ZOOLOGICAL RESEARCH Species delimitation based on diagnosis and monophyly, and its importance for advancing mammalian taxonomy EliécerE.Gutiérrez1,2,*,GuilhermeS.T.Garbino3 1Pós-GraduaçãoemBiodiversidadeAnimal,DepartamentodeEcologiaeEvolução,CentrodeCiênciasNaturaiseExatas,Universidade FederaldeSantaMaria,SantaMaria,RS97105-900,Brazil 2DivisionofMammals,NationalMuseumofNaturalHistory,SmithsonianInstitution,WashingtonDC20013-7012,USA 3Pós-graduação,DepartamentodeZoologia,InstitutodeCiênciasBiológicas,UniversidadeFederaldeMinasGerais,BeloHorizonte,Minas Gerais31270-901,Brazil ABSTRACT INTRODUCTION A recently proposed taxonomic classification for extant Arecentlyproposedtaxonomicclassificationofextant ungulates (Groves & Grubb, 2011) sparked a series of ungulates sparked a series of publications that publications criticizing the species concept upon which the criticize the Phylogenetic Species Concept (PSC) classification was based, i.e., Phylogenetic Species Concept claiming it to be a particularly poor species concept. (PSC) (Heller et al., 2013; Zachos et al., 2013; Zachos, 2013,2015;Zachos&Lovari,2013),albeitpreviouspublished These opinions reiteratively stated that (1) the two opinionshadalreadypresentedsomeofthesamearguments fundamental elements ofthe "PSC", i.e., monophyly against the PSC (e.g., Frankham et al., 2012; Garnett & and diagnosability, do not offer objective criteria as Christidis, 2007; Isaac et al., 2004; Tattersall, 2007). Two to where the line between species should be drawn; main claims about the PSC have been reiteratively used to and (2) that extirpation of populations can lead to highlight it as a particularly poor species concept: (1) the artificial diagnosability and spurious recognitions of two fundamental elements of the PSC, i.e., monophyly and diagnosability, do not offer objective criteria as to where the species. This sudden eruption of criticism against linebetweenspeciesshouldbedrawn;and(2)theextirpation the PSC is misleading. Problems attributed to ofpopulationscanleadtoartificialdiagnosabilityandspurious the PSC are common to most approaches and recognitionsofspecies. Moreover,thesecriticismsportraythe conceptsthatmodernsystematistsemploytoestablish use of the PSC as detrimental to conservation efforts. We species boundaries. The controversial taxonomic argue that the problems attributed to the PSC are common to most methodological approaches to species limits and to propositionsthatsparkedcriticismagainstthePSCare themostcommonlyusedspeciesconceptsthathavebeenthe indeedhighlyproblematic,notbecauseofthespecies basisforthetaxonomicclassificationsofmammalscurrentlyin conceptuponwhichtheyarebased,butbecauseno use. Furthermore, we present evidence that the PSC based evidence(whatsoever)hasbecomepublictosupport on diagnosability and monophyly as operational criteria has a substantial portion of the proposed classification. helped to substantially advance mammalian systematics. In We herein discuss these topics using examples addition, we show that the recent criticism against Groves & Grubb’s(2011)ungulatetaxonomyismistakenlyfocusedonan from mammals. Numerous areas of biological "allegedpoverty"ofthePSC(seeabriefcommentrelevantto research rest upon taxonomic accuracy (including thisgeneraltopicbyTsangetal.,2016,p. 529),whereasthe conservationbiologyandbiomedicalresearch);hence, realcauseoftaxonomicinflationinthatproposedclassification itisnecessarytoclarifywhatare (andwhatarenot) therealsourcesoftaxonomicinaccuracy. Received: 14December2017; Accepted: 12February2018; Online: Keywords: Alpha taxonomy; Phylogenetic Species 08March2018 Concept; Species concepts; Taxonomic inertia; *Correspondingauthor,E-mail:[email protected] Taxonomicinflation DOI:10.24272/j.issn.2095-8137.2018.037 SciencePress ZoologicalResearch39(5):301–308,2018 301 laysonnumerousempiricalproblems. With regard to the dPSC (sensu Cracraft, 1983; see also Eldredge & Cracraft, 1980; Nixon & Wheeler, 1990; Wheeler PHYLOGENETICSPECIESCONCEPT(PSC) &Meier,2000andreferencestherein),weagreewithprevious Before we discuss these matters, we must clarify that the criticisms (Heller et al., 2013, 2014; Zachos & Lovari, 2013, name “Phylogenetic Species Concept” has been associated Zachos, 2016a) in that this concept is prone to promote to various concepts (e.g., McKitrick & Zink, 1988; Nixon spuriousrecognitionofmeregeographic(includingsubspecies) & Wheeler, 1990), two of which are central in the or even individual variants of a single species as if such above-mentioned debate. These concepts can be better variants were each a valid species. This is due to the high regardedassetsofcriteriaforspeciesdelimitationratherthan degree of subjectivity and arbitrariness implicit in the task "concepts",asproposedbydeQueiroz(2007);however,herein of judging what characteristics are to be deemed adequate werefertothesesetsofcriteriaas"concepts"onlytofacilitate to diagnose species and distinguishing such characteristics communication by using the same terminology employed by fromthosethatwouldsimplyleadtodiagnosesofpopulations, authors of previous articles. These concepts are as follows or groups thereof, within a single species (but see Wiens (seesummariesbyGrovesetal.,2017;Zachos,2016a): & Servedio, 2000). Species are not phenotypically and genotypically homogeneous across geography, therefore it is Phylogenetic species concept, diagnosis-based alwaysthecasethatvariouspopulationswithinasinglespecies version (dPSC): “The smallest diagnosable cluster can be diagnosed. These diagnoses by themselves must of individual organisms within which there is a not be a justification to regard such populations as different parentalpatternofancestryanddescent”(Cracraft, species. This limitation of the dPSC is exacerbated when 1983, p.170). Subsequently formulated as “... sample sizes are small, as is often the case for medium and the smallest aggregation of populations (sexual) largemammals.Inthesecases,aresearchermayerroneously or lineages (asexual) diagnosable by a unique infertheexistenceofphenotypicdiscontinuityandthepresence combination of character states in comparable of characteristics enabling the diagnosis of a sample—the individuals (semaphoronts)” (Nixon & Wheeler, latter based on a set of specimens that at the time were 1990). A more recent version states that species perceived as worthy of species-level recognition. However, are “... the smallest population or aggregation as more samples are obtained, individuals with intermediate ofpopulationswhichhasfixedheritabledifferences phenotypeswithrespecttotheputativenewspeciesandother from other such populations or aggregations” geographic samples may be found. This would render the (Groves&Grubb,2011;seealsoGroves,2017). putative new species, which was previously thought to be diagnosable, conspecific with an already recognized species Phylogenetic species concept, monophyly-based (e.g.,Peresetal.,1996).Examplesoftheseplausibleproblems version (mPSC) “... a geographically constrained are abundant in the proposed classification of ungulates by groupofindividualswithsomeuniqueapomorphous Groves & Grubb (2011) (see below), but are by no means character, is the unit of evolutionary significance” exclusive to it (e.g., Díaz et al., 1999, 2002; Fonseca & (Rosen,1978,p.176). Pinto,2004; Solari,2004; vanRoosmalenetal.,2000,2007); numerousexamplesexistinearlycontributionstomammalian Athirdconceptthatmustbeincorporatedinthediscussion taxonomy(e.g.,Miller,1912; Pocock,1941; Robinson&Lyon, isasfollows(seeGrovesetal.,2017): 1901),andeventhelastdecadehasseenclaimsadvocatingfor therecognitionofaspeciesmadeonthebasisofphenotypic Phylogenetic species concept, diagnosis-and- diagnosesofasfewasoneortwospecimens—e.g.,Meijaard monophyly-based version (dmPSC), defined as et al., 2017 p. 513; see also Mantilla-Meluk (2013) for a “... the smallest diagnosable cluster of individual