FIG. 1.

A gene tree and a species tree with a matching labeled topology t. (A) A tree t of size 6 isomorphic to the gene tree and species tree depicted in (B, C). Tree t is characterized by its shape and by the labeling of its taxa. It is convenient to label the internal nodes of t. We identify each lineage (edge) of t by its immediate descendant node, so, for example, lineage g results from the coalescence of lineages a and b. (B) A possible realization R₁ of the gene tree in (A) (dotted lines) in the species tree with a matching topology (solid lines). The ancestral configuration at species tree node is . The configuration at node m is . (C) A different realization R₂ of the gene tree in (A) in the matching species tree. The configurations at species tree nodes and m are and , respectively.