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If you go back far enough, whales are a type of fish, just like us humans
Either that or fish aren't a phylogenetic group. You decide
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You can't evolve out of a clade, or so they say.
Of course it's not helpful to call humans and whales fishes in common parlance. But in phylogenetics, why not?
Because that would make fish and vertebrates synonyms so why not drop the former altogether?
Words can mean more than one thing thought, and depending on the context it can be useful to convey the fact that the vertebrates all evolved from fish-like ancestors, or that whales are more closely related to some fish than those fish are related to other fish.
And it's totally valid to use it that way even though I wouldn't. That's the "You decide" part of my first comment