monkeysubspeciesnamedonthebasisofmorphometricdata organismsformingamonophyleticgroupwithinwhich andpelagecolorationfromonlyfourspecimens.Unfortunately, there is a parental pattern of ancestry and descent” in some cases descriptions of species have been carried out (Mayden,1997,p.407;McKitrick&Zink,1988). notonlywithunacceptablysmallsamplesizesbutalsomerely based on images (illustrations, photos, or both) and lacking DO THESE VERSIONS OF THE PSC OFFER OBJECTIVE preserved type specimens (see Pine & Gutiérrez, 2018 for a CRITERIA AS TO WHERE THE LINE BETWEEN SPECIES reviewofcasesandproblemsassociatedtothisphenomenon). SHOULDBEDRAWN? Althoughnodataexisttosupportthenotionthatthecollection of a single individual (for it to properly serve as a preserved The short answer is “no”, but “no” would also be the answer holotype) significantly increases the probability of an already if the question were asked with regard to any other species endangeredspeciestobecomeextinct,someresearchersmay concept, including the Biological Species Concept (BSC) prefer not to carry out such collection (e.g., Donegan, 2008; whenappliedtoallopatricpopulations(seeGroves,2012and butseeDubois&Nemésio,2007;Dubois,2009),oritmaybe references therein). However, the three phylogenetic species unfeasible due to impediments in obtaining collection permits. concepts described above differ importantly regarding the In such cases, a wide survey of museum specimens might degreeofobjectivitywithwhichtheycanbeapplied. 302 www.zoores.ac.cn lead to the discovery and subsequent use of specimens in 2003;Nixon&Wheeler,1990;deQueiroz&Donoghue,1988; taxonomic descriptions. Undertaking comprehensive surveys Zachos,2014b;Zander,2007;seealsoFunk&Omland,2003). of museum specimens may be disregarded by describers of Discussing these views requires a much more extensive text new species, but the possible data yielded, which may be andwoulddistractfromtheaimofthisperspectivepiece—i.e., coupled with photos of living animals, might ameliorate the clarifying that despite the recent criticisms made against detrimentaleffectsofextremelysmallsamplesizesandhelpin PSCs,atleastoneoftheseconceptshasservedtopositively unveilinggeographicandnon-geographic(ontogenetic,sexual) advance mammalian systematics. By requiring monophyly, variation(e.g.,Garbinoetal.,2016). the application of the dmPSC secures that a phylogenetic Thesecondconcept,themPSC,underwhichspeciesmust inferenceisconductedtodescribeorrevalidateaspecies,thus be both monophyletic and geographically restricted, seems decreasingthechancesthatpolyphyleticgroupsofpopulations indefensible. Within a species there can be large numbers wouldbenamedasaspecies. Thesephylogeneticestimates ofmonophyleticgroupsthataregeographicallyrestrictedonly alsoprovideframeworksforevaluatingalternativesincasesin due to recent changes in their environment. An example which the description or recognition of a clade as a species of this is the populations of brocket deer of the Cordillera wouldrenderanalreadyrecognizedspeciesasparaphyletic.In de Mérida, Venezuela, which for decades were recognized suchcases,researchersmightsimplynotdescribeorformally as a valid species, Mazama bricenii. This recognition was recognize that clade at all—which might be acceptable, as based on no data whatsoever and on the assumption of not every clade in a phylogenetic tree represents a taxon a plausible differentiation due to its supposed geographic worth naming—or describe it at the subspecies level—with isolation. However, a recent study that employed ecological paraphyly persisting at a lower taxonomic rank—or describe niche modeling found that, if these populations were truly itandacceptparaphyleticspeciesasvalid, inwhichcasethe isolated in modern time, such isolation commenced not long researchercouldnotinvokeanyspeciesconceptthatrequires ago (Gutiérrez et al., 2015). The study also found that while monophyly (including the dmPSC) upon