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1、智课网TOEFL备考资料商洛托福阅读tpo文本分享到:今天和智课教育托福小编学习一篇商洛托福阅读tpo文本的文章。托福TPO阅读文本SpeciationinGeographicallyIsolatedPopulationsEvolutionarybiologistsbelievethatspeciation,theformationofanewspecies,oftenbeginswhensomekindofphysicalbarrierarisesanddividesapopulationofasinglespeciesintoseparatesubpop
2、ulations.Physicalseparationbetweensubpopulationspromotestheformationofnewspeciesbecauseoncethemembersofonesubpopulationcannolongermatewithmembersofanothersubpopulation,theycannotexchangevariantgenesthatariseinoneofthesubpopulations.Intheabsenceofgeneflowbetweenthesubpopulations,gen
3、eticdifferencesbetweenthegroupsbegintoaccumulate.Eventuallythesubpopulationsbecomesogeneticallydistinctthattheycannotinterbreedevenifthephysicalbarriersbetweenthemwereremoved.Atthispointthesubpopulationshaveevolvedintodistinctspecies.Thisroutetospeciationisknownasallopatry(“allo-”m
4、eans“different”,and“patria”means“homeland”).Allopatricspeciationmaybethemainspeciationroute.Thisshouldnotbesurprising,sinceallopatryisprettycommon.Ingeneral,thesubpopulationsofmostspeciesareseparatedfromeachotherbysomemeasurabledistance.Soevenundernormalsituationsthegeneflowamongth
5、esubpopulationsismoreofanintermittenttricklethanasteadystream.Inaddition,barrierscanrapidlyariseandshutoffthetrickle.Forexample,inthe1800samonstrousearthquakechangedthecourseoftheMississippiRiver,alargeriverflowinginthecentralpartoftheUnitedStatesofAmerica.Thechangeseparatedpopulat
6、ionsofinsectsnowlivingalongoppositeshore,completelycuttingoffgeneflowbetweenthem.Geographicisolationalsocanproceedslowly,overgreatspansoftime.Wefindevidenceofsuchextendedeventsinthefossilrecord,whichaffordsglimpsesintothebreakupofformerlycontinuousenvironments.Forexample,duringpast
7、iceages,glaciersadvanceddownthroughNorthAmericaandEuropeandgraduallycutoffpartsofpopulationsfromoneanother.Whentheglaciersretreated,theseparatedpopulationsofplantsandanimalscameintocontactagain.Somegroupsthathaddescendedfromthesameparentpopulationwerenolongerreproductivelycompatibl
8、e—theyhadevolvedintosepara
