Field of Science

How to Recognise a Species

On a couple of previous occasions, I've discussed the use of the Biological Species Concept (BSC) versus the Phylogenetic Species Concept* (PSC) - see posts here and here, the first of which defines the two terms if you're not familiar with them. One question that someone, somewhere will always bring up whenever I broach the subject is whether use of the PSC would require the division of humans into separate species, so this post is partially in response to that. From a more general perspective, I thought that some of you might be interested simply in knowing how a taxonomist recognises if they have a new species.

*The more I cover this topic, though, the less I like the term "phylogenetic species concept", because the PSC isn't determined solely, or even primarily, by phylogeny. I prefer the term "diagnostic species concept". Real, honest-to-goodness phylogenetic species concepts, where species are defined by holophyly, have been proposed before (e.g. Donoghue, 1985) and I think the term should be reserved for them. Such concepts have never been particularly popular, though, because frankly they're shit.

The first point, that I think I've said before but can never be stressed enough, is that whatever species concept you are using, a species represents a hypothesis. Sadly, specimens do not come with convenient labels saying to what species they belong. If they did, I'd probably be out of a job (not that I won't soon be out of a job - that thesis deadline looks like it's getting closer every day*). A taxonomist presented with an array of specimens will divide them up into what appear to be distinct species, but that division will always be subject to further revision. Intermediate specimens may be found that blur the boundaries between previously distinct morphologies. Two different "species" may hatch out of eggs laid by a single parent. That sort of thing. No taxonomic revision can ever be final.

*Oh, wait...

Two species of the planktic nudibranch Glaucus (G. atlanticus on the left and G. marginatus on the right). Photo by Gary Cobb (yes, I've used this one before - good, isn't it?).

What most taxonomists are looking for when distinguishing species are discrete, discontinuous characters. Whatever your favoured species concept, ultimately what you are trying to identify are populations that are genetically isolated from others. To give an example, Megalopsalis sp. A is found in Western Australia (I'm calling it "sp. A" here because 'tain't published yet). It differs from other species of Megalopsalis because the femur of the pedipalp (a little leg-like appendage in front of the legs that's used for feeding, visible in the photo below of Protolophus by Robert Pearson) has a row of spines running along the top (dorsal) edge. Over in Victoria and South Australia lives Megalopsalis hoggi, a closely related species. Specimens of M. hoggi never have spines on top of the pedipalp, but they may or may not have spines underneath it (ventrally).

You may be wondering why I regard presence or absence of dorsal spines as a character distinguishing species, but not presence or absence of ventral spines. After all, both are equally distinct. The difference is a matter of geography. In the specimens that I've seen of Megalopsalis sp. A, all of them have dorsal spines on the pedipalp, and I've not seen that character in Megalopsalis specimens outside the range of species A. Specimens with dorsal spines are geographically discontinuous from specimens without, indicating that they represent a distinct population. However, in the case of M. hoggi, specimens with ventral spines on the pedipalp are found in the same locality as specimens without ventral spines. The two are not discontinuous, suggesting that in this case that they are not genetically isolated.

It is possible that the ventrally-spined and ventrally-spineless individuals do represent separate species, but with overlapping distributions (and indeed, western specimens of M. hoggi tend to have spines while eastern specimens tend not to). If so, then study of other character complexes (such as population genetics) may reinforce their distinction. It is also possible that future collections may uncover overlapping variation in the Western Australian population that I've called "species A", and it may not be so distinct after all. But the recognition of two species, one variable and one not, is the most consistent with the information I have on hand so far.

Which leads up to my second point - pretty much any character could potentially distinguish a species, but pretty much any character could also potentially not distinguish a species. The ultimately important factor isn't the characters themselves - it's what they say about the gene flow leading to their current distribution. A single widespread species may encompass more variation within its distribution than a whole complex of closely-related species with restricted ranges. The difference is that character distributions will overlap in the former case but not in the latter.

So to get back to the question of whether use of the PSC would lead to recognition of separate species - personally, not having studied human variation to any great extent*, I don't actually know (really, it's exactly the same question as whether human races have any biological validity, just phrased using different terminology). I suspect that it wouldn't - while humans do vary considerably over their extraordinarily wide range, most of that variation is probably clinal rather than falling into discrete populations. It doesn't reflect underlying barriers to gene flow. This is especially the case in the modern world, where increased transportation and migration is breaking down what geographical barriers there may once have been.

*As organisms go, primates are mostly fairly dull.

As an unrelated postscript, in both of the earlier posts I've referred to a "tendency" of using the PSC to lead to recognition of more species than the BSC. Which got me thinking - are there any situations where the PSC would recognise less species than the BSC? Some possibilities that occured to me were situations involving polyploidy, or cases where interfertility was determined by the action of a single gene such as chirality reversals in gastropods. In these cases, diploid vs. polyploid or dextral vs. sinistral individuals might not be directly interfertile (which would mean they belonged to distinct biological species), but mutation from one state to the other might mean that there was still a reasonably steady gene flow going on (so they would not be distinct phylogenetic species). Any thoughts?


Donoghue, M. J. (1985). A critique of the biological species concept and recommendations for a phylogenetic alternative. Bryologist 88: 172-181.


  1. a species represents a hypothesis

    No, I don't agree. It represents an observed pattern. The hypothesis is that it is formed by such and so a process. The claim that all or most species are formed by that set of processes is a theory.