which to base the thefocalpopulationformedamonophyletichaplogroup,itwas description. Whichever of these alternatives the researcher embedded within a larger (yet shallow) clade whose terminal prefers,andattemptstojustify,duetophilosophical,pragmatic, branches corresponded to Mazama rufina. Results from that or both considerations, the fact that the dmPSC requires a studyshowedthatwhatwasonceknownasMazamabricenii phylogeneticestimateisanadvantageoverotherconceptsthat actuallycorrespondstoMazamarufina(Gutiérrezetal.,2015), donot,includingthedPSCandtheBSC. andillustratehowtheapplicationofthemPSCwouldhaveled The requirement that a candidate species must also be tomistakenlyrecognizeM.bricenii asifitwereavalidspecies. diagnosable in order to be recognized under the dmPSC is Such taxonomic recognition would be a mPSC-based artifact indispensable.Aspeciesmusthaveaseriesofgeneticallyfixed causedby(1)thefactthatsequencesobtainedfromspecimens characteristicsthatarecommontoitsmembersandthatserve fromtheCordilleradeMérida(wherethepopulationstowhich todistinguishitfromothersuchspecies. However,asalready the name M. bricenii would apply occur) were recovered in a discussed (see above), diagnosability alone is, in general, an monophyletic haplogroup; and (2) because those populations inadequateapproachtoestablishspecieslimits. mightbegeographicallyisolatedinmoderntime. Acknowledging the existence of the dmPSC is important The third concept, dmPSC, for which species must be becauseitdoesrepresentoneofthemostexplicitmethodsto both monophyletic and diagnosable, has been useful for inferspecieslimits—contraauthorsthatignoredtheexistence improving mammalian taxonomy. As previously noted, many of this concept in their arguments against or in favor of monophyletic groups are found within a single species, and the "PSC" (e.g., Gippoliti et al., 2018; Groves, 2012, 2017; that monophyly per se does not constitute a criterion to Heller et al., 2013; Zachos & Lovari, 2013; Zachos et determine where the line between species should be drawn. al., 2013; Zachos, 2013, 2014a, 2015, 2016a). Some However, it is also true that assessing whether a candidate operationalstepsindelimitingspecieswillalwaysbearbitrary. species is monophyletic or not provides a fundamental basis In this sense, the advantage of the dmPSC over other for its potential recognition as a valid species. Recognizing concepts is that its operational criteria for recognition of a polyphyletic taxon as if it were a valid species would be speciescanbeobjectivelytested. Inotherwords, monophyly absurd. On the other hand, in some situations (e.g., species and diagnosability are, in general, more easily testable for originatingfromperipheralisolation)acandidatespeciesmight allopatric populations than reproductive barriers (BSC), and meetthecriteria(i.e.,monophylyanddiagnosability)forvalidity more objectively demonstrated than the central criteria upon under the dmPSC, but its recognition renders the species in which other concepts define species, such as “ecological whichthecandidatehadthusfarbeenincludedasparaphyletic. roles” in the Ecological Species Concept (Van Valen, 1976). No consensus has been reached as to whether taxonomists When applied based on sufficient geographic and taxonomic should accept paraphyletic species as valid, or, alternatively, sampling, and, ideally (but not strictly necessary; see if taxonomists should recognize as valid only those that are below), employing phylogenetic inferences using data from monophyletic(seeCarteretal.,2015;Crisp&Chandler,1996; independentsources(e.g.,DNAsequencedataobtainedfrom Dias et al., 2005; Ebach et al., 2006; Freudenstein, 1998; independently inherited genes), the dmPSC has improved Funk & Omland, 2003; Hörandl, 2006, 2007; Nelson et al., the taxonomic classifications of various groups of mammals, ZoologicalResearch39(5):301–308,2018 303 some of which remained problematic for decades. Among Dorcatragus