Why Humans are Fish [If that word refers to anything 'real' at all]

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So, I originally made a tongue-in-cheek response to @Rumraket correcting him on his remark that humans are not fish.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:

  1. Cladistic
  2. Phenetic
  3. Evolutionary

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:

  1. Fish are an objectively real group: but that means that tetrapods are fish too.
  2. 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.
  3. 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.


@Rumraket granted we are fish in a cladistic sense. That’s true.

There are other meanings of the word though, other senses.

Thanks for the lengthy reply but I couldn’t find anywhere that you explain in what sense a group of organisms is real. But you’ve used the world real many times in your reply, but only in sentences where you merely assert the group, clade, fish is real. What is it about the clade that makes it real?

I think you’re actually contradicting yourself without having realized it. Particularly where in the previous thread you responded to me like this:

So the clade fish is only recognizable to us now as a distinct clade because a long line of it’s ancestors have gone extinct, leaving us with this illusion that only this particular set of characters seemingly sufficiently different from members outside of the clade, now exist only among members of the clade. Had those ancestors still existed, it would much easier to see how arbitrary it is where you put the line and say “that population right there, that’s objectively the common ancestor of fishes, all it’s decendants belong to the fishes, and all it’s ancestors do not”. Clades, too, are arbitrary distinctions. There isn’t anything that compels us to the clade fish in particular, other than quirks of fossilization and paleontological discovery. That is one of the problems with the old concept of taxonomic rankings into “domains”, “kingdoms”, “phyla” etc.

And I think this is relevant to the previous discussion because it raises the issue of whether we can say that a group of entities sharing certain characters is “real” or not. You have implicitly admitted that we can group things by certain characters and that those groups really do refer to something physically real, even if that realness doesn’t exist at a fundamental physical level.

Of course, I said a lot more prior to that where I laid down the argument for why cladistics is objective (unlike phenetic and evolutionary taxonomy) to support that ‘fish’ - as a clade - is a real group. See here (emphasis mine):

To summarise: There is a REAL natural phylogenetic hierarchy. Cladistic taxonomy represents this hierarchy as monophyletic taxa. i.e. clades. Hence why cladistics is objective. Ergo, clades are ‘real’ groups, unlike those that are paraphyletic or polyphyletic.

You misunderstood what I was saying there. Using your words, but making a few edits, what I meant by that statement is the following:

So the common paraphyletic sense of fish is only recognizable to us now as a distinct group because a long line of ancestors have gone extinct, leaving us with this illusion that the particular set of characters of some descendants (tetrapods) makes them seemingly sufficiently different from members of the fish group. Had those ancestors still existed, it would much easier to see how arbitrary it is where you put the line and say “these descendants right there, that’s objectively a new group distinct from - and not part of - the fish group. All it’s decendants don’t belong to the fishes, even while all it’s ancestors do”.

But to address the point that you were making here; about the fact that we couldn’t pint out the very first member of a group by looking at them. Not even theoretically, because at every point the direct ancestors / descendants would still look pretty much the same. But this has little to do with. cladistics. A clade is not defined by looks (phenotype), but in terms of phylgenetic relationships, i.e. a common ancestor + all descendants. That group is real. It exists independent of what we think about it. It exists regardless whether we are theoretically able to identify that specific ancestor (or ancestral population) among all the ancestors or not. Your point here (It’s impossible to point out the ‘first’ of a group based on phenotype) is actually a point against phenetics, NOT cladistics.

As explained previously, clades are not arbitrary. But seeing how you are referring to Linnaean ranks as an example of something that has the same problem, I think I can see the issue you are referring to. It’s true that assigning ‘ranks’ to groups, even if the groups are clades, is arbitrary. It is similarly arbitrary as to what name we assign to each clade. But the issue of picking a name for a clade is something I have already addressed preemptively in the very last paragraph of the top comment. Regardless what name (or rank) we give to the clade, it doesn’t affect the realness of the clade.

I’d think if the grouping represents something objectively real, then yes. Although, I don’t think I have admitted that we can group things specifically by “certain characteristics”. At least not here, as I have specifically argued against phenetics. Sure, a clade is inferred by identifying the synapomorphies, but that is how we infer the group, it’s not how the group is defined.

EDIT: This part below, I will copy paste it back to the previous thread since that is relevant to the original topic.

