Monday, July 15, 2013

Why bother about cladism?

A topic that I return to again and again is the sadly never-ending controversy around phylogenetic systematics, or cladism. There are, I would say, perhaps three major positions in the debate:
  1. The cladist one, according to which all formally named supraspecific taxa should be monophyletic.
  2. That of "evolutionary" systematists, according to which formally named taxa should be allowed to be paraphyletic (they generally don't understand the part about "supraspecific" and why it is important).
  3. One that we could perhaps call post-cladist, held by people who would say something like this: "Who cares about formally named taxa? As long as we have a phylogenetic tree, we know what the relationships of species are, and that gives us all the information we need right here, e.g. for plant breeding or designing biological pest controls. And we can use the phylogenies for studies in historical biogeography, evolutionary biology, and many other areas. So will you stop shouting at each other? Nobody gives a damn."
Readers of this blog may well take the third of these positions and wonder whether I am not a bit weird for investing so much energy into promoting the first.

The thing is, I am actually quite sympathetic to the post-cladist stance. As long as it is understood that one cannot make sense of the evolution of Banksia without considering Dryandra to be part of it, I don't care if somebody speaks of Dryandra and Banksia despite the former genus formally having been sunk into the latter. As long as it is understood that examining the historical biogeography of Primula does not make sense without including Dodecatheon, it really does not matter if somebody still says Dodecatheon despite that genus formally having been made a synonym of Primula. So why do I care?

There are two reasons. The first is that, pace the post-cladist dismissal of their importance, formally named taxa do matter because they have the potential to mislead people. Not everybody will refer back to the phylogenetic trees when they want to conduct a biological study or just understand the diversity of the natural world. If all taxonomists and systematists accept a formal taxon Banksia that does not include the species of Dryandra then some end-users of taxonomic research will be mislead into thinking that Banksia is a natural group. And that means that they remain unaware of the fact that some Banksias are more closely related to Dryandra than they are to other Banksias, and depending on what they do in their ignorance it may have adverse consequences. Taxonomy potentially does have an impact beyond ivory tower squabbles.

The second reason why I care, and why I promote the cladist position, is of course the existence of its opponents, the continued attempts of "evolutionary" systematists to reverse the conceptual progress of biological classification in the 20th century. It would be one thing if there were only cladists and post-cladists, if it were understood how important it is to base decisions and study design on a knowledge of the correct phylogenetic relationships. In that situation, the cladist's constant harping on the importance of rejecting all non-monophyletic taxa might arguably appear a tad obsessive, and "we get it already, but who cares about the formal circumscription of that small genus that nobody knows anyway" might arguably be an understandable reaction.

It is quite another thing, however, if there is still a dozen or so professional botanists out there who actively want to destroy phylogenetic systematics and return the practice of the field to where it was ca. 1930 while, to add insult to injury, using arguments that have repeatedly been shown to be completely misguided. It is the same as with all too many other issues, be it something political such as women's rights or something scientific such as the acceptance of evolution: It is naive to assume that progress cannot be undone. If one side rests on its laurels while the other is constantly pushing back then it can and will be undone. I at least intend to do my bit to not let that happen if it can be avoided, and while the other two issues I just mentioned are clearly more important, phylogenetic systematics is relevant for my chosen profession.


  1. The English edition of Henning came out in 1966,a year after I received my PhD. Hennig's ideas where very strongly promoted by a number of ichthyologists I know personally. The most vocal being the late Donn Rosen, from AMNH. A number of his students and associates were also strongly supportive. Their ardor was almost evangelical, and people remarked on the "cladistic religion". Evolutionary taxonomists of the time had classifications comprising a mixture of grade and clade assessments. A number of us, particularly the older workers, did not like being told that their life work was based on faulty methodology. I think Ernst Mayer was one example.

    I understood the cladistic arguments and was well aware of the controversy. Even so, it took me from 1966 to 1995 to publish my first cladistic phylogeny (later not supported by DNA analysis).

    I am a strong supporter of Linnean taxonomy and think it offers no necessary impediment to expressing cladistic relationships. It is a legalistic system with some of the warts and foolishnesses that go along with that. I don't think respected scholars should have to waste paper and ink arguing whether a specific epithet should be harti or hartii, for example. Some of the post-cladists may think the Linnean system a dinosaur, and not care to learn, understand, or use it. The cladists I have know over the years have been well versed, and comfortable with the system.

  2. "Did not like being told our life work was based on faulty methodology" is a very personal way of seeing it. Things are moving fast, and some of the papers I published less than ten years ago I would do completely different today. That is the way it goes, all part of learning and improving.

    But it is understandable that it took time to persuade, and that especially many older colleagues have still not accepted phylogenetic systematics. I can live with that. I have much more empathy for the refusal to accept the approach itself than for the habit of some participants in the discussion to continually regurgitate the same easily refuted arguments. If somebody honestly says, I just prefer to classify like this, then there is not much one can say. But if they keep arguing against straw cladism in paper after paper it gets annoying.