Circularity, evolution, systematics … and circularity

Many have argued strongly that incorporation of evolutionary theory into systematics is dangerously circular, while others have maintained that such an integrated approach increases the accuracy of phylogenetic inference. Here, it is demonstrated that such blanket statements regarding exclusion or inclusion of evolutionary principles in systematics fail to distinguish between two very different types of principles. ‘Phylogeny‐neutral’ evolutionary principles are those inferred without any recourse to specific phylogenetic hypotheses (e.g. via developmental genetics, biomechanics). In contrast, ‘phylogeny‐dependent’ principles are those which can only be inferred on the basis of specific phylogenetic hypotheses (e.g. character associations detected via ‘comparative methods’). Inclusion of phylogeny‐neutral principles in systematic studies as a priori assumptions can be justified, since these principles have (often strong) external empirical support from other spheres of investigation. However, inclusion of phylogeny‐dependent principles in systematic studies is circular, since such principles have no external empirical support but are themselves derived from systematic studies. Advocating inclusion or exclusion of all (or as many as possible) evolutionary principles from phylogenetic analysis is therefore misguided. Rather, phylogeny‐neutral principles are independently supported and can be included, while phylogeny‐dependent principles are unjustified assumptions and should be excluded to avoid circularity. However, integration of complex phylogeny‐neutral principles in systematics can create operational problems, even though there are no methodological reasons against their inclusion.