Adaptive speciation theory.docx

Adaptive speciation theory.docx

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Adaptivespeciationtheory

Adaptivespeciationtheory:

aconceptualreview

Abstract：

Speciation—theoriginofnewspecies—isthesourceofthediversityoflife.Atheoryofspeciationisessentialtolinkpoorlyunderstoodmacro-evolutionaryprocesses,suchastheoriginofbiodiversityandadaptiveradiation,towellunderstoodmicro-evolutionaryprocesses,suchasallelefrequencychangeduetonaturalorsexualselection.Animportantquestioniswhether,andtowhatextent,theprocessofspeciationis‘adaptive’,i.e.,drivenbynaturaland/orsexualselection.Here,wediscusstwomainmodellingapproachesinadaptivespeciationtheory.Ecologicalmodelsofspeciationfocusontheevolutionofecologicaldifferentiationthroughdivergentnaturalselection.Thesemodelscanexplainthestablecoexistenceoftheresultingdaughterspeciesinthefaceofinterspecificcompetition,buttheyareoftenvagueabouttheevolutionofreproductiveisolation.Mostsexualselectionmodelsofspeciationfocusonthediversificationofmatingstrategiesthroughdivergentsexualselection.Thesemodelscanexplaintheevolutionofprezygoticreproductiveisolation,buttheyaretypicallyvagueonquestionslikeecologicalcoexistence.Bymeansofanintegratedmodel,incorporatingbothecologicalinteractionsandsexualselection,wedemonstratethatdisruptiveselectiononbothecologicalandmatingstrategiesisnecessary,butnotsufficient,forspeciationtooccur.Toachievespeciation,matingmustatleastpartlyreflectecologicalcharacteristics.Theinteractionofnaturalandsexualselectionisalsopivotalinamodelwheresexualselectionfacilitatesecologicalspeciationevenintheabsenceofdivergingfemalepreferences.Inviewoftheseresults,itiscounterproductivetoconsiderecologicalandsexualselectionmodelsascontrastingandincompatibleviewsonspeciation,onebeingdominantovertheother.Instead,anintegrativeperspectiveisneededtoachieveathoroughandcoherentunderstandingofadaptivespeciation.

Keywords:

Speciationmodels.Prezygoticisolation.Postzygoticisolation.Reinforcement.Disruptiveselection.Evolutionarybranching.Competitivespeciation.Sexualselection.Fisherianrunawayprocess.Good-genesmodels.Condition-dependentornament

Introduction:

Recently,thescientificworldcelebratedthe150thanniversaryofthereleaseofCharlesDarwin'sseminalbookOntheOriginofSpeciesbyNaturalSelection.Darwin'sideas（Darwin1859;Darwin1871）onnaturalandsexualselectionhaveinspiredgenerationsofbiologists,andwehavenowafairlygoodcomprehensionofhowselectionactswithinpopulationsandhowpopulationsaretransformedundertheinfluenceofselectionpressures.Itisimportanttorealise,however,thatmuchofourunderstandingofselection-inducedchangesrelatestotheprocessofanagenesis,thegradualevolutionofwholepopulations.Asimilarunderstandingoftheroleofselectionincladogenesis,thesplittingofspeciesintoreproductivelyisolatedunits,islargelylacking.Despiteofitstitle,theOrigindidactuallynotcontributemuchtoresolvingthequestionwhetherand

howspeciationisdrivenbynaturalselection.Asoundunderstandingofspeciationisofkeyimportanceforevolutionarytheory,sincethebirthofnewspeciesisthecruciallinkbetweenmicro-evolution（thatmainlyoccursatorbelowthespecieslevel）andmacro-evolutionaryprocesseslikeadaptiveradiationsthatlargelyoccurabovethespecieslevel.Ifspeciationtendstobeadaptive,thatis,drivenbydirectionalforceslikenaturalorsexualselection,thenone

couldhopeforachievinganoverarchingadaptivetheoryincludingbothmicro-andmacro-evolution.

Darwin（Origin,chapter4）envisagedspeciationastheresultoftwoprocesses:

selectionfordiversificationallowingtheexploitationofpreviouslyunusedopportunities,andtheextinctionofintermediateformsasaconsequenceofseverecompetitionamongtheseforms.Hence,accordingtoDarwin,selectionplaysamajorroleinthespeciationprocess.Infact,hisviewcomesclosetomodernideasoncompetitivespeciationtobediscussedbelow.However,Darwin'sverbalargumentsareoftenvagueandnotalwaysconvincing,partlybecauseofhispre-Mendelianideasoninheritance.Itispartlyforthisreasonthatthefoundingfathersofthe‘ModernSynthesis’largelydiscardedDarwin'sviewonspeciation（MayrandProvine1998）,givingnonselectivefactorslikegeographicisolationamuchmoreprominentrolethanselection.

