托福TPO42阅读文本.docx

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GeographicIsolationofSpecies

BiologistErnstMayrdefinedaspeciesas“anactuallyorpotentiallyinterbreedingpopulationthatdoesnotinterbreedwithothersuchpopulationswhenthereisopportunitytodoso.”Akeyeventintheoriginofmanyspeciesistheseparationofapopulationwithitsgenepool（allofthegenesinapopulationatanyonetime）fromotherpopulationsofthesamespecies,therebypreventingpopulationinterbreedingWithitsgenepoolisolated,aseparatepopulationcanfollowitsownevolutionarycourse.Intheformationofmanyspecies,theinitialisolationofapopulationseemstohavebeenageographicbarrierThismodeofevolvingnewspeciesiscalledallopatricspeciation.jKz4rZB。

Oqe9SKJ。

Manyfactorscanisolateapopulationgeographically.Amountainrangemayemergeandgraduallysplitapopulationoforganismsthatcaninhabitonlylowlandlakes,certainfishpopulationsmightbecomeisolatedinthisway.Similarity,acreepingglaciermaygraduallydivideapopulation,oralandbridgesuchastheIsthmusofPanamamayformandseparatethemarinelifeintheoceanwatersoneitherside.EThA0BA。

lU9rZiL。

Howformidablemustageographicbarrierbetokeeppopulationsapart?

Itdependsontheabilityoftheorganismstomoveacrossbarriers.Birdsandcoyotescaneasilycrossmountainsandrivers.Thepassageofwind-blowntreepollenisalsonothinderedbysuchbarriers,andtheseedsofmanyplantsmaybeearnedbackandforthonanimalsIncontrast,smallrodentsmayfindadeepcanyonorawideriveraneffectivebarrier.Forexample,theGrandCanyon,inthesouthwesternUnitedSlates,separatetherangeofthewhile-tailedantelopesquirrelfromthatofthecloselyrelatedHarris’antelopesquirrel.Smaller,withashortertailthatiswhiteunderneath,thewhite-tailedantelopesquirrelinhabitsdesertsnorthofthecanyonandwestoftheColoradoRiverinsouthernCaliforniaHams'antelopesquirrelhasamorelimitedrangeindesertssouthoftheGrandCanyon.CjMcnf4。

pPMCa1H。

Geographicisolationcreatesopportunitiesfornewspeciestodevelop,butitdoesnotnecessarilyleadtonewspeciesbecausespeciationoccursonlywhenthegenepoolundergoesenoughchangestoestablishreproductivebarriersbetweentheisolatedpopulationanditsparentpopulation.Thelikelihoodofallopatricspeciationincreaseswhenapopulationissmallaswellasisolated,makingitmorelikelythanalargepopulationtohaveitsgenepoolchangedsubstantially.Forexample,inlessthantwomillionyears,smallpopulationsofstrayanimalsandplantsfromtheSouthAmericanmainlandthatmanagedtocolonizetheGalapagosIslandsgaverisetoallthespeciesthatnowinhabittheislands.pGm60P2。

bp1Mu2f。

Whenoceanicislandsarefarenoughaparttopermitpopulationstoevolveinisolation,butcloseenoughtoallowoccasionaldispersionstooccur,theyareeffectivelyoutdoorlaboratoriesofevolution.TheGalapagosislandchainisoneoftheworldsgreatestshowcasesofevolutionEachislandwasbornfromunderwatervolcanoesandwasgraduallycoveredbyorganismsderivedfromstraysthatrodetheoceancurrentsandwindsfromotherislandsandcontinents.Organismscanalsobecarriedtoislandsbyotherorganisms,suchasseabirdsthattravellongdistanceswithseedsclingingtotheirfeathers.5qDlp3l。

TCrwtyy。

ThespeciesontheGalapagosIslandstoday,mostofwhichoccurnowhereelse,descendedfromorganismsthatfloated,flew,orwereblownovertheseafromtheSouthAmericanmainlandForinstance,theGalapagosislandchainhasatotalofthirteenspeciesofcloselyrelatedbirdscalledGalapagosfinchesThesebirdshavemanysimilaritiesbutdifferintheirfeedinghabitsandChx39g0。

5AZTj8B。

theirbeaktype,whichiscorrelatedwithwhattheyeatAccumulatedevidenceindicatesthatallthirteenfinchspeciesevolvedfromasinglesmallpopulationofancestralbirdsthatcolonizedoneoftheislands.Completelyisolatedontheislandaftermigratingfromthemainland,thefounderpopulationmayhaveundergonesignificantchangesinitsgenepoolandbecomeanewspecies.Later,afewindividualsofthisnewspeciesmayhavebeenblownbystormstoaneighboringisland.Isolatedonthissecondisland,thesecondfounderpopulationcouldhaveevolvedintoasecondnewspecies,whichcouldlaterrecolonizetheislandfromwhichitsfoundingpopulationemigrated.TodayeachGalapagosislandhasmultiplespeciesoffinches,withasmanyastenonsomeislands.jgeODg4。

