There is a one-eyed species problem in philosophy too. So people can be monists about the biological accounts, or pluralists, and they can be monists philosophically, or pluralists. We have to realise that the philosophical problems are not about what species the taxa are, but about what species the category is. This distinction is called, in philosophy, the Use-Mention distinction. When I talk about, say, Gorilla gorilla , I am using the notion of species. When I talk about the species concept, I am mentioning it - in effect the word “species” is put inside quotation marks as a kind of indirect speech. The difference between categories and taxa is an easy one to confuse. It is part of a broader problem in philosophy of relating words, ideas and things. To set things up for our discussion of species concepts, let’s look at a helpful little diagram based on the ideas of a 19thC American philosopher named Peirce:
We have already discussed some mistakes people make when thinking about biology. Let me refresh your memory. First of all, when we define something we typically say it has a set of essential properties or characters. Definition has to cover all, and only, those things we want to define. These characters are often referred to as “necessary and sufficient” criteria for meeting the definition. We will call this “essentialism” later on. Then we noted that biology is composed of many gradually changing properties, instead of being either-or. This is sometimes called “black and white thinking”, and it rarely applies simply in biology. Finally, let’s look again at lineages . Remember that these are causal chains formed by either replication, in the case of genes and the like, or reproduction, in the case of populations. The sense we shall use it in here is of a population that is reproduced over time, so that each generation is the “child” of an earlier generation, and the “parent” of the next. Okay, with this technical stuff in place, let’s move on.
Let me begin by telling you the “received view” about species. You will encounter this in many textbooks and articles. The term “ eidos ” in Greek means various things. It is the term from which we get “idea”, “idol”, and so on. It meant literally “appearance” or “seeming”. Plato discussed it under the term “form” and it was the core problem of western philosophy for two thousand years.
On the Received History (not the one of S&G, but here of “species” only), Aristotle produced an “essence-based” logic by dividing up general terms (genera) into special terms (species). Both were founded on their essences. Linnaeus, the founder of taxonomy, defined species as essences that could not change, and Darwin changed both the essentialism and the static nature of Linnaeus’ species. Darwin’s insights led to the idea that species were reproductively isolated groups in a gene pool, and so essentialism was replaced by “population thinking”. On this view, before Darwin everybody thought species were things that had essences - something every member of the species had that made them the same species, and which no other organism had if it wasn’t a member of the species. The modern view is that a species is what is propagated by reproduction; the older view that species were classes without variation. The received history is wrong in almost every detail. It is wrong historically, as the idea that species exhibited variation is as old as Aristotle and Epicurus, and in that the claim that species were essences actually firmed up after Darwin, not before. But it is also wrong philosophically. Philosophers just prior to Darwin knew that the species of biology were not the same as the species of logic. But that is not really something we need to deal with here. There is enough truth in the received history if you take it as the view of those who opposed evolution shortly after the Origin , and so it forms the alternative view of the opposition.
Why are there so many species concepts? Including replacement concepts for the term “species”, and treating the seminal 1942 book by Ernst Mayr - Systematics and the Origin of Species - as the commencement date, there are at present around 26 species concepts in play. I want to suggest that the reason for there being so many concepts is not because biologists are stupid, or because they love playing with words (at least, no more than philosophers do), nor because it doesn’t matter. I would like to suggest that each species concept is designed to deal with the ways the biological world sorts itself into groups of organisms. In short, species concepts are driven by what biologists know about biology. Let’s briefly discuss the concept classes, and then discuss the sorts of exceptional biological cases they have to deal with. Ignoring some minor variations, there are around five main classes of species concept. Each of them is designed to classify living beings that cannot be classified completely in terms of the other concepts. Often, the 26 species concepts are mixtures, or compound concepts, of these five basic kinds. The “original” concept is the one put into place at the beginning of the period in which biology was a distinct science - the morphological concept. There are several versions of this, including the “essentialist” version, where a species has some necessary and sufficient properties that is found in each of its members and nowhere else, and the cluster versions, in which a species is a “cloud” of properties in some measurement space which is shared mostly, by the members. “Morphology” can mean the form of some body part, or the form of the genome, or the form of some matrix of characters. In one way or another, all species concepts are morphological, if they are ever used to diagnose the species of organisms. Let’s go through each one now.
