tpo42三篇托福阅读TOEFL原文译文题目答案译文背景知识.docx

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tpo42三篇托福阅读TOEFL原文译文题目答案译文背景知识

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GeographicIsolationOfSpecies

①BiologistErnstMayrdefinedaspeciesas“anactuallyorpotentiallyinterbreedingpopulationthatdoesnotinterbreedwithothersuchpopulationswhenthereisopportunitytodoso.”Akeyeventintheoriginofmanyspeciesistheseparationofapopulationwithitsgenepool(allofthegenesinapopulationatanyonetime)fromotherpopulationsofthesamespecies,therebypreventingpopulationinterbreeding.Withitsgenepoolisolated,aseparatepopulationcanfollowitsownevolutionarycourse.Intheformationofmanyspecies,theinitialisolationofapopulationseemstohavebeenageographicbarrier.Thismodeofevolvingnewspeciesiscalledallopatricspeciation.

②Manyfactorscanisolateapopulationgeographically.Amountainrangemayemergeandgraduallysplitapopulationoforganismsthatcaninhabitonlylowlandlakes,certainfishpopulationsmightbecomeisolatedinthisway.Similarly,acreepingglaciermaygraduallydivideapopulation,oralandbridgesuchastheIsthmusofPanamamayformandseparatethemarinelifeintheoceanwatersoneitherside.

③Howformidablemustageographicbarrierbetokeeppopulationsapart?Itdependsontheabilityoftheorganismstomoveacrossbarriers.Birdsandcoyotescaneasilycrossmountainsandrivers.Thepassageofwind-blowntreepollenisalsonothinderedbysuchbarriers,andtheseedsofmanyplantsmaybecarriedbackandforthonanimals.Incontrast,smallrodentsmayfindadeepcanyonorawideriveraneffectivebarrier.Forexample,theGrandCanyon,inthesouthwesternUnitedStates,separatetherangeofthewhite-taile