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  See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/228014710 Species Concepts Chapter  · April 2001 DOI: 10.1038/npg.els.0001744 CITATIONS 3 READS 8,496 1 author:Some of the authors of this publication are also working on these related projects: Olfaction   View projectMichael T. GhiselinCalifornia Academy of Sciences 223   PUBLICATIONS   6,085   CITATIONS   SEE PROFILE All content following this page was uploaded by Michael T. Ghiselin on 23 May 2015. The user has requested enhancement of the downloaded file.  Species Concepts Michael T Ghiselin, California Academy of Sciences, San Francisco, California, USA The species is one of the most fundamental units in biology. However, there are differentconcepts of what is meant by the term ‘species’ that reflect diverse goals and prioritieson the part of scientists. Introduction The species, like the cell and the organism, is one of themost fundamental units in biology. It is fundamental intwo very important ways. On the one hand it is a basicpopulational and evolutionary unit. Species becometransformed through time. They undergo speciation,giving rise to separate and distinct lineages that diversifythrough time. On the other hand, it is also a basic unit inclassification. Systematics is often defined as the study of biodiversity, but this is a bit too broad. Systematicsemphasizes those aspects of biodiversity that are the resultof evolution. Its results are expressed as taxonomy, orformal biological classification. Each species is given aname that serves as a unique identifier and has a place in ahierarchical arrangement that at once reflects its evolu-tionary relationships and provides a basis for biologists’language. Essentialism or typology This dual role of species creates serious problems becauseordinary ways of classifying the objects around us are notalways appropriate to dealing with an evolving universe.Systematic biology had its origins in the language of everyday life, and the modern science of taxonomy thatgoes back to Linnaeus was pre-evolutionary. It wassupposed that ‘kinds’ of plants and animals are asimmutable as are kinds of minerals. They might varywithin certain limits, but for one species to change intoanother was contrary to the kind of philosophy that wasacceptedatthetime.Agroupwassupposedtohavewhatiscalled an ‘essence’ or ‘nature’ that made it what it is.Classification meant discovering this essence, and defininggroups on the basis of supposedly ‘essential’ properties.Much of the history of evolutionary thinking reflects astruggle to get rid of what is called ‘essentialism’ or‘typology’. Essentialism presupposes the reality of es-sences, leading people to think in terms of stereotypes andto screen out that which is unique or variable. Nominalism and realism Even if we reject essentialism, the species presentsdifficulties for an evolutionist. If a species gradually splitsinto two descendant species, for example, there is nomomentintimeatwhichonecansaythatwenowhavetwospecies instead of one. One way of coping with suchsituations has been to adopt a philosophy called ‘nomin-alism’,whichviewsclassificationassomethingarbitraryorconventional. It often takes the form of claims that speciesare not ‘real’, and has been opposed to ‘realism’. Thetraditional claim of nominalists was that individuals arereal, but classes are not, and that species are classes andtherefore not real. A third possibility, which is that speciesare not classes, but individuals in the sense of supraindi-vidualwholes(populations),hasgraduallywonacceptanceduringthelatterhalfofthetwentiethcentury.Accordingtothis view, species are like baseball teams, groups of organisms, but not kinds of organisms. Here ‘individual’isusedinamoregeneralsensethan‘organism’andmeansaconcrete particular thing. It is a technical term in thebranch of philosophy called ‘ontology’, which deals withsuch issues. Species Concepts and Definitions Scientists and philosophers have thus taken quite a varietyof positions about what it means when we say thatsomething is, or is not, a species. The philosophicalpositions just mentioned have sometimes been calledessentialist, nominalist and individualistic species con-cepts. When, however, we speak of species concepts wegenerallymeansomethingmoreparticular,forexamplethebiological species concept, which is just one example of anindividualistic concept. A conceptual analysis of thebiological species concept and its possible alternativesallows us to understand the ongoing debates amongbiologists and philosophers. What we mean by a conceptisobscure,andherewewillfocusonwhattoallintentsandpurposes is the same thing, the definition of a term.In any such discussion it is crucial to make it clear bothwhataspeciesconcept,ordefinition,isandwhatitisnot.Aspeciesdefinitionis adefinitionofageneralterm,‘species’. Article Contents Introductory article .  Introduction .  Species Concepts and Definitions .  TheBiologicalSpeciesConcept:SpeciesasPopulations .  Recognition, Cohesion and Evolutionary SpeciesConcepts .  Species as Kinds, or Classes .  Kinds of Species: Polymorphic, Polytypic and Cryptic(Sibling) .  