智课网TOEFL备考资料大同托福阅读tpo文本解析分享到:今天和智课教育托福小编学习一篇大同托福阅读tpo文本解析的文章。托福TPO阅读文本Speciation in Geographically Isolated PopulationsEvolutionary biologists believe that speciation, the formation of a new species, often begins when some kind of physical barrier arises and divides a population of a single species into separate subpopulations. Physical separation between subpopulations promotes the formation of new species because once the members of one subpopulation can no longer mate with members of another subpopulation, they cannot exchange variant genes that arise in one of the subpopulations. In the absence of gene flow between the subpopulations, genetic differences between the groups begin to accumulate. Eventually the subpopulations become so genetically distinct that they cannot interbreed even if the physical barriers between them were removed. At this point the subpopulations have evolved into distinct species. This route to speciation is known as allopatry (“allo-” means “different”, and “patria” means “homeland”).Allopatric speciation may be the main speciation route. This should not be surprising, since allopatry is pretty common. In general, the subpopulations of most species are separated from each other by some measurable distance. So even under normal situations the gene flow among the subpopulations is more of an intermittent trickle than a steady stream. In addition, barriers can rapidly arise and shut off the trickle. For example, in the 1800s a monstrous earthquake changed the course of the Mississippi River, a large river flowing in the central part of the United States of America. The change separated populations of insects now living along opposite shore, completely cutting off gene flow between them.Geographic isolation also can proceed slowly, over great spans of time. We find evidence of such extended events in the fossil record, which affords glimpses into the breakup of formerly continuous environments. For example, during past ice ages, glaciers advanced down through North America and Europe and gradually cut off parts of populations from one another. When the glaciers retreated, the separated populations of plants and animals came into contact again. Some groups that had descended from the same parent population were no longer reproductively compatible they had evolved into separate species. In other groups, however, genetic divergences had not proceeded so far, and the descendants could still interbreed for them, reproductive isolation was not completed, and so speciation had not occurred.It prevents most groundwater from circulating.A.describes.托福tpo阅读文本Westward MigrationThe story of the westward movement of population in the United States is, in the main, the story of the expansion of American agriculture-of the development of new areas for the raising of livestock and the cultivation of wheat, corn, tobacco, and cotton. After 1815 improved transportation enabled more and more western farmers tescape a self-sufficient way of life and enter a national market economy. During periods when commodity prices were high, the rate of westward migration increased spectacularly. “Old America seemed tbe breaking up and moving westward,“ observed an English visitor in 1817,during the first great wave of migration. Emigration tthe West reached a peak in the 1830s. Whereas in 1810 only a seventh of the American people lived west of the Appalachian Mountains, by 1840 more than a third lived there.Why were these hundreds of thousands of settlers-most of them farmers, some of them artisans-drawn away from the cleared fields and established cities and villages of the East? Certain characteristics of American society help texplain this remarkable migration. The European ancestors of some Americans had for centuries lived rooted tthe same village or piece of land until some religious, political, or economic crisis uprooted them and drove them across the Atlantic. Many of those whexperienced this sharp break thereafter lacked the ties that had bound them and their ancestors ta single place. Moreover, European society was relatively stratified; occupation and social status were inherited. In American society, however, the class structure was less rigid; some people changed occupations easily and believed it was their duty timprove their social and economic position. As a result, many Americans were an inveterately restless, rootless, and ambitious people. Therefore, these social traits helped tproduce the nomadic and daring settlers whkept pushing westward beyond the fringes of settlement. In addition, there were other immigrants whmigrated west in search of new homes, material success, and better lives.The West had plenty of attractions: the alluvial river bottoms, the fecund soils of the rolling forest lands, the black loams of the prairies were tempting tNew England farmers working their rocky, sterile land and tsoutheastern farmers plagued with soil depletion and erosion. In 1820 under a new land law,