The Extinction and De-Extinction of Species

Linfield University DigitalCommons@Linfield

Faculty Publications Faculty Scholarship & Creative Works

2017

The Extinction and De-Extinction of Species

Helena Siipi University of Turku

Leonard Finkelman Linfield College

Follow this and additional works at: https://digitalcommons.linfield.edu/philfac_pubs

Part of the Biology Commons, and the Philosophy of Science Commons

DigitalCommons@Linfield Citation Siipi, Helena and Finkelman, Leonard, "The Extinction and De-Extinction of Species" (2017). Faculty Publications. Accepted Version. Submission 3. https://digitalcommons.linfield.edu/philfac_pubs/3

This Accepted Version is protected by copyright and/or related rights. It is brought to you for free via open access, courtesy of DigitalCommons@Linfield, with permission from the rights-holder(s). Your use of this Accepted Version must comply with the Terms of Use for material posted in DigitalCommons@Linfield, or with other stated terms (such as a Creative Commons license) indicated in the record and/or on the work itself. For more information, or if you have questions about permitted uses, please contact [email protected]. The extinction and de-extinction of species 

I. Introduction  WhendeathcameforCelia,ittooktheformofƒtree.Heedlessofthedangerposed bybranchesoverladenwithsnow,CeliawanderedthroughthelandscapeofSpain’s OrdesanationalparkinJanuary2000.branchfellonherskullandcrushedit.So deathcameandtookher,leavingƒbodytobefoundbyparkrangersandƒlegacyto bemournedbyconservationistsaroundtheworld.  Theconservationistsmournednotonlythedeathoftheorganism,butalsoan attendantdecreaseinbiodiversity.Celiawasthelastmemberofthesubspecies CaprapyrenaicapyrenaicaǡbetterknownasthePyreneanibex;whenshedied,so toodidthetaxonbecomeextinct.  Wheremoststorieswouldend,Celia’sstory—oratleastthestoryofherDNA—had justbegun.Biologistshadcollectedtissuesamplesfromherbodyshortlybeforeher passing.Usingcellsculturedfromthosesamples,scientistsworkingforthe companyAdvancedCellTechnologysetaboutcloningCelia.Thefruitoftheirlabor wasbornin2009andsurvivedforsevenminutesbeforesuccumbingtolung problems.Celia’sclonedidrepresentƒmaterialtriumphofresurrectionbiology, morepopularlyknownasde-extinction(Folchetal2009;Pina-Aguilaretal2009).  Atthemoment,numerousresearchgroupsaroundtheworldareworkingtowards de-extinctionofdifferentspecies.Effortsareunderway,forexample,toengineerƒ passengerpigeon(EctopistesmigratoriusȌfromthegenomeoftherelatedband- tailedpigeon(PatagioenasfasciataȌ(Zimmer2013;O’Connor2015).Woolly mammothsmightbeclonedfromthetissuespreservedinthepermafrostofthe Siberiantundra(Loietal2011).Alternatively,anIndianelephant(Elephas maximusȌgenomemightserveasƒtemplate(Salsberg2000).Severalgroupsare workingtowardscloningTasmaniantigers(Greer2009).  Worriesregardinganthropogenicextinctionhavethisfar,atleastpartly,beenbased onƒviewaboutirreversibilityofextinction.AsJohan-WolfgangWägele(2014)puts it,“[…]thepoliticalsupportforactionsthatcanmitigatebiodiversitylossesishalf- heartedandinadequate.Thisisextremelydangerous,becauselossofspeciesis irreversibleǤ[…]lifeforms,oncelost,cannotregenerate.”Thedevelopmentsin resurrectionbiologyquestionthisvarybasictenetofconservation.Resurrection biologyistakentoimplythat“therevivalofanextinctspeciesisnolongerƒfantasy” (Zimmer2013)orthat“extinctionmightnotbeforever[…]”(Redfordetal.2013). Consequently,developmentsinresurrectionbiologyareoftenmetwithenthusiasm.  Yet,thepossibilityoffuturesuccessinresurrectionbiologyraisesmanyquestions. Someareempirical:canresurrectedspeciesbereintroducedintothewild?What aretheenvironmentalcostsandbenefitsofspeciesresurrection?Otherquestions arephilosophical.Inthisessayweaddresssomeofthesephilosophicalquestions.In particular,weconsiderwhetherthegoalsofresurrectionbiologyareconceptually coherent.Ourinquiryanalysestworelatedconceptsrelevanttotheresurrection: theconceptofextinctionandtheconceptofspecies.Throughtheanalysesofthem wedemonstrate,first,theimplicationsthatresurrectionbiologymayhaveforthe conceptualfoundationsoflifesciences,andsecond,implicationsthatdifferent speciesconceptsandunderstandingsofextinctionmayhaveonthestatusof animalsproducedbyde-extinctiontechnologies.  Beforeengagingourinquiry,wewillfirstbrieflydescribe“de-extinction”techniques andtechnologiesinPartIIbelow.InPartIIIwepresentfourdifferentspecies conceptsandthreedifferentwaysofunderstandingextinction.Wealsoraisethe challengeofseeingextinctionassomethingnecessarilyfinal.InPartIVweapplythe presentedconceptstoresurrectionbiologyanddiscussthepossiblewaysoffitting resurrectionbiologytogetherwiththeideaoffinalityofextinction.Inpartwe discussthespeciesandextinctionconceptsthatarecompatiblewithresurrection biologyifextinctionisnotseenasnecessarilyfinal.Finally,weconcludeinPartVI withthoughtsonfurtherimplicationsofouranalysis.

