生物专业英语（英文）

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1、cyclicLesson9Theoriginofspecies1.ThebiologistsdefineaspeciesModernbiologistsgenerallydefineaspeciesasgroupofactuallyorpotentiallyinterbreedingpopulationsthatisreproductivelyisolatedfromothersuchgroups.Membersofspeciescaninterbreedwitheachother,buttheycannotbreedwithorganismsbelongingtoanothe

2、rspecies.Oneadvantageofthestandardofreproductiveisolationisthatitisveryprecise.Notice,however,thatitcanonlybeappliedtoorganismsthatreproducesexually.Asexualreproducers,includingmostprokaryotes,manyplants,andsomeanimals,mustbeclassifiedintospeciesonthebasisofphysical(biochemicalormorphologica

3、l)traits.2.PreventinggeneexchangeTwogeneraltypesofmechanismsoperatetoblocktheexchangeofgenesbetweenindividualsofrelatedgroups.Thefirstgeneraltypeismadeupofprezygoticisolatingmechanisms-mechanismsthatpreventtheformationofzygotes.Prezygoticisolationfallsintotwocategories:ecologicalandbehaviora

4、l.Inthefirstcase,tworelatedgroupmaybecomeadaptedtoslightlydifferentenvironments_perhapsvaryingsoiltypesorfoodsources.Overtime,thesegeneticdifferencesbecomesogreatthatsuccessfulcross-fertilizationcannolongertakeplace.Inbehavioralisolation,relatedgroupsevolvedifferingbehaviors-suchasspeciesmat

5、ingrituals-thatrestricttheexchangeofgenestomembersofthesamegroup.-----PreventinggeneexchangeSometimesthedifferencesthatproduceprezygoticisolationinvolvemechanicalisolation.Thatis,matingisphysicallyimpossiblebetweenmembersofdifferentspeciesbecausegenitalsofmalesandfemalesarestructurallyincomp

6、atibleorbecausemoleculesonthesurfacesofspermandeggfailtobind.Afinaltypeofprezygoticmechanismistemporalisolation,inwhichtime-relatedenvironmentalcuesthattriggerreproductiveprocessesaredifferentforrelatedspecies.Inpostzygoticisolatingmechanismsmatingoccurs,buttheresultinghybridorganismisinviab

7、leorsterile.Inaspecialcaseofhybridsterilitytermedhybridbreakdown,thesecondandsubsequentgenerationsafteracrossshowreducedreproductivesuccess.Inpostzygoticisolatingmechanismsmatingoccurs,buttheresultinghybridorganismisinviableorsterile.Inaspecialcase