When it comes to the previous discussion, we have agreed (unless I am mistaken) that sex is a spectrum, with male and female representing the opposite ends of the spectrum, and intersex or DSD representing the middle. In this sense, male and female are real. However, if you treat ‘male’ and ‘female’ as ‘distinct (binary) groups’, I would say that they are not real. In this case, these are subjective constructs, similar to what portions of the spectrum of visible light constitutes the color ‘red’ or ‘blue’.

My concern is not cladistics. It’s with the idea that the lack of sharp boundaries is supposed to undermine the idea of the “realness” of some group. I am not objecting to the biological system of classification (I’d be happy to agree the clade fish is real), I am arguing for the idea of some sort of physical realness to methods of categorization even despite it having to involve some degree of arbitrariness at the boundaries. No, not at the fundamental level. At the fundamental level everything is just quantum fields or whatever, but things don’t have to be real at the fundamental level to be real, or to matter to us, or to have real-world consequences we have to deal with and can’t just pretend isn’t of any value or consequence and we just have to convince ourselves is some sort of illusion of our perception.

You argued at length by drawing from an analogy using colors that color groups aren’t real, because they can in principle blend into each other over infinitely many gradations. That is the reason you stated for why color groups aren’t real. And so the decision of where one group begins and another ends would have to be totally arbitrary. The idea seems to be that if there is not sharp dividing line in principle then the group isn’t real.

But I do think some groups are real, even if there is no actual sharp dividing line between them. At the very least there is something physically real about the phenomenon in question that lends itself to a more intuitive categorization. How things in reality are colored, for example, seems to lend itself to some sort of categorization: As I argued before, many objects in our surroundings are relatively uniformly colored, and while colors are part of some huge space of possible colors that don’t have obvious or clear boundaries, rarely do we find real things that are perfectly uniformly colored all possible colors in equal amounts.

The world is in many cases such that while there isn’t a perfectly sharp boundary between things (areas in light and areas in shade), there is a relatively narrow transition zone where one part shifts gradually into another but much larger area in relatively uniform illumination or shade. In such cases the distribution of light lends itself in an intuitive sense to categorization. This part A is “in shade” provided by the trunk of that tree, that part B is not, and the transition zone C between them occupies a much smaller area.


The exact dividing line is an arbitrary decision to a degree, but I think any reasonable person would have to admit it would not make sense to put the dividing line right in the middle of the letters A or B, and while the exact spot you pick for a dividing line is arbitrary to a degree, it would be “natural” to put it somewhere in C (which could be even more narrow).

In other instances the natural world is much more blurry, and the areas in more uniform illumination are much smaller and the transition zones much broader. In some instances there seems to only be these areas where no obvious boundary is discernible. The shadow on the left of B has a much more blurry edge. Presumably one could have endless arguments about where to draw the line there, and reasonable people have lots of space for disagreement. So not everything lends itself obviously and intuitively to categorization. And it comes in degrees. Some things are totally ambiguous, some are less so, and some are such that essentially everyone who isn’t mentally ill can agree on the distinctions.

So if we return to the case of fish, we can see that even here there is some sense of arbitrariness involved. Even here we have to decide on how far back that clade should go, and we do that by picking characters shared among some collection of organisms and not shared by others. And we can know that this apparent dividing line of characters is a product of fossilization and extinction. That there was, once upon a time, organisms in existence that would have blurred out that dividing line between ancestor and descendant. Hence just like with colors, organisms exist on some conceivably infinite spectrum of anatomical traits that usually doesn’t exist in reality. Nevertheless, despite this arbitrariness of where exactly the clade fish first began, we nevertheless do have the distribution of characters in extant and fossil organisms that we do(in a way similar to how things in our surroundings are colored), and this distribution lends itself to categorization. So the clade fish is real. At least real enough that we can reliably identify it’s members and non-members.

And I think we can say similar things about the categories male and female. Going back to the shadow analogy, I think many more people fall into the A and B parts than into C, and while the distinction would ultimately have to be arbitrary, I think that for purposes where having such categories becomes important (for matters of law, policy, health care and what have you), we’re going to have to decide where to put that line. And just flat out denying that those categories refers to something real isn’t a reasonable alternative.


But…that’s the whole argument I am making (regarding fish that is). Remember, you asked me "in what sense you think the cladistic sense of the word fish is real…What do you mean by it being real, where some other category is not? What is the “realness” of it that some other category, for example one that corresponds to the everyday sense of the word, is not?"

