  Rare events have played an increasing role in molecular phylogenetics as
potentially homoplasy-poor characters.In this contribution we analyze the
phylogenetic information content from a combinatorial point of view by
consid-ering the binary relation on the set of taxa defined by the existence of
a single event separating two taxa. We showthat the graph-representation of
this relation must be a tree. Moreover, we characterize completely the
relationshipbetween the tree of such relations and the underlying phylogenetic
tree. With directed operations such as tandem-duplication-random-loss events in
mind we demonstrate how non-symmetric information constrains the position ofthe
root in the partially reconstructed phylogeny.
