Aubert's analysis of phylogenetic terminology, part 5: is cladism anti-realist?

Continuing the discussion of this paper from here, here, here, and here, and working through the main claims of the paper as I see them:

• The various definitions provided in the paper are in some way better than the ones that are currently accepted.

• There is no relevant difference between the systematics-relevant relationships and structures existing at any level of the diversity of life. (E.g. mother > daughter is completely equivalent to bony fish > land animals - they can all be drawn as diamonds and arrows, right?)

• A strictly phylogenetic classification is formally impossible.

• Cladism is part of structuralism and therefore characterised by "anti-realism and a metaphysical way of thinking".

• Cladism is built on biologically unrealistic assumptions that have been empirically falsified.

• There exists an objective approach to delimiting paraphyletic groups.

• It would be preferable to have two parallel classifications, one of clades and one that includes taxa that are allowed to be non-monophyletic.

What assumptions is cladism based on?

The section of Aubert's paper most relevant to the claim of cladism being anti-realist and metaphysical is #10, Analysis of the Cladist Doctrine. But before we come to the supposed structuralist influence, it is interesting to examine how the cladist "doctrine" is described:

Cladism as it was founded by Hennig

Okay, let's stop right here for a moment. Aubert wants to recognise paraphyletic taxa, and mainstream systematics today recognises only monophyletic ones. Question is now, are we talking about cladism as founded by Hennig, or are we talking about Phylogenetic Systematics as practised today, by hundreds of people publishing papers to circumscribe groups as monophyletic? Because finding a mistake made by Hennig personally will not necessarily address the underlying disagreement with all the latter, who still want to make taxa monophyletic even as they may or may not agree with Hennig's species concept and phylogenetic methodology.

This is a bad start, comparable to a creationist rejecting modern evolutionary biology because of Darwin's now outdated ideas on inheritance. Let's see where this is going, but it is important to realise that today's phylogenetic systematists do not treat Willi Hennig like an infallible prophet. Practises have changed since 1966.

Onwards:

is based on three key assumptions. The first is that cladograms' nodes do represent real speciation events through splitting (Hennig 1966; see Figure 20C).

Cladogram nodes simply represent speciation events full stop. But whether that speciation was through splitting or budding is irrelevant for a cladogram representation of the tree of life (although it would not be for a phylogram representation). One suspects that some conflation between at least three different issues is going on here: (1) how speciation is assumed to work, (2) how to display the tree of life, as a cladogram, phylogram or whatever, and (3) how to classify organisms into species.

The second is that it is extremely unlikely, given the continual transformation of the living world and the vagaries of fossilization, that a species we know is the ancestor of another one. Even if by chance this ever happens, it would be anyway impossible to prove this parental link.

The point of Hennig's Internodal Species Concept is not that all speciation events are assumed to be splitting, but that the descendant species should both be treated as individually different from the ancestral species even if speciation was through budding, as should be unmistakably clear from reading his book. This is not about identity of ancestor and one of two descendants being "unlikely"; it is about such identity being nonsensical.

The chimpanzees are not our ancestors from 7 Myr ago, they are our sister lineage. The chimpanzees of today cannot be our ancestors, as they exist at the same time as we do and not 7 Myr ago. In reality, the ancestral species has turned into both humans and chimps and is thus identical to both together. Same for all other lineage splits. So arguing that many speciation events are really budding is missing the point.

What is more, there are even cladists who would agree with Aubert's species concept. I do not necessarily share their point of view, but they are still cladists and can still recognise monophyletic supraspecific taxa.

The third, more implicit, is that the evolution of species is occurring at a more or less constant rate (uniformitarianism, not to be confused with actualism). This would imply that the more recent is the divergence between two species, the more similar these two species are. Clades would therefore be the kind of groups with the maximum content in phenetic informations.

Cladism is not about maximising phenetic information content; phenetics is. That is why the latter is called phenetics and the former isn't, just in case that wasn't clear. Cladism is about reflecting relatedness, evolution, common descent. Thus all the rest is irrelevant. It is really interesting how often the text of the present paper not only assumes that phenetic similarity is the only criterion we should ever care about and argues from there, which is problematic enough, but also that it is already the only one that everybody does care about. That is just not the case.