megalotis, Eudorcas nasalis, Eudorcas tilonura, studiesthatexemplifyhowthedmPSChashelpedtoadvance Gazella acaciae, Gazella karamii, Gazella shikarii, Lama mammalian systematics, even if some of them used this mensalis, Madoqua hararensis, Mazama fuscata, Mazama species concept without explicitly or correctly invoking it, are jucunda, Mazama trinitatis, and Redunca cottoni; (3) no thoseondidelphidmarsupials(e.g.,Díaz-Nieto&Voss,2016; published phylogenetic information seems to be the basis of Giarlaetal.,2010; Gutiérrezetal.,2010; Martínez-Lanfranco mostoftheirtaxonomicpropositions;(4)ingeneral,nodetailed etal.,2014;Pavanetal.,2017;Vossetal.,2018),rodents(e.g., discussions were presented on whether recognizing a taxon Hawkins et al., 2016; do Prado & Percequillo, 2017; Rogers as a valid species was more appropriate and justifiable than & González, 2010; Voss et al., 2013), bats (e.g., Baird et al., regarding it as a subspecies, and it seems that the objective 2008; Molinari et al., 2017; Moras et al., 2016; Velazco et of the authors was to merely recognize as valid species as al., 2010), and medium and large mammals (e.g., Bornholdt manytaxaaspossible,withoutcriticalevaluationofalternatives etal.,2013; Gutiérrezetal.,2015; Helgenetal.,2009, 2013; (e.g., recognizing subspecies when appropriate); (5) a list Janecˇka et al., 2008; Koepfli et al., 2008; Miranda et al., of the specimens examined was not provided, and hence it 2017; do Nascimento & Feijó, 2017 (and references therein is difficult for the scientific community to evaluate the authors’ for phylogenetic evidence)). These studies have not only assertionsonspecimenmorphologiesbasedonthesamematerial unraveled the true-species nature of previously unrecognized withcertainty;(6)nodataweremadeavailablethatwouldenable species, but in many cases have shown that taxa considered reproductionandtestabilityoftheanalysesthatwerethebasisof asvalidspeciesfordecadesarenotvalidspeciesatall. thetaxonomicpropositions;(7)althoughtheauthorscitepublished TheapplicationofanyPSCcanpromoterampanttaxonomic studies for some of the taxonomic changes they proposed, for inflation when applied without sufficient rigor. In our opinion, otherstheydidnotandnowhereintheirmonographcanbefound this inflation is caused less by philosophical aspects and results from any quantitative analyses. We cannot understand properties of the PSCs and more by empirical shortcomings. why Groves & Grubb (2011) failed to publish the results of their In several studies, geographic and individual variation do not quantitativeanalysesgivencurrentpossibilitiestodoso(seebelow). appeartobesatisfactorilyaddressed,andnamesareappliedto Unfortunately,thisproblemisnotuniquetotheproposedungulate what could be intraspecific variants (see examples in Tattersall, taxonomicclassificationofGroves&Grubb. Animportantvolume 2007). Onotheroccasions,monophyleticgroupsrecoveredfrom on mammals of South America (Patton et al., 2015) contained molecular phylogenies based on sequence data from a single the first modern taxonomic treatments of various rodent groups locus promptly receive new or revalidated names (e.g., Boubli (e.g.,familySciuridae,generaAepeomys,Oecomys,Rhipidomys, etal.,2012; Thinhetal.,2010). Nevertheless, itisimportantto Thomasomys), but results from analytical procedures assessing notethatwhentheestablished,traditionaltaxonomicclassification geographicandnon-geographicvariationofthosegroupshavenot of a focal group is the result of dogmatic acceptance of expert been published (in the book or elsewhere), and in some cases opinions (often past-century authorities), without support from it seems unlikely they will ever be published. Luckily, several data (see Gutiérrez & Helgen, 2013), then even the use of unpublished Ph.D. dissertations that served as the basis for the limitedevidence—e.g., analysesofsequencedatafromasingle book sections treating those taxa have been privately shared