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.
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.