托福TPO42阅读文本Word格式.docx

托福TPO42阅读文本Word格式.docx

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Manyfactorscanisolateapopulationgeographically.Amountainrangemayemergeandgraduallysplitapopulationoforganismsthatcaninhabitonlylowlandlakes,certainfishpopulationsmightbecomeisolatedinthisway.Similarity,acreepingglaciermaygraduallydivideapopulation,oralandbridgesuchastheIsthmusofPanamamayformandseparatethemarinelifeintheoceanwatersoneitherside.

Howformidablemustageographicbarrierbetokeeppopulationsapart?

Itdependsontheabilityoftheorganismstomoveacrossbarriers.Birdsandcoyotescaneasilycrossmountainsandrivers.Thepassageofwind-blowntreepollenisalsonothinderedbysuchbarriers,andtheseedsofmanyplantsmaybeearnedbackandforthonanimalsIncontrast,smallrodentsmayfindadeepcanyonorawideriveraneffectivebarrier.Forexample,theGrandCanyon,inthesouthwesternUnitedSlates,separatetherangeofthewhile-tailedantelopesquirrelfromthatofthecloselyrelatedHarris’antelopesquirrel.Smaller,withashortertailthatiswhiteunderneath,thewhite-tailedantelopesquirrelinhabitsdesertsnorthofthecanyonandwestoftheColoradoRiverinsouthernCaliforniaHams'

antelopesquirrelhasamorelimitedrangeindesertssouthoftheGrandCanyon.

Geographicisolationcreatesopportunitiesfornewspeciestodevelop,butitdoesnotnecessarilyleadtonewspeciesbecausespeciationoccursonlywhenthegenepoolundergoesenoughchangestoestablishreproductivebarriersbetweentheisolatedpopulationanditsparentpopulation.Thelikelihoodofallopatricspeciationincreaseswhenapopulationissmallaswellasisolated,makingitmorelikelythanalargepopulationtohaveitsgenepoolchangedsubstantially.Forexample,inlessthantwomillionyears,smallpopulationsofstrayanimalsandplantsfromtheSouthAmericanmainlandthatmanagedtocolonizetheGalapagosIslandsgaverisetoallthespeciesthatnowinhabittheislands.

Whenoceanicislandsarefarenoughaparttopermitpopulationstoevolveinisolation,butcloseenoughtoallowoccasionaldispersionstooccur,theyareeffectivelyoutdoorlaboratoriesofevolution.TheGalapagosislandchainisoneoftheworldsgreatestshowcasesofevolutionEachislandwasbornfromunderwatervolcanoesandwasgraduallycoveredbyorganismsderivedfromstraysthatrodetheoceancurrentsandwindsfromotherislandsandcontinents.Organismscanalsobecarriedtoislandsbyotherorganisms,suchasseabirdsthattravellongdistanceswithseedsclingingtotheirfeathers.

ThespeciesontheGalapagosIslandstoday,mostofwhichoccurnowhereelse,descendedfromorganismsthatfloated,flew,orwereblownovertheseafromtheSouthAmericanmainlandForinstance,theGalapagosislandchainhasatotalofthirteenspeciesofcloselyrelatedbirdscalledGalapagosfinchesThesebirdshavemanysimilaritiesbutdifferintheirfeedinghabitsand

theirbeaktype,whichiscorrelatedwithwhattheyeatAccumulatedevidenceindicatesthatallthirteenfinchspeciesevolvedfromasinglesmallpopulationofancestralbirdsthatcolonizedoneoftheislands.Completelyisolatedontheislandaftermigratingfromthemainland,thefounderpopulationmayhaveundergonesignificantchangesinitsgenepoolandbecomeanewspecies.Later,afewindividualsofthisnewspeciesmayhavebeenblownbystormstoaneighboringisland.Isolatedonthissecondisland,thesecondfounderpopulationcouldhaveevolvedintoasecondnewspecies,whichcouldlaterrecolonizetheislandfromwhichitsfoundingpopulationemigrated.TodayeachGalapagosislandhasmultiplespeciesoffinches,withasmanyastenonsomeislands.

paragraph1

BiologistErnstMayrdefinedaspeciesas“anactuallyorpotentiallyinterbreedingpopulationthatdoesnotinterbreedwithothersuchpopulationswhenthereisopportunitytodoso.”Akeyeventintheoriginofmanyspeciesistheseparationofapopulationwithitsgenepool（allofthegenesinapopulationatanyonetime）fromotherpopulationsofthesamespecies,therebypreventingpopulationinterbreedingWithitsgenepoolisolated,aseparatepopulationcanfollowitsownevolutionarycourse.Intheformationofmanyspecies,theinitialisolationofapopulationseemstohavebeenageographicbarrierThismodeofevolvingnewspeciesiscalledallopatricspeciation.

