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648SYSTEMATICBIOLOGYVOL.51

Syst.Biol.51(4):648-652,2002

DOI:10.1080/10635150290102320

UnitsinBiogeography

BERNHARDHAUSDORF

ZoologischesInstitutundZoologischesMuseumderUniversit¨atHamburg,Martin-Luther-King-Platz3, D-20146Hamburg,Germany;E-mail:hausdorf@zoologie.uni-hamburg.de

Inthepasttwodecades,vicariancebio-

geographyhasbecomeamajorsubdiscipline ofbiogeography.Thesubjectofvicariance biogeographyisthestudyofarearela- tionships(e.g.,NelsonandPlatnick,1981;

MorroneandCrisci,1995;Humphriesand

Parenti,1999).Animportantstepinthein-

vestigationofarearelationshipsisthetrans- formationofataxoncladogramintoan areacladogram.Differentdelimitationsof "unitareas"canresultindifferentoutcomes concerningarearelationships(Henderson,

1991).Therefore,areadelimitationisacru-

cialissue.Mostauthorsagreethatareasofen- demismshouldbetreatedasunits.However, thede?nitionandthedelimitationofareasof endemismarecontroversial.Whereasmost authorsconsideredanextensivesympatryof atleasttwospeciesasafundamentalrequire- ment(NelsonandPlatnick,1981;Platnick,

1991;Morrone,1994;Linder,2001),Harold

andMooi(1994)didnotrequiresympa- trybutratherusedcongruenceamongarea cladogramsasarecognitioncriterion.

AREASOFENDEMISM:DEFINITION

Althoughthereisgeneralagreementthat

areasofendemismaretheunitsinbiogeogra- phy,hardlyanyclearde?nitionofthatterm canbefoundintheliterature(Henderson,

1991;butseeHaroldandMooi,1994;Linder,

2001).Beforerecognitioncriteriaandthede-

limitationofareasofendemismcanbedis- cussed,thetermareaofendemismshould bede?nedclearly.Ade?nitionofareasof endemismcanbederivedfromthevicari- ancemodel,becausethetermhasbeenused mainlywithinthevicariancebiogeography framework.

Accordingtothevicariancemodel,an

ancestralbiotawasfragmentedbytheap- pearanceofabarrier.Thebarrierlimited orobviatedthegene?owbetweenpopula- tionsseparatedbythebarrier.Thisvicari- anceeventresultedinallopatricspeciationof manyofthespeciesformerlyconstitutingthe ancestralbiota.Inthisway,twonewbiotas emerged,whichareseparatedbythebarrier.

Thecausallinkbetweentheappearanceof

thebarrierandtheformationofnewbiotas isessentialinthevicariancemodel.Byrepeti- tionsofthisprocess,smallareaswithdistinct biotas,thatis,withmanyspeciesrestricted toindividualareas,emerge.Theseareareas ofendemism.Thus,areasofendemismcan bede?nedasareasdelimitedbybarriers,the appearanceofwhichentailstheformationof speciesrestrictedbythesebarriers.

Usually,vicarancecannotbeobserveddi-

rectlybecausethisprocessgenerallytakesge- ologictimeperiods.Therefore,itisimportant toderivepredictionsaboutobservablepat- ternsfromthemodel.

Onepredictionthatcanbederivedfrom

thevicariancemodelconcernsthespatial similaritiesofdistributionareasoriginating byvicariance.Afteravicarianceevent,there shouldbeagroupofspeciesrestrictedtoone sideofabarrierandagrouprestrictedto theothersideofthebarrier.Onaverage,the rangeofaspecieswillbemoresimilartothe rangesofotherspecieslivingonthesame sideofthebarrier,thatis,inthesameareaof endemism,thantotherangesofspeciesliv- ingontheothersideofthebarrier.Thus,two groupsoftaxa,eachwithsimilarranges,orig- inatefromavicarianceevent.Suchgroupsof taxahavebeencalledfaunalor?oristicele- ments.Therangesoftwospecieslivingonthe samesideofthebarriermaynotnecessarily overlap,especiallywhenthesespeciesdiffer ecologicallyandthusarerestrictedtodiffer- entportionswithintheareaofendemism.

