Polyphyletic Taxa.
In contrast, some taxa are made up of the descendants of two or more lines (hence 'polyphyletic'). Animals with many features in common may prove not to be as closely related as their classification would suggest. For example, if all fishes were grouped into a class Pisces (as they once were), this would be polyphyletic, for it would include sharks and other cartilaginous fishes. They however have their own class (Chondrichthyes) to distinguish them from bony fishes. The bony fishes themselves have long been grouped into the class Osteichthyes, but even this is polyphyletic: it includes two distinct groups, the actinopterygians (ray-finned fishes) and sarcopterygians (lungfishes, coelacanths and extinct lobe-finned fishes), which are sufficiently distinct to be considered classes. |
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