TPO阅读42文本答案翻译.docx
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TPO-42
GeographicIsolationofSpecies
BiologistErnstMayrdefinedaspeciesas“anactuallyorpotentiallyinterbreedingpopulationthatdoesnotinterbreedwithothersuchpopulationswhenthereisopportunitytodoso.”Akeyeventintheoriginofmanyspeciesistheseparationofapopulationwithitsgenepool(allofthegenesinapopulationatanyonetime)fromotherpopulationsofthesamespecies,therebypreventingpopulationinterbreeding.Withitsgenepoolisolated,aseparatepopulationcanfollowitsownevolutionarycourse.Intheformationofmanyspecies,theinitialisolationofapopulationseemstohavebeenageographicbarrier.Thismodeofevolvingnewspeciesiscalledallopatricspeciation.
Manyfactorscanisolateapopulationgeographically.Amountainrangemayemergeandgraduallysplitapopulationoforganismsthatcaninhabitonlylowlandlakes,certainfishpopulationsmightbecomeisolatedinthisway.Similarly,acreepingglaciermaygraduallydivideapopulation,oralandbridgesuchastheIsthmusofPanamamayformandseparatethemarinelifeintheoceanwatersoneitherside.
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Howformidablemustageographicbarrierbetokeeppopulationsapart?
Itdependsontheabilityoftheorganismstomoveacrossbarriers.Birdsandcoyotescaneasilycrossmountainsandrivers.Thepassageofwind-blowntreepollenisalsonothinderedbysuchbarriers,andtheseedsofmanyplantsmaybecarriedbackandforthonanimalsIncontrast,smallrodentsmayfindadeepcanyonorawideriveraneffectivebarrier.Forexample,theGrandCanyon,inthesouthwesternUnitedStates,separatetherangeofthewhile-tailedantelopesquirrelfromthatofthecloselyrelatedHarris’antelopesquirrel.Smaller,withashortertailthatiswhiteunderneath,thewhite-tailedantelopesquirrelinhabitsdesertsnorthofthecanyonandwestoftheColoradoRiverinsouthernCaliforniaHams'antelopesquirrelhasamorelimitedrangeindesertssouthoftheGrandCanyon.
Geographicisolationcreatesopportunitiesfornewspeciestodevelop,butitdoesnotnecessarilyleadtonewspeciesbecausespeciationoccursonlywhenthegenepoolundergoesenoughchangestoestablishreproductivebarriersbetweentheisolatedpopulationanditsparentpopulation.Thelikelihoodofallopatricspeciationincreaseswhenapopulationissmallaswellasisolated,makingitmorelikelythanalargepopulationtohaveitsgenepoolchangedsubstantially.Forexample,inlessthantwomillionyears,smallpopulationsofstrayanimalsandplantsfromtheSouthAmericanmainlandthatmanagedtocolonizetheGalapagosIslandsgaverisetoallthespeciesthatnowinhabittheislands.
Whenoceanicislandsarefarenoughaparttopermitpopulationstoevolveinisolation,butcloseenoughtoallowoccasionaldispersionstooccur,theyareeffectivelyoutdoorlaboratoriesofevolution.TheGalapagosislandchainisoneoftheworld’sgreatestshowcasesofevolution.Eachislandwasbornfromunderwatervolcanoesandwasgraduallycoveredbyorganismsderivedfromstraysthatrodetheoceancurrentsandwindsfromotherislandsandcontinents.Organismscanalsobecarriedtoislandsbyotherorganisms,suchasseabirdsthattravellongdistanceswithseedsclingingtotheirfeathers.
ThespeciesontheGalapagosIslandstoday,mostofwhichoccurnowhereelse,descendedfromorganismsthatfloated,flew,orwereblownovertheseafromtheSouthAmericanmainland.Forinstance,theGalapagosislandchainhasatotalofthirteenspeciesofcloselyrelatedbirdscalledGalapagosfinches.Thesebirdshavemanysimilaritiesbutdifferintheirfeedinghabitsandtheirbeaktype,whichiscorrelatedwithwhattheyeat.Accumulatedevidenceindicatesthatallthirteenfinchspeciesevolvedfromasinglesmallpopulationofancestralbirdsthatcolonizedoneoftheislands.Completelyisolatedontheislandaftermigratingfromthemainland,thefounderpopulationmayhaveundergonesignificantchangesinitsgenepoolandbecomeanewspecies.■Later,afewindividualsofthisnewspeciesmayhavebeenblownbystormstoaneighboringisland.■Isolatedonthissecondisland,thesecondfounderpopulationcouldhaveevolvedintoasecondnewspecies,whichcouldlaterrecolonizetheislandfromwhichitsfoundingpopulationemigrated.■TodayeachGalapagosislandhasmultiplespeciesoffinches,withasmanyastenonsomeislands.■
1.Theword“key”inthepassageisclosestinmeaningto
A.early
B.crucial
C.noticeable
D.frequent
2.Theword“initial”inthepassageisclosestinmeaningto
A.best
B.usual
C.first
D.actual
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3.Accordingtoparagraph1,allopatricspeciationispossiblewhen