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The unfalsifiability of cladograms and its consequences

Popper's falsificationism provides the normative reference system in recent discussions regarding theory and methodology of systematics. According to Popper, the falsifiability of a hypothesis represents a necessary precondition for its corroborability. It is shown that cladograms, independent of “strict”, “methodological” or “sophisticated” falsification, are not falsifiable in principle. No present observation is prohibited by any tree hypothesis and, thus, no Popperian test of cladograms exists. It is shown that the congruence test, which is commonly said to represent a Popperian test of cladograms, instead tests sets of apomorphy hypotheses. Three different strategies that have been proposed to circumvent this problem are discussed and refuted: (1) referring to Popper's convention to renounce ad hoc maneuvers; (2) referring to Popper's treatment of probability hypotheses; and (3) decoupling corroboration from falsification. As a consequence, within a Popperian framework the unfalsifiability of cladograms implies that cladograms cannot explain any present day observation and, thus, represent metaphysical hypotheses.