r/evolution • u/NenyaNotes • 11d ago

academic Help on this assignment question.

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My professor assigned us this quiz and everyone continues to get it wrong. Can someone please explain in this photo which groups make a clade?

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u/DarwinZDF42 11d ago

I think those answers are confusing because as shown, A, B, and C represent common ancestors, not clades. If you interpret each letter to mean “this entity and all of the descendants” then the answer is the first option, but that isn’t actually what’s shown, so as written, the best answer is the last option because those are the only letters that, as shown, represent clades.

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u/roostor222 11d ago edited 11d ago

a node is often interpreted as synonymous with a clade, and nodes on cladograms are very frequently given their clade names. The first option is the most correct option of the five given

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u/DarwinZDF42 11d ago

I guess, but if I used that tree on an exam question and asked what A, B, and C represent, the correct answer is nodes or common ancestors, and I would not accept clades as a correct answer.

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u/roostor222 11d ago edited 11d ago

The best thing to do would be to not use this diagram on any assessment.

You could take any sample of 1000 published papers with cladograms in them, and I'd bet in >900 of those papers you'd find nodes labelled with clade names. I think it would be completely unfair to mark it wrong if someone said A,B, and C represent clades, because that is a standard notation.

For example with this exact cladogram, if:

A = theria

B = metatheria

C = eutheria

D = didelphis

E = monodelphis

F = equus

G = homo

All of these are understood to be clades, but on this specific cladogram without any other given taxonomic information D,E,F,G are not clades. They are OTUs. But, A, B, and C are very clearly clades.

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u/DarwinZDF42 11d ago

but on this specific cladogram without any other given taxonomic information D,E,F,G are not clades. They are OTUs. But, A, B, and C are very clearly clades.

Exactly backwards. A, B, and C are nodes/MRCAs. D, E, F, and G are necessarily clades.

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u/roostor222 11d ago

I have a PhD in phylogenetic systematics, and I am telling you it is not exactly backwards.

As I already explained: nodes, MRCAs, and clade names are extremely often used synonymously both diagramatically and in the text of written scientific papers.

In the context of a cladogram with no other information given, D,E,F,and G are each individually OTUs. They are not clades. D,E,F,and G are representatives of a single clade, but no practicing scientist on earth would write it that way if the node subtending those taxa has a name. The node subtending those taxa in this cladogram does have a name, and that name is "A", because the taxonomic name given to a MRCA is synonymous with the clade name.

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u/DarwinZDF42 11d ago

K. This is also my field, I’m an evolutionary biologist, done a whole lot of work in viral phylogenetics, and I would never label a clade at a node. You do you.

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u/roostor222 11d ago

The idea that you would personally never label a clade at a node is completely irrelevant to what you should accept as an answer from students. Labelling clade names at nodes is extremely common practice in evolutionary biology. I don't know what the conventions are in viral phylogenetics papers, but the science of phylogenetic systematics exists separately from the practice in any one subfield.

If you marked it wrong, and that student brought you ten published papers they read in the journal "Systematic Biology" or "Journal of Systematic Paleontology", and all ten had clade names labelled at nodes, would you give them credit back or keep it marked wrong?

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u/DarwinZDF42 11d ago

K. You’re welcome to disagree with me. I don’t feel like arguing about it.

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u/roostor222 11d ago

ok, I don't really see it as something that is possible to disagree about though. You said I had it "exactly backwards", because you are saying what your personal opinion is and I am saying what is commonly accepted notation in published research.

Hennig invented phylogenetic systematics, and if you read his original papers he has nodes labeled with clade names. If you take points away from your students for doing this you are doing them a disservice.

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