1.Theword“key”inthepassageisclosestinmeaningto

A.earlyB.crucialC.noticeableD.frequent

2.Theword“initial”inthepassageisclosestinmeaningto

A.bestB.usualC.firstD.actual

3.Accordingtoparagraph1.allopatricspeciationispossiblewhen

A.apopulationcontainsallthedifferentgenespresentinaspeciesataparticulartime

B.apopulationbecomesisolatedduetothepresenceofageographicbarrier

C.geneticmixingbeginstooccurinpreviouslyseparatepopulationsofaspecies

D.aspeciesissuccessfulincrossingageographicbarrier

paragraph1&

2

Manyfactorscanisolateapopulationgeographically.Amountainrangemayemergeandgraduallysplitapopulationoforganismsthatcaninhabitonlylowlandlakes,certainfishpopulationsmightbecomeisolatedinthisway.Similarity,acreepingglaciermaygraduallydivide

apopulation,oralandbridgesuchastheIsthmusofPanamamayformandseparatethemarinelifeintheoceanwatersoneitherside.

4.Howisparagraph2relatedtoparagraph1?

A.Paragraph2pointsoutanumberofwaysinwhichthephenomenonofgeographicisolationmentionedinparagraph1canoccur

B.Paragraph2identifiesdiscoveriesthatledtotheconclusionpresentedinparagraph1thatgeographicisolationhasplayedaroteintheoriginofmanyspecies

C.Paragraph2providesevidencesupportingthestatementinparagraph1thatapopulationcanfollowitsownevolutionarycourseonceitsgenepoolbecomesisolated

D.Paragraph2explainswhytheterm“allopatric”wasadoptedtodescribethemethodofspeciationdescribedinparagraph1

paragraph3

Itdependsontheabilityoftheorganismstomoveacrossbarriers.Birdsandcoyotescaneasilycrossmountainsandrivers.Thepassageofwind-blowntreepollenisalsonothinderedbysuchbarriers,andtheseedsofmanyplantsmaybeearnedbackandforthonanimalsIncontrast,smallrodentsmayfindadeepcanyonorawideriveraneffectivebarrier.Forexample,theGrandCanyon,inthesouthwesternUnitedSlates,separatetherangeofthewhile-tailedantelopesquirrelfromthatofthecloselyrelatedHarris’antelopesquirrel.Smaller,withashortertailthatiswhiteunderneath,thewhite-tailedantelopesquirrelinhabitsdesertsnorthofthecanyonandwestoftheColoradoRiverinsouthernCaliforniaHams'

5.Inparagraph3,theauthorcontrastsavarietyoforganismstoillustratewhichofthefollowingpoints?

A.Geographicbarriersarelesslikelytokeepapartpopulationsofplantsthanpopulationsofanimals.

B.Geographicbarriersaremorelikelytokeepapartpopulationsoflargeorganismsthanpopulationsofsmallorganisms

C.Somemembersofaspeciesareabletocrossgeographicbarriers,whileothermembersofthesamespeciesarenot.

D.Theeffectivenessofgeographicbarriersinkeepingorganismsapartdependsonanorganism’sabilitytomoveacrossbarriers.

6.Paragraph3supportstheideathatwhite-tailedantelopesquirrelsandHams'

antelopesquirrelshavewhichofthefollowingincommon?

A.TheyarethetwosmallestrodentsnowfoundinthesouthwesternUnitedStates.

B.Theyhavewhitecoloringunderneaththeirtails

C.TheycannotcrosstheGrandCanyon

D.Theycannotsurviveindesertconditions

paragraph4

Geographicisolationcreatesopportunitiesfornewspeciestodevelop,butitdoesnotnecessarilyleadtonewspeciesbecausespeciationoccursonlywhenthegenepoolundergoesenoughchangestoestablishreproductivebarriersbetweentheisolatedpopulationanditsparentpopulation.Thelikelihoodofallopatricspeciationincreaseswhenapopulationissmallaswellasisolated,makingitmorelikelythanalargepopulationtohaveitsgenepoolchangedsubstantially.Forexample,inlessthantwomillionyears,smallpopulationsofstrayanimalsandplantsfromtheSouthAmericanmainlandt