tpo 31文本.docx
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tpo 31文本.docx
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tpo31文本
SpeciationinGeographicallyIsolatedPopulations
1Evolutionarybiologistsbelievethatspeciation,theformationofanewspecies,oftenbeginswhensomekindofphysicalbarrierarisesanddividesapopulationofasinglespeciesintoseparatesubpopulations.Physicalseparationbetweensubpopulationspromotestheformationofnewspeciesbecauseoncethemembersofonesubpopulationcannolongermatewithmembersofanothersubpopulation,theycannotexchangevariantgenesthatariseinoneofthesubpopulations.Intheabsenceofgeneflowbetweenthesubpopulations,geneticdifferencesbetweenthegroupsbegintoaccumulate.Eventuallythesubpopulationsbecomesogeneticallydistinctthattheycannotinterbreedevenifthephysicalbarriersbetweenthemwereremoved.Atthispointthesubpopulationshaveevolvedintodistinctspecies.Thisroutetospeciationisknownasallopatry(“allo-”means“different”,and“patria”means“homeland”).
Para.2Allopatricspeciationmaybethemainspeciationroute.Thisshouldnotbesurprising,sinceallopatryisprettycommon.Ingeneral,thesubpopulationsofmostspeciesareseparatedfromeachotherbysomemeasurabledistance.Soevenundernormalsituationsthegeneflowamongthesubpopulationsismoreofanintermittenttricklethanasteadystream.Inaddition,barrierscanrapidlyariseandshutoffthetrickle.Forexample,inthe1800samonstrousearthquakechangedthecourseoftheMississippiRiver,alargeriverflowinginthecentralpartoftheUnitedStatesofAmerica.Thechangeseparatedpopulationsofinsectsnowlivingalongoppositeshores,completelycuttingoffgeneflowbetweenthem.
Para.3Geographicisolationcanalsoproceedslowly,overgreatspansoftime.Wefindevidenceofsuchextendedeventsinthefossilrecord,whichaffordsglimpseintothebreakupofformerlycontinuousenvironments.Forexample,duringpasticeages,glaciersadvanceddownthroughNorthAmericaandEuropeandgraduallycutoffpartsofpopulationsfromoneanother.Whentheglaciersretreated,theseparatedpopulationsofplantsandanimalscameintocontactagain.Somegroupsthathaddescendedfromthesameparentpopulationwerenolongerreproductivelycompatible–theyhadevolvedintoseparatespecies.Inothergroups,however,geneticdivergenceshadnotproceededsofar,andthedescendantscouldstillinterbreed–forthem,reproductiveisolationwasnotcompleted,andsospeciationhadnotoccurred.
Para.4AllopatricspeciationcanalsobebroughtbytheimperceptiblyslowbutcolossalmovementsofthetectonicplatesthatmakeupEarth’ssurface.◎About5millionyearsagosuchgeologicmovementscreatedthelandbridgebetweenNorthAmericaandSouthAmericathatwecalltheIsthmusofPanama[巴拿马地峡].◎Whilepreviouslythegapbetweenthecontinentshadallowedafreeflowofwater,nowtheisthmuspresentedabarrierthatdividedtheAtlanticOceanfromthePacificOcean.◎Thisdivisionsetthestageforallopatricspeciationamongpopulationsoffishesandothermarinespecies.◎
Para.5Inthe1980s,JohnGravesstudiedtwopopulationsofcloselyrelatedfishes,onepopulationfromtheAtlanticsideofisthmus,theotherfromthePacificside.Hecomparedfourenzymesfoundinthemusclesofeachpopulation.GravesfoundthatallfourPacificenzymesfunctionbetteratlowertemperaturesthanthefourAtlanticversionsofthesameenzymes.ThisissignificantbecausePacificseawateriftypically2to3degreescoolerthanseawaterontheAtlanticsideofisthmus.Analysisbygelelectrophoresisrevealedslightdifferencesinaminoacidsequenceoftheenzymesoftwoofthefourpairs.Thisissignificantbecausetheaminoacidsequenceofanenzymeisdeterminedbygenes.
Para.6Gravesdrewtwoconclusionsfromtheseobservations.First,atleastsomeoftheobserveddifferencesbetweentheenzymesoftheAtlanticandPacificfishpopulationswerenotrandombutweretheresultofevolutionaryadaptation.Second,itappearsthatcloselyrelatedpopulationsoffishesonbothsidesoftheisthmusarestartingtogeneticallydivergefromeachother.BecauseGraves’studyofgeographicallyisolatedpopulationsofisthmusfishesoffersaglimpseofthebeginningofaprocessofgradualaccumulationofmutationsthatareneutraloradaptive,divergencesheremightbeevidenceofallopatricspeciationinprocess.
1.Theword“promotes”inthepassageisclosestinmeaningto
ODescribes
OEncourages
ODelays
ORequires
2.Accordingtoparagraph1,allopatricspeciationinvolveswhichofthefollowing?
OThedivisionofapopulationintosubspecies
OThereunitingofseparatedpopulationsaftertheyhavebecomedistinctspecies
OThemovementofapopulationtoanewhomeland
OTheabsenceofgeneflowbetweensubpopulations
3.Whydoestheauthorprovidetheinformationthat“subpopulationsofmostspeciesareseparatedfromeachotherbysomemeasurabledistance"?
