potentially interbreeding population that does not interbreed with other such populations when there is opportunity to do so.” A key event in the origin of many species is the separation of a population wi
2、th its gene pool (all of the genes in a population at any one time) from other populations of the same species, thereby preventing population interbreeding. With its gene pool isolated, a separate population can follow its own evolutionary course. In the formation of many species, the initial isolat
3、ion of a population seems to have been a geographic barrier. This mode of evolving new species is called allopatric speciation.Many factors can isolate a population geographically. A mountain range may emerge and gradually split a population of organisms that can inhabit only lowland lakes, certain
4、fish populations might become isolated in this way. Similarly, a creeping glacier may gradually divide a population, or a land bridge such as the Isthmus of Panama may form and separate the marine life in the ocean waters on either side.Too many words you dont know? Look them up in新托福TPO阅读词汇速查速记!Wec
5、hat: geeqi0805How formidable must a geographic barrier be to keep populations apart? It depends on the ability of the organisms to move across barriers. Birds and coyotes can easily cross mountains and rivers. The passage of wind-blown tree pollen is also not hindered by such barriers, and the seeds
6、 of many plants may be carried back and forth on animals In contrast, small rodents may find a deep canyon or a wide river an effective barrier. For example, the Grand Canyon, in the southwestern United States, separate the range of the while-tailed antelope squirrel from that of the closely related
7、 Harris antelope squirrel. Smaller, with a shorter tail that is white underneath, the white-tailed antelope squirrel inhabits deserts north of the canyon and west of the Colorado River in southern California Hams antelope squirrel has a more limited range in deserts south of the Grand Canyon.Geograp
8、hic isolation creates opportunities for new species to develop, but it does not necessarily lead to new species because speciation occurs only when the gene pool undergoes enough changes to establish reproductive barriers between the isolated population and its parent population. The likelihood of a
9、llopatric speciation increases when a population is small as well as isolated, making it more likely than a large population to have its gene pool changed substantially. For example, in less than two million years, small populations of stray animals and plants from the South American mainland that m
10、anaged to colonize the Galapagos Islands gave rise to all the species that now inhabit the islands.When oceanic islands are far enough apart to permit populations to evolve in isolation, but close enough to allow occasional dispersions to occur, they are effectively outdoor laboratories of evolution
11、. The Galapagos island chain is one of the worlds greatest showcases of evolution. Each island was born from underwater volcanoes and was gradually covered by organisms derived from strays that rode the ocean currents and winds from other islands and continents. Organisms can also be carried to isla
12、nds by other organisms, such as sea birds that travel long distances with seeds clinging to their feathers.The species on the Galapagos Islands today, most of which occur nowhere else, descended from organisms that floated, flew, or were blown over the sea from the South American mainland. For insta
13、nce, the Galapagos island chain has a total of thirteen species of closely related birds called Galapagos finches. These birds have many similarities but differ in their feeding habits and their beak type, which is correlated with what they eat. Accumulated evidence indicates that all thirteen finch
14、 species evolved from a single small population of ancestral birds that colonized one of the islands. Completely isolated on the island after migrating from the mainland, the founder population may have undergone significant changes in its gene pool and become a new species. Later, a few individuals
15、 of this new species may have been blown by storms to a neighboring island. Isolated on this second island, the second founder population could have evolved into a second new species, which could later recolonize the island from which its founding population emigrated. Today each Galapagos island ha
16、s multiple species of finches, with as many as ten on some islands. 1. The word “key” in the passage is closest in meaning toA. earlyB. crucialC. noticeableD. frequent2. The word “initial” in the passage is closest in meaning toA. bestB. usualC. firstD. actualYou enjoy the convenience of having all
17、vocabulary questions listed as a separate part in新托福TPO阅读词汇速查速记.3. According to paragraph 1, allopatric speciation is possible whenA. a population contains all the different genes present in a species at a particular timeB. a population becomes isolated due to the presence of a geographic barrierC.
18、genetic mixing begins to occur in previously separate populations of a speciesD. a species is successful in crossing a geographic barrier4. How is paragraph 2 related to paragraph 1?A. Paragraph 2 points out a number of ways in which the phenomenon of geographic isolation mentioned in paragraph 1 ca
19、n occurB. Paragraph 2 identifies discoveries that led to the conclusion presented in paragraph 1 that geographic isolation has played a role in the origin of many speciesC. Paragraph 2 provides evidence supporting the statement in paragraph 1 that a population can follow its own evolutionary course
20、once its gene pool becomes isolatedD. Paragraph 2 explains why the term “allopatric” was adopted to describe the method of speciation described in paragraph 15. In paragraph 3, the author contrasts a variety of organisms to illustrate which of the following points?A. Geographic barriers are less lik
21、ely to keep apart populations of plants than populations of animalsB. Geographic barriers are more likely to keep apart populations of large organisms than populations of small organismsC. Some members of a species are able to cross geographic barriers, while other members of the same species are no
22、tD. The effectiveness of geographic barriers in keeping organisms apart depends on an organisms ability to move across barriers6. Paragraph 3 supports the idea that white-tailed antelope squirrels and Hams antelope squirrels have which of the following in common?A. They are the two smallest rodents
23、now found in the southwestern United States.B. They have white coloring underneath their tailsC. They cannot cross the Grand CanyonD. They cannot survive in desert conditions7. The word “undergoes in the passage is closest in meaning toA. experiencesB. allowsC. preventsD. causes8. Which of the sente
24、nces below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential informationA. Geographic isolation is sometimes but not always the reason for the creation of reproductive barriers between a
25、parent population and the populations descended from itB. Genetic changes in a geographical isolated population do not necessarily make the population look different enough from its parent population to be considered a new speciesC. Geographical isolation allows the separated populations to evolve i
26、ndependently of each other and so can lead to the formation of new speciesD. Geographic isolation can lead to new species only if the gene pool of the isolated population changes enough to prevent it from reproducing with the parent population9. According to paragraph 4, why does the size of a popul
27、ation affect the likelihood of allopatric speciation?A. Because smaller populations are more likely than larger ones to become geographically isolatedB. Because the gene pool of a small isolated population is more likely to undergo substantial change than is the gene pool of a larger populationC. Be
28、cause an isolated population can become a new species with substantially less change to its gene pool than would be required by a larger populationD. Because smaller populations are more likely to be made up of stray animals or plants than larger populations are10. The word “managed” in the passage
29、is closest in meaning toA. were ableB. were forcedC. arrivedD. expanded11. Paragraph 5 supports the idea that the Galapagos island chain was able to become “one of the worlds greatest showcases of evolution” primarily because ofA. the richness of the volcanic soil of each of the islands in the chain
30、B. the distance of the individual islands from each other and from the mainlandC. the relativity long time it took for the islands to become covered by organismsD. the outdoor laboratories that scientists have built on the islands to study evolution12. According to paragraph 6, what is true about th
31、e thirteen species of Galapagos finches?A. All thirteen species are now found on most of the Galapagos IslandsB. All thirteen species are descended from the same population of ancestral birdsC. All thirteen species evolved on the island that was originally colonized by finches from the mainland.D. A
32、ll thirteen species occur only in small, completely isolated populations.13. Look at the four squares that indicate where the following sentence could be added to the passageThis process of speciation and colonization could have been repeated over and over again, gradually involving all the islands in the chain.Where would the sentence best fit? Click on a square to add the sentence to the passage.14. Directions: An introductory sentence for a brief summary of the passage is provided bel