Monism versus Pluralism with Respect to SpeciesConcepts 1 ENCYCLOPEDIA OF LIFE SCIENCES © 2001, John Wiley & Sons, Ltd. www.els.net  Itdefinitelyis notadefinitionof  Homosapiens oranythingelse that might be given as an example of the general term.It is a definition of it, not of one of them. Somemisunderstanding has resulted from the fact that ‘species’can be read as either singular or plural. The practice of saying ‘the species’ when one means species in generalsometimes helps. However, modern taxonomy offers amore satisfactory solution. It treats the general term asreferringtoalevelinahierarchicalsystemofclassification.These levels are called ‘categories’ and the groups thatoccupy the levels are called ‘taxa’ (singular ‘taxon’). Thusthe genus is a category, and each and every group that hasthatrank,suchas Homo ,isataxon.Likewise,thespeciesisa category, and each and every species, such as  Homosapiens , is a taxon. So when we want to make sure that ourmeaning is clear, we say that a species definition is adefinition of the species category.Becausethespeciescategoryis aclass,weshouldbeableto define it in a straightforward manner, by listing theproperties that are necessary and sufficient for somethingtobeaspecies(thedefiningproperties).Asweshallsee,thisisindeedpossible,butbynomeanseasy.Howonedefinesaspeciestaxon,however,isanentirelydifferentmatter.Italldepends on one’s definition of the species category, andthat in turn may depend upon one’s underlying philoso-phical assumptions. If one believes that species taxa are,like the species category, classes, then it should be possibleto define them in the sense of listing defining properties. If one rejects that, however, and says that particular speciesare individuals, then, just like individual organisms, theyhavenodefiningproperties.Theycanbedescribed,butnotdefined.Muchconfusionresultsbecausetheterm‘define’issometimes used in a loose sense, where to be precise wereally ought to say ‘describe’ or ‘diagnose’. The Biological Species Concept:Species as Populations The biological species concept that is generally, if some-times reluctantly, accepted by evolutionary biologiststreats species as reproductively isolated populations. Suchpopulations may consist of lesser groups that are alsopopulations, including local ones. A (biological) species isreproductively isolated from all other species, and there-fore it cannot interbreed with them. The componentpopulations of a species, however, have the capacity,eitherdirectlyorindirectly,tointerbreedwithoneanother.Such interbreeding is not a property of the componentorganisms. It is the exchange of genetic material, or geneflow, throughout the population as a whole. As aconsequence of interbreeding a species is a reproductivecommunity that is held together by sex.Thefollowingformaldefinitionofthespeciescategoryisthatof Mayr:Groupsofinterbreeding naturalpopulationsthat are reproductively isolated from other such groups.Such formal definitions are apt to be misleading,especially since their meaning depends upon the definitionof other terms, and upon the theoretical context in whichthey are used. For this reason some background wasprovided before giving the quotation, but that is only abeginning. The biological species concept is linked to thetheory of species formation, or speciation. When a speciesspeciates, it gives rise to populations that are considereddistinctspeciesonlywhentheyhavecometodifferinawaythat would keep them separate, even if they occupied thesame geographical area.Itis generallyacceptedthatunderordinary circumstances, populations must be geographi-cally isolated (allopatric) before such divergence can takeplace to the point that they will remain distinct even whenthey are in the same place (sympatric). The evolution of suchintrinsicisolatingmechanismsistherebyequatedwithspeciation, and serves as the basis for defining the speciescategory in terms of reproductive isolation. The biologicalspecies concept does not, however, exclude the possibilityof sympatric speciation, i.e. without geographical isola-tion.However, the biological species concept has manydifficulties, some imagined, others real. Reproductiveisolation between entire species is often confused with theinability of any two organisms within a species to produceoffspring. The definition covers populations under naturalconditions:artificialcrossesproducedinthelaboratoryareirrelevant.Likewiseacertainamountofhybridizationdoesnot contradict the definition, so long as the species remaindistinct andcan evolve separately. A problemnot so easilydismissed arises because allopatric populations are con-sidered parts of the same species so long as they arepotentially, not just actually, capable of interbreeding.Again,isolationbetweenpopulationsrefersonlytoagivenmoment in time, or to the so-called ‘nondimensional’situation. Changes that would isolate contemporarypopulations can evolve within a single population over along series of generations, and some persons would like touse such changes as a criterion for delimiting ‘chronos-pecies’inthefossilrecord.Thebiologicalspeciesdefinitionprovides no criterion for dividing up a single lineage intoso-calledchronospecies.Nordoesittelluswhathappenstothe ancestral species when it splits into daughter species,perhaps by budding off a small peripheral one. Does it, ordoes it not, cease to