Thede?nitionofareaofendemismin

thesenseofNelsonandPlatnick(1981:468),

Platnick(1991),Morrone(1994),andLinder

(2001)isbasedonspatialsimilaritiesofdistri- butionareas,thatis,relativelyextensivesym-

patryofatleasttwospecies(Platnick,1991).Downloaded from https://academic.oup.com/sysbio/article/51/4/648/1698781 by guest on 25 August 2023

2002POINTSOFVIEW649

FIGURE1.Hypotheticaldistributionsforthediscussionofthede?nitionofareaofendemism.(a)Twogroups oftwospecieseachwithrelativelyextensivesympatrydescribetwoareasofendemism.(b)Thedistributionarea ofoneofthreegroupsoftwospecieseachwithrelativelyextensivesympatryoverlapsthedistributionareaofthe othergroups.Thus,thesethreegroupsofspeciescannotdescribethreeareasofendemism.

Linder(2001:893)presentedanexplicitedef-

initionofareaofendemismaccordingto whichareasofendemismare"areasdelim- itedbythecongruentdistributionofatleast twospeciesofrestrictedrange."Icallthisthe standardde?nitionofareaofendemism.The hypotheticalexampledepictedinFigure1a demonstratestheprinciple.Therangesof fourspeciesareshown.Thedistributionsof species1and2,and3and4,respectively, aremoreorlesscoincident.Thus,theyde- ?netwoareasofendemism.However,the exampledepictedinFigure1bshowsthat thestandardde?nitionisinsuf?cienttode- cidewhichareasshouldbeconsideredareas ofendemism.Therearethreegroupsoftwo specieswithlargelycongruentranges.The rangesofspecies5and6overlapwiththose ofallotherspecies.Accordingtothede?ni- tionabove,therewouldbethreepartlyover- lappingareasofendemism.Thisinterpre- tationisnotcompatiblewiththevicariance model,whichstatesthatareasofendemism arenonoverlapping.

Asecondpatternthatcanbepredicted

basedonthevicariancemodelconcernsthe phylogeneticrelationshipswithinthetaxain- habitingtheareasofendemism.Becauseof thecausallinkbetweentheappearanceof barriersandtheoriginofnewspecies,area cladogramsbasedontaxonomiccladograms ofindividualtaxashouldbecongruentand shouldre?ecttheuniquehistoryofthear- eas(withtheexceptionofsomespeciesnot respondingtosomeofthevicarianceevents; seePlatnickandNelson,1978;Nelsonand

Platnick,1981).Thispatternisthebasisof

thede?nitiongivenbyHaroldandMooi (1994:262):anareaofendemismis"ageo- graphicregioncomprisingthedistribution oftwoormoretaxathatexhibitaphylo- geneticanddistributionalcongruenceand havingtheirrespectiverelativesoccurringin othersuch-de?nedregions."Contrarytothe suggestionsofNelsonandPlatnick(1981),

Platnick(1991),Morrone(1994),andLinder

(2001),thisde?nitiondoesnotrequirethat allspeciesoccurinalargeportionofthe areaofendemism;therangesofsomespecies maynotoverlapwithinanareaofendemism (e.g.,becausetheyarerestrictedtodifferent habitats).

WhereasHaroldandMooi"s(1994)de?ni-

tionandmyde?nitionofareasofendemism donotnecessarilyrequiretherelativeexten- sivesympatrydemandedbyPlatnick(1991), therequirementthattheareasofendemism bedelimitedbybarriers,theappearanceof whichentailedtheformationofspeciesre- strictedbythebarriers,ismorestringentthan

Platnick"s(1991)requirementofcongruent

distributionallimitsoftwoormorespecies.

