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.