With the rise of genetics, an increasing role was assigned to the ability of organisms to exchange genes or not. One of these was dubbed the “biological” species concept by Ernst Mayr, meaning that a species was something that depended on the properties of the biological organisms rather than on the properties of the biologist’s and their recognition abilities. This is abbreviated as the “biospecies” concept, or BSC. The mechanisms of genetic isolation are diverse under this concept. More to the point, the modes of isolation are diverse. Some biospecies concepts require hybrid infertility. Some require only a lowered hybrid fitness. Some require a lessened or halted gene flow between populations. Here is Mayr’s view of speciation that underlies it [diagram]. A recent form of the biospecies concept has relied on the ability of the organisms themselves to recognise each other as potential mates. All of them involve the evolution of reproductive isolating mechanisms (RIMs) It follows that a biospecies concept applies only to organisms that are sexually-reproducing. Organisms that are asexual (that is, which are part of the tree of life branches that never evolved sex) or which are secondarily asexual (which lost sexual reproduction at some evolutionary point) cannot be biospecies. Another criticism of biospecies is that it fails as a way of identifying species in two ways. One, because we rarely if ever actually test reproductive isolation; the other, that what we do to diagnose species under any concept is use morphology anyway (and this is true of genetic information, which is a kind of morphology). Finally, biospecies therefore exist only when you can show, or it is true anyway, that they will not interbreed with another close relative, which implies That biospecies rely on potential isolation, That it is a theoretical rather than an operational view, That they only exist in one time and place (or you get species that are not self-fertile over long periods as they evolve.
This brings us neatly to a more modern species concept. Darwin himself realised that species are transitory phenomena due to evolution, and remain distinct because of extinction of the intermediate forms. A concept that takes this into account is the evolutionary species concept - a species is a distinct lineage that remains distinct over evolutionary time. Evolutionary species can change over time and remain a species. In older days, especially amongst paleontologists, if the form diverged sufficiently, even if it had possibly not split, it would be considered to be a “chronospecies”. The problem with this is that there is no agreed “amount” of divergence that marks out a species, and so it ends up being a conventional and arbitrary way to classify species. On evolutionary accounts, so long as the species are separate, they are a single species no matter how much they evolve while distinct. Criticisms of evospecies lie in the vagueness of terms like “evolutionary fate”. How much change is enough to show that an evospecies really is distinct from others? If the lineage merges back into the ancestral stock, was it not an evospecies during that time?
Phylogenetic species concepts are based on the cladistic taxonomic rules I introduced in my last lecture, of groups (called “clades”) being monophyletic - which is to say, a single group and all its descendants. One concept was the way Willi Hennig, the father of cladistics, divided it. He proposed a convention that when a single species split into two, then logically that species would be considered to have gone extinct even if the “parent” was left unchanged. Hence, if the cladogram is understood to be an evolutionary tree, a species is an unbroken line between two nodes. This is not very popular except in some European departments, mainly German. A view known now as “process” cladism treats a cladogram as an evolutionary tree, and the top of the tree is therefore a slice at a given time. A species is thus any lineage of the tree that has a single population as an ancestor that no other species has - what cladists call a “Monophyletic species”. However, it is ambiguous on a number of criteria, such as whether species can continue to exist after speciation (for then they are not monophyletic) and whether they can form more than once (the “respeciation problem”). A third account relies on cladograms not being, in themselves, evolutionary trees, but instead a way to represent the relationships of organisms independently of history. This is called “pattern cladism”, and it treats cladograms as ways to test evolutionary trees. On the third view, species are the smallest diagnosable group of organisms. In cladospeak, this is called an “autapomorphic” clade, the branch that is defined by a set of unique characters or traits. Unlike the Monophyletic concept, the Autapomorphic concept merely needs a single diagnosable group, and it does not need to be compared to other species except in so far as it is part of a cladogram. A simple way to recall these accounts is that Hennig makes a species name extinct when lineages split; the second gives species names only for currently unsplit lineages; and the third makes species names for anything you can’t divide further in your diagnosis.