Thatareasaredelimitedbybarriers,theap-

pearanceofwhichentailedtheformationof speciesrestrictedbythebarriers,isahypoth- esisthatmustbetested.HaroldandMooi (1994)proposedtotestthepredictionthatthe areacladogramsofdifferenttaxaarecongru- ent.Theproblemwiththisapproachisthat theunitsofthebiogeographicanalysismust bede?nedbeforethetaxoncladogramscan beconvertedintoareacladograms.Thepro- posedtestcanbeusedtodeterminewhether theprede?nedunitareasareareasofen- demism.Obviously,theunitsmustbede- ?nedbeforeareasofendemismcanberec- ognized.Thus,areasofendemismasde?ned hereorbyHaroldandMooi(1994)cannotbe theunitsofbiogeographicanalyses.

Areasofendemismarenotusedasbasic

unitsintheprocedureproposedbyHarold andMooi(1994).Intheirprocedure,areasof occurrencearethebasicunits.Axelius(1991) showedthattheuseofdistributionareasas basicunitsfortheconstructionofareaclado-

gramscanresultinparadoxicalconclusionsDownloaded from https://academic.oup.com/sysbio/article/51/4/648/1698781 by guest on 25 August 2023

650SYSTEMATICBIOLOGYVOL.51

(e.g.,anareaismorerelatedtoadifferent areathantosomepartofitself)whendis- tributionareasoverlap.HaroldandMooi (1994)avoidedthisproblembycombiningall (partly)overlappingareas.Thus,theywere unabletorecognizeareasofendemismwhen dispersalhadoccurred.

AREASOFENDEMISM:DELIMITATION

Althoughthestandardde?nitionofareaof

endemismisbasedondistributionpatterns ofspecies(moreorlesscongruentdistribu- tionalboundariesofatleasttwospecies),ge- omorphologicalorpalaeogeographicalunits (e.g.,islands,continents,mountainranges) areconsideredareasofendemisminmost empiricalstudieswithoutanexplicitanaly- sisofdistributionpatterns.Thisassumption isprobablymadebecauseofproblemsinde- limitingareasofendemism.

Thedelimitationofareasofendemismis

notproblematicwhenspeciesoriginateby vicarianceandthereisnodispersal.Under theseconditions,therearegroupsofspecies havingseparateranges(asinFig.1a).How- ever,thedelimitationofareasofendemism becomesproblematicwhendispersaloc- curs.Theoccurrenceofdispersalhasnever beendoubtedbyvicariancebiogeographers (PlatnickandNelson,1978).Withoutdisper- sal,therewouldbecontinuitywithinand discontinuitybetweenareasofendemism.

Thisconditionhasbeencitedasarecognition

criterionofbioticprovinces(Peters,1955).

However,changesinspeciescompositionare

gradualshiftsoverlargezonesratherthan sharpbreaksbetweenneighboringhomoge- neousareas(Peters,1955;Kaiseretal.,1972).

Considerthedistributionareasofthe

10speciesdepictedinFigure2.Assumethat

thesituationistheresultofavicarianceevent separatingeasternandwesternspeciesand subsequentdispersal.Howlargewerethear- easofendemism?Weretheyaslargeasthe distributionareasofspecies3and9or2and

7,orweretheyevensmallerthanthedis-

tributionareasofspecies5and6?Thebio- geographicaldataaloneareinsuf?cientfor delimitingtheareasofendemism.Dispersal decouplesthehistoryofspeciesfromthehis- toryofareas.Thebest-knownrealexample ofthegeneralproblemdepictedinFigure2 isthedelimitationoftheOrientalandthe

Australianregions.Althoughmanyauthors

haveaddressedthissubject,itisnotpossible

FIGURE2.Hypotheticaldistributionsof10species

demonstratingthedif?cultyinvolvedindelimiting areasofendemismbasedonbiogeographicaldataalone whendispersalhasoccurred. todeterminethelimitsofthesetworegions onthebasisofbiogeographicaldata(Mayr,

1944;HollowayandJardine,1968;Simpson,

1977;Vane-Wright,1991).Mayr(1944)pro-

poseddrawingtheborderlinebetweenthe twozoogeographicregionsintheareawhere thefaunalelementsofthetworegionsinter- minglesuchthatthefaunalelementofone regionprevailsononesideoftheborderline andthefaunalelementoftheotherregion prevailsontheotherside.However,Mayr admittedthatthislinewouldbeanarbitrary separationofacontinuousseriesofvalues atthehalfwaypointbetweentheextremes andthatthelinecouldbedifferentfordif- ferenttaxa.Ifitisnotpossibletoestablish theborderbetweentworegionsseparatedby severalhundredkilometersofoceanonthe basisofbiogeographicaldata,itwillhardly bepossibletodelimitareasofendemismsep- aratedonlytemporarilybyclimaticbarriers onacontinent,forexample.

