This is a continuation of a sub-discussion that I had with @Rumraket on another thread. I felt that I have accidentally side-tracked the conversation away from the original topic, which is why I made a new topic here.
So, I originally made a tongue-in-cheek response to @Rumraket correcting him on his remark that humans are not fish. However, I would argue that we actually are fish. Specifically, I am making the argument for why all humans, and all other tetrapods, are fish according to a cladistic usage of that term. I will explain this in more in detail later.
Here I provide a few back back and forths for some context:
And this is where I have moved the sub-topic here. The answer to that question goes back to the whole debate between the different schools of biological classification (taxonomy).
The original taxonomical method which continues to hold some influence to this day is Linnaean taxonomy devised by Carl Linnaeus over almost 3 centuries ago. In addition to establishing the current naming convention (binomial nomenclature), he classified the species within taxa forming a nested hierarchy, each taxon based on physical traits that are shared by its members. The fact that shared traits between species produces a nested hierarchy is significant, since that pattern is exactly what one would expect if different species had common ancestry. Taxonomy formed one major line of evidence that Darwin used to support his proposition.
However, Linnaean taxonomy is very subjective. Taxonomic ranks are arbitrary and it has no rigorous method to determine which shared traits are important or not to unite members in a taxon. That is why Linnaean made many errors (e.g. sloths as primates). Nevertheless, despite such mistakes, Linnaeus managed to guess the ‘tree of life’ (without trying, since he didn’t thought of it in terms of common descent) with a surprising degree of accuracy. Still, a better methodology was needed and in the last half of the 20th century there were a couple of competing schools of taxonomy:
Cladistic taxonomy (aka phylogenetic systematics, or simply cladistics) is based on the phylogenetic relationships of organisms. According to cladistics, only taxa that are monophyletic (consisting of a common ancestor + all descendants) is considered to be valid. Monophyletic taxa are also called ‘clades’ (Greek for ‘branch’). The phylogenetic relationships are inferred by distinguishing ‘synapomorphies’ (shared derived traits) from ‘plesiomorphies’ (ancestral traits) and ‘homoplasies’ (convergent traits). Synapomorphies are used to identify the clades.
Left (monophyletic) based on ‘synapomorphies’ (shared derived traits)
Middle (polyphyletic) based on ‘homoplasies’ (convergent traits)
Right (paraphyletic) based on ‘plesiomorphies’ (ancestral traits).
On the other hand, Phenetic taxonomy is similar to the tradition of Linnaean taxonomy in that it is based on the similarities that are observed directly. However, it involves quantifying the physical traits, as many as possible, and using statistical methods to calculate the degrees of similarities between different species. Basically, each species would be classified according to it’s overall phenotype (hence why it is called phenetics) and each taxon includes members with overall phenotype that are most similar to each other. These specific traits that make them more similar to each other may indeed be synapomorphies, and the taxon would be a clade, but only incidentally. Phenetics doesn’t really care. Some taxa may be based on plesiomorphies, in which case the taxa would be ‘paraphyletic’ (consisting of a common ancestor + some but not all descendants) or others are based on homopolasies, in which case it would be ‘polyphyletic’ (consisting of groups, but not their shared ancestor).
Evolutionary taxonomy is based on the evolutionary history of groups. Here, traits that evolved at some points during the evolutionary history are used to define taxa. If all descendants of the ancestors among which the evolved trait first appeared are included, the taxon is be monophyletic. However, some descendants may lose these traits later on and could be excluded by evolutionary taxnomists, thereby making the taxon paraphyletic. Evolutionary taxonomy is basically a compromise between cladistics and phenetics. It accepts monophyletic and paraphyletic taxa, but not those that are polyphyletic. It accepts the phenetic argument of overall similarity regarding paraphyletic groups, but it rejects polyphyletic groups based on the cladistic argument of phylogenetics.
There was a fierce debate between these schools regarding which is the best one and on what criteria. There was one criteria that phenetics used to argue for their own favor, but one advocate of cladistics actually showed that - even according that criteria of the advocates of phenetics - cladistics is better. Another criteria, one that is more relevant to the question that was asked to me, is objectivity; i.e. which classification system produces groups that represents a real and unambiguous property of nature. In order for phenetics to be objective, the identified taxa must represent some sort of natural phenetic hierarchy; and members of each taxon defining an ‘overall similarity’ which represents an ‘idealism’ of form for each taxon. However, there is no such thing as a real phenetic hierarchy in nature. Phenetic taxonomy is fundamentally subjective. This was one major critique aimed at phenetics from the evolutionary taxonomy side. This is ironic, since the same argument could be used against the paraphyletic taxa that evolutionary taxonomy considers to be valid. Evolutionary taxonomy may exclude certain descendant groups based on whether they have evolved to the point of being ‘different enough’ from their ancestors. However, how do we determine whether any descendants should left? And if yes, how do we determine which should be left out and which should stay? How ‘different’ is ‘different enough’? That is arbitrary. Thus, paraphyletic taxa are not objective (they don’t represent real properties of nature).
What about the phylogenetics based cladistics, with their monophyletic taxa (clades)? Well, unlike with phenetics, there actually is a natural phylogenetic hierarchy. It’s is a plain fact that organisms are more closely related to some than they are to others. Phylogenetic relationships are REAL, independent on what we think about them. Thus cladistics, specifically monophyletic taxa, are objective as they are representative of something that is real.
What has all of this do to with ‘fish’? Well, the word ‘fish’ - as it is commonly used - is paraphyletic.
You and all other tetrapods are deeply nested within the fish group.
Some descendants of the last fish common ancestor are not considered to be fish, i.e. the tetrapods; and some fish are more closely related to tetrapods than they are to other fish. To put it in other words; the coelacanth is more closely related to you than it is to salmon, all of these are more closely related to each other than they are to sharks, and all of those previously mentioned are more closely related to each other than they are to hagfish.
The implication of the cladistic argument regarding this leads us to the following options:
- Fish are an objectively real group: but that means that tetrapods are fish too.
- Fish are an objectively real group: but if one wants to exclude tetrapods, then they have to make a bizarre argument, e.g. claiming that only “ray-finned fish” are fish; while hagfish, lamprey, sharks, rays and Coelacanths are NOT fish. However, the only reason one would be doing that is just for the sake of excluding tetrapods, so that would be cheating.
- Fish are NOT an objectively real group.
Lastly, some preemptive responses to common counter arguments:
I don’t dispute the fact that the term “fish” is more commonly used in a non-cladistic / paraphyletic sense. What I am saying is that - if the word ‘fish’ is to refer to a real group, while preserving as much of its common meaning as possible, i.e. everything universally accepted as a fish remains a fish (e.g. sharks, lampreys, salmon, coelacanth), then it follows that tetrapods (incl. humans) are fish too. If anything else, then ‘fish’ aren’t a real group.
A common response I encounter is the suggestion that I shouldn’t use the word “fish” like this. I should use a “scientifically correct word” (whatever that means) instead. However, this is simply an aversion to the word itself, not about what I am actually referring to when I use that word.
In any case, "A rose by any other name would smell… just as fishy"
At least, I think that’s how the old saying goes.