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Why is the traditional group called fishes not considered a single evolutionary lineage?

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Illustration showing examples of vertebrates: a turtle for reptiles, a songbird for birds, a bluegill fish, a salamander for amphibians, and two mammals-a whale and a groundhog. — Vertebrate The major groups of vertebrates include fishes, amphibians, reptiles, birds, and mammals.

The traditional group known as "fishes" is not considered a single evolutionary lineage because it is a paraphyletic group. In biological classification, a monophyletic group (or clade) must include a common ancestor and all of its descendants. The traditional grouping of fishes fails this criterion because it excludes tetrapods—the group of limbed vertebrates that includes amphibians, reptiles, birds, and mammals.12

Because tetrapods evolved from a specific group of fleshy-finned fishes, they are, in a strict evolutionary sense, descendants of fish. By grouping "fishes" together while excluding the tetrapods that descended from them, the category leaves out a portion of the lineage, making it paraphyletic rather than a true evolutionary clade.21

Furthermore, the term "fish" encompasses highly diverse groups—such as jawless fishes, cartilaginous fishes, and bony fishes—that do not share a single, exclusive common ancestor that is not also an ancestor to other non-fish vertebrates.3