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Comment by wrnr | Hacker News Reader

I like this story by Bred Weinstein where he asks is a whale a fish or a mammal. If you say fish because it swims in the water you are ignorant of the fact they are mammals who evolved back to live in the water. When you say mammal for this reason you are ignorant of how to read the phylogenic tree of life where subclasses are always part of the superclass. Most fish are Osteichthyes, a group that also includes all mammals. But sharks are not part of this group making us humans closer related to salmon then salmon are to sharks. Its a bad question but if forced to answer you can only be wrong in your own educated way.

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Interesting information about sharks!

But it's wrongfully pedantic to pretend that monophyletic groups are the only valid classification system. Paraphyletic groups are fine too. Vertebrata or Craniata, minus Tetrapoda, is more or less the definition of "fish".

Seems like that supports the point. :)

Not really, as far as I can tell. The argument was not just that you're picking between choices, but that both choices are fundamentally wrong in a technical way. But taxonomic categories don't technically force you to be wrong. That part of the argument is just broken.

Biologically whales are clearly mammals and not fish. Whether bony or cartilaginous fish are closer to mammals does not put that fact into question.

If we're measuring distance by MRCA, the question is equivalent to whether bony fish are closer to mammals or to cartilaginous fish. That framing does different things to intuition, whatever definitions we've agreed on.

> Its a bad question but if forced to answer you can only be wrong in your own educated way.

You can give an answer that spells out all of that. Alternatively, if you are limited to a single word, there is a "correct" answer to "is a whale a fish or a mammal": "yes."

Does this mean my intuition in this case has actually been right all these years? I used to run around telling everybody that sharks weren't really fish, just like Pluto wasn't really a planet (way back when I was unquestionably considered one). I had sound knowledge behind the Pluto idea, behind the shark one, not so much - I just understood them to be an ancient line, and to be obviously built after a different blueprint than anything known to me as a fish.

No, sharks are fish. They are cartilaginous fish. Terrestrial vertebrates descend from (and therefore are, in one technical sense) lobe-finned fish, which are a part of the bony (=not cartilaginous) fish, but that doesn't make sharks not fish, it just makes sharks more distant from mammals (etc.) thanks bony fish.

I stand corrected. In all humility.

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