榆林托福阅读tpo.docx

智课网 TOEFL 备考资料榆林托福阅读 tpo 分享到:今天和智课教育托福小编学习一篇榆林托福阅读 tpo 的文章。托福 TPO 阅读文本 Speciation in Geographically Isolated Populations Evolutionary biologists believe that speciation, the formation ofa new species, often begins when some kind of physical barrier arises and divides a population ofa 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, ic differences between the groups begin to accumulate. Eventually the subpopulations e so ically 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 mon. 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 thro