Journal of Classification

, Volume 9, Issue 1, pp 91-116

First online:

The complexity of reconstructing trees from qualitative characters and subtrees

  • Michael SteelAffiliated withZ.I.F. der Universität Bielefeld

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In taxonomy and other branches of classification it is useful to know when tree-like classifications on overlapping sets of labels can be consistently combined into a parent tree. This paper considers the computation complexity of this problem. Recognizing when a consistent parent tree exists is shown to be intractable (NP-complete) for sets of unrooted trees, even when each tree in the set classifies just four labels. Consequently determining the compatibility of qualitative characters and partial binary characters is, in general, also NP-complete. However for sets of rooted trees an algorithm is described which constructs the “strict consensus tree” of all consistent parent trees (when they exist) in polynomial time. The related question of recognizing when a set of subtrees uniquely defines a parent tree is also considered, and a simple necessary and sufficient condition is described for rooted trees.


Trees Qualitative characters Compatibility Resolved quartets Clusters Strict consensus tree