II. De-extinction  Thegoalofresurrectionbiologyisdeceptivelysimpleinitsarticulation:itistomake extinctspeciesextantoncemore.Aswewillsee,however,thissimpleformulation begsƒnumberoftheoreticalquestions.Phrasedmoreneutrally,resurrection biologyaimstoproduceanimalsthatare(toƒhighdegree)similartomembersof extinctspecies.Thesekindsofanimalscanbecreatedeitherthroughselective breedingorthroughdifferentapplicationsofcloningtechnologies.  Resurrectionbiology’searliestattemptstooktheformof“back-breeding”(Oksanen ƬSiipi2014).Throughwell-practicedmethodsofhusbandry,skilledbreedersmay crossextantlineagestowardstheendofreplicatingphenotypes,andperhapseven genotypes,ofclosely-relatedextinctlineages.Thismethodisstructurallyidenticalto otherformsofartificialselection,withonekeydifference:insteadofproducingnew organismsinexistingbreedsornewtypesofbreeds,thegoalistoproduceanimals thataresimilartomembersofextinctbreeds.Morerecently,conservationistshave consideredthepossibilitythatthistechnique,alsoknownas“lineagefusion,”could resurrectextinctsubspeciesofGalápagostortoises(Poulakakisetal2008;Garrick etal2014).  Themorepublicizedversionsofresurrectionbiologyarebasedonapplyingthe cloningtechnologies(Zimmer2013).Infact,therearetwomethodsbywhichthe goalofresurrectioncanbeachieved:throughsomaticcellnucleartransferor throughgeneticengineering(SherkowƬGreely2013;OksanenƬSiipi2014).  Resurrectionbysomaticcellnucleartransfer(abbreviatedtoSCNT)isquitesimilar to“ordinary”cloningexemplifiedinthefamedsheepDolly.SCNT’susein resurrectionbiologyispopularizedbyworkssuchasJurassicParkanditis sometimescalledcross-speciescloningǤ(Wilmutetal1999;Zimmer2013.)The resurrectionprocessbeginswiththecultivationofƒsomaticcell’snucleusfromthe tissueoftheextinctspecies.Thatnucleus,includingitsfullcomplementofgenetic material,istheninsertedintoanenucleatedeggofanotherspecies.memberof thatspeciesworksasƒsurrogatemother.Whencarriedtoterm,theprocedure producesnearly-identicaltwinstothemembersoftheextinctspecies;theonly geneticdifferenceswouldbeintheorganisms’mitochondrialandimmunecellDNA (Hiendlederetal2004).  SCNTisthemeansbywhichCeliawascloned.Thismethodisalsostudiedasƒ meansofresurrectingspeciessuchasgastricbroodingfrogs(RheobatrachussilusȌ (Archer2013),woollymammoths(MammuthusprimigeniusȌ(Loietal2011),and Tasmaniantigers(ThylacinuscynocephalusȌ(Greer2009).  Asƒmethodofresurrectionbiology,geneticengineeringissimilartoSCNTinthat anorganismisproducedfromtheimplantationofƒmodifiedembryointoƒ surrogatemother;thedifferencebetweenthemethodsliesintheoriginofthe embryo’sgeneticmaterial.De-extinctionthroughgeneticengineeringbeginswith thecultivationofgeneticmaterialfromƒmemberofƒsistertaxontothetarget extinctspecies,ratherthanfromƒtissueoftheextinctspeciesitself.Whengenomic differencesbetweentheextinctspeciesanditssistertaxoncanbeidentified,the geneticmaterialfromthedonororganismcanbemodifiedtomatchtheextinct species’genome.Themodifiedgeneticmaterialisplacedintothedonornucleus, whichistheninsertedintoanenucleatedegg,whichisinturnimplantedintothe surrogatemother.Thismethodhastheadvantageofpotentiallyincreasingthe geneticsimilaritybetweencloneanddonororganism:engineersmightreconstitute thedonor’smitochrondrialDNA,therebyoffsettingpotentialproblemsintheSCNT processsuchasthoseoutlinedbyHiendlederetal(2004).  passengerpigeon(EctopistesmigratoriusȌmightberesurrectedthiswayfromthe genomeoftherelatedband-tailedpigeon(PatagioenasfasciataȌ(Zimmer2013; O’Connor2015).HornerƬGorman(2009)goasfarassuggestingthatnon-avian dinosaursmightbeengineeredfromthegenomeofƒdomesticchicken;Bhullaretal (2015)havemadeprogressinderivingƒdinosaurphenotypefromthechicken’s genotype1Ǥ  1Thereproductionofnonaviandinosaurtraitsdiffersfromtheresurrectionofthe passengerpigeoninoneimportantrespect:thelatterwouldbetheresultof replicatinganorganism’sentiregenomewhereastheformerwouldbetheresultof replicatinganorganism’sgenesequences.Nevertheless,thisisƒdifferenceof degreeratherthanofkind;certainly,ifƒsufficientnumberofnonaviandinosaur traitsweretobereplicatedthentheentiretyofƒnonaviandinosaurphenotype  Eachofthesemethodsofresurrectionbiologyfacespracticalobstaclesand limitations.Towit:back-breedingcanonlyresurrectextincttaxabelowthespecies category;SCNTandgeneticengineeringareviableonlyforextincttaxawhose developmentalenvironmentsweresufficientlysimilartothoseofsurrogates; geneticengineeringassumesantecedentknowledgeoftheextincttaxon’sgenome. Nevertheless,manyoftheobstaclesarecontingentonthestateofourtechnological art.Thesuccessofresurrectionbiologymaynotbeinevitable,butitmaybelikelier thannot(Stone2003;Zimmer2013).  Inadditiontothesepracticalobstacles,however,therearesignificantconceptual challengestobemet.Inthispaperwequestionthecommonviewsregardingthe successofresurrectionbiology.Itisnotself-evidentthatanimalscreatedbyabove describedmethodsaremembersoftheextinctspecies(e.g.Pyreneanibex,woolly mammoth,passengerpigeon,orgastricbroodingfrog).Oneneednottakeitfor grantedthattheirreversibilityofextinctioncan(orwill)bechangedbythe technologicaldevelopmentsdescribedabove.

