托福TPO42阅读文本.doc

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1、托福TPO42阅读文本+题目+答案下载GeographicIsolationofSpeciesBiologistErnstMayrdefinedaspeciesas“anactuallyorpotentiallyinterbreedingpopulationthatdoesnotinterbreedwithothersuchpopulationswhenthereisopportunitytodoso.”Akeyeventintheoriginofmanyspeciesistheseparationofapopulationwithitsgenepoo

2、l(allofthegenesinapopulationatanyonetime)fromotherpopulationsofthesamespecies,therebypreventingpopulationinterbreedingWithitsgenepoolisolated,aseparatepopulationcanfollowitsownevolutionarycourse.Intheformationofmanyspecies,theinitialisolationofapopulationseemstohavebeenageograph

3、icbarrierThismodeofevolvingnewspeciesiscalledallopatricspeciation.Manyfactorscanisolateapopulationgeographically.Amountainrangemayemergeandgraduallysplitapopulationoforganismsthatcaninhabitonlylowlandlakes,certainfishpopulationsmightbecomeisolatedinthisway.Similarity,acreepinggl

4、aciermaygraduallydivideapopulation,oralandbridgesuchastheIsthmusofPanamamayformandseparatethemarinelifeintheoceanwatersoneitherside.Howformidablemustageographicbarrierbetokeeppopulationsapart?Itdependsontheabilityoftheorganismstomoveacrossbarriers.Birdsandcoyotescaneasilycrossmo

5、untainsandrivers.Thepassageofwind-blowntreepollenisalsonothinderedbysuchbarriers,andtheseedsofmanyplantsmaybeearnedbackandforthonanimalsIncontrast,smallrodentsmayfindadeepcanyonorawideriveraneffectivebarrier.Forexample,theGrandCanyon,inthesouthwesternUnitedSlates,separatetherang

6、eofthewhile-tailedantelopesquirrelfromthatofthecloselyrelatedHarris’antelopesquirrel.Smaller,withashortertailthatiswhiteunderneath,thewhite-tailedantelopesquirrelinhabitsdesertsnorthofthecanyonandwestoftheColoradoRiverinsouthernCaliforniaHams'antelopesquirrelhasamorelimitedrange

7、indesertssouthoftheGrandCanyon.Geographicisolationcreatesopportunitiesfornewspeciestodevelop,butitdoesnotnecessarilyleadtonewspeciesbecausespeciationoccursonlywhenthegenepoolundergoesenoughchangestoestablishreproductivebarriersbetweentheisolatedpopulationanditsparentpopulation.T

8、helikelihoodofallopatricspeciationincreaseswhen