26 an introduction to genetic analysis.docx
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26 an introduction to genetic analysis.docx
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26anintroductiontogeneticanalysis
Chapter26
EvolutionaryGenetics
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.
MESSAGE
Darwin'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.
MESSAGE
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
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