I have answered that question in detail, and now - suddenly - cladistics no longer concerns you??

I will take that as a “thanks you for answering my question on why you think the cladistic sense ‘fish’ is real as opposed to the common sense”. {EDIT: I retract this statement}

Again, I already addressed this with regarding the names we assign to the clades is indeed arbitrary, but clades themselves are not arbitrary.

Basically you are pointing out that, since there are (practically speaking) an infinite number of ancestors, we theoretically could define a clade based on each of them. Hence, there is nothing special about the particular clade that we may give the name of ‘fish’. That’s just an accident of the surviving species and the extinct fossil species we happened to discover. If we were to find a new fossil species that for all intents and purposes looks like a fish, and it is sister to the fish we knew, we may reassign the name ‘fish’ to a more inclusive (parent) clade. However, let’s say that happens. Guess what? That newly characterized clade that gained the name ‘fish’ is also a real group. As I have pointed out before, it doesn’t matter what we call any clade. Conversely, it doesn’t matter to what clade we assign the word ‘fish’. The clade remains a real group. That is what I mean when I say “in the cladistic sense, fish is a real group”. I am not saying that it is objective give the name ‘fish’ to that or any clade. So, you are just talking past the point I was making.

Looking at the rest of your comment, that’s all relevant to the original topic, so I will answer that in the original thread. Regarding the topic right here, I think we’re done.

If we’re eukaryotes, and bilaterians, and deuterostomes, and chordates, and vertebrates, and chondrichthyans, and sarcopterygians, and…(skipping a few levels here)…and mammals, and primates, and simians, and apes, then we’re 100% fish.

If you generally buy cladistics, we’re fish. If not, then we’re not.


Then you have started this thread on a misunderstanding of what I was asking in the original thread. I picked those statements out of your long-winded reply because it’s obvious they’re crucial to the whole (original)discussion. So rather than get sidetracked by a lot of tangential issues, I want to go straight to the heart of the matter. What makes a group be real?

You happen to have brought up the case of the cladistic sense of fish as an example of a group you think is real, while you also attempted to undermine an argument for realness I made, by saying (using colors) that because there’s in principle an infinite number of gradations between any colors we’d have to make an arbitrary decision on where to draw the line, then presumably because of this arbitrariness no color group is real.
I have pointed out an inconsistency in these two points of yours by bringing your attention to the arbitrariness of where the clade fish began. So if your argument from the color analogy is successful, that there being arbitrariness involved undermines color groups, then it undermines your argument for the realness of the fish-clade.

Now I happen to agree that the clade fish refers to something real real but as a consequence of a historical contingency. It is real in the sense that because the organisms that would have undermined the boundary between the clade fish and a more inclusive clade ancestral to it, are extinct and left no descendants, then this makes it a lot easier(reduces the amount of arbitrariness) involved in deciding where the clade begins. That there is something real about how characters are distributed in extant organisms and in fossils that lends itself to a particular depth for this clade. Incidentally I don’t care about the name “fish” either. It’s not about names of clades (be they “fish” or “thingamabob”, and it’s not about ultimately about cladistics. Right now the example of fish is merely a point of contention because of how another argument you’ve been making contradicts your insistence that fish is real.

To explain how the degree of arbitrariness, despite still being present, nevertheless can allow for some sort of real categorization, I used an analogy with shadows. The relevant attributes in this analogy was how much area was taken up by the bright, dark, and transitional zones. If the transitional zone is small enough in relation to the sizes of the dark and bright zones, then this makes the distinction more obvious, and therefore lends itself to categorization.

From where I am sitting I think that, to be consistent and preserve the realness of the clade fish, you have to drop your argument that there being arbitrariness involved in making distinctions means those distinctions don’t refer to real groups. I think it is actually a matter of degree. I don’t think there being arbitrariness involved necessarily undermines the realness of a group.

Are you up to speed now or do you need further elaboration?

It’s not about cladistics (it was never about cladistics), it’s about realness. I could have asked this same question for any grouping of objects you think is “real”. What do you mean by saying it is real?

I believe I have given an answer to such a question myself. I think that when we say some group of things is real we are often times referring to the fact that there is something about how things we put in the group are different from things outside the group (even if only in degree) that lends itself to such a categorization, that makes it obvious that there is some sort of boundary where one gives way to another.

Then you’re mistaken because you don’t seem to have answered it at all. Remarkably I think I’ve done a better job than you have of explaining how it makes sense to call it real.