OToindicatehowscientistsareabletodeterminewhethersubpopulationsofaspeciesareallopatric
OTodefinewhatitmeansforagroupofanimalsorplantstobeasubpopulation
OTosuggestthatallopatricspeciationisnottheonlyroutetospeciation
OTohelpexplainwhyallopatricspeciationisacommonwayfornewspeciestocomeabout
4.Theword“accumulate”inthepassageisclosestinmeaningto
OBecomemoresignificant
OOccurrandomly
OGraduallyincreaseinnumber
OCausechanges
5.Inparagraph2,whydoestheauthormentionthatsomeinsectpopulationswereseparatedfromeachotherbyachangeinthecourseofMississippiRivercausedbyanearthquake?
OTomakethepointthatsomekindofphysicalbarrierseparatesthesubpopulationsofmostspecies
OTosupporttheclaimthattheconditionofallopatrycansometimesariseinashorttime
OToprovideanexampleofasituationinwhichgeneflowamongthepopulationsaspecieshappensataslowrate
OToexplainwhyinsectlivingalongoppositeshoresoftheMississippiRiverareverydifferentfromeachother
6.Accordingtoparagraph3,separationofsubpopulationsbyglaciersresultedinspeciationinthosegroupsofplantsandanimalsthat
OWerereproductivelyisolatedevenaftertheglaciersdisappeared
OHadadjustedtotheoldconditionscausedbytheglaciers
OWereabletosurvivebeingseparatedfromtheirparentpopulation
OHadexperiencedsomegeneticdivergencesfromtheirparentpopulation
7.Theword“colossal”inthepassageisclosestinmeaningto
OConsistent
OGradual
OEnormous
OEffective
8.Accordingtoparagraph4,whichofthefollowingistrueofthegeologicmovementsthatbroughtabouttheIsthmusofPanama?
OThemovementsbroughtpopulationsofcertainfishesandmarineorganismsintocontactwithoneanotherforthefirsttime.
OThemovementstransferredpopulationsoffishesandothermarineanimalsbetweenthePacificandAtlanticOceans.
OThemovementcreatedconditionsthatallowedwatertoflowmorefreelybetweenthePacificandAtlanticOceans.
OThemovementcreatedconditionsfortheformationofnewspeciesoffishesandothermarineanimals.
9.Theword“sequence”inthepassageisclosestinmeaningto
OQuality
OOrder
OFunction
ONumber
10.Accordingtoparagraph5,bycomparingtheenzymesfromtworelatedgroupsoffishesonoppositesidesoftheisthmus,Gravesfoundevidencethat
OTherewereslightgeneticdivergencesbetweenthetwogroups
OTheAtlanticgroupoffishesweredescendedfromthePacificgroupoffishes
OThetemperatureofwateroneithersideoftheisthmushadchanged
OGeneticchangesintheAtlanticgroupoffishesweremorerapidanfrequentthaninthePacificgroupoffishes
11.Itcanbeinferredfromparagraph5and6thatthereasonGravedconcludedthatsomeofthedifferencesbetweenthePacificandAtlanticenzymeswerenotrandomwasthat
OEachofthePacificenzymesworksbetterincoolerwaters
OTheenzymesoftheAtlanticfishpopulationshadnotchangedsincetheformationoftheIsthmusofPanama
OGelelectrophoresisshowedthatthechangesbenefitedboththeAtlanticandthePacificfishpopulations
OThedifferencesbetweentheenzymesdisappearedwhenthetwofishpopulationswereexperimentallyswitchedtoothersideoftheisthmus
12.Whichofthesentencebelowbestexpressestheessentialinformationinthehighlightedsentenceinthepassage?
Incorrectchoiceschangethemeaninginimportantwaysorleaveoutessentialinformation.
OGraves’sstudyprovidesevidencethatisthmusfishesareintheprocessofbecominggeographicallyisolated.
OGraves’sstudyofmutatingisthmusfishesyieldsresultsthatdifferfromresultsofotherstudiesinvolvingallopatricspeciation.
OGraves’sstudyofisolatedpopulationsofisthmusfishesprovidessomeevidencethatallopatricspeciationmightbebeginning.
OGraves’sstudyindicatesthatwhenisolated,populationsofisthmusfishesregisterneutraloradaptivemutations.
13.Lookatthefoursquares【】thatindicatewherethefollowingsentencecanbeaddedtothepassage.
Theformationoftheisthmushadimportantconsequencesforglobalpatternsofoceanwaterflow.
Wherewouldthesentencebestfit?
14.ProseSummary
Allopatricspeciationtakesplacewhenphysicallyseparatedpopulationsofasinglespeciesgraduallydivergegeneticallytothepointofbecomingunabletointerbreed.
A.Allopatricspeciationiscommonbecausethegeneflowbetweensubpopulationsisgenerallylimitedandthebarriersthatcompletelyseparatesubpopulationscanariseinavarietyofways.
B.Duringpasticeages,some,butnotall,subpopulationsseparatedbyglaciersevolvedintodistinctspecies.
C.Speciationdoesnotneedtotakeplacethroughallopatrybecausesubpopulationswillformdistinctspecieswheneverthe
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