locus(despitethewell-knownshortcomingsofthisapproach;see among colleagues. Clearly, making these Ph.D. dissertations Dávalos&Russell,2014; Knowles&Carstens,2007; Maddison, (andotherunpublishedmaterial)digitallyavailabletothescientific 1997),ideallycoupledwithqualitativeand/orquantitativeanalyses communityfreeofchargefromarepositoryontheInternet(e.g., of morphological data—can well justify taxonomic changes if Dryad,Figshare,InternetArchive,ResearchGate,Zenodo),iftheir based on adequate sampling (e.g., Gutiérrez et al., 2010, 2015, authorsgrantauthorization,shouldbeconsideredbytheeditorsof 2017;Vossetal.,2013;contraZachos,2009,2016b). thisbook,andsimilaractionsshouldbeconsideredbyauthorsand The proposed ungulate taxonomic classification that editorsoffuturemonographsintroducingtaxonomicclassifications. sparked the recent series of criticism against the PSC CANEXTIRPATIONOFPOPULATIONSLEADTOARTIFICIAL is particularly problematic, but not because it was based DIAGNOSABILITY AND SPURIOUS RECOGNITIONS OF on the dPSC. Most controversial aspects of this proposed SPECIESUNDERTHEdmPSC? classificationarenotatallassociatedtoanyspeciesconcept, asitmightseemifonereadstherecentdebatebetweenFrank The short answer is "it can", but again the same answer Zachos and Colin Groves and their co-authors with regard would apply if the question were asked about other species to the "PSC", but rather to more practical aspects of such a concepts, including the BSC. In their criticism of the "PSC", monograph, to name just a few (see also Heller et al., 2013; Zachos&Lovari(2013)claimedthatthefactthatextirpationof Holbrook, 2013; Zachos, 2014a, p. 1): (1) Groves & Grubb populationscanleadtoartifactualdiagnosabilityandmonophyly (2011)didnotassessgeographicvariationatallformostofthe isoneoftheweaknessesofthePSC that makes this concept species they recognized; (2) unfortunately, for some species aparticularlypoorone. Theystatedthat“Thereisyetanother recognized by these authors, the sample size employed was line of argumentation that clearly shows the shortcomings of not indicated nor any published study cited to support the both diagnosability and monophyly as yardsticks for species taxonomicproposals,whereasformanyotherallegedspecies delimitationandthatwebelieveisanothercoupdegrâceforthe the sample sizes were extremely low—e.g., Alcelaphus tora, PSC.Diagnosability (just likereciprocalmonophyly) can and 304 www.zoores.ac.cn oftendoesoccurasaconsequenceofextinctionofintermediate Let us imagine a species, species A, with wide distribution forms...”. Unfortunately, Zachos&Lovari(2013)didnotrealize andshowinggeographicvariationbywayofaclineinseveral thatextirpationofintermediatepopulationsisoneofthenatural qualitativecranialtraitsbelievedtobetaxonomicallyimportant, causesofspeciation. Theseeventsleadtospeciation,affecting i.e., used by most authors to distinguish species within the gene flow and ecological adaptation of extreme phenotypes in corresponding genus. We will illustrate these traits as color populations that previously were genetically connected by the and shape in polygons that represent populations of species existence of intermediate populations. In fact, it could be said A (Figure 1, panel 1). If recent extirpations of intermediate thatlivingtaxathatcanbevalidlyrecognizedontheplanetexist populations take place and only those populations occurring asseparatebiologicalentitiesandastaxonomicallydiagnosable attheoppositeextremesofspeciesA’srangeremainasextant, units only because of the extinction of intermediate forms. If then these populations will become allopatric and it would be all intermediate forms that have lived since the beginning of highly likely that they would be considered as members of life on Earth were still with us, then all living organisms, from differentspeciesundertheBSC(Figure1,panel2). TheBSC prokaryotes to eukaryotes and from plants to animals, would would fail to regard these populations as conspecific, even