26 an introduction to genetic analysisWord文件下载.docx

26 an introduction to genetic analysisWord文件下载.docx

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KeyConcepts

Evolutionconsistsofcontinuousheritablechangeoforganismswithinasinglelineofdescent（phyleticevolution）andthedifferentiationbetweendifferentlinesofdescenttoformdifferentspecies（diversification）.

TheDarwinianmechanismofevolutionrestsonthreeprinciples:

（1）organismswithinaspeciesvaryfromoneanother,

（2）thevariationisheritable,and（3）differenttypesleavedifferentnumbersofoffspringinfuturegenerations.

Bothphyleticchangeanddiversificationaretheresultoftheinteractionbetweenthedirectionalforceofnaturalselectionandrandomevents.

Naturalselectionisthedifferentialreproductionofdifferentgenotypesthatisaconsequenceoftheirdifferentphysiological,morphological,andbehavioraltraits.

Randomeffectsincludethesamplingofgameteseachgenerationinfinitepopulationsandtherandomoccurrenceofmutations.

Aconsequenceoftherandomfactorsinevolutionisthatthesameforcesofnaturalselectiondonotleadtothesameevolutionaryresultinindependentlinesofdescent.

Speciesarereproductivelyisolatedpopulationsoforganismsthatcanexchangegeneswithinthegroupbutnotwithotherspecies,becausethegroupsarephysiologically,behaviorally,ordevelopmentallyincompatible.

EvolutionarynoveltiesarepossiblebecausenewDNAisacquiredeitherbyduplicationandsubsequentdifferentiationofDNAalreadypresentinthespeciesorbytheintroductionofnovelDNAfromotherspecies.

Introduction

ThemoderntheoryofevolutionissocompletelyidentifiedwiththenameofCharlesDarwin（1809–1882）thatmanypeoplethinkthattheconceptoforganicevolutionwasfirstproposedbyDarwin,butthatiscertainlynotthecase.Mostscholarshadabandonedthenotionoffixedspecies,unchangedsincetheirorigininagrandcreationoflife,longbeforepublicationofDarwin'

sTheOriginofSpeciesin1859.Bythattime,mostbiologistsagreedthatnewspeciesarisethroughsomeprocessofevolutionfromolderspecies;

theproblemwastoexplainhowthisevolutioncouldoccur.

Darwin'

stheoryofthemechanismofevolutionbeginswiththevariationthatexistsamongorganismswithinaspecies.Individualsofonegenerationarequalitativelydifferentfromoneanother.Evolutionofthespeciesasawholeresultsfromthedifferentialratesofsurvivalandreproductionofthevarioustypes,sotherelativefrequenciesofthetypeschangeovertime.Evolution,inthisview,isasortingprocess.

ForDarwin,evolutionofthegroupresultedfromthedifferentialsurvivalandreproductionofindividualvariantsalreadyexistinginthegroup—variantsarisinginawayunrelatedtotheenvironmentbutwhosesurvivalandreproductiondodependontheenvironment.

MESSAGE

Darwinproposedanewexplanationtoaccountfortheacceptedphenomenonofevolution.Hearguedthatthepopulationofagivenspeciesatagiventimeincludesindividualsofvaryingcharacteristics.Thepopulationofthenextgenerationwillcontainahigherfrequencyofthosetypesthatmostsuccessfullysurviveandreproduceundertheexistingenvironmentalconditions.Thus,thefrequenciesofvarioustypeswithinthespecieswillchangeovertime.

ThereisanobvioussimilaritybetweentheprocessofevolutionasDarwindescribeditandtheprocessbywhichtheplantoranimalbreederimprovesadomesticstock.Theplantbreederselectsthehighest-yieldingplantsfromthecurrentpopulationand（asfaraspossible）usesthemastheparentsofthenextgeneration.Ifthecharacteristicscausingthehigheryieldareheritable,thenthenextgenerationshouldproduceahigheryield.ItwasnoaccidentthatDarwinchosethetermnaturalselectiontodescribehismodelofevolutionthroughdifferentialratesofreproductionofdifferentvariantsinthepopulation.Asamodelforthisevolutionaryprocess,hehadinmindtheselectionthatbreedersexerciseonsuccessivegenerationsofdomesticplantsandanimals.

WecansummarizeDarwin'

stheoryofevolutionthroughnaturalselectioninthreeprinciples:

1.Principleofvariation.Amongindividualswithinanypopulation,thereisvariationinmorphology,physiology,andbehavior.

2.Principleofheredity.Offspringresembletheirparentsmorethantheyresembleunrelatedindividuals.

3.Principleofselection.Someformsaremoresuccessfulatsurvivingandreproducingthanotherformsinagivenenvironment.

