【正文】 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 are word “managed” in the passage is closest in meaning to able forced paragraph 5When oceanic islands are far enough apart to permit populations to evolve in isolation, butclose enough to allow occasional dispersions to occur, they are effectively outdoor laboratories of evolution. The Galapagos island chain is one of the world s 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 islands by other organisms, such as sea birds that travel long distances with seeds clinging to their feathers. 5 supports the idea that the Galapagos island chain was able to bee “one of the world39。2Biologist 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 with 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 isolation 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 fish populations might bee isolated in this way. Similarity, a creeping glacier may gradually dividea 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. 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 can occurB. Paragraph 2 identifies discoveries that led to the conclusion presented in paragraph 1 that geographic isolation has played a rote 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 once its gene pool bees isolatedD. Paragraph 2 explains why the term “allopatric” was adopted to describe the method of speciation described in paragraph 1paragraph 3How 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 windblown tree pollen is also not hindered by such barriers, and the seeds of many plants may be earned 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 Slates, separate the range of the whiletailed antelope squirrel from that of the closely related Harris’ antelope squirrel. Smaller, with a shorter tail that is white underneath, the whitetailed antelope squirrel inhabits deserts north of the canyon and west of the Colorado River in southern California Hams39。托福TPO42閱讀文本+題目+答案下載Geographic 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 with 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 isolation 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 mountain range may emerge and gradually split a population of organisms that can inhabit only lowland lakes, certain fish populations might bee isolated in this way. Similarity, 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.How formidable must a geographic barrier be to keep populations apart? It depends on the ability of the organisms to move across and coyotes can easily cross mountains and rivers. The passage of windblown tree pollen is also not hindered by such barriers, and the seeds of many plants may be earned 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 Slates, separate the range of the whiletailed antelope squirrel from that of the closely related Harris’ antelope squirrel. Smaller, with a shorter tail that is white underneath, the whitetailed antelope squirrel inhabits deserts north of the canyon and west of the Colorado River in southern California Hams39。 antelope squirrel has a more limited range in deserts south of the Grand Canyon.Geographic 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 likelihood of allopatric 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 l