Cutting tree branches to pick OTUs: a novel method of provisional species delimitation

Delimitation of species is crucial in all studies of biodiversity, its geographic patterns and evolutionary dynamics as well as in the corresponding conservation applications. In practice, operational taxonomic units (OTUs) are often used as provisional surrogates of the species, whose evidence-based and robust delimitation requires too extensive data and complex analyses. The novel method for this provisional species delimitation is suggested, which uses any phylogenetic tree with meaningful branch lengths as an input and delimits OTUs on it by identification of branches whose removal significantly changes structure of the tree. Such branches are considered to reflect interspecific differentiation that is assumed generally more erratic than intraspecific branching. It is called branch-cutting method as it evaluates structural importance of the branch by its cutting (shrinking to zero length) and inspecting impact of this operation on the average pairwise distances between tree tips. Tree tips can be also constrained to be either conspecific or heterospecific which allows the method to achieve more robust and informed delimitations and to focus on particular phylogenetic scale. Usefulness of the method is demonstrated on four empirical examples and comparison with similar methods is performed.