Clearly,aselectiveprocesscanproducechangeinthepopulationcompositiononlyiftherearesomevariationsamongwhichtoselect.Ifallindividualsareidentical,noamountofdifferentialreproductionofindividualscanaffectthecompositionofthepopulation.Furthermore,thevariationmustbeinsomepartheritableifdifferentialreproductionistoalterthepopulation'

sgeneticcomposition.Iflargeanimalswithinapopulationhavemoreoffspringthandosmallonesbuttheiroffspringarenolargeronaveragethanthoseofsmallanimals,thennochangeinpopulationcompositioncanoccurfromonegenerationtoanother.Finally,ifallvarianttypesleave,onaverage,thesamenumberofoffspring,thenwecanexpectthepopulationtoremainunchanged.

sprinciplesofvariation,heredity,andselectionmustholdtrueifthereistobeevolutionbyavariationalmechanism.

TheDarwinianexplanationofevolutionmustapplytotwodifferentaspectsofthehistoryoflife.Oneisthesuccessivechangeofformandfunctionthatoccursinasinglecontinuouslineofdescenttime,phyleticevolution.Figure26-1showssuchacontinuouschangeoveraperiodof40millionyearsinthesizeandcurvatureoftheleftshelloftheoyster,Gryphea.Theotheristhediversificationthatoccursamongspecies:

inthehistoryoflifeonearth,therearemanydifferentcontemporaneousspecieshavingquitedifferentformsandlivingindifferentways.Figure26-2showssomeofthevarietyofbivalvemolluscformsthatexistedatvarioustimesinthepast130millionyears.Everyspecieseventuallybecomesextinctandmorethan99.9percentofallthespeciesthathaveeverexistedarealreadyextinct,yetthenumberofspeciesandthediversityoftheirformsandfunctionshaveincreasedinthepastbillionyears.Thusspeciesnotonlymustbechanging,butmustgiverisetonewanddifferentspeciesinthecourseofevolution.Bothoftheseprocessesaretheconsequencesofheritablevariationwithinpopulations.Heritablevariationprovidestherawmaterialforsuccessivechangeswithinaspeciesandforthemultiplicationofnewspecies.Thebasicmechanismsofthosechanges（asdiscussedinChapter24）aretheoriginofnewvariationbyvariouskindsofmutationalmechanisms,thechangeinfrequencyofallelesbyselectiveandrandomprocesses,thepossibilityofdivergenceofisolatedlocalpopulationsbecausetheselectiveforcesaredifferentorbecauseofrandomdrift,andthereductionofvariationbetweenpopulationsbymigration.Fromthosebasicmechanisms,populationgenetics,asdiscussedinChapter24,derivesasetofprinciplesgoverningchangesinthegeneticcompositionofpopulations.Theapplicationoftheseprinciplesofpopulationgeneticsprovidesanarticulatedtheoryofevolution.

Evolution,undertheDarwinianscheme,istheconversionofheritablevariationbetweenindividualswithinpopulationsintoheritabledifferencesbetweenpopulationsintimeandinspace,bypopulationgeneticmechanisms.

Asynthesisofforces:

variationanddivergenceofpopulations

Inevolution,thevariousforcesofbreedingstructure,mutation,migration,andselectionareallactingsimultaneouslyinpopulations.Weneedtoconsiderhowtheseforces,operatingtogether,moldthegeneticcompositionofpopulationstoproducebothvariationwithinlocalpopulationsanddifferencesbetweenthem.

Thegeneticvariationwithinandbetweenpopulationsisaresultoftheinterplayofthevariousevolutionaryforces（Figure26-3）.Generally,asTable26-1shows,forcesthatincreaseormaintainvariationwithinpopulationspreventthedifferentiationofpopulationsfromeachother,whereasthedivergenceofpopulationsisaresultofforcesthatmakeeachpopulationhomozygous.Thus,randomdrift（orinbreeding）produceshomozygositywhilecausingdifferentpopulationstodiverge.Thisdivergenceandhomozygosityarecounteractedbytheconstantfluxofmutationandthemigrationbetweenlocalities,whichintroducevariationintothepopulationsagainandtendtomakethemmorelikeeachother.

WhenDarwinarrivedintheGalapagosIslandsin1835hefoundaremarkablegroupoffinchlikebirdsthatprovidedaverysuggestivecaseforthedevelopmentofhistheoryofevolution.TheGalapagosarchipelagoisaclusterof29islandsandisletsofdifferentsizeslyingontheequatorabout600milesoffthecoastofEcuador.Figure26-4showsthe13Galapagosfinchspecies.Finchesaregenerallyground-feedingseedeaterswithstoutbillsforcrackingthetoughoutercoatsoftheseeds.TheGalapagosspecies,thoughclearlyfinches,haveanimmensevariationinhowtheymakealivingandintheirbillshapesandtheirbehaviors,whichunderlytheseecologicaldifferences.Forexample,thevegetariantreefincheatsfruitsandleaves,theinsectivorousfinchhasabillwithabitingtipforeatinglargeinsects,and,mostremarkableofall,thewoodpeckerfinchgraspsatwiginitsbeakandusesittoobtaininsectpreybyprobingholesintrees.Thisdiversityofspeciesarosefromanoriginalpopulationofaseed-eatingfinchthatarrivedintheGalapagosfromthemainlandofSouthAmericaandpopulatedtheislands.Thedescendantsoftheoriginalcolonizersspreadtothedifferentislandsandto