    Neither does a species represent an explanation, for the same reason (contra Fitzhugh):

    Fitzhugh, Kirk. 2009. Species as Explanatory Hypotheses: Refinements and Implications. Acta Biotheoretica 57 (1):201-248.

  2. K, so what about the other 99% of life, which are mostly asexual? =D

    They do seem to fall into more or less discrete patterns... perhaps reflecting stable points in the 'design space', but rigid pop gen definitions don't work very well there...

    My head hurts. Why does our brain insist on categorising everything to death? Seriously, as if learning about the whole infinite electron distribution cloud wasn't painful enough... XP

  3. John, I will accept your verdict of sloppy usage of the word "hypothesis" with all due apologies. Perhaps a more accurate way to put it is that a species represents an observed pattern (as you said), and the publication of a species represents a prediction (is that more correct) that further observations will continue to fit that pattern? Anyway, the point I was trying to make was that any species identification is always open to challenge.

    I certainly don't see how a species could be an explanation, any more than "I am wearing a red shirt" is an explanation.

    Psi - phylogenetic species concepts don't have as much of a problem with asexual taxa as biological species concepts, because the PSC recognises a species as a diagnostic cluster. It's probably still not perfect, though, because depending on how you define "diagnostic" you could potentially end up with some very small species.

  4. First off, only one of the three "phylogenetic" species concepts is diagnostic (autapomorphic). The other is a lineage conception that is (usually) held to be monophyletic (not always, though), which I call the phylogenetic taxon conception. The third, rarely used, is the Hennigian conception in which species are interspeciation segments.

    As to asexual organisms, they only do not fall into species if you a priori define species as necessarily the outcome of a certain kind of process (reproductive compatibility). But maybe there are many kinds of species (Wilkins 2003 :-) )...

    And maybe there's an inverse relationship between gene exchange and ecological adaptation that explains why asexual organisms fall into clusters (Wilkins 2008)...

    Wilkins, John S. 2003. How to be a chaste species pluralist-realist: The origins of species modes and the Synapomorphic Species Concept. Biology and Philosophy 18:621-638.
    ———. 2007. The Concept and Causes of Microbial Species. Studies in History and Philosophy of the Life Sciences 28 (3):389-408.

  5. Are the mathematical concepts of metrics and metric balls applicable?

    A metric is a function which takes two objects as its arguments and returns the distance between them as a nonnegative real number. For example, an organism metric would quantify some sort of "distance" between two organisms.

    A metric ball is the set of all objects less than a certain distance from a "central" object. For example, the 1000000-ball centered at me in metric d is the set of all organisms, x, such that d(x, me) < 1000000. The radius of this ball is 1000000.

    The concept of a metric ball seems very similar to the concept of a typological, rank-based taxon to me. Type ≅ center and radius ≅ rank. The only differences are: 1) metric balls with different centers and the same radius can overlap, and 2) the type of a rank-based taxon is not necessarily "at the center".

    There may be ways of dealing with the differences. The question I'm interested in is, which metrics are 1) informative, and 2) practical?

  6. In every economy, people spend their time doing one thing instead of another, make one thing instead of another, deliver things here instead of there. If you ask people why, they will talk about money, but the real reasons go deeper. Money was invented to help everyone keep track.

    Species were invented to help you keep track. It's a grave error to think they're any more real than money.

  7. Biological variation is strongly discontinuous. Sometimes the variation is as distinct as distant islands, sometimes it is more like nearby islands united at low tide, and sometimes it is like hills connected by valleys. This biological variation is real and worth describing and discussing.

    Humans need words to discuss things, to store and retrieve information, even to think about things. So we need to name biological variation.

    We need our words, our species names, to refer to mutually exclusive units. Biology isn't that neat.

    Therefore, there will always be a tension between our human need for mutually exclusive words and the variable (discontinuous or continuous) nature of real biological diversity. In other words, the species we describe will be more or less biologicaly real, but in some cases they will also be more or less arbitrary human constructs.

    We define species concepts in terms of biological reality, but in part (only in part!) our species concepts result from our need to categorize. Therefore, we can never find a truly final, completely useful definition of the species.

    What to do? Searching for the perfect definition of the species can only get one so far.

    As a practicing plant taxonomist, I choose to take species names seriously -- they do categorize real, important biological variation -- but also lightly. I use the species concept that seems to me best for describing variation in the group I'm working with (usually some variation of the BSC or PSC). Some species names clearly refer to real biological units, no matter what species concept I use. Others are the merely the best I can do at the time.

    I consider the species names I use to be hypotheses about similarity, patterns of diversity, and relationships among population. These hypotheses will be tested both by academic botanists and by field workers. The names will be accepted or rejected on the basis of their utility for describing and reporting variation and (in well studied groups) for expressing relationships. Most species ideas will be pretty stable (even if they wander from genus to genus!). Some will alternate forever between an inclusive and a more finely split definition.

    So it goes.

    (And now it's time for me to go apply some more or less useful names to some real diversity.)

  8. The cases where new species arise because copulation is not possible anatomically between different groups as in dextral & sinistral snails are also problematic, because the differences between the 2 groups (species) could be due to just one gene. Along these lines, can great danes & chihuahua mate? If not, they should be considered 2 separate species.


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