CYkUHMD。

paragraph1

BiologistErnstMayrdefinedaspeciesas“anactuallyorpotentiallyinterbreedingpopulationthatdoesnotinterbreedwithothersuchpopulationswhenthereisopportunitytodoso.”Akeyeventintheoriginofmanyspeciesistheseparationofapopulationwithitsgenepool（allofthegenesinapopulationatanyonetime）fromotherpopulationsofthesamespecies,therebypreventingpopulationinterbreedingWithitsgenepoolisolated,aseparatepopulationcanfollowitsownevolutionarycourse.Intheformationofmanyspecies,theinitialisolationofapopulationseemstohavebeenageographicbarrierThismodeofevolvingnewspeciesiscalledallopatricspeciation.tlNm4iv。

G7eSSbl。

1.Theword“key”inthepassageisclosestinmeaningtozbkS6Jz。

Z1M8CYq。

A.earlyB.crucialC.noticeableD.frequent

2.Theword“initial”inthepassageisclosestinmeaningtovRNFyIn。

Bo7jeHe。

A.bestB.usualC.firstD.actual

3.Accordingtoparagraph1.allopatricspeciationispossiblewhenHsfq9Iz。

dC2kwLC。

A.apopulationcontainsallthedifferentgenespresentinaspeciesataparticulartimepVPyf9D。

KTkjdD1。

B.apopulationbecomesisolatedduetothepresenceofageographicbarriereLMEPwe。

UmHnpBM。

C.geneticmixingbeginstooccurinpreviouslyseparatepopulationsofaspeciesb1r84o1。

m7Zh0tq。

D.aspeciesissuccessfulincrossingageographicbarrieriXo4FaM。

pBn0kUe。

paragraph1&2

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

KHRLP2J。

Manyfactorscanisolateapopulationgeographically.Amountainrangemayemergeandgraduallysplitapopulationoforganismsthatcaninhabitonlylowlandlakes,certainfishpopulationsmightbecomeisolatedinthisway.Similarity,acreepingglaciermaygraduallydividebNleNCT。

nDEVZO3。

apopulation,oralandbridgesuchastheIsthmusofPanamamayformandseparatethemarinelifeintheoceanwatersoneitherside.HGC79Iq。

9CGokx2。

4.Howisparagraph2relatedtoparagraph1?

A.Paragraph2pointsoutanumberofwaysinwhichthephenomenonofgeographicisolationmentionedinparagraph1canoccurak1NzPb。

uobYb7T。

B.Paragraph2identifiesdiscoveriesthatledtotheconclusionpresentedinparagraph1thatgeographicisolationhasplayedaroteintheoriginofmanyspeciesTvSoN6u。

BQFfHXa。

C.Paragraph2providesevidencesupportingthestatementinparagraph1thatapopulationcanfollowitsownevolutionarycourseonceitsgenepoolbecomesisolatedqZwXeEJ。

S3x5YlI。

D.Paragraph2explainswhytheterm“allopatric”wasadoptedtodescribethemethodofspeciationdescribedinparagraph16yv7Dan。

7S7efCd。

paragraph3

Howformidablemustageographicbarrierbetokeeppopulationsapart?

Itdependsontheabilityoftheorganismstomoveacrossbarriers.Birdsandcoyotescaneasilycrossmountainsandrivers.Thepassageofwind-blowntreepollenisalsonothinderedbysuchbarriers,andtheseedsofmanyplantsmaybeearnedbackandforthonanimalsIncontrast,smallrodentsmayfindadeepcanyonorawideriveraneffectivebarrier.Forexample,theGrandCanyon,inthesouthwesternUnitedSlates,separatetherangeofthewhile-tailedantelopesquirrelfromthatofthecloselyrelatedHarris’antelopesquirrel.Smaller,withashortertailthatiswhiteunderneath,thewhite-tailedantelopesquirrelinhabitsdesertsnorthofthecanyonandwestoftheColoradoRiverinsouthernCaliforniaHams'antelopesquirrelhasamorelimitedrangeindesertssouthoftheGrandCanyon.qLtu0EL。

UlHtCwg。

5.Inparagraph3,theauthorcontrastsavarietyoforganismstoillustratewhichofthefollowingpoints?

ga2VUIw。

mgLmxtO。

A.Geographicbarriersarelesslikelytokeepapartpopulationsofplantsthanpopulationsofanimals.fr38Jrv。

1WITG2B。

B.GeographicbarriersaremorelikelytokeepapartpopulationsoflargeorganismsthanpopulationsofsmallorganismsTad9fQd。

jFwyOuK。

C.Somemembersofaspeciesareabletocrossgeographicbarriers,whileothermembersofthesamespeciesarenot.FImc5Me。

tq4SNBG。

D.Theeffectivenessofgeographicbarriersinkeepingorganismsapartdependsonanorganism’sabilitytomoveacrossbarriers.lEW7Z4l。

HHoRwkG。

6.Paragraph3supportstheideathatwhite-tailedantelopesquirrelsandHams'antelopesquirrelshavewhichofthefollowingincommon?

AiAR26u。

LSiREoA。

A.TheyarethetwosmallestrodentsnowfoundinthesouthwesternUnitedStates.5zB9040。

BDKxpsq。

B.Theyhavewhitecoloringunderneaththeirtails

C.TheycannotcrosstheGrandCanyon

D.Theycannotsurviveindesertconditions

paragraph4

Geographicisolationcreatesopportunitiesfornewspeciestodevelop,butitdoesnotnecessarilyleadtonewspeciesbec