III. Concepts of species and extinction  III.1.Speciesconcepts  Oneofthemostintractableissuesinthephilosophyofbiologyisthespecies problem.Wedonotexpecttoresolveherethequestionofhowtoconceiveand delineatespecies,butwedobelievethattheforegoingdiscussioncarries implicationsthatshoulddirectfuturedebates.theorist’sviewofresurrection biologyshouldbecompatiblewiththetheorist’schoiceofspeciesconcept.  Onereasonthatdebateoverthespeciesproblemremainssovigorousisthat disputantscannotagreeonwhatexactlyisatstake.Thenumberofspeciesconcepts currentlyunderconsiderationmayrangeanywherebetweenone(Wilkins2009) andmorethantwodozen(Mayden1997).Wewillnotenumeratethevarietyof speciesconceptshere.Instead,wefollowOkasha(2002)indistinguishingfourbasic categoriesofspeciesconcepts.Thisschemashouldbebroadenoughto accommodateanyreader’schosenconcept;itshouldalsoremaininformative enoughtodemonstratehowone’sviewsregardingthesuccessandpossibilityof resurrectionbiologyandthechoiceofspeciesconceptarenecessarilyclosely connectedtoeachother.  Okashaarguesthatspeciesconceptsmaybefundamentallycategorizedasphenetic, biological,ecological,orphylogenetic.Pheneticconceptsdefinespeciesmembership intermsofoveralltraitsimilarity.Conceptsfallingunderthiscategory,including Boyd’shomeostaticpropertyclustertheory(1999)orDevitt’snewbiological  wouldfollow.Whetherornotscientistscanknowwhentheyhavesucceededin replicatingƒdinosaurphenotypeisƒseparateepistemologicalquestion. essentialism(2008),tendtotreatspeciesasnaturalkindsoratleastassetsof organisms(Kitcher1984).Biologicalconcepts,tracingmostexplicitlybackto Dobzhansky(1937),definespeciesmembershipintermsofintrinsicreproductive isolatingmechanisms.Organismsarethenseentobelongtoƒsamespeciesifitis actuallypossibleforthemtointerbreed.2Ecologicalconcepts,bycontrast,define speciesmembershipintermsofextrinsicreproductiveisolatingmechanisms. Speciesasconceivedbiologicallyorecologicallyarecountedasunitsinthe biologicalontology,eitherasindividuals(Ghiselin1987)orassets(Kitcher1984; seealsoEreshefsky2010).Phylogeneticconceptsdefinespeciesmembershipin termsofgenealogicalhistory,asinthelineageconceptsendorsedbySimpson (1961)andDeQuieroz(1998).  Thedescriptionofspeciesconceptsrarely,ifever,includesconditionsforƒspecies’ extinction.Ghiselin(1987)arguesthattheextinctionofƒspeciesisanalogouswith thedeathofanorganism,butthisisnotanespeciallyinformativeclaimgiventhe vaguenessoforganismaldeath(Zandtetal2011).Towit:onemayendorse Dobzhansky’sbiologicalspeciesconcept—asGhiselindoesandcollege-leveltexts oftendo—andthereforeconceivespeciesasunits,butthiscommitmentofferslittle helpinclassifyingpoorCeliaatthetimeofherdeath.Ontheonehand,thelastC. pyrenaicapyrenaicawasobviouslynotpartofanypopulationcapableofproducing viableoffspringthroughnaturalmeansofreproduction3Ǣontheotherhand,itseems primafacieabsurdtoassertthatthelastC.pyrenaicapyrenaicawasnot,infact,ƒ memberofthetaxonC.pyrenaicapyrenaicaǤAtƒminimum,thechoiceofƒspecies conceptisambiguouswithrespecttothedefinitionof“extinction.”   2Biologicalspeciesconceptsmaybeoperational,i.e.testableinpractice,or theoretical(Mayden1997).ErnstMayr,themostvocalproponentofbiological concepts,distinguishedbetweenthesesensesofhisbiologicalspeciesconceptby referenceeithertoactualinterbreedingbetweenorganisms(intheoperational sense)ortopotentialinterbreedingbetweenorganisms(inthetheoreticalsense) (Stamos2003).Inordertorecognizeorganisms’potentialtointerbreed,ƒconcept mustspecifysomesalienttraitsimilaritybetweenthoseorganisms,suchas Paterson’sspecificmaterecognitionsystem(1985)oroverallgeneticsimilarity(see Mayden1997).Inthissense,thetheoreticalsenseofbiologicalspeciesconceptsis functionallyindistinguishablefrompheneticspeciesconcepts.Whenwereferto biologicalconcepts,then,wemeanspecificallytheoperationalsenseofthose concepts. 3OnemightarguethatCeliaboretraitsthatdemonstratedherreproductive compatibilitywithothermembersofthesubspecies,andthatwecouldtherefore recognizeherasƒmemberofthattaxon;butdoingsowouldentailrejectingan operationalviewofthebiologicalspeciesconcept(seenote2).GivenCelia’s separationintimefrommembersofthetaxonC.pyrenaicapyrenaicaǡshecouldnot actuallyinterbreedwithanyorganisminthattaxon;itisforthisreasonthat Simpson(1961)andMayrarguethatthebiologicalspeciesconceptisnot operationalacrossthetemporaldimension(seeStamos2003). Whilespeciesconceptchoicedoesnotimplyanyparticulardefinitionofextinction, theconversedoesnothold.Howonedefinesextinctiondoescarryimplicationsfor whichspeciesconceptonemaychoose.Thedebateoverde-extinctionmay thereforecontributeinformationthatadvancesthespeciesproblem.  III.2.Extinctionconceptsandthechallengeoffinality  Theterm‘extinction’canrefertofourdifferentkindsofdisappearanceofƒspecies. speciescanvanishbyhybridizationwithanotherinterfertilespecies.Itcanalso givebirthtotwo(ormore)newdaughterspeciesthroughallopatricspeciation. Sometimesƒspeciesslowlytransformsitselfintoƒnewspecies.However,whatis underinterestinresurrectionbiologyisƒtypeofextinctioninwhichspeciesvanish fromtheontologicalledgerwithoutevolvingintoormergingwithsomeothertaxon. (Delord2007;Delord2014.)Extinctionisthensaidtocomethroughterminationof ƒlineage(RaupandStanley1971).Accordingtoourknowledge,ongoing resurrectionprojectsconcernanimalsthatareextinctinthislastmentionedsense.4 AsJulienDelord(2014)putsit,“peoplegetexcitedbythepossibilityofrecreationof ƒtotallyextinctspecies,andnotonspeciesthatwereonlysubmittedtoƒprocessof anaphyleticchangeofspecification”.Thetypeofextinctionunderinterestin resurrectionbiologyhasinliteraturealsobeencalled“finalextinction”(Delord 2007;Delord2014)and“trueextinction”(Raup1991).  Theenthusiasmforresurrectionbiologyisoftenbasedonconvictionthatfinalityof ‘finalextinction’or‘trueextinction’canbechangedbythetechnologiesdescribed above.Resurrectionbiologyiscommonlypresentedassimilartoadvancesin medicine:justaswemaycurediseasesthatwereformerlyterminal,resurrection biologymaychangeextinctionfromƒterminalstatetoƒtemporaryone.Theidea thenisthatextinctionhasthisfarbeenirreversiblemerelybecausescientistshave lackedpracticaltoolsforrevisingit.Thismightbecalledpracticalirreversibility.  Sometheoristsdisputethisview.AlistairGunnisamongtheopponents.Heargues that  extinctalsosayssomethingaboutfutureoftheclassȂthatonceitbecomesƒ nullclass,itcannevercometohavemembersagain.Itmayevenbeclaimed thatthisiswhatextinctmeans.Ifso,thenthequestion,“Canextinctspecies berecreated?”isanswerednegativelybyƒresorttowhatissometimescalled “definitionalstop”.  Wildlifepreservationistarealwaystellingus,aswarning,that“Extinctionis forever!”.Perhapsthiswarningtellsusnomoreabouttheworldthan “Bachelorsareunmarried!”[…]Anextinctspeciesisonethathascometoan

 4Theprojectsalsoseemtoconcerncharismaticanimalsandanimalsthathavegone extinctbecauseofhumanactivities. end,hasdiedout,ispermanentlyƒnullclass.Asweusethewordextinctǡit seems,therecreationofanextinctspeciesisƒlogicalimpossibility.(1991)  AccordingtoGunn,finalityofextinctionisƒconceptualnecessity:forhimextinction isirreversiblenotjustinpracticebutalsoinprinciple.CanGunn’sviewbeaccepted? Ifextinctionisnecessarilyfinal,howshouldtheanimalsbornfromde-extinction proceduresbeunderstood?Ideasaboutresurrectionbiologymakingextinctionnon- finalrestsoncertainviewsregardingspeciesandextinction,butalsoGunn’sview hasstrongtiestocertain(other)speciesandextinctionconcepts.Wewillnext presentthreewaysoffittingGunn’sviewtogetherwithcurrenttechnological developmentsinresurrectionscience.Ofeachofthethreealternativeswepresent itsimplicationsregardingconceptsofspeciesandextinction.Inorderto demonstratetheseimplications,wewillemploytheexampleofCeliaandher clone—let’scallthelatterDelia.5