exist as a single morphological and reproductive continuum: employing the approach presented by Tobias et al. (2010; no distinguishable taxa would exist! Logically, in cases in see also Brooks & Helgen, 2010), which uses the degree of whichextirpationshavetakenplacefairlyrecently(e.g.,dueto differentiation known to exist between different but sympatric human-related causes) no speciation may have yet occurred. species (i.e., species A and B in Figure 1) as a standard In such cases, the extirpation of populations can potentially to assess the taxonomic relevance of differentiation between leadtoartifactualrecognitionsofspeciesunderthePSCs,but allopatric populations (i.e., populations of species A in North thisissueisfarfrombeingassociatedonlytothePSCs;rather, andSouthAmericainFigure1)—inorderwords,thisapproach it is a problem that can potentially affect most, if not all, usestheformerdegreeofdifferentiationasathresholdatwhich species concepts currently in use. For example, this issue (or above) allopatric populations could be treated as different can lead to artifactual recognition of species under the BSC. speciesundertheBSC. Figure1Illustrationofhowpopulationextirpationscanpromoteartifactualrecognitionofpopulationsofasinglespeciesasifthey weredifferentspeciesundertheBiologicalSpeciesConcept(andmanyotherconcepts) Figure 1 Illustration of how population extirpations can promote artifactual recognition of SpeciesA,whichisrepresentedbyquadrilaterals(rectangleandrectangle-likepolygons),possessesawidedistributionandgeographicvariationbywayofaclinein severalqualitativecranialtraitscopnospiduelraetidontasx oofn ao msiincgallely sipmepcoiersta anst ;ifh ethreeyin wtheerse editrfafeitrsenatr espilelucsietrsa utenddears tchoel oBrioalnodgsichaal pSepoecfitehse polygons(panel1).SpeciesB,which isrepresentedbycircles,isrestrictedtoNorthAmerica,whereitoccursinsympatrywithsomeNorthAmericanpopulationsofspeciesA(panel1).Ifextirpationof Concept (and many other concepts) populationsofspeciesAtakesplaceandonlythosepopulationsoccurringattheoppositeextremesofspeciesA’srangeremainasextant,thenthesepopulationswill Species A, which is represented by quadrilaterals (rectangle and rectangle-like polygons), becomeallopatric(panel2).Inthatscenario,itwouldbehighlylikelythattheremainingextantpopulationswouldbeconsideredasmembersofadifferentspecies undertheBSC(panel2).TheBSpCoswseosusleds faa iwltiodere dgiastrrdibtuhteisoen panodp uglaeotiognraspahsicc voanrsiapteiocnifi cb,ye wveany eomf ap lcolyinineg inth seevaeprparlo qaucahliptaretisveen tedbyTobiasetal.(2010;seealso Brooks&Helgen,2010),whichusesthedegreeofdifferentiationknowntoexistbetweendifferentbutsympatricspecies(i.e.,speciesAandB)asastandardto cranial traits considered taxonomically important; herein these traits are illustrated as color and assessthetaxonomicrelevanceofdifferentiationbetweenallopatricpopulations(i.e.,populationsofspeciesAinNorthandSouthAmerica). shape of the polygons (panel 1). Species B, which is represented by circles, is restricted to North CONCLUSIONS America, where it occurs in sympatry with some Noruths eAsmebriocatnh podpiualgatnioonss iosf sapencdies mA onophyly (dmPSC) to delimit species has been, and will continue to be, important for (panel 1). If extirpation of populations of species A takes place and only those populations Major differences exist among the different concepts labeled positively advancing mammalian taxonomy. Our preceding as the "Phylogenetic SpeocciceusrrinCgo ant tcheep otp"p,osaitne dexttrhemeeso onfe spethciaest A's randgeis rceumsasinio ans esxthaontu, ltdhenre thcetsifey pompuislautniodnes rstandings that could arise will become allopatric (panel 2). In that scenario, it would be highly likely that the remaining extant populations would be considered as members of a different speciesZ uonodloerg tihcea BlRSCe s(epaanreclh 39(5):301–308,2018 305 2). 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