Aubert's analysis of phylogenetic terminology, part 4: cladism impossible?

Continuing the discussion of this paper from 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.

So today is going to be about the claim that a strictly cladistic classification is formally impossible.

Staying at first at the abstract level, imagine a group Aides. Its ancestral species A split into species B and C. Subsequently, B split into species D and E, while C split into F and G. The following is a strictly cladistic classification, and it does not look impossible:

Aides = { A, Bides = { B, D, E }, Cides = {C, F, G } }

Realistically, we will probably not know the ancestral species in most cases, so in practice we will be dealing with synchronous classifications like this:

Aides = { Bides = { D, E }, Cides = {F, G } }

Again, this is a strictly cladistic classification, and it is not self-evidently impossible. So what is going on?

What is going on is this: A strictly cladistic classification is impossible if and only if all the following premises are accepted:

1. We have fossils that we can characterise sufficiently well that we can really talk about 'species', as opposed to, say, merely pollen fossils that belong somewhere into our group.

2. We want an asynchronous classification of all species that have ever existed, as opposed to one classification for every major time-slice.

3. We insist on classifying the fossil species as ancestral if they look the way our analyses imply the ancestor would perhaps more or less have looked like, as opposed to classifying them as another terminal.

4. We insist on using the Linnean ranked categories, and here in particular binominal species names, as opposed to finding a different solution that is less artificial and pre-evolutionary.

The key is #4, because binominal species names consist of genus name and specific epithet. So if an ancestral species needs a genus name, that genus will automatically be non-monophyletic to all its descendant genera unless all descendants are collapsed into it. This is the same as Richard Brummitt's argument, so nothing new.

Note, however, that for the argument to work, all four premises have to be accepted; reject a single one, and it will fail.

#1: Most systematic work on the planet has to make do without any good fossils, so the problem never arises in the first place. That, however, is admittedly the strongest link in the chain as additional fossils could always turn up.

#2: For practical purposes, most humans care only about extant species except in a "dinosaurs are cool" sense, so it is not immediately clear that we should worry about accommodating fossils in the same classification as we use for managing weeds, food plants and protected species. Similarly, one could easily produce a classification for the flora and fauna of the Cretaceous without worrying about the descendants of some of the species in it. It is at least not self-evident that an asynchronous classification is required.

#3: Most cladists and, I would argue, also most practising systematists who are agnostic about cladism, treat all fossils as side branches. In fact phylogenetic analyses are generally designed to work that way, and it would be difficult to do it any differently without begging the question. This is where Aubert argues that cladists are anti-realists for excluding the possibility of a fossil being a real ancestor. More on that perhaps another time, but I would argue it is a case of pragmatism and prudence instead of anti-realist philosophy.

#4: In practice, the creators of modern classifications have become very pragmatic about leaving groups unassigned at some of the Linnean ranks, and I see no problem with that. Conversely, many researchers increasingly use informal clade names such as Euasterids II, not least because there are simply not enough Linnean ranks. There is no good solution yet for leaving species unassigned at genus rank, but that is only because of the weight of tradition. Some are ready to seriously consider moving beyond the state of the art of the 1750ies.

What I want to stress is that while e.g. the late Brummitt and here Aubert simply assume(d) all four premises to be obviously indisputable, many practising systematists would not agree with that assessment, and consequently they have no reason to believe that a phylogenetic classification is impossible.

But even to those who do insist that all of them are indisputable the question must at some point arise whether the supposed impossibility of phylogenetic classification is the fault of an ill fit of cladism to empirical reality, as the paraphylists believe, or the fault of an ill fit of the artificial Linnean system to reality.

Or to look at it from another angle, at some point there is no way around the realisation that any asynchronous classification that tries to treat species as static diamonds-in-jelly will produce absurdities. The reason is simply that ancestral species were not actually static but have over time gradually evolved and diversified. The descendants aren't more diamonds-in-jelly that were specially created to fit into somebody's graph, but they are what the ancestors turned into.

And here is the rub: Cladists and, whatever other downsides their attempt may have, the people behind PhyloCode, are asking frankly how we need to change our approach to classification to accurately model and describe the products of evolution. On the other hand, 'evolutionary' systematists and, whatever other advantages the traditional system may have, the people who take it for granted that we should never under any circumstances relax the demands of Linnean ranks and binominals, seem to be asking themselves how best to force a recalcitrant reality into abstract, fixed categories that were invented still under the assumption of special creation.

To repeat what I wrote in a reply under my last post on the issue, the latter approach looks a bit too much like trying to hammer a square peg into the round hole of an ideal. We can't do anything about the peg, because changing reality is not an option. Perhaps we should try a different hole.