EversinceDarwin,theoreticalargumentshaveplayedanimportantroleindebatesonthecausesofspeciation.Inthesecondhalfofthe20thcentury,morethan100mathematicalmodelshavebeendevelopedtostudytheroleofselectioninspeciationwithgeneflow（reviewedinKirkpatrickandRavigné2002）.Althoughmanyofthesemodelsdemonstratethatselection-drivenspeciationispossibleinprinciple,theytypicallyleadtotheconclusionthatselection-drivenspeciationwillonlyoccurunderhighlyspecificconditionsorforratherextremeparametercombinations（Felsenstein1981）.Accordingly,speciationmodelsseemedtosuggestthatadaptivespeciationisarareandunlikelyphenomenon.Thisconclusionischallengedbytworecentdevelopmentsinspeciationtheory,whichseemtosuggestthatnaturalandsexualselectioncanbemorepowerfulinthecreationofnewspeciesthanthetraditionalmodelsseemtosuggest.First,avarietyofecologicalspeciationmodelshasbeendeveloped（Dieckmannetal.2004）thatshowthatDarwin'sintuitivenotionofcompetitivespeciationcanbegivenatheoreticalunderpinning.Thesemodelsarebasedonadynamicviewofnaturalselection,allowingselectiontoswitchfromstabilisingtodisruptiveinthesamecoherentframework.TherealisationthatDarwinianfitnessishighlycontextdependentanddynamic,andthatinadiversityofsettings,naturalselectioncandriveapopulationtowardaregimeofongoingdisruptiveselection,makesselection-drivenspeciationmoreplausiblethanintraditionalmodels.Second,varioussexualselectionmodelsofspeciation（Ritchie2007）givedisruptivesexualselectionaprominentplaceinthespeciationprocess.Severalstudies（e.g.,TurnerandBurrows1995;Higashietal.1999）demonstratethat,underspecificcircumstances,sexualselectionmayleadtothedivergenceoffemalepreferenceswithinasinglepopulation,eventuallyleadingtoreproductiveisolation.

Bothdevelopmentshaveinitiatedafiercedebateinthescientificliterature.Modelsofspeciationdrivenbydisruptiveselectionareinherentlycomplexandthereforehavetomakemanysimplifyingassumptions.Thequestionthereforeariseswhethertheresultsofthesemodelsaregeneralandrepresentativeforreal-worldsystems,orwhetherinstead,theymainlyreflectmodellingdetailsorthechoiceofparametersandinitialconditions.Incaseofecologicalspeciationmodels,theanalysisisoftenbasedonconceptsofadaptivedynamicstheory,whichhavebeenheavilycriticised（e.g.,WaxmanandGavrilets2005）anddefended（e.g.,DoebeliandDieckmann2005）.Therepresentativenessofsimulationmodelsofecologicalspeciationhavebeenquestioned（e.g.,Gavrilets2004;butseeDoebeliandDieckmann2005）becauseoftheirassumptionsonthegeneticarchitectureoftraits,theirchoiceofparameterslikemutationrateorpopulationsize,andtheirassumptionsunderlyingtheevolutionofassortativemating（Matessietal.2001）.Likewise,modelsofspeciationthroughdisruptivesexualselectionhavebeencriticised（e.g.,VanDoornandWeissing2001;ArnegardandKondrashov2004;VanDoornetal.2004）becauseoftheirhighlyspecialinitialconditionsandthelackofstabilityoftheincipientspeciesinthefaceofecologicalcompetition.Fornon-specialists（andnotonlyforthem!

）,itisincreasinglydifficulttojudgethescopeandvalidityoftheargumentsandcounterargumentsinthisdebate.Thisispartlyaconsequenceofthehighdegreeoftechnicalsophisticationrequiredforsettingupandanalysingadaptivespeciationmodels.Moreover,proponentsandopponentstendtobasetheirviewsonmodelvariantsthatdiffertosuchanextentintheirassumptionsthatthemodeloutcomesarenotdirectlycomparable.

Insituationslikethese,itisoftenimpossibletosaythatonemodelvariantis“inherently”betterthananalternativeone.Selection-drivenspeciationresultsfromtheintricateinterplayofprocessesatthephenotypiclevel（wherenaturalandsexualselectionareoperating）andatthegeneticlevel（where,forexample,linkagedisequilibriahavetoevolvebetweenecologicalandmatingcharacters）,andmodelsfocusingsimultaneouslyonbothlevelstendtobeintractable.Themoretraditionalspeciationmodelstendtohavetheirfocusongeneticprocessesandareoftenquitesophisticatedinthisrespect.Onthedownside,theyoftenmakesimplisticassumptionsonselectionandmating,whichcannoteasilybegivenamechanisticinterpretationandwhichtypicallylackapopulationdynamicalunderpinning.Incontrast,theadaptivedynamicsschooltendstoderivefitnessfromfirstprinciples;fitness“emerges”frommechanisticandpopulationdynamicalconsiderations（Metz2008）.Onthedownside,theassumptionsontheunderlyinggeneticsareoftensimplisticandunrealistic.Itisimportanttorealisethatbothapproacheshavetheirvirtuesandtheirshortcomings;andthateachapproachshedslightondifferentaspectsoftheprocessofspeciation.Althoughthediversityofmodelsandmodeloutcomesmaylookconfusing,apluralisticapproachmayactuallybethebestresearchstrategyforachievingrobustinsights.Speciationtheoryistheprototypeexampleofaresearchfieldwherescientifictruthcanonlybeapproachedt