There are quite a number of other kinds of species concepts, but these are the main ones. Nearly all species concepts since the year dot have included some ecological aspect - species have what is variously called a “station”, “habitat”, “country”, “niche” and so on. Recently, species have been proposed to be whatever occupies a local ecosystem in the same manner - that is, what can be replaced by close relatives. Hence, subspecies that live in the same ecological relation as varieties elsewhere are treated as species. It is very rare to find anyone who thinks all species are just ecospecies and nothing else; it is usually allied in some way to biospecies. Paleospecies are a special application of morphospecies to the fossil record. For example, we are not sure at present if Homo ergaster and Homo erectus are the same biospecies or not - there is sufficient difference in morphology to put them in different species, but this difference is not enough to rule out them being variant forms of a single biospecies. The conception of species for those organisms that do not reproduce using sex (such as some bacteria and algae) forms a real problem for systematics. It is usually noted that these forms are grouped in arbitrary ways. Some authors want to replace the notion of “species” for them altogether. Since A. J. Cain’s book Animal Species and their Evolution, in 1954, though, they have been called “agamospecies” - species without gametes. Prior to that, many systematists would either overlook asexuals or deny they existed. Agamospecies is a trashcan category, though. There are many different kinds of asexual organisms and lifecycles, so we have to try to collect them into natural groups. For example, there is little reason to isolate whiptail lizards in the southern USA that reproduce sexually from those that are “parthenogenic” (i.e., breed only females) and put them in the same class as Hydra or bacteria. Aristotle warned us of the dangers of privative definitions, defining something in terms of what it is not . Photocopies also reproduce without sex. Are they agamospecies? Finally there is the widely ridiculed taxonomic species concept - a species is whatever a competent taxonomist chooses to call a species. Kitcher calls this the “cynical” species concept. It is not so cynical as it might appear, though. It was an attempt late in the 19thC to overcome the professional chaos of taxonomy using many different species concepts in favour of making species work based on authorities in the fields. In short, it took specialists seriously. Moreover, there is an argument to make, I believe, that the pattern recognition abilities of trained observers will tend to isolate groups correctly in the right circumstances. If true, this is a kind of phenetic concept of species.
Peirce’s “triad” is the relationship between what is written or spoken - the “symbol”, what is in our heads as a thought or concept, and what the words and thoughts both relate to. The Symbol, either written down or spoken (or done as a piece of art, and so on, but we can leave these to one side) symbolises the thought or reference. The thought refers to the thing in the world, and so indirectly the symbol stands for the thing. “ Species” is a symbol. It symbolises some ideas we have in our head, which are either just thoughts, or are thoughts that refer to something real. Either we have thoughts about species that are just artifacts of our thinking, or which really do refer to actual things in the world. We shall see this pop up under the issue of whether species are real or ideal. But, as I mentioned, species can be a category or it can be a taxon. As a category, “species” is a way of uniting many different things or ideas under one thought and one word. As a taxon, a species is a particular biological entity such as Gorilla gorilla . So let’s see how Peirce’s triad deal with this distinction.
As a category “species” is a symbol that symbolises whatever idea we have in our head about the smallest biological kinds. If these refer to a class of things in the world, then the thought refers to reality, and the word refers to a real class of living things.
As a taxon, a species is a name, symbolising a definition or diagnosis we use to refer to an actual species like (in this case Homo sapiens ). It is important to keep categories and taxa separate. If we do not, we will make the mistake of thinking that whatever it is that makes one or a limited number of species distinct will be what it is that defines the species category . Many biologists, and not a few philosophers, have committed just this mistake. We can avoid this error by referring to the concept of “species”, and to individual species.
[Read out] Gargantua here, by the way, is the name of an individual gorilla. Hull goes one step further - he says that the category or concept of species refers to a type of individual itself. In short, species are individuals, like organisms, and the class-level concept is the species category . His view, which he coauthored with a fellow named Michael Ghiselin, a biologist, is referred to as the Individuality Thesis. It is still being debated, and it is hard to separate all the threads of the discussion. What I am going to do now is treat this as four independent issues or questions about the ontology of species. This leaves us with eight possible positions, and four “kinds” of individuals species may be.
If we are not scientific monists - that is, if we do not assume that only one kind of species concept applies to all living things, we may still want to be a philosophical monist. This would mean that we justify all the scientific concepts we like on some philosophical basis. One of these, which we shall see, is the Individuality Thesis. Another is the Homeostatic Property Cluster concept. A third is the General Lineage Account. The first thing we have to decide is if we think species are real things or not. This is not a simple issue - it mirrors some longstanding philosophical and logic arguments, that go back to the beginning of the middle ages, whether general terms exist in the world or only in our minds. This is called the “nominalism” debate, which we shall cover later. Another issue has to do with how we classify the world - are species classes, or in philosophical jargon, “natural kinds”, or are they just particular individuals. This is covered under the Individuality Thesis. Then we need to decide is species act like systems, and what kind, or if they are just names we give to phenomena - this bird appears like that bird, so they are the same “species”. It is worthwhile remembering that the word species means “appearance” in the original Latin, translating a Greek word with the same meaning. One bit of technical philosophy I have to introduce here is “ontology”. This is a term of that branch of philosophy call Metaphysics, and it applies to the sorts of things that exist. A scientific ontology is, therefore, the sorts of things that a scientific theory or set of theories asserts do exist. We need to see if species are what biological theories say must exist or not. A lot of people think that evolution does not require species. Some think Darwin denied they existed, which, by the way, he did not.