IV. Finality of extinction and implications of the concepts  IV.1.Firstalternative:replication  Accordingtothefirstalternative,animalscreatedbyresurrectionbiologydonot belongtotheoriginalspecies(suchasthePyreneanibex,woollymammoth,and passengerpigeon.)6ǤEvenwhenmorphological,geneticandbehavioralpropertiesof theanimalsproducedbyresurrectionbiologyaretoƒgreatextentsimilarto membersoftheextinctspeciesinquestiontheycannotandshouldnotbe consideredasmembersofthatspecies.  Thisalternativerequiresthatclassificationofanorganismdoesnotdepend(atleast solelyormainly)onitsfeaturesandproperties.Twoanimalscanbelongtoƒ differentspecieseventhoughtheyaretoƒveryhighextentsimilarwithrespectto their(genetic)properties.Bythisstandard,itispossibleforDeliatobeclassified outsidethetaxonC.pyrenaicapyrenaicaevenifCelia—theorganismfromwhich Delia’sgenomewasderived—isƒmemberofthattaxon.  Toacceptthisviewrequiresrejectionofpheneticspeciesconcepts.Ifspeciesare definedbysimilaritiesbetweenorganismsthentwofullydevelopedorganisms sharingthesamegenomeoughttobemembersofthesamespecies;giventhe  5ThecaseofCeliaandDeliaconcernsextinctionandde-extinctionofƒsubspecies. Yet,inthepapertheinterestisintheextinctionofspeciesandinthespecies concept.ForthesakeoftheargumentandinordertoincludeƒreallifecaseofCelia andDelia,wepresupposethatextinctionofsub-speciesisaninstanceofreal extinction.Asƒresult,wealsopresupposethatquestionspresentedaboutde- extinctioninthispapercanandshouldbeansweredsimilarlyregardingspeciesand sub-species.Thesepresuppositions,however,arenotdetrimentaltoourgeneral argumentregardingspeciesconceptandspeciesextinction. 6FordiscussionofthissuggestionseeSiipi2014;Garvey2007;Delord2014. assumptionthatCeliaandDeliaaremembersofdifferentspecies,wewouldhaveto concludethatspeciesarenotdefinedbysimilaritiesbetweenorganisms.According toGunnthisisƒdesirableoutcome:“classification[ofspecies]issupposedtoexhibit evolutionaryrelationships,notmeremorphologicalsimilarities:toprovide explanationsandnotmerelydescriptions”(1991).  Thecentralproblemofthefirstalternativeisthatitdoesnotseemconsistentwith viewsgenerallyheldregardingclones.Dolly,theclonedsheep,isgenerallyaccepted tobeƒsheep(Siipi2014).Gunn(1991)andDelord(2014)arecertainlyrightin claimingthatƒcloneofanyindividualisnotidenticaltothatindividual. hypotheticalcloneofCharlesDarwin,forexample,isnotCharlesDarwin,butƒ numericallydistinctindividualgeneticallyidenticaltohim.Nevertheless,even thoughDarwin’scloneisnotDarwin,hiscloneisquitegenerallyacceptedtobeƒ humanbeing(Garvey2007;Siipi2014).Thus,thesupportersofthisfirstalternative haveseveralquestionstoanswer.Whatisthatcrucialdifferencebetween “ordinary”cloningandresurrectionbiologybecauseofwhichoutcomesofthelatter failtobemembersofthetargetspecies?IfDeliaisnotƒC.pyrenaicapyrenaicaǡwhat isshe?  Thereareatleastfourpossibilities.First,ananimalproducedbyde-extinction proceduresmightbeconsideredasƒhybridorganismȂforexample,ƒhybrid betweenƒmammothandanelephant.secondpossibilityistoseetheorganisms producedaschimeras.(Delord2014.)However,thesealternativesconsiderably stretchthecurrentmeaningsoftheterms‘hybrid’and‘chimera’.Thus,adoptionofƒ newtermdenotingmerelytomixesofspeciesproducedbycross-speciescloning mightbeƒthird,andbetter,alternative.Seenintheseways,de-extinction proceduresmightstillbeseenaspartlysuccessful.Eventhoughtheseprocedures cannotbringextinctspeciesbacktolife,theyareabletocreatesomekindofmixes betweentheextinctspeciesandother(closelyrelated)species.Bythislogic,the extinctspeciesremainsextinct.  Fourth,theanimalsproducedbyde-extinctionproceduresmightbeseentobelong tonewhumancreatedspeciesthatmaybeunderstoodasƒcopyof(andthus distinctfromthe)theoriginalspecies(Garvey2007;Delord2014;Siipi2014).We callthisthirdpossibilityreplicationǤCanitbeacceptedandwhatareits implications?  Onemight,followingGunn(1991)andDelord(2014)argueforreplicationbynoting thattheenvironmentandecologicalrolesofthemembersoftheextinctspeciesand oftheanimalscreatedbyresurrectionbiologyarelikelytodifferƒlot.Forexample, ƒwoollymammothcreatedbyresurrectionbiologywouldhavetolivewith elephants(oraloneor,atbest,inƒsmallgroupwithothersimilarlyproduced animals)andinanenvironmentwidelydifferentfromtheoneoftheoriginalwoolly mammoths.Accordingtothislineofthought,speciesmaywell,asnotedbyRobert Elliot(1994)havemembersthatareartificiallycreated.Yet,thelivingconditionsof membersoftheoriginalspeciesandtheanimalsbornthroughde-extinction proceduresdifferdramatically.Moreover,despitetheirgeneticsimilarity,the animalscreatedbyresurrectionbiologydifferfromtheextinctspecieswithrespect totheircellularmachinery(mitochondria,ribosomesetc)(Zimmer2013).These twoissuesdonotholdwithrespectto(ordinary)clones.Theyandtheirsurrogate mothersbelongtosamespeciesandtheyliveincommunitiesofthespeciesfrom whichtheyarecloned.Yet,thesedifferencesmaynotbesufficientforjustifyingthe alternativeofreplicationǤEpigeneticvariationsperseareinsufficientfor distinguishingspecies(Bentleyetal2004;cfDelord2014).Neitherareevengreat differencestoso-called‘normal’behaviorusuallyconsideredasthreatstospecies status.  Onemightclaimthatevenifitwerepossibletocreateorganismsthatarenotjust geneticallybutalsomorphologicallyandbehaviorallytotallysimilartomembersof theoriginalspecies,theseanimalsstillmayfailtomembersoftheoriginalspecies. Thereasonisthatthecreatedanimalslackthenecessaryconnectionwiththe originalspecies.Whatiscrucialforspeciesmembershipisƒcontinuityandlineage betweenitsmembers.Accordingtothislineofthought,“aspecieswentdefinitely extinctattheendofthereproductiveorlivingprocess,andsothereisno resurrectionatall:weonlywitnesstheresultsofhumanmodificationontheliving processofthebearingspecies,whichisanotherstreamoflifethantheextinctone” (Delord2014).  ThiswayofclassifyingDeliaasƒmemberofƒspeciesdistinctfromC.pyrenaica pyrenaicaisconsistentwithbiologicalandecologicalspeciesconcepts.Since biologicalconceptsareintendedtobeoperational—thatis,testableinactual practice—thenitfollowsthatCeliaandDeliashouldbeclassifiedindifferent species,sincetheirspatio-temporalseparationwouldmakeitimpossibletotestthe compatibilityofanyrelevantintrinsicmechanismsforreproductiveisolation7Ǥ Ecologicalconceptsallowthatspatialdistancemaybesufficientforspecies distinctionsbyecologicalstandards(VanValen1976);temporaldistanceisaneven moreeffectiveextrinsicmechanismforreproductiveisolation.   7Proponentsofbiologicalspeciesconceptsoftendenythattheirconceptsare operationalacrossthetemporaldimension(seesupranote3).Itisforthisreason thatSimpson(1961)arguedforsupplementingbiologicalspeciesconceptswithhis “successionalspeciesconcept”forthepurposesofphylogeneticreconstruction.One mayarguethatCeliaandDeliahavetraitsthatgivethemthepotentialtointeract withinthesamereproductivelyisolatedpopulation;however,Hullarguesthat ‘unrealizedpotentialitiesdon’tcount’indiagnosingspeciesmembershipby biologicalstandards(1965).EvenlaboratorytestsperformedusingCelia’s preservedgeneticinformationwouldthereforebeinsufficienttodemonstratethe conspecificityofCeliawithclones:iftwoorganismsdonotactuallybearthe appropriaterelationprescribedbybiologicalconceptsthentheyareinfactnot membersofthesamespecies.Whetherthisshouldbeconsideredƒvirtueorƒvice ofbiologicalconceptsislefttothereadertodecide. Phylogeneticspeciesconceptsmayormaynotbeconsistentwithclassificationof CeliaandDeliaindifferentspecies.Thekeypointishowonedefinesƒbiological lineage.Ghiselin,forexample,conceiveslineagesasconcreteentitiesdelimitedin spaceandtimebyƒparticularkindofconnectednessbetweenspeciesmembers (1987).LineagesofthiskindprecludetheclassificationofDeliawithCelia,given theirspatio-temporalseparation.However,onemightalternativelyconceiveof lineagesintermsofthetransmissionofgeneticinformation,inwhichcasethe identificationofDelia’sgeneticinformationwithCelia’swouldentailtheir classificationinthesamespecies(Garvey2007;Delord2014).  IV.2.Secondalternative:re-creation  Thesecondalternativeforfittingresurrectionsciencetogetherwithfinalityof extinctionistoclaimthatanimalscreatedaremembersofthespecies(e.g.Pyrenean Ibexǡwoollymammoth,PassengerPigeon,etc)thatoncewentextinct.Yet,despite theirexistence,thespeciesremainsextinct.Thisalternativeiscalledre-creationǤBy thisstandard,CǤpyrenaicapyrenaicawouldbeanextinctsub-speciesevenfollowing thecreationofitsnewmember,Delia.Thealternativeofre-creationmaysound surprisingforitrequiresƒpossibilityofƒspeciesbeingextincteventhoughithas livingmembers.Couldsuchstatementbeintellectuallyacceptable?  Non-existenceiscertainlyconceptuallycentraltoextinction.Itmightevenbe suggestedthatitfollowsfromthedefinitionofextinctionthatifƒspeciesisextinct, thentherearenolivingorganismsthatbelongtoit.Buthasthenon-existencehave topermanent?AsnotedbyDavidM.Raup,theword‘extinct’isanadjective(1991). AnimalsaresaidtogoextinctortobecomeextinctǤThealternativeofre-creationis basedonunderstandinggoingorbecomingextinctasƒprocessǤItisƒprocessin whichƒspeciesbecomesƒnullclassthroughthedeathofthelastmemberofthe species.Whenthatprocessiscompletethespeciesisextinctand,cruciallytothe alternativeofre-creation,remainsextinctwhateverhappenslater.Resurrection biologyisunabletochangespeciesstatusfromextincttonon-extinctbecausethe processofextinctionhasalreadybeencompleted.Onceƒspeciesbecomesƒnull classitremainsextinct,eventhoughitmaybecausedtohavenewmemberslater (andconsequentlyisnolongerƒnullclass).Ifresurrectionbiologybecomes successfulwewillhavetwokindsofextinctspeciesinthefuture:onesthathave beenandothersthathavenotbeenre-created.  Thissecondalternative,ofcourse,toƒgreatextentreliesoncertainviewsregarding extinction.Firstandforemost,itrequiresseeingextinctionasƒprocess:ƒspecies becomingƒnullclassbythedeathoftheonlymembersofthespecies.The alternativeisnotcompatiblewithseeingextinctionasƒpropertyofƒspeciesbeing ƒnullclass.Thealternativemayseemodd,butitisalsopossiblethatthisoddityis onlyapparentandfollowsfromthefactthatuntilnowtheprocessofextinctionhas alwaysledtoƒpermanentnon-existenceofmembersofthespeciesinquestion. Whatthesuccessofresurrectionbiologymaychangeisthiscontingentconnection betweenbecomingandbeingƒnullclass.However,theterm‘final’isusuallytaken torefertostatesofaffairsofenumeratedobjects,nottoprocesses.Thus,ifthegoal istosavefinalityofextinction,andifitissenselesstoclaimprocessestobefinal, thenclaimingthatextinctionisƒprocessisnotƒsolutionatall.  Themoreseriousproblemofthealternativeofre-creationisthatitdoesnotseem tobeconsistentwithanyspeciesconceptpresented.Pheneticspeciesconcepts woulddemandthatDeliabeclassifiedwithCelia,butpheneticconcepts—wherein speciesmembershipisdefinedbytheinstantiationofindividuallynecessaryor jointlysufficientproperties—treatspeciesasnaturalkinds(Devitt2008;Ereshefsky 2010).Ifƒkindhasmembers,thenitmostcertainlyexists;ifDeliaisƒmemberofC. pyrenaicapyrenaicaǡandifC.pyrenaicapyrenaicaisƒnaturalkind,thenthatsub- speciesmustbeextant.  Thealternativeofre-creationisalsoinconsistentwithbiologicalandecological speciesconcepts.Sincebothofthesespeciesconceptstreatspeciesasconcrete populations,andsinceallmembersofC.pyrenaicapyrenaicahaddiedatsomepoint inthepast,itwouldfollowthatthespecieswasextinct(Hull1965;Ghiselin1987). Moreover,DeliacannotbeclassifiedasƒmemberofC.pyrenaicapyrenaicabythese conceptssinceshecannotbeconsideredƒmemberofnow-vanishedpopulations.  Similarly,thissuggestionisinconsistentwithphylogeneticspeciesconcepts.By theseconcepts,ƒspeciesisextinctifthelineagehasendedandanorganismisƒ memberofƒspeciesifitispartofthespecies’lineage.TosaythatC.pyrenaica pyrenaicaisextinctwouldthereforeimplythattherecanbenofuturemembersof thelineage,butthiswouldbecontradictedbyDelia’sclassificationasƒmemberof thesub-species.  Thus,eitheralternativeofre-creationwouldhavetorelyonsomenewkindofview ofspecies,oritsincompatibilitywithspeciesconceptscanbetakentoimplythe alternativetobeinconsistentand,thus,notsuccessful.  IV.3.Thirdalternative:non-extinction  ThethirdalternativeforfittingGunn’sviewtogetherwithresurrectionbiologyisto claimthatinsuccessfulcasesofcross-speciescloningextinctionactuallynevertook place.Accordingtothislineofthought,animalsproducedbycross-speciescloning belongtothetargetspecies.Delia,forexample,isƒmemberofC.pyrenaica pyrenaica.However,exactlybecauseitispossibletocreatetheseanimals,extinction didnothappenwhenallmembersofthespeciespopulationdied.Wecallthis alternativenon-extinctionǤ  Thenon-extinctionalternativeisbasedontheviewthatextinctioncanbe conceptualizedandunderstoodinseveraldifferentwaysthatdifferwithrespectto thepointinwhichextinctionisseentotakeplace.Mostcommonly,extinctionis seentotakeplacewhenallmembersofƒspecieshavedied.Asnotedabove, sometimesextinctionisassociatedwithendofreproductivepossibilitiesȂthatis, withthedisappearanceofthereproductivepopulation.Bythisunderstanding,ƒ speciesmightbeconsideredextincteventhoughthereremainlivingorganisms classifiedwithinthespecies.Thismaybethecaseifalllivingmembersbelongtothe samesex,oriftheyareallsterile,orifthepopulationistoosmalltomaintainthe requisitegeneticdiversity(Tilmanetal1994).Thiskindofextinctionissometimes calledfunctionalextinctionǤFinally,andrelevantlytoresurrectionbiology,itmay alsobepossibletoassociateextinctionwithƒlossofinformationsufficientfor producinganindividualwithcharacteristicsofthespecies.(Delord2007.) Accordingtothislineofthought,extinctionisnotƒquestionaboutwhetherƒ specieshaslivingmembers;rather,itisƒfunctionofpropagatinggenetic(orother relevant)information.Inthecaseofsuccessfulresurrectionbiology,then,extinction couldneverhaveactuallytakenplace:informationnecessaryforproducing individualswasnotlostanditwaspossibletobringthemintoexistencebycloning methods.  Thethirdalternativeimpliesthatresurrectionbiologyisnot,strictlyspeaking,de- extinctionorresurrection.Sinceextinctiondidnottakeplace,neithercande- extinctionhappen.Oneoddityofthealternativeisthatitrequiresthatƒspeciesmay failtobeextinctevenwhennoanimalbelongingtoitisalive.However,thismaynot, beasstrangeasitfirstsounds.TheplantspeciesSilenestenophyllaǡforexample, persistedfor30,000yearsinwell-preservedseedsbeforebeingcultivatedagainin themodernday(Yeshinaetal2012).  Thenon-extinctionalternativeisconsistentwithpheneticspeciesconceptsbecause thoseconceptstreatspeciesasnaturalkinds.Hull(1980)arguesthatnaturalkinds mayhavegapsintimeandspacebetweenmemberswhilethekinditselfpersists.In hisexample,anatomicelementsuchasgoldwouldcontinuetoexistevenifevery individualgoldatomintheuniverseweretodisappear,becausenewatomswiththe appropriateatomicnumbermayyetbecreated.Theatomicelementitselfisƒclass thatmaypersistdespiteƒtemporaryabsenceofmembers.Suchwouldbethecase withspeciessubjecttode-extinction.C.pyreneicapyreneicamighthaveseemedto beextinctfollowingCelia’sdeath,butthelatercreationofDelia,ƒnewmemberof Celia’sspecies,demonstratesthatthespecieswasnotinfactextinct.  Phylogeneticspeciesconceptsmaybelikewiseconsistentwithnon-extinction, providedthatlineagesaredefinedappropriately.Ifwetakeƒlineagetobeƒ sequenceofindividualswhopropagatesomeinformationaboutthespecies—beit geneticorphenotypic—regardlessofthemeansofpropagation(assuggestedinDe Queiroz1998),thenƒlineageoflivingorganismsmayhavegapsinspaceortimeso longastheimportantinformationispreserved.Deliapropagatesinformationthat originatedinCeliaaboutC.pyreneicapyreneicaǢDeliaandCeliaaremembersofthe samespeciesforthisreason,andthepreservationofrelevantinformationbetween thedeathofthelatterandthebirthoftheformerwouldallowforthepersistenceof thespeciesinthattimeframe.Solongasthelineageisƒlineageofsomekindof information,ratherthanƒlineageofindividualorganisms,thenphylogenetic speciesconceptsmayaccommodatethenon-extinctionview.  Bycontrast,neitherbiologicalnorecologicalconceptscanbeconsistentwiththis suggestion.Bothtreatspeciesaspopulationswhoseboundariesintimeandspace aredelimitedbyparticularrelations,eitherbetweenmembersofthepopulations witheachotherorbetweenmembersofthepopulationsandtheirenvironments.In ourchosenexample,Deliasimplydoesnotstandinthoserelationstoany populationthatwouldhaveincludedCelia,norcouldshe:thosepopulationsaretoo distantinspaceandtimeforDeliatoenterintotheappropriaterelationswithany membersofthosepopulations.

