Abstract

ields to graphically represent various types of hierarchical relationships, including evolutionary relationships between species, divergent patterns between subpopulations and evolutionary relationships between genes. These trees are generally rooted and semi-labeled, i.e., they descend from a single node called the root, bifurcate at lower nodes and end at terminal nodes, called tips or leaves; the leaves are labeled by the names of the species, subpopulations or genes being studied. In biological studies the latter are called operational taxonomic units (OTU's). Traditionally, trees were inferred form morphological similarities among the OTU's. To build an evolutionary species tree, or phylogenetic tree, two species which shared the most characteristics were classified as `siblings' and assumed to share a common ancestor which is not the ancestor of any other species. Such `siblings' are said to be homologous, and it is this basic homo

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