托福TPO42阅读文本（附答案）

《托福TPO42阅读文本（附答案）》由会员上传分享，免费在线阅读，更多相关内容在教育资源-天天文库。

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

2、itsgenepool(allofthegenesinapopulationatanyonetime)fromotherpopulationsofthesamespecies,therebypreventingpopulationinterbreedingWithitsgenepoolisolated,aseparatepopulationcanfollowitsownevolutionarycourse.Intheformationofmanyspecies,theinitialisolationofapopulationseem

3、stohavebeenageographicbarrierThismodeofevolvingnewspeciesiscalledallopatricspeciation.Manyfactorscanisolateapopulationgeographically.Amountainrangemayemergeandgraduallysplitapopulationoforganismsthatcaninhabitonlylowlandlakes,certainfishpopulationsmightbecomeisolatedin

4、thisway.Similarity,acreepingglaciermaygraduallydivideapopulation,oralandbridgesuchastheIsthmusofPanamamayformandseparatethemarinelifeintheoceanwatersoneitherside.Howformidablemustageographicbarrierbetokeeppopulationsapart?Itdependsontheabilityoftheorganismstomoveacross

5、barriers.Birdsandcoyotescaneasilycrossmountainsandrivers.Thepassageofwind-blowntreepollenisalsonothinderedbysuchbarriers,andtheseedsofmanyplantsmaybeearnedbackandforthonanimalsIncontrast,smallrodentsmayfindadeepcanyonorawideriveraneffectivebarrier.Forexample,theGrandCa

6、nyon,inthesouthwesternUnitedSlates,separatetherangeofthewhile-tailedantelopesquirrelfromthatofthecloselyrelatedHarris’antelopesquirrel.Smaller,withashortertailthatiswhiteunderneath,thewhite-tailedantelopesquirrelinhabitsdesertsnorthofthecanyonandwestoftheColoradoRiveri

7、nsouthernCaliforniaHams'antelopesquirrelhasamorelimitedrangeindesertssouthoftheGrandCanyon.Geographicisolationcreatesopportunitiesfornewspeciestodevelop,butitdoesnotnecessarilyleadtonewspeciesbecausespeciationoccursonlywhenthegenepoolundergoesenoughchangestoestablishre

8、productivebarriersbetweentheisolatedpopulationanditsparentpopulation.Thelikelihoodofallopatricspeciationincreaseswhen