V. Non-final extinction: literal resurrection  WhatifGunn’sviewaboutfinalityofextinctionisnotaccepted?Wecallliteral resurrectiontheviewthatresurrectionbiologyactuallycanbesuccessfulin reversingextinctionthroughthecreationofnewmembersofspeciesthatoncewent extinct.Accordingtothislineofthought,resurrectionbiologyaccomplishestwo goals:first,theproductionofanimalsthatbelongtoƒspeciesthatis(ormore precisely,hasthusfarbeen)extinct;second,ƒchangeofextinctionfromfinalto non-finalstate.Theorganismsproducedbyresurrectionbiologyarethenclassified asthemembersoftheextinctspecies,andtheirexistencechangesthespecies’ statusfromextincttoextant.Followingthislogic,DeliaisƒmemberofC.pyrenaica pyrenaicaandwhenshewasbornthesub-specieswas(forƒmoment)resurrected fromthespeciescategory’sequivalentofdeath.  Literalresurrectionismostobviouslycompatiblewithpheneticspeciesconcepts, butthisconsistencymustbequalified.Therelevantquestionhereisnotaboutthe speciestaxon:asrepeatedlynotedabove,pheneticconceptswouldholdthatCelia andDeliamustbothbemembersofC.pyreneicapyreneicagiventheirnear-complete geneticandphenotypicsimilarity.Instead,therelevantquestionisaboutthespecies category:arespeciesclassesorsets?Iftheformer,thenƒspeciesmaypersistevenif allofitsmembersaredead;ifthelatter,thenƒspecies’existencedependsonthe existenceofitsconstituentmembers(Stamos2003).  Intheprevioussectionwediscussedthepossibilitythatspeciesarenaturalkind classes:inthatcase,C.pyreneicapyreneicawouldnothavereallybeenextinct betweenCelia’sdeathandDelia’sbirth.Ifspeciesarenaturalkindsets,however,the implicationsaredifferent.Setsonlyexistgiventheexistenceoftheset’smembers (seeKitcher1984);sincethesetassociatedwithC.pyreneicapyreneicahadno membersbetweenCelia’sdeathandDelia’sbirth,itwouldfollowthatthespecies wasextinct.SinceCeliaandDeliamustbemembersofthesamespecies,however, therelevantsetwouldhavetoreappearuponDelia’sbirth.  Thealternativeofliteralresurrectioniscertainlyinconsistentwithbiological speciesconceptsandprobablyinconsistentwithecologicalones.Biologicalconcepts definespeciesintermsofrelationsintrinsictothespecies,andaswehavealready discussedƒclonesuchasDeliacannotenterintotherequisiteintraspecificrelations withmembersofanextinctspecies,bydefinition8ǤEcologicalspeciesconcepts, whichdefinespeciesintermsofrelationsbetweenmembersofƒspeciesandsome extrinsicfactor,couldallowforliteralresurrectioninprincipleȂassumingthatthe relevantextrinsic(i.e.,ecological)factorsremainconstant.Inpractice,thisis exceedinglyunlikely.Theconstancyofecologicalfactorsassociatedwithƒspeciesis contingentontheabsenceofecologicalcascadeeffectsfollowingthespecies’ extinction,butresearchshowsthatsucheffectsmayberapidacrossƒvarietyof differentecosystems(Paceetal1999).DeliamightbeƒmemberofC.pyreneica pyreneicaiftheecologyofOrdesanationalparkhadremainedunchangedfollowing Celia’sdeath;however,extinctionofC.pyreneicapyreneicaitselflikelyensured someecologicalchange.  Phylogeneticconceptsalsoseeminconsistentwithtrueresurrection.Ifƒlineageis definedbothbyhistoricalendpointsandconnectednessbetweenthosepoints,then speciescannotreturnfromextinctionexhypothesiǡasGunnargued.Bythis standard,oneofthenecessaryconditionsofbeingƒmemberofC.pyrenaica pyrenaicawouldbeinclusionwithinanhistoricallineagethatendedwiththedeath ofCelia;sinceDeliawasbornafterCelia’sdeath,itfollowsthatDeliacannotbeƒ memberofC.pyrenaicapyrenaicaandthatsub-speciesmustremainextinct.Even thoughphylogeneticconceptsmaybeconsistentwithCeliaandDeliabelongingto thesamespecies,asshownaboveinthealternativeofnon-extinction,thisis possibleonlyprovidedthatnoextinctionhastakenplacebetweenthegenetic donor’sdeathandtheclone’sbirth.