Suppose we are shopping in Plato’s Delicatessen for the tastiest species concept. What do we want of it? What would count as a good resolution to all these differing flavours biologists have developed? I suggest that we want a concept that is real, covers all these cases as best as it can, and eliminates problematic ones, deals primarily with facts about the biology, not facts about the biologists, and asks questions of putative species that can only be answered by empirical investigation. We do not want to make this entirely a philosophical matter of synthetic a priori ideas. This has to be about science.
There are four major issues at play in the philosophical debate over species, as I see it. The first, which I have alluded to before, is whether the category of species refers to real things or just to ideas (leaving aside whether ideas real real or not). In the Peircean triad, this is to ask if the thought has a referent. The second is to ask if the category refers to classes, or kinds, or whether it refers to particulars. This is a technical distinction I will explain in a bit. The third question is whether species form cohesive functional entities. The fourth is to ask if we can recognise species or not; that is, if they are identifiable or diagnosable (remember that I alluded to this before, too).
By “real things” we mean something like “concrete objects”. That is, are species taxa things that exist in space and time, with beginnings and endings? This question was asked of the logical notion of species at the beginning of the middle ages, and the subsequent debate defined how we came to think of the world, and led to the beginnings of modern science. It is sometimes called the “nominalism” debate. If these things exist only in our minds as ideas, or conceptions, then the reality is that only individual organisms are real. Many people have held this view, including a good many evolutionists such as T.H. Morgan, the great geneticist. Species names are only therefore useful to us as a way to communicate, a view argued in favour of by John Locke, the philosopher whose ideas helped establish empiricism. Sometimes people say that realism means you think things are objects, while idealism means they are not. This is a misuse of the language, so we will not use “object” here, as it gets too confusing. I also mention, but do not like to use, the term “species nominalism”. Nominalism is the view that general words and ideas do not refer to anything. But I will use the phrase “idealism” here. Historically, idealism is the view that only ideas are real, but today we tend to mean that things are only ideas.
Our second question is whether the category of species refers to things (taxa) that are abstract or concrete. If they are abstract, then the taxa are eternal concepts. This is what Hull rejected. If they are bounded in space and time, then they are concrete things. Things that are general kinds, which are abstract, were called in the Nominalism Debate “universals”. These were types that existed independently of whether or not anything existed as a concrete token - if no human being lived now, the species Homo sapiens would remain. Things that exist individually are referred to as “particulars”. A particular Homo sapiens is John Wilkins. Another is Karen Jones. The Individuality Thesis is that species themselves are particulars, or rather, the Individuality Thesis as interpreted in one way.
A good many species concepts assume that there is something causal about species. What makes us a separate species from, say, chimps is that we cannot, do not or will not interbreed with them. This is the class of Reproductive Isolation concepts of species. It assumes some functional identity, some causal role for mechanisms that make species what they are. Other functional cohesion concepts are ecological, or evolutionary. Each of these presumes that species are caused to be unique and distinct. We can think of a species like a gun. It has a mechanism that must work if it is to be an instance of the category - “gun” or “species”. This example, taken from a book by one of the originators of the Individuality Thesis, has some interesting ramifications, as we will see.
Finally, we must ask if a species is something that we can observe or identify. For example, a cloud is an identifiable object, but arguably, not a functional one. If we say that a species is an identifiable, or salient, object, then any species that is not identifiable by some actual or ideal observer is not a species. This is sometimes confused with the operational or practical issue of whether we can find diagnostic criteria for identifying an organism as a member of the same or a different species to some other sample. These four issues define the species debate in philosophy. Let us look briefly now at what the claims are in the literature, and then relate them to these issues.