Association fallacy

This misunderstanding of phylogenetic systematics does not bode well for the rest of the section. It could also be added that cladism = structuralism = anti-realism is not actually an argument for cladism = wrong or cladism = unacceptable. Maybe cladism is wrong, maybe anti-realism is bad. Personally, I would disagree with the former and agree with the latter. The point is that even if cladism = anti-realism, at least one of these two claims would have to be demonstrated independently from the present argument. In the absence of such independent demonstration it is at best something like an association fallacy.

Is cladism nominalist, anti-realist, metaphysical?

What is now the argument for cladism being anti-realist? On pages 33 and 34, it proceeds as follows:

The presumed speciation mechanism requires taxonomists not to consider any species as ancestral to another one, and thus to classify them only as sister groups. This dogma according to which reality is not accessible to us is characteristic of a nominalist philosophy of knowledge: science aims to model and predict manifestations of reality but not to understand reality itself.

As I have explained above, this is simply not about speciation mechanisms. In addition, cladism does recognise species as ancestral; indeed it recognises only species as ancestral where 'evolutionary' systematics, in marked contrast to all of evolutionary biology, also considers genera, families, orders and classes as ancestral. Cladists just don't consider one of the daughter lineages to be identical to the ancestral lineages, that's all.

Admittedly, there are cladists who would consider ancestry to be un-knowable (see blog post here), and so at least that minority could be considered nominalist. I must admit that I do not understand their view, and it appears at first sight similar to certain postmodernist armchair philosophers claiming that science can't know anything ever about anything. In my eyes we can always draw tentative conclusions given the best evidence currently available and wait for somebody to come up with contrary evidence.

There are certainly some cases, especially in groups like diatoms, where the fossil record is so ridiculously complete that it is feasible to reconstruct direct ancestry. So I am a cladist who would not rule ancestral species out in principle, and thus the charge of nominalism does not apply to me. More importantly, the same is true for many other cladists, indeed the majority as far as I can tell. Consider E.O. Wiley for example, who describes how he would classify ancestral species in the Linnean system in one of his papers (Wiley 1979, Syst. Zool. 28: 308-337). And he just happens to have written what is perhaps the textbook of Phylogenetic Systematics.

That being said, this is mostly a theoretical consideration. Taking into account the whole diversity of life, cases like the aforementioned diatoms or even complete skeletons are rare. I would really like to know how the 'evolutionary' systematists who insist on classifying ancestral species would do that in practice given only what is usually available, for example a pollen fossil or half a jaw-bone. At least in my eyes, somebody who claims that these fragments provide sufficient proof for an ancestor-descendant relationship at the species level is over-reaching.

Finally, note that the idea that we cannot truly understand the nature of reality but only describe it as best as possible using our limited minds and language is not self-evidently wrong. In fact it might be self-evidently correct, regardless of how we treat ancestral species. I don't think most, if any, of us can really comprehend 13 billion years or gravity, we can just tell stories and draw analogies. Either way, in my eyes at this point the argumentation has already collapsed under the weight of its misunderstanding of cladism.

Hennig did recognize the material nature of reality and its independence of the subject. This can be seen through his attachment to the fact that successive branchings of cladograms do represent an evolutionary process. This is clearly a materialist position.

I see no problem here.

Finally, the Hennigian conceptual framework can be described as metaphysical since it is both rigidly static and reductionist. Indeed, its phyletic arrangement does not reflect any transformation (no species becomes another, no group emerges from another group, etc.) and is reduced to represent only the structure of the process supposed to be at work (the splitting of a mother species into two daughter species).

The difference between cladism and 'evolutionary' systematics lies precisely in the cladist understanding that the ancestral species is becoming the descendant clade. It is the 'evolutionary' systematist who has a rigid and static, pre-evolutionary understanding of systematics, as when they consider a small part of a diversifying clade to be the unchanged ancestor.

And that's it. That was the argument. In summary, I am not convinced that cladism as such is structuralist, anti-realist, metaphysical, or whatever other association fallacies one would like to throw at the approach.