Field of Science

The Phylogenetic Species Concept: is there such a thing as too much?

A recent editorial in the open-access journal PLoS Biology (you can tell it's online because of the acronym title ;-P ) comments on the recent widespread adoption of the Phylogenetic Species Concept as opposed to the Biological Species Concept, and the resulting 'taxonomic inflation' in the number of recognised species around the world. The authors feel that this is a Bad Thing, because increased number of species by definition means more endangered taxa, which means less effective conservation. This is not the first time this point has been made, and I feel the need to put my own oar in on the subject. To do this, I'm going to have to address the two arguments that are really at stake here - I'm going to call them the idealistic and the pragmatic sides of the argument.

The idealistic point is that we should not be using the Phylogenetic Species Concept in recognising species. I wrote a page on Palaeos a while back explaining some species concepts, and I'll refer you there if you want a more detailed description. For now, it'll suffice to say that the 'Biological Species Concept' (BSC) defines a species as a cluster of individuals 'potentially interbreeding to produce viable, fertile offspring', while the 'Phylogenetic Species Concept' (PSC) defines a species as the 'smallest diagnosable population of organisms sharing a distinct pattern of descent'. The latter concept has also been referred to as the LITU (Least Inclusive Taxonomic Unit) by authors who do not necessarily feel that an LITU is equivalent to a 'species'.

One side-effect of using the PSC rather than the BSC is that the latter is indeed generally predisposed to recognise more species. Under the PSC, there is really no place for the concept of 'subspecies' because, after all, if a population can be distinguished as a subspecies then it fulfils the requirements for a species. For vertebrates in particular the subspecies rank has a long history of use, and so application of the PSC results in all those subspecies becoming recognised as species. [Invertebrate workers have, on the whole, been less inclined to recognise subspecies (with the notable exception of lepidopterologists), so this is less of an issue there. I'm not familiar enough with the situation in plants to comment.] Also, there is no theoretical lower limit on how fine distinctions between 'species' can be, so long as those distinctions are accurate.

The BSC, however, has its own problems. Despite being the most commonly quoted species concept, it is usually recognised more in the breach than the practice. The requirements for actually testing whether populations can interbreed soon become prohibitive, and the BSC is difficult to apply in the case of populations that are separated geographically (to ask a strawman question, are two individuals 'potentially interbreeding' if they're never going to meet?) Recognising species using the BSC, therefore, generally depends on the individual author's own subjective judgements about whether or not two populations are conspecific.

In my opinion, the balance weighs overall in favour of the Phylogenetic Species Concept. In practice, the PSC is more objective and falsifiable than the BSC. As an extreme example of taxonomic inflation, the authors of the PLoS Biology editorial refer to Merriam's 1918 classification of North American brown bears into 82 species, and quote George Gaylord Simpson's critique that the distinguishing characters of these species were so finely defined that "On such a system twin bear cubs could be of different species". There are two replies to this: (1) why not recognise 82 species if there are 82 distinguishable populations?, and (2) Simpson's complaint indicates that Merriam's species would not be valid under the PSC, because the PSC requires that a species be a population (the 'distinct pattern of descent' referred to above). If a character varies within a population to the extent that it overlaps with other populations, it is not usable in distinguishing them as species.

In regards to the pragmatic side of the argument, the PLoS authors have more of a point. As the number of species supposedly requiring conservation increases, funds to conserve them become more stretched and the effectiveness of conservation goes down. However, the practicalities deriving from species recognition should be divorced from species recognition itself. Populations of organisms exist in nature independently of whether or not humanity chooses to recognise them, and the role of science is to try and identify what is, not what should be. It is true that as the number of recognised species increases, it becomes increasingly evident that we will not be able to conserve them all. Nevertheless, the appropriate response is probably to try and work out how to direct funds in such a way that the maximum amount of biodiversity is saved. This will probably require some sort of prioritising of conservation targets rather than attempting to save everything, and thinking in terms of ecosystems and taxon groups rather than individual species. It is still better that we keep a clearer view of what is out there than masking diversity within expansive categories.


  1. Good article.

    As a marine invert systematics grad student myself. I take offense to the constant phylogenetic reduction of vertebrate species. I recognize that they are threatened, but it seems to that vertebrate systemacists are looking for more and more ways to subdivide smaller and smaller populations of species. It is almost as if they can't accept the fact that have identified a vast majority of terrestrial vertebrates.

    Statistical cut-offs for practitioners of the phylogenetic species concept need to be more strict and need to use a multiple gene consensus tree that takes into account all populations and closely related species. THEN, I would be convinced. Granted, I haven't read the actual paper on the leopard, just the press releases and other news info, I have run into other papers dealing with similar issues in other groups of animals. Many authors seem to feel that every branch point is a new species, but each branch point need to be evaluated in light of all the branches and the correct statistical method(s) applied. For instance, nucleotide divergence of 1 or 2 genes may say that a leopard species is more than one species, but in the feline phylogeny how do those branch points (hopefully individuals from different population?) look in the entire phylogeny of felines, or mammals? Is the LITU a species or a population? You said it right when you said "If a character varies within a population to the extent that it overlaps with other populations, it is not usable in distinguishing them as species." This is how I determine if I have a new species when I'm IDing individuals.

    Morphological taxonomists have attempted to overcome the biological species concept's difficult rules by arguing for suites of characters that are sufficiently different from all other closely related species or populations. This seems to work for below the family level as many molecular phylogenies concur with morphology-based phylogenies, at least in the invertebrate organisms I have dealt with. It is at the higher classifications that molecular phylogenies have made some fascinating discoveries, for instance that Buddenbrockia situation you talked about in a previous post and that I have posted about on my blog as well.

  2. I don't have a problem with the inflation of vertebrate species - as finer tools for distinguishing populations are applied, it becomes inevitable that the number will go up significantly.

    I think my advisor when I was working on my MSc put it best when he pointed out that ultimately any "species" identification is a hypothesis, and like any other hypothesis it can stand or fall as further data become available.

  3. A question. If the PSC is applied to human classification, would it not mean that several human species would be identified? I mean it's hard enough explaining to racialists that there are no human races, without the PSC coming along and trying to say that humans divide into different species based on derived characteristics.

  4. Before you could divide modern humans into phylogenetic species, you would have to demonstrate distinct, non-overlapping characters between races. This would not be an easy thing, as most if not all human races do overlap considerably. Cameron McCormick wrote a post a little while ago that demonstrated some of the arbitrariness of "racial divisions".

    In fact, the potential for division under the PSC you asked about is almost exactly the same as the division into races that has occurred in the past - just different names for the same thing. If there are indeed "no human races", then there are no divisions under the PSC.

    This "political loading" (for want of a better term) that is sometimes attached to the word "species" is another reason why some authors have preferred to use less connotative terms such as Least Inclusive Taxonomic Unit.

  5. I completely get the pragmatic argument about conservation money. But this bothers me at a deep level anyway. We are in the business of systematics to understand how the living world is organized. Therefore, it is problematic that we are willing to alter species concepts for what really come down to political issues.

  6. I am very concerned with political considerations, even noble ones like conservation, being used as a criterion for species concepts. If you attack the PSC based on scientific grounds, go right ahead. But if it does describe the realities of the way the world of living things is organized and you reject it for political reasons, this is a bad precedent.


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