The Species are Classes thesis is, in effect, the default view. It’s what we historically thought species were before this debate got going. It has three claims: That all members of a species must share in some universal set of characters or traits. That a particular species is a natural kind - that it is something that is not dependent on our conceptualisation, and which has an essential definition, or, that there is an essence to being a member of a species. Essences are either causal powers, or they are verbal N&S criteria (in which case this is the same as point 1, so we will call an essence a causal power). That species are abstract objects - they are not “in” space and time, but that individual organisms “instantiate” or “realise” them. On the Classes view, a species can be reborn or reconstituted. Jurassic Park scientists might revive the species long after it is dead.
The Individuality Thesis also has three major claims. One is that species are historical objects - what we have been calling “concrete” objects. They are spatiotemporally bounded. The next is that species, the taxa, do not have members like a class does, but it has parts. If species were classes, then they could have or not have members, but if species are composed of parts, take away all the parts and you have no species left. The third is the idea that a species is, or can be, a functionally cohesive entity. Let’s look at each in turn.
The idea that species are spatiotemporally bounded, or concrete, is the claim that a species is a historical object like an organism is. We can therefore ask some analogous questions about it. For example, does it act like an organism - do species “age”? Does it solve problems? Must it have a sharp beginning? Can it, once extinct, revive or revolve? Can it begin more than once. On the Individuality Thesis, a species has a singular historical birth and death. Jurassic Park science will not revive the same species. There are stickleback fish, for example, in British Columbia in Canada that are deep water species and shallow water species. They mate with those that share their environment and morphology. It seems they have more than once evolved. Once they evolve shallow water forms from deep water forms, they are able to interbreed with earlier shallow water forms. Is this species constantly re-evolving? If so, isn’t it true that the species “shallow water sticklebacks” is a kind rather than a historical individual? The Respeciation Problem is a difficulty for the Individuality Thesis.
So we find that we have
Michael Ghiselin, one of the founders, with Hull, of the Individuality Thesis, also believes that species are functional individuals. He uses the metaphor of a gun - you can take it apart, and then it is not a working gun. Disrupt the gene flow or cohesion of the gene pool, and it is no longer behaving like the original species. This leaves us a cute problem. Hobbes, long ago, proposed a problem of human identity - if we age and replace our physical parts, in what sense are we the same person when old as when young? He used the analogy of Theseus’ ship - each time it is drydocked, part of it is replaced - the keel, planks, a mast. Eventually it is entirely made of new parts. Is it the same ship? A species, if it is made of parts that function together, suffers from this problem, for if it is the way in which it works that makes it that species, and not the parts, isn’t it the case that there is something else - “being a gorilla species” for example - that makes it a gorilla species, and isn’t that just a class concept? Finally the question arises if a species that is not functionally organised can be an individual. I think so - we have already answered this in dividing logical, functional and phenomenal or observational identity from each other. Let’s see where we have got to.
These are the alternatives in play. We divide them into real and ideal (question 1), and then into the logical, or metaphysical, functional, or cohesion, and phenomenal, or observational, senses of species. You will see this gives us a binary number from 1 to 8, or as I have lettered them, from A to H. Ask yourself, when you read a claim that a species is an individual or a class, what is the actual claim, and is the person being consistent. I have given A to H names: Idea, Appearance, Effect, Apparent Effect, Particular, Apparent Particular, System and Full individual. If they are not helpful, think up your own.
The Individuality Thesis was enthusastically adopted by two groups in biology when it was proposed. One was the Punctuated Equilibrium Theory group - they believe that evolution typically occurs in fits and starts, followed by periods in which no real change occurs. This fit well with both the historical and the functional notions of species as individuals, although I would suggest it really ties best with the phenomenal conception. It seems to me that the IT doesn’t need sharp edges in time and space. If organisms can have vague boundaries and be individuals, so can species.
Remember I mentioned the notion of “species nominalism” that I wasn’t going to use? I lied. Here is a statement about nominalism from the major philosopher W. V. O. Quine that happens to use biological species as a case. He thought that if we used the reproductive isolation conception, we were committed to the abstract notion of species. I hope that by now you can see why he was wrong, or why we are equally committed to an abstract notion of “organism”. Quine was a nominalist - he rejected universals. I think we can be too if we need to, and still speak of species as individual groups.
So we can accept the idea that a species taxon is a group of organisms, and not be forced into thinking they are universals. In other words, species are groups not classes. Most nominalisms are actually conventionalisms, like the way Maynard Smith and Locke thought of them.