InsituationssuchasthatinFigure2,the

operationalmethodsproposedforidentify- inganddelimitingareasofendemismby

Morrone(1994)andLinder(2001;seealso

LinderandMann,1998)cannotbeused

to?ndthelimitsofareasofendemism.

Moreover,thereasonsforusingparsimony

algorithmsdesignedtoreconstructadi- chotomoussplitsequencetodelimitareasof endemismareunclear,becausethedistribu- tionoftaxaingridquadratsisnottheresultof suchasplitsequencebetweenthequadrats.

Existingmethodsalsodonottestwhethera

distributionpatternisnonrandom.

Hovenkamp(1997)triedtoevadethe

problemofdelimitingareasofendemism byfocusingonsupposedvicarianceevents

insteadofareasofendemism.However,inDownloaded from https://academic.oup.com/sysbio/article/51/4/648/1698781 by guest on 25 August 2023

2002POINTSOFVIEW651

situationssuchasthatshowninFigure2, thisapproachisnosolutionbecausethepo- sitionofthebarriercausingthevicariance eventcannotbelocatedbecauseofsubse- quentdispersal(atleastnotwithbiogeo- graphicaldata).

BIOTICELEMENTSASBIOGEOGRAPHIC

UNITS

ReconsiderthesituationdepictedinFig-

ure2.Itisnotpossibletodeterminebyan analysisofdistributionareaswhichspecies dispersedhowfarortodelimittheareas ofendemismwiththeavailableinformation.

However,bycomparingthedistributionar-

easofthevariousspecies,twogroupsof specieswithsimilarrangescaneasilyberec- ognized.Suchgroupshavebeencalledfau- nalor?oristicelementsorgroups(e.g.,Rebel,

1931;deLattin,1957;Mayr,1965;Holloway

andJardine,1968;Udvardy,1969;Jardine,

1972;Birks,1987;Frey,1992;Dennisetal.,

1998).Ide?nebioticelementasagroupof

taxawhoserangesaresigni?cantlymoresim- ilartoeachotherthantothoseoftaxaofother suchgroups.Acomputationalmethodfor theanalysisofbioticelementswasdescribed byHausdorfandHennig(unpubl.).

Theexistenceofbioticelementsispre-

dictedbythevicariancemodel(seeabove), andincontrasttoareasofendemism,biotic elementscanbedeterminedbyusingdistri- butiondataalone.Hence,bioticelementsare suitableforuseasbiogeographicunits.

Areasofendemismaresuitableforuseas

biogeographicunitsonlywhenspeciesorig- inatebyvicarianceandthereisnodisper- sal.However,thesepresuppositionsarenot testedinvicariancebiogeographicstudies.

Vicariancebiogeographyregardsdispersal

asasecondaryprocessthatcreatesnoisein thedata.However,bioticelementsdonot presupposeaspeci?cspeciationmodeand aresuitableunits,evenwhenextensiverange expansionsorrangeshiftshaveoccurred, suchasinresponsetoclimaticchanges(Frey,

1992).Theconceptofbioticelementstriesto

summarizeallgeneralitiesofthegeographi- caldistributionsoforganisms.

Thedelimitationofbioticelementsisonly

the?rststepinabiogeographicanalysis.

Whether(partsof)bioticelementsarehis-

toricalunitsandhowfartheyarein?uenced byecologicalfactorsmustbeinvestigatedby furtheranalyses.

