Why we don't consider supraspecific taxa as ancestral to others

I am currently reading Richard Zander's recently published book A Framework for a Post-Phylogenetic Systematics. As a cladist, phylogeneticist and regular user of molecular sequence data I would call the work a target rich environment and could write quite a lot about it. The problem is, it comes in at 214 pages and is written in an extremely dense, jargon-laden style. Discussing it in a way that does it justice would require an effort similar to Slacktivist's famous deconstruction of the Left Behind novels or Adam Lee's page-by-page review of Atlas Shrugged.

I am neither willing nor able to invest that amount of time, and surely much fewer people would find such an enterprise interesting enough to follow it than in the above two cases. What I will do, therefore, is limit myself to a few unsystematic discussions of individual topics or arguments encountered while reading the Framework. Today we will explore what appears to be its author's greatest frustration with phylogenetic systematics.

Interestingly, every proponent of paraphyletic taxa in botany has a different main argument. Elvira Hörandl, for example, is convinced that 'evolutionary' classifications have greater information content than phylogenetic systems. The late Richard Brummitt argued that Linnean binary taxonomy and phylogenetic systematics are incompatible when we try to classify ancestors. And Richard Zander finds it simply unacceptable that phylogenetic systematics does not allow the treatment of supraspecific taxa as ancestral to others at the same or higher rank.

Before we discuss whether or not it would make sense to treat supraspecific taxa as ancestral it might help to ask ourselves why he considers the issue so important. Given his style of writing and argumentation, it is actually very hard to extract that information from the book, but I think I have finally pieced his reasoning together.

It appears as if he believes that postulating taxa as ancestral to other taxa is the entire purpose and definition of evolutionary biology. An oft repeated mantra in the book is "characters do not evolve, taxa do", and it is heavily implied that therefore it does not make sense to see evolutionary biology as the study of character evolution, for example. Instead, it is about inferring and describing how taxa turn into other taxa.

I found that a really astonishing insight once the coin dropped: Zander is convinced that unless we allow, for example, that a family of plants can be the ancestor of a different family of plants there is no evolutionary biology. (Imagine the last four words underlined, bold and flashing in neon colours.)

In other words, he first defines evolutionary biology in a way that suits him and then reasons from there to conclude that phylogenetic systematics is obviously absurd because it makes (his idiosyncratic definition of) evolutionary biology impossible. This is slightly jarring because the book repeatedly accuses phylogeneticists of trying to win discussions by definition.

Note also that phylogeneticists would not have any problem with considering species as ancestral; obviously clades must descend from species. There are also phylogeneticists who have no problem with allowing species to be paraphyletic groups of internodes on a phylogenetic tree, so that contemporary species could be seen as the ancestors of other contemporary species. I have discussed this in greater detail elsewhere (species don't have to be monophyletic; internodal species concept; composite species concept).

Suffice to say for present purposes that such a practice does not pose problems for phylogenetic classification because (a) its principles apply only to supraspecific taxa and clonal lineages, not to sexually reproducing species, and (b) if an asynchronous composite species existed along multiple branches of a phylogeny then only because it did not acquire any apomorphies during that time (if it had, it would be considered to transform into a new species through anagenesis at that point), but in the absence of apomorphies along the branch we cannot decide on order of appearance of its side branches, making it impossible to define clades that would be made paraphyletic by said species. In other words, this species concept cannot pose any problems for cladism unless we cheat and circumscribe 'clades' without evidence that they are clades.

(Confused now? Sorry.)

Anyway, the point is that the issue is only with taxa above the species level. So now, why do phylogenetic systematists not classify supraspecific taxa as ancestral to others? What do we see as the problems with that idea?

The first and perhaps most obvious issue is that it seems absurd to consider one contemporary group of organisms to be the ancestor of another contemporary group of organisms. They are, well, contemporary, aren't they? In reality, they both had a common ancestor perhaps millions of years ago. A well known example is with us and monkeys: today's monkeys are not our ancestors, instead we shared an ancestor with today's monkeys a long time ago.

The second issue is that seeing, say, a paraphyletic family of 2,000 species as the ancestor of another family of 500 species does not describe reality well in another way, independent from the question of contemporaneity: The ancestor of any natural group of organisms is precisely one ancestral species. It simply does not make sense to say that an entire paraphyletic family comprising this ancestral species and 1,999 other, related species around it is "the ancestor" because those 1,999 others did not contribute anything to the 500 descendants. They are simply NOT their ancestors.

And finally, there are of course the other theoretical considerations that motivate phylogenetic systematics. Among them is the realization that due to the gradual nature of evolutionary change through time there is no objective cut-off to be had between a non-monophyletic group A and a monophyletic subgroup B nested within it. In addition, there is the problem that a non-monophyletic A would always contain a species that is more closely related to B than to anything else in A.

For these reasons, we cannot objectively define B as different from A, it is instead a subgroup of A. That means that a supraspecific taxon never terminates anywhere along the tree of life except where the branches end, that is where its member species go extinct. It is important to understand this:

Once your ancestor was a fish, you are a fish no matter how much you have changed It is not the case that a fish was your ancestor and now you are not a fish any more. We humans are a specialized, land-living group of bony fish. The same for birds and dinosaurs, Dryandra and Banksia, and so on.

Species are ancestors of clades, but clades never end. They merely acquire new subclades.