To conclude, allow me to assert my own opinion here. I think species, the taxa, are indeed individuals, in the sense of being particulars - they are logical individuals . I hold that the species category is, though, a class concept - species are whatever is separated causally from other species.
Systematics and speciesJohn Wilkins
Lecture 1: ClassificationThe mind is finite in its powers of comprehension; theobjects, on the contrary, which are presented to it are,in proportion to its limited capacities, infinite in number.… How can the infinity of nature be brought down tothe finitude of man? This is done by means ofClassification.[Sir William Hamilton, Lectures on Metaphysics, lect.XXXIV]
Classification• The basic statement of classification:– A is more closely related to B than eitheris to CABC
Characters• Symbols for the traits of organisms• Homologous - similar by descent orancestry (homo- = same + logy)– Molecular homology: orthology– Serial homology• Homoplasious - similar in form (homo- +plasty = form)– Convergent evolution– Not informative of evolutionary past
Hierarchy• Ever since Aristotle, all properclassification is hierarchical• That is: nested setsNever this
Representing hierarchies• Tree diagrams• Indented lists
Systematics approaches (sect6.3)We can relate organisms:• By similarity– Morphological (form)– Phenetic (clusters)– Essentialist (essences)• By genealogy– Cladistic– Evolutionary
Similarity relations taxa• Morphological– using a key characters of form, or several– Problems: arbitrary choice of characters• Phenetic– Using any characters, plotting in a graph to findclusters– Problems: sensitive to characters used• Essentialist– Necessary and sufficient criteria– Problems: species are usuallypolymodal/polytypic– Can be species with no N&S criteria (siblingspecies)
Cladism• Cladism draws branching trees• A “natural” group is monophyletic
Evolutionary systematics• Genealogy plus grade• Can be paraphyletic or polyphyleticMole Wolf ThylacineConverginggroupLizard Crocodile Bird“Primitive”group
Clouds, clades and grades• Which is the “right” way to proceed?• Monist: My way or the highway• Pluralist: Whatever gets you through thenight• Clouds - phenetic clusters of similar things,or essentialist classes of identical things• Clades: branches in pattern or process• Grades: what the observer thinks istheoretically important
Theory, observation and taxa• Theory-free classification– Operationalism– Instrumentalism or conventionalism• Theory-relative classification– Is evolution the foundation for hierarchy?• Theory-testing classification– Should a classification test other aspectsof the theory of a field?
Summary slide• Two sides to it: what scientists doand say (mostly one-eyed), andthe philosophical issues (multiplePoVs)• Some [false] history - theReceived View of Species• Five kinds of species conceptsused by scientists
Review• Definitions - necessary and sufficient(N&S) criteria• Gradual thinking versus black andwhite thinking• Lineages - series of parent-childentities (populations)
The [False] Received Historyof SpeciesPlato defined Form (eidos, trans. species) assomething that had an essence, and Aristotle set up a wayof dividing genera (genē) into species (eidē) so that eachspecies shared the essence of the genus, and each individualin the species shared the essence of the species. Linnaeustook this idea and made species into constant andessentialistic types. Darwin overcame this essentialism.Later naturalists, under the influence of genetics,discovered the biological species concept, in which speciesare found to be populations without essences, but withcommon ancestry. Population thinking replaces typologicalessentialism.
The Received History ofSpecies, briefly• In older view a species has an essenceshared by all its members• Shares in the essence of the genus• Linnaeus took the logical species andapplied it to biology• Morphological conception (based on form)• Darwin discovered population thinking• Others later invented the “biological”species concept
• Morphological (Linnaean)• Reproductive isolation (Biological)• Evolutionary• Phylogenetic (3? kinds)• EcologicalSpecies concepts in play
• Biospecies or BSC• Speciation by evolution of RIMs• Hybrid infertility• Gene pool concepts• Shared mating mechanismsBiological species concepts
• AKA evospecies or ESC.• Independent lineages -unique evolutionary fate(B, C, and D)• Chronospecies - speciesacross time and form (Ato B) not 2 but 1evolutionary species!Evolutionary speciesconcepts
HennigianMonophyletic• Phylospecies or PSC• Between two nodes(Hennigian concept)• Descended from asingle population(Monophyletic concept)• Defined by a unique setof characters(Autapomorphicconcept – amorphologicalconcept?)Phylogenetic speciesconceptsAutapomorphic
• Ecological species concepts(Ecospecies)• Paleontological species concept(Paleospecies)• Asexual species concepts(Agamospecies)• Taxonomic species concept (Cynicalspecies)Other concepts
Summary of concepts• Older morphological view based onessences• Modern “biological” concepts based onreproduction and genetic exchange• Evolutionary and phylogenetic conceptsbased on history• Ecological and other conceptions
Categories and TaxaQuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.X“X”
QuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.Species Categories
Species TaxaQuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.