Ifbiotasevolveaccordingtothevicariance

modelwithoutdispersal,eachbiotawould consistofasinglebioticelementandthe bioticelementswouldnotoverlap.Justas sympatryofsistergroups,sympatryofbiotic elementsisevidencefordispersal.Ifabiota consistsofasinglebioticelement,thereis noevidenceforvicarianceeventswithinthat biota.Onlywhendifferentbioticelements canbedistinguishedisitmeaningfultoin- vestigatehowtheyhavebeenin?uencedby vicarianceanddispersal.

Bioticelementsarenotnecessarilygen-

eratedbyvicarianceevents.Bioticelements canalsooriginatewhenanareaiscolonized fromdifferentsourceareasacrosspreexist- ingbarriersbychancedispersalandwhen thepopulationsintheareaunderconsider- ationevolveintonewspecies.Anotherpos- siblescenarioisthatspeciesthatoriginated inseparateareascolonizethesamelargerre- gion,eitheraftertheremovalofbarriersor bychancedispersal.

Insuchcases,bioticelementsaregeo-

graphicalbutnothistoricalunits.Whether bioticelements(orpartsofthem)arehis- toricalunitscanbetested,suchasbya comparisonofthecladogramsoftherespec- tivegroups.Thetaxoncladogramscanbe convertedintoelementcladogramsbyre- placingthetaxonnameswiththerespec- tivebioticelement(analogoustothearea cladogramapproach).Theelementclado- gramsofthosegroupsthatformhistorical unitsshouldmatch.Thepartsofthebiotic elementsthatformhistoricalunitscanbe termedvicarianceelements.

Someproblemswiththeareacladogram

approachcanbesolvedbyusingbioticel- ementsasunitswiththeelementclado- gramapproach.Oneimportantissuecon- cernsgroupswithdifferenthistoriesinthe samearea.Forexample,theterrestrialmol- luscfaunaofnortheasternAfricaisdom- inatedbyPalearcticgroups,whereasthe freshwatermolluscfaunaisdominatedby

Ethiopiangroupsthatcolonizednortheast-

ernAfricathroughtheNile(Pallary,1909).

Areacladogramsbasedonterrestrialsnails

implythatnortheasternAfricaisrelatedto otherMediterraneancountries,whereasarea cladogramsbasedonfreshwatermolluscsin- dicatethatnortheasternAfricaisrelatedto partsofsubsaharanAfrica.Aconsensustree ofsuchcladogramswouldbeunresolved.

TheareacladogramapproachdoesnotrevealDownloaded from https://academic.oup.com/sysbio/article/51/4/648/1698781 by guest on 25 August 2023

652SYSTEMATICBIOLOGYVOL.51

informationonthehistoryofnortheastern

Africabecausethefaunaiscomposedofdif-

ferentelementswithdifferenthistoriesthat cannotbeshowninasingleareacladogram.

Ananalysisofthebioticelementsisamore

appropriateŽrststeptowardunderstanding thegenesisofcompositebiotas.

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Syst.Biol.51(4):652-664,2002

DOI:10.1080/10635150290102311

ResolutionofaSupertree/SupermatrixParadox

JOHNGATESY,1CONRADMATTHEE,2ROBDESALLE,3ANDCHERYLHAYASHI1

1DepartmentofBiology,UniversityofCalifornia,Riverside,California92521,USA

2DepartmentofZoology,UniversityofStellenbosch,Stellenbosch7602,SouthAfrica

3DepartmentofInvertebrates,AmericanMuseumofNaturalHistory,CentralParkWestat79thStreet,

NewYork,NewYork10024,USA

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Firstsubmitted9May2001;reviewsreturned

22March2002;?nalacceptance2May2002

AssociateEditor:PeterLinder

fortheuseof"supertrees"insystemat- ics(Sandersonetal.,1998).Inthisframe- work,individualcharactersarenotinter- pretedasphylogeneticevidence.Instead, topologiessupportedbydifferentpublished

studies,thatis,sourcetrees,areencodedintoDownloaded from https://academic.oup.com/sysbio/article/51/4/648/1698781 by guest on 25 August 2023

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