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SpeciationinGeographicallyIsolatedPopulationsEvolutionarybiologistsbelievethatspeciation,theformationofanewspecies,oftenbeginswhensomekindofphysicalbarrierarisesanddividesapopulationofasinglespeciesintoseparatesubpopulations.Physicalseparationbetweensubpopulationspromotestheformationofnewspeciesbecauseoncethemembersofonesubpopulationcannolongermatewithmembersofanothersubpopulation,theycannotexchangevariantgenesthatariseinoneofthesubpopulations.Intheabsencesofgeneflowbetweenthesubpopulations,geneticdifferencesbetweenthegroupsbegintoaccumulate.Eventuallythesubpopulationsbecomesogeneticallydistinctthattheycannotinterbreedevenifthephysicalbarriersbetweenthemwereremoved.Atthispointthesubpopulationshaveevolvedintodistinctspecies.Thisroutetospeciationisknownasallopatry(“allo-”means“different”,and“patria”means“homeland”)Allopatricspeciationmaybethemainspeciationroute.Thisshouldnotbesurprising,sinceallopatryisprettycommon.Ingeneral,thesubpopulationsofmostspeciesareseparatedfromeachotherbysomemeasurabledistance.Soevenundernormalsituationsthegeneflowamongthesubpopulationsismoreofanintermittenttricklethanasteadystream.Inaddition,barrierscanrapidlyariseandshutoffthetrickle.Forexample,inthe1800samonstrousearthquakechangedthecourseoftheMississippiRiver,alargeriverflowinginthecentralpartoftheUnitedStatesofAmerica.Thechangeseparatedpopulationsofinsectsnowlivingalongoppositeshore,completelycuttingoffgeneflowbetweenthem.Geographicisolationalsocanproceedslowly,overgreatspansoftime.Wefindevidenceofsuchextendedeventsinthefossilrecord,whichaffordsglimpsesintothebreakupofformerlycontinuousenvironments.Forexample,duringpasticeages,glaciersadvanceddownthroughNorthAmericaandEuropeandgraduallycutoffpartsofpopulationsfromoneanother.Whentheglacierretreated,theseparatedpopulationsofplantsandanimalscameintocontactagain.Somegroupsthathaddescendedfromthesameparentpopulationwerenolongerreproductivelycompatible—theyhadevolvedintoseparatespecies.Inothergroups,however,geneticdivergenceshadnotproceededsofar,andthedescendantscouldstillinterbreed—forthem,reproductiveisolationwasnotcompleted,andsospeciationhadnotoccurred.AllopatricspeciationcanalsobebroughtbytheimperceptiblyslowbutcolossalmovementsofthetectonicplatesthatmakeupEarth’ssurface.■About5millionyearsagosuchgeologicmovementscreatedthelandbridgebetweenNorthAmericaandSouthAmericathatwecalltheIsthmusofPanama.Theformationoftheisthmushadimportantconsequencesforglobalpatternsofoceanwaterflow.■Whilepreviouslythegapbetweenthecontinentshadallowedafreeflowofwater,nowtheisthmuspresentedabarrierthatdividedtheAtlanticOceanfromthePacificOcean.■Thisdivisionsetthestageforallopatricspeciationamongpopulationsoffishesandothermarinespecies.■Inthe1980s,JohnGravesstudiedtwopopulationsofcloselyrelatedfishes,onepopulationfromtheAtlanticsideofisthmus,theotherfromthePacificside.Hecomparedfourenzymesfoundinthemusclesofeachpopulation.GravesfoundthatallfourPacificenzymesfunctionbetteratlowertemperaturesthanthefourAtlanticversionsofthesameenzymes.ThisissignificantbecausePacificseawateristypically2to3degreescoolerthanseawaterontheAtlanticsideofisthmus.Analysisbygelelectrophoresisrevealedslightdifferencesinaminoacidsequenceoftheenzymesoftwoofthefourpairs.Thisissignificantbecausetheaminoacidsequenceofanenzymeisdeterminedbygenes.Gravesdrewtwoconclusionsfromtheseobservations.First,atleastsomeoftheobserveddifferencesbetweentheenzymesoftheAtlanticandPacificfishpopulationswerenotrandombutweretheresultofevolutionaryadaption.Second,itappearsthatcloselyrelatedpopulationsoffishesonbothsidesoftheisthmusarestartingtogeneticallydivergefromeachother.BecauseGraves’sstudyofgeographicallyisolatedpopulationsofisthmusfishesoffersaglimpseofthebeginningofaprocessofgradualaccumulationofmutationsthatareneutraloradaptive,divergencesheremightbeevidenceofallopatricspeciationinprocess.1.Theword“promotes”inthepassageisclosestinmeaningtoA.describesB.encouragesC.delaysD.requires2.Accordingtoparagraph1,allopatricspeciationinvolveswhichofthefollowing?A.Thedivisionofapopulationintosubspecies.B.Thereunitingofseparatedpopulationsaftertheyhavebecomedistinctspecies.C.Themovementofapopulationtoanewhomeland.D.Theabsenceofgeneflowbetweensubpopulations.3.Whydoestheauthorprovidetheinformationthat“thesubpopulationsofmostspeciesareseparatedfromeachotherbysomemeasurabledistance”?A.Toindicatehowscientistsareabletodeterminewhethersubpopulationsofaspeciesareallopatric.B.Todefinewhatitmeansforagroupofanimalsorplantstobeasubpopulation.C.Tosuggestthatallopatricspeciationisnottheonlyroutetosubpopulation.D.Tohelpexplainwhyallopatricspeciationisacommonwayfornewspeciestocomeabout.4.Theword“accumulate”inthepassageisclosestinmeaningtoA.BecomemoresignificantB.OccurrandomlyC.GraduallyincreaseinnumberD.Causechanges5.Inparagraph2,whydoestheauthormentionthatsomeinsectpopulationswereseparatedfromeachotherbyachangeinthecourseofMississippiRivercausedbyanearthquake?A.Tomakethepointthatsomekindofphysicalbarrierseparatesthesubpopulationsofmostspecies.B.Tosupporttheclaimthattheconditionof