VI. Conclusion  Literalresurrectionseemstobetheviewofresurrectionbiologyclosesttopopular conceptions(cf.Zimmer2013).Assumingthatextinctionisƒresultofthe disappearanceofƒspeciespopulation,andthatextinctionisnotnecessarilyƒfinal state(asliteralresurrectionpresupposes),itfollows(forreasonsstatedabove)that onemustadoptƒpheneticspeciesconcepttomaintainlogicalconsistency.  However,thetechnologicaldevelopmentsofresurrectionbiologydonotimplythat wemustacceptthepheneticspeciesconceptortheassumptionsbehindliteral resurrection.Asshownabove,onemayadoptƒviewofresurrectionbiologythat rejectstheliteralresurrectionviewthatextinctionisnotƒterminalstate.Onemay accommodateƒviewofextinctionasfinalifoneiswillingeither(a)togiveupthe viewthatCeliaandDeliabelongtothesamespecies,aswouldbethecaseinspecies  8Theinconsistencyofliteralresurrectionwithbiologicalspeciesconceptsrestson theassumptionthatbiologicalconceptsareoperationalratherthantheoretical(see footnote2).Ifƒbiologicalconceptistakentobetheoretical—thatis,ifitdefines speciesintermsofcounterfactualrelationsinadditiontorealones—thenDelia mightbeƒmemberofC.pyreneicapyreneicaǤHowever,thispossibilitycanonlybe acceptedbyreferencetopheneticsimilarity,andsotherelevantspeciesconcept underwritingtheargumentwouldbepheneticratherthanbiological. replicationor(b)togiveuptheviewofextinctionasdisappearanceofƒspecies population,aswouldbethecasewiththenon-extinctionalternative.  Itisnotablethatnospeciesconceptisconsistentwithallpossibleviewsof resurrectionbiology,norisanyviewofresurrectionbiologyconsistentwithall speciesconcepts.Pheneticconceptsareincompatiblewithreplicationandre- creation;biologicalconceptsareincompatiblewithre-creation,non-extinction,and literalresurrection;ecologicalconceptsareincompatiblewithre-creationand literalresurrectioninprinciple,andwithnon-extinctioninpractice;phylogenetic conceptsareincompatiblewithre-creationandliteralresurrection.  What,then,doestheprogressofresurrectionbiologyimplyforbiologicaltheory?It maybesignificantthatbiologicalandecologicalconceptsareleastcompatiblewith viewsofde-extinction.Phylogeneticconceptsmaybemoreeasilycompatiblewith viewsofde-extinction,buttheirinconsistencywithliteralresurrectionrequires significantchangetoreceivedconceptsofextinction.Itisfortheseandsimilar reasonsthatDelord(2014)arguesthatpheneticconceptsaremostconsistentwith speciesresurrection.Weagree.  Achievementsinresurrectionbiologymaysooninfluencetheintellectualbasesof lifesciencesandespeciallytheconceptsofspeciesandextinction.Atthesametime, viewsregardingthoseverysamebasesshouldeffectonhowoneseesand understandstheoutcomesofresurrectionbiology.Thiswouldmarkƒuseful cognitiveshiftamongconservationistsandbiologists,butprogressinresurrection biologycouldalsoproduceanevenrarerjewel:conceptualchangeamong philosophers.Wemayclingtooldviewsanddevaluethisdevelopingtechnologyand researchorwemayembracethescienceandrevisehowitsimportantconceptsare defined.Thatchoiceislefttothereader. 