I am deeply mystified as to how you’ve come to the idea that the names of clades is an issue, but nothing relevant to names is in what you quote of mine.

Okay, great. I think that completely contradicts your argument for the non-realness of color groups. Because by this same principle you’d have to say every additional gradation you include in a color group would correspond to a real group too. That looks to me like an obvious example of self-contradictory thinking on your part.

Is there some post here with my name attached to it that is invisible to me, where someone pretending to be me has started arguing about the names, the labels, of clades? I doubt it. Whatever has caused you to come under the misapprehension that this is about labels (as if the name “fish” somehow should irk me), be assured that it isn’t.

Which was never in contention, as I made clear with my very first reply in this thread.

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First, it is the right of the people (not scientists) to fix the meaning of ordinary-language words. And the popular definition of the word “fish” is not based on ancestry, but anatomy: roughly, a fish is defined as “a limbless cold-blooded vertebrate animal with gills and fins living wholly in water” (Oxford Languages), or “[an] aquatic, craniate, gill-bearing animal that lacks limbs with digits” (Wikipedia).

Second, it is silly to say that only categories that are valid across space and time are real. Ordinary-language taxa were never intended to apply to “deep time”; they are merely a handy way of slicing and dicing that sliver of reality that we call the present. As such, they do an admirable job.

Finally, if you want to use words that express our inter-connectedness with other animals, you can simply say that we are chordates, or (more precisely) craniata. Those are scientific words, and scientists have every right to define them cladistically.

Saying “We are fish” is no more justifiable than saying “We are bacteria.” It’s an abuse of language.

Well, of course we aren’t bacteria; bacteria are a clade that eukaryotes don’t belong to. More importantly, you ignore the pedagogic value of using “fish” in an evolutionary sense on appropriate occasions. Saying “we are fish” is no less justifiable than saying “we are mammals” or “we are animals” or “we are monkeys”.

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Obviously it would be silly to just toss the term out there and expect everyone to understand it if they haven’t ever come across the topic of evolution, common descent, nested hierarchies, or branching trees of relationships before, so one would have to offer some explanation.

What do we mean what we say that? Well we mean something similar to what we would mean if we said we are primates, mammals, vertebrates, and so on. Many people seem to know what mammal mean, so it’s analogous to that.

Assuming one is willing to offer some explanation to stave off confusion (the word “fish” does of course have that other colloquial sense), of course it’s entirely justifiable to say we are fish, just as we are mammals etc.

The philological issue here is that eukaryotes, chordates, tetrapods, mammals, and primates, are all essentially cladistic words with the intention to include the whole group or membership in the group, even when used by lay people. The popular usage of fish refers to present aqueous species most closely resembling ancestral fish. The fish market does not sell turkey. Fish and chips does not serve up steak. The colloquial usage expressly excludes terrestrial descendants. So while children’s books such as Grandmother Fish and popular books such as Shubin’s Your Inner Fish may be helpful to establish common descent among the public, the general usage of “fish” is not going to change anytime soon. And there you have your reason for technical jargon in the sciences.

I have already addressed these kinds of objections preemptively in the first comment:

Or to be specific just to make sure there are no misunderstanding:

  • I am not arguing that everyone to update the meaning of the word fish.
  • I am not saying that no other sense of the word ‘fish’ has no utility (I myself am forced to use the word ‘fish’ in the common sense all the time, just like I am forced to treat tomatoes as vegetables when I am trying to find tomatoes in the grocery store among where they devide fruit and vegetable in different areas)
  • What I am saying is that IF ‘fish’ refers to a ‘real’ group or organisms, it must refer to a clade. Otherwise, it is not a real group, but it may still have utility regardless (see previous points).

Also, I second what @John_Harshman said. We don’t belong to the bacteria clade @vjtorley so the statements “we are fish” and “we are bacteria” are not equally justifiable.

Already answered, several times:

Basically, if the group can be objectively defined. A clade is one example, since it exists regardless of what we think about it (as was already explained).

And I have explained, also several times (incl. last reply), that you are talking past my point. Your argument here (again) boils down to the arbitrariness of what particular clade you assign the word “fish”, and that is indeed the case. However, that has no bearing on the ‘realness’ of the clades. The arbitrariness of how to call a clade does not make the clade itself arbitrary. Hence, this doesn’t undermine my argument for the realness of the ‘fish’ clade whatsoever, and I don’t have to drop my argument about the color analogy in order to be consistent.