exist? Furthermore, many organismsare not parts of (sexually reproducing) populations. Theyare asexual, and not just on a temporary basis, butpermanently asexual. This circumstance burdens taxono-mistswithaseriousproblemofwhattodowiththeasexuallineages or clones. Nomenclature proceeds as if each andevery organism belongs to one and only one species;evolutionarytheorytellsusthattheybelongtoatmostonespecies. There are also many practical problems. It is very Species Concepts2  difficult to study populations when all one has is a singlemuseum specimen. Palaeontologists likewise often mustdeal with quite fragmentary fossil remains. Recognition, Cohesion andEvolutionary Species Concepts Some alternatives to the biological species concept aremerely variations on the same populational theme. Let usconsidersomeofthesebeforeproceedingtomorphologicaland other concepts that treat species as classes of similarorganisms rather than as populations. Recognition concept The recognition concept emphasizes what holds speciestogether,inplaceofwhatkeepsthemapart.Theorganismsare supposed to share a ‘specific mate recognition system’,or SMRS, which enables them to recognize conspecifics of the appropriate sex. According to this concept, species arereproductive populations, and on the whole they areequivalent to biological species. However, there are twoimportant differences. First, a population, in principle atleast, might vary geographically in how mates arerecognized. Second, a single lineage might evolve adifferent SMRS. Would it then be a different species? Cohesion concept Thecohesionconceptemphasizesthepointthatsomethingdoes in fact hold species together. This provision of cohesionispreciselytheroleofsexinthebiologicalspeciesconcept. It has been suggested that something other thansex might play this role. However, no convincing evidencehas yet been provided that such a thing actually exists. Evolutionary concept Theevolutionaryspeciesconceptemphasizesthepointthatspecies are not just populations as they exist at any giveninstantintime:theyarealsolineages,andthefactthattheyevolve is very important. Thus evolutionary species arebasically the same things as biological species, butconsidered from a somewhat different point of view. Cladistic species concepts Cladistic species concepts have been advocated by thosewhorejectthebiologicalspeciesastheappropriateunitforthe study of relationships among lineages. There is noagreementastowhatthisunitshouldbe,butingeneralitisa part of a biological species. Advocates of this conceptargue that what others call subspecies or local populationsshould have their taxonomic rank elevated to that of full-blownspecies.Thismovewouldhavemanyseriouseffects.The number of species names would be vastly increased.Also, human beings would not all belong to the samespecies. Species as Kinds, or Classes Folk taxonomy and common sense suggest that speciestaxa are kinds of organisms. However, the biologicalspecies concept does not treat them as kinds of organismsor for that matter as kinds of anything. Rather they arewholes, with organisms as parts. In this respect they arevery much like organizations. A university is made up of departments and professors, but it is not a kind of department or a kind of professor. Likewise when we saythatsomebodyisahumanbeing,wedonotmeanthatheorshe is ‘a  Homo sapiens ’ but rather that he or she is anorganism-level component of   Homo sapiens . This mayseem to contradict common usage, but it really does not.Most people would consider a human sperm cell a part of our species, even though it is not a human being.Some species concepts do treat species as kinds. Itdeserves emphasis that these concepts treat species taxa,andnotjustthespeciescategory,askinds.Accordingtothebiologicalspeciesconceptthespeciescategoryisakind,butthe taxa are not kinds. If species taxa are kinds, then thespecies category is a kind of kind. We shall now explainwhat these concepts are and consider their merits. Species as natural, or objective kinds The physical sciences, such as chemistry, generally classifymaterial objects upon the basis of properties thatsupposedly reflect the laws of nature that govern suchobjects. The periodic table of the elements, for example,consists of groups such as the halogens that encompasslesser groups, such as chlorine and bromine. Chlorine hasthe properties that it does because the laws of naturenecessitate that whenever matter assumes a given config-uration those properties simply must be present. Suchkindsareveryimportantinchemicaltheory,forthelawsof nature allow us to predict the properties of moleculeswheneverandwherevertheyoccur,irrespectiveoftimeandplace. And in principle at least it should be possible todefine such kinds in terms of those properties: e.g. byspecifying the atomic number of each element.Thespeciescategorymaylikewisebeinterpretedassucha natural kind. The biological species concept picks out akind of population about which laws of nature may beformulated, at least in principle. These might include lawsfor speciation, for example. Likewise a good case can bemade out for the existence of natural kinds that are in factkinds of organisms, for example autotrophs and hetero- Species Concepts3
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