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1、智课网TOEFL备考资料阿勒泰托福阅读tpo文本解析托福TPO阅读文本，下面是智课教育小编为同学们准备的阿勒泰托福阅读tpo文本解析，备考期间同学们可以把本套托福TPO阅读文本下载下来，好好的学习一下。托福TPO阅读文本SpeciationinGeographicallyIsolatedPopulationsEvolutionarybiologistsbelievethatspeciation,theformationofanewspecies,oftenbeginswhensomekindofphysic
2、albarrierarisesanddividesapopulationofasinglespeciesintoseparatesubpopulations.Physicalseparationbetweensubpopulationspromotestheformationofnewspeciesbecauseoncethemembersofonesubpopulationcannolongermatewithmembersofanothersubpopulation,theycannotexchange
3、variantgenesthatariseinoneofthesubpopulations.Intheabsenceofgeneflowbetweenthesubpopulations,geneticdifferencesbetweenthegroupsbegintoaccumulate.Eventuallythesubpopulationsbecomesogeneticallydistinctthattheycannotinterbreedevenifthephysicalbarriersbetweent
4、hemwereremoved.Atthispointthesubpopulationshaveevolvedintodistinctspecies.Thisroutetospeciationisknownasallopatry(“allo-”means“different”,and“patria”means“homeland”).Allopatricspeciationmaybethemainspeciationroute.Thisshouldnotbesurprising,sinceallopatryis
5、prettycommon.Ingeneral,thesubpopulationsofmostspeciesareseparatedfromeachotherbysomemeasurabledistance.Soevenundernormalsituationsthegeneflowamongthesubpopulationsismoreofanintermittenttricklethanasteadystream.Inaddition,barrierscanrapidlyariseandshutoffth
6、etrickle.Forexample,inthe1800samonstrousearthquakechangedthecourseoftheMississippiRiver,alargeriverflowinginthecentralpartoftheUnitedStatesofAmerica.Thechangeseparatedpopulationsofinsectsnowlivingalongoppositeshore,completelycuttingoffgeneflowbetweenthem.G
7、eographicisolationalsocanproceedslowly,overgreatspansoftime.Wefindevidenceofsuchextendedeventsinthefossilrecord,whichaffordsglimpsesintothebreakupofformerlycontinuousenvironments.Forexample,duringpasticeages,glaciersadvanceddownthroughNorthAmericaandEurope
8、andgraduallycutoffpartsofpopulationsfromoneanother.Whentheglaciersretreated,theseparatedpopulationsofplantsandanimalscameintocontactagain.Somegroupsthathaddescendedfromthesameparentpopulationwerenolongerrepro