On the traditional view, the species category is a class ofclasses defined in terms of the properties which particularspecies possess … and particular organisms are individuals… The relation between organisms, species and the speciescategory is membership. An organism is a member of itsspecies and each species is a member of the speciescategory. On the view being urged [by Ghiselin and Hull],both particular species and the species category must bemoved down one category level. Organisms remainindividuals, but they are no longer members of their species.Instead an organism is part of a more inclusive individual, itsspecies, and the names of both particular organisms (likeGargantua) and particular species (like Gorilla gorilla)become proper names. The species concept is no longer aclass of classes but merely a class. [David Hull 1976: 174f]Philosophical issues
Four major issues1. Realism versus idealism2. Class versus group3. Functionally coherent or not?4. Identifiable or not?
First issue: Realism• Species are either real things, or they arenot• If not, then they are conceptions in themind only (i.e.,the concepts do not refer toanything)• Conventionalism (Locke, T. H. Morgan,Maynard Smith)• Real things are objects, mere concepts areideas. Hence, “species realism” vs “speciesidealism [nominalism]”
Second issue: Class orgroup?• Issue whether or not species areabstract entities that are timeless, orwhether they are groups of organismsthat begin and end and have alocation (but not nec. one singlelocation).• Philosophically, the problem of“universals” (both in the definitions,and in the essences) versus“particulars”
Third issue: functional ornot?• Issue of whether a species must havesome kind of functional identity -whether genes flow between allpopulations, or whether there are“developmental constraints”, orwhether they occupy a particularniche that means they are maintainedby selection.
Fourth issue: identifiable ornot?• Issue of whether or not a species mustbe salient to human observers or theirtests• Often confused with whether or not aspecies has “key characters” and canbe diagnosed
Species as classes - 3claims1. That species have universalcharacters2. That species are natural kinds3. That species are abstractions• Each of these suggests that aspecies can be “reborn” afterextinction (The Jurassic Park Scenario)
Species as individuals - 3claims1. A species has a historical duration(birth and death) and a physicallocation (the historical individualthesis)2. A species has parts rather thanmembers (organisms are components)3. A species has a causal cohesion(the functional individual thesis)
Historical speciesDo:• Species act like an individualorganism?• Species reproduce?• Species have sharp beginning andendings?Species, once extinct, cannot berevived (otherwise a new historicalspecies) - the Respeciation Problem
Functional species• Some argue that species are functionalentities, the same way a gun is a functionalentity (not just parts, but parts organised ina functioning way). The gene pool would beseen as a functional individual.• Ship of Theseus problem of identity• If species are not functionally organised,are they still individuals?
Summary of speciesontologyIndividual type Logical Functional PhenomenalIdealA. Pure abstraction [Idea]B. Phenomenal individual [Appearance]C. Integrative individual [Effect]D. Phenomenal, Integrative individual[Apparent Effect]RealE. Historical individual [Particular]F. Historical, Phenomenal individual[Apparent Particular]G. Historical, Integrative individual[System]H. Full individual
Punctuated Equilibrium Theoryand the Individuality Thesis• The idea that species have abeginning and end fit well with PE• The Individuality Thesis doesn’t need“sharp” edges in time or space
Species nominalismWe may say, for example, that some dogs are whiteand not … commit ourselves to recognizing eitherdoghood or whiteness. … On the other hand, whenwe say that some zöological species are cross-fertilewe are committing ourselves to recognizing theseveral species themselves, abstract though they are… at least until we devise some way of paraphrasingthe statement as to show that the seeming referenceto species … was an avoidable manner of speaking.”- Quine “On what there is” (1948)
Nominalism? Conventionalism• Nominalism denies the reality ofabstract general terms, that covermany objects• So-called species nominalists do notdeny that the species exist, but onlythat they are not classes (do not haveN&S properties)• Species are named for convenience
Conclusion• Species can be one of four kinds ofindividuals• They can be real individuals, or idealindividuals, or real classes or idealclasses, and so on