Acknowledgements  Wethanktwoanonymousrefereesfortheirusefulcommentsontheearlierversion ofthepaper.  Works Cited

Archer, M. (2013). Second chance for Tasmanian tigers and fantastic frogs. http://longnow.org/revive/tedxdeextinction. Accessed 29 November 2015.

Bentley, R. A., Hahn, M. W., & Shennan, S. J. (2004). Random drift and culture change. Proceedings of the Royal Society of London B: Biological Sciences, 271(1547), 1443- 1450.

Bhullar, B. A. S., Morris, Z. S., Sefton, E. M., Tok, A., Tokita, M., Namkoong, B., Camacho, J., Burnham, D. A., & Abzhanov, A. (2015). A molecular mechanism for the origin of a key evolutionary innovation, the bird beak and palate, revealed by an integrative approach to major transitions in vertebrate history. Evolution, 69(7), 1665- 1667.

Boyd, R. (1999). Homeostasis, species, and higher taxa.

De Queiroz, K. (1998). The general lineage concept of species, species criteria, and the process of speciation.InR.Wilson(ed.),Species:NewInterdisciplinaryEssays(pp.49– 90).Cambridge,MA:TheMITPress.

Delord, J. (2007). The nature of extinction. Studies in History and Philosophy of Biological and Biomedical Sciences 38 (3), 656-667.

Delord, J. (2014). Can We Really Re-create an Extinct Species by Cloning? A Metaphysical Analysis. In M. Oksanen & H. Siipi (eds.), The Ethics of Animal Re- creation and Modification: Reviving, Rewilding, Restoring (pp. 22-39). Houndmills: Palgrave Macmillan.