Correction: The fact that a clade may have been assigned the name of “fish”, THAT is a consequence of historical contingency and is arbitrary. How a clade (and every other clade) is defined as it consisting of a common ancestor + all descendants is NOT a consequence of historical contingency nor is it arbitrary. The clade remains real, regardless what name we give it. It remains real even if we don’t think about it.

And as I have pointed out, these are actually distinct (geometrically defined) zones. There is no arbitrariness here. This is nothing like the color categories. It’s a false analogy.

Again…YOU asked me why I think the cladistic sense of the word fish is real. And I have to explain cladistics over and over again in order to address your argument about me thinking that clades are real categories while colors are not are somehow inconsistent with each other. So, don’t complain when I bring up cladistics to prove my point.

I think that the word “obvious” is doing some heavy lifting here. What is obvious to you isn’t obvious to everyone. How “obvious” must it be for a group / boundaries to be defined? More importantly, it seems like you think the ‘realness’ of a group is implied based on how “we” make the categorization of putting things into groups with boundaries that “we” find “obvious”. Basically, a group is real or implied to be real if we recognize it, especially if we find it “obvious”. If we don’t recognize it or find it “obvious”, then it is not real, or it may not be, real. Did I get that right, or not? (serous question, not put words in your mouth, just asking if I got this correct).

If you think so, then please point out where I failed to sufficiently explain why monophyletic groups are real while paraphyletic and polyphyletic groups are not? And where did you explain that better than I did? So far, regarding the ‘fish’ clade, all I have seen is you trying to make the counterargument that the ‘fish’ clade has a degree of arbitrariness in order to claim that I hold contradictory beliefs; while I have explained why clades don’t have arbitrariness at all.

It’s when you make the objections that there is some arbitrariness involved with the ‘fish’ clade. THAT arbitrariness you keep referring to, that is all about how a clade gets a name. Of course, you didn’t say verbatim “how a clade gets its name is arbitrary” to claim that the fish clade has arbitrariness. What I am pointing out that this is what your objections boils down to, but you don’t realize this yourself. I tried to explain this to you, but you didn’t get that and you got confused, thinking that I am putting words in your mouth.

False, because (in that quote right there) I did not say that every gradation constitute a real group. I said that every clade is a real group. Clades are not gradations. Clades are distinct groups; a common ancestor + all descendants. Gradations are not. See the difference??

I am expecting you to follow-up on this by arguing that each clade is indeed a gradation, because if we were to look at every ancestor that defines every clade, each ancestor would just look like like a gradation that is part of a continuum. However, here you would be thinking in terms of phenetics, NOT cladistics, thus you would be completely talking past the point of what makes a clade a real group.

This is where your argument goes off the rails.


  1. There are other legitimate definitions of the word that are internally consistent.

  2. These other definitions have real utility, so much so that you are using the word by these definitions all the time.

Therefore the clade-based definition is not the only “real” grouping of fish.

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For any two species, the clade they belong to is their common ancestor and all of the descendants of that common ancestor. We are not going back to an arbitrary point. We are going back to the common ancestor. We are also not arbitrarily excluding or including species in the group, which is what happens in Linnaean classifications. ALL of the descendants from that common ancestor must be included in the clade. Common ancestor? Not arbitrary. All descendants? Not arbitrary.

What you may be hinting at is the difficulty of detecting which species belong in a clade. I think this is a problem of measurement and not the product of an arbitrary system. The evidence we are working from is the morphology of living species, the morphology of extinct species, and the genomes of living species, and these data sets may not be robust enough to accurately detect which species belong in a clade.

The only real problem I can see with cladistics is if there is a significant amount of horizontal genetic transfer. This would muddy our definition of common ancestry.


You assume that the criteria for “real” are consistency and utility. But why should that be so? Is every conceivable set a real group? Are trees a real group? Blue animals? Dogs other than chihuahuas?

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Those are NOT mutually exclusive points. Note that I said “IF” (in capital letters no less) in the first quote. That is why (as clarified in the second quote) I am not dictating that we should use that word such that it is referring to a ‘real’ group, nor am I saying we should not use that word to refer to a ‘non-real’ group.

I second the point @John_Harshman made. You seem to equate “realness” with “utility”. However, I do not. The reason why I acknowledge the utility of the colloquial usages in every day life is simply a matter of conforming to long and deeply established cultural traditions.

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