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1、SpeciationinGeographicallyIsolatedPopulationsEvolutionarybiologistsbelievethatspeciation,theformationofanewspecies,oftenbeginswhensomekindofphysicalbarrierarisesanddividesapopulationofasinglespeciesintoseparatesubpopulations.Physicalseparationbetweensubpopulationspromotest
2、heformationofnewspeciesbecauseoncethemembersofonesubpopulationcannolongermatewithmembersofanothersubpopulation,theycannotexchangevariantgenesthatariseinoneofthesubpopulations.Intheabsencesofgeneflowbetweenthesubpopulations,geneticdifferencesbetweenthegroupsbegintoaccumulat
3、e.Eventuallythesubpopulationsbecomesogeneticallydistinctthattheycannotinterbreedevenifthephysicalbarriersbetweenthemwereremoved.Atthispointthesubpopulationshaveevolvedintodistinctspecies.Thisroutetospeciationisknownasallopatry(“allo-”means“different”,and“patria”means“homel
4、and”)Allopatricspeciationmaybethemainspeciationroute.Thisshouldnotbesurprising,sinceallopatryisprettycommon.Ingeneral,thesubpopulationsofmostspeciesareseparatedfromeachotherbysomemeasurabledistance.Soevenundernormalsituationsthegeneflowamongthesubpopulationsismoreofaninter
5、mittenttricklethanasteadystream.Inaddition,barrierscanrapidlyariseandshutoffthetrickle.Forexample,inthe1800samonstrousearthquakechangedthecourseoftheMississippiRiver,alargeriverflowinginthecentralpartoftheUnitedStatesofAmerica.Thechangeseparatedpopulationsofinsectsnowlivin
6、galongoppositeshore,completelycuttingoffgeneflowbetweenthem.Geographicisolationalsocanproceedslowly,overgreatspansoftime.Wefindevidenceofsuchextendedeventsinthefossilrecord,whichaffordsglimpsesintothebreakupofformerlycontinuousenvironments.Forexample,duringpasticeages,glac
7、iersadvanceddownthroughNorthAmericaandEuropeandgraduallycutoffpartsofpopulationsfromoneanother.Whentheglacierretreated,theseparatedpopulationsofplantsandanimalscameintocontactagain.Somegroupsthathaddescendedfromthesameparentpopulationwerenolongerreproductivelycompatible—th
8、eyhadevolvedintoseparatespecies.Inothergroups,however,geneticdivergenceshadnotproceededso