Devitt, M. (2008). Resurrecting Biological Essentialism. Philosophy of Science, 75(3), 344-382.

Dobzhansky, T. (1937). Genetics and the Origin of Species (No. 11). Columbia University Press.

Elliot, R. (1994). Extinction, Restoration, Naturalness. Environmental Ethics, 16(2), 135- 144.

Ereshefsky, M. (2010). Species. URL = http://plato.stanford.edu/archives/spr2010/entries/species. Accessed 29 November 2015.

Folch, J., Cocero, M. J., Chesné, P., Alabart, J. L., Domínguez, V., Cognié, Y., Roche, A., Fernandez-Arias, A., Marti, J.I., Sanchez, P., Echegoyen, E., Beckers, J. F., Sanchez Bonastre, A., & Vignon, X. (2009). First birth of an animal from an extinct subspecies (Capra pyrenaica pyrenaica) by cloning. Theriogenology, 71(6), 1026-1034.

Garrick, R. C., Benavides, E., Russello, M. A., Hyseni, C., Edwards, D. L., Gibbs, J. P., Tapia, W., Ciofi, C., & Caccone, A. (2014). Lineage fusion in Galápagos giant tortoises. Molecular ecology, 23(21), 5276-5290.

Garvey, B. (2007). Philosophy of Biology. Stocksfield: Acumen Publishing.

Ghiselin, M. T. (1987). Species concepts, individuality, and objectivity. Biology and Philosophy, 2(2), 127-143.

Greer, A. (2009). Cloning the thylacine. Quadrant, 53(7-8), 28-39.

Gunn, A. (1991). The restoration of species and natural environments. Environmental Ethics 13(4), 291-312.

Hiendleder, S., Zakhartchenko, V., & Wolf, E. (2004). Mitochondria and the success of somatic cell nuclear transfer cloning: from nuclear–mitochondrial interactions to mitochondrial complementation and mitochondrial DNA recombination. Reproduction, Fertility and Development, 17(2), 69-83.

Horner, J. R., & Gorman, J. (2009). How to build a dinosaur: extinction doesn't have to be forever. Penguin.

Hull, D. L. (1965). The effect of essentialism on taxonomy: two thousand years of stasis. British Journal for the Philosophy of Science, 15: 2-32, 16: 1-18.

Hull, D. L. (1980). Individuality and selection. Annual review of ecology and systematics, 11, 311-332.

Kitcher, P. (1984). Species. Philosophy of Science, 51(2): 308-333.

Loi, P., Wakayama, T., Saragustry, J., Fulka, J., & Ptak, G. (2011). Biological time machines: a realistic approach for cloning an extinct mammal. Endangered Species Research, 14(3), 227-233.

Mayden, R. L. (1997). A hierarchy of species concepts: the denouement in the saga of the species problem.Claridge, M. F., Dawah, H. A., & Wilson, M. R. (eds). Species: the units of biodiversity (pp. 381-423). London: Chapman & Hall Ltd.

O’Connor, M.R. (2015). Resurrection Science. New York: St. Martin’s Press.

Okasha, S. (2002). Darwinian metaphysics: Species and the question of essentialism. Synthese, 131(2), 191-213.

Oksanen, M., & Siipi, H. (2014). The Ethics of Animal Re-creation and Modification: Reviving, Rewilding, Restoring. Houndmills: Palgrave Macmillan.

Pace, M. L., Cole, J. J., Carpenter, S. R., & Kitchell, J. F. (1999). Trophic cascades revealed in diverse ecosystems. Trends in ecology & evolution, 14(12), 483-488.

Paterson, H. G. H. (1985). The recognition concept of species. In E.S. Vrba (ed.), Species and Speciation. Pretoria: Transvaal Museum Monograph no. 4.

Pina-Aguilar, R. E., Lopez-Saucedo, J., Sheffield, R., Ruiz-Galaz, L. I., de J. Barroso- Padilla, J., & Gutiérrez-Gutiérrez, A. (2009). Revival of extinct species using nuclear transfer: hope for the mammoth, true for the Pyrenean ibex, but is it time for “conservation cloning”?. Cloning and stem cells, 11(3), 341-346.

Poulakakis, N., Glaberman, S., Russello, M., Beheregaray, L. B., Ciofi, C., Powell, J. R., & Caccone, A. (2008). Historical DNA analysis reveals living descendants of an extinct species of Galápagos tortoise. Proceedings of the National Academy of Sciences, 105(40), 15464-15469.

Raup, D.M. & Stanley, S.M. (1971). Principles of Paleontology. San Francisco: W.H. Freeman and Company.

Raup, D.M. (1991). Extinction: Bad genes or bad luck? London: W.W. Norton.

Redford, K.H., Adams, W & Mace, G.M. (2013). Synthetic biology and conservation of nature: Wicked problems and wicked solutions. PLOS Biology 11(4), 1-4.

Salsberg, C. A. (2000). Resurrecting the woolly mammoth: science, law, ethics, politics, and religion. Stanford Technology Law Review, 1, 1-30.

Sherkow, J. S., & Greely, H. T. (2013). What if extinction is not forever?. Science, 340(6128), 32-33.

Siipi, H. (2014). Authenticity of animals. In M. Oksanen & H. Siipi (eds.), The Ethics of Animal Re-creation and Modification: Reviving, Rewilding, Restoring (pp. 22-39). Houndmills: Palgrave Macmillan.

Simpson, G. G. (1961). Principles of animal taxonomy. New York: Columbia University Press.

Stamos, D. N. (2003). The Species Problem. Maryland: Lexington Books.

Stone, R. (2003). Mammoth: The Resurrection of an Ice Age Giant. Fourth Estate.

Tilman, D., May, R. M., Lehman, C. L., & Nowak, M. A. (1994). Habitat destruction and the extinction debt. Nature 371(1), 65-66.

Wilmut, I., Schnieke, A. E., McWhir, J., Kind, A. J., & Campbell, K. H. S. (1999). Viable offspring derived from fetal and adult mammalian cells. Clones and clones: facts and fantasies about human cloning, 21.

Van Valen, L. (1976). Ecological species, multispecies, and oaks. Taxon, 25(2/3), 233- 239.

Wägele, J.-W. (2014). The necessity of biodiversity research: we are responsible for the quality of life of coming generations. In D. Lanzerath & M. Friele (eds.) Concepts and Values in Biodiversity. London: Routledge.

Wilson, R. A. (1999). Species: New interdisciplinary essays. Cambridge, MA: The MIT Press.

Wilkins, J. S. (2009). Species: a history of the idea. Berkeley, CA: University of California Press.

Yashina, S., Gubin, S., Maksimovich, S., Yashina, A., Gakhova, E., & Gilichinsky, D. (2012). Regeneration of whole fertile plants from 30,000-y-old fruit tissue buried in Siberian permafrost. Proceedings of the National Academy of Sciences, 109(10), 4008- 4013.

Zandt, B. J., ten Haken, B., van Dijk, J. G., & van Putten, M. J. (2011). Neural dynamics during anoxia and the “wave of death”. PLoS One, 6(7), e22127.

Zimmer, C. (2013). Bringing them back to life. National Geographic, 223(4), 28-41.