The problem with phenetics

The phenetic principle fails to meet the two requirements of any classification system: it is neither natural nor objective:

A phenetic classification is only natural in the sense that it is true of a very large number of characters: it must be, because all the characters it is true for were used to define it. As more and more characters are used to define groups, the number of non-defining characters decreases, so that in the extreme case of a classification based on all possible characters, there would be no distinction between a natural and artificial classification.

However, there is no reason to suppose that characters not used in the classification will fit into the same groups. For characters not studied a phenetic classification may well be artificial.

Michael Donoghue discusses the problem with the phenetic principle.