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#A brief introduction to phylogeny and systematics.  In order to identify to which organism a 16S rDNA sequence belongs, as well as to provide an evolutionary context for your organism of interest, we recommend inferring a phylogenetic tree to compare the new 16S rDNA sequence to other 16S rDNA sequences (see Section 11). Building such a phylogenetic tree is (relatively speaking) the easy part. Intelligent interpretation of the tree will require an investment of time, similar to the investment required to learn the basics of UNIX. Fortunately, there are a number of resources available for this purpose. We recommend [this online tutorial (http://evolution.berkeley.edu/evolibrary/article/phylogenetics_02)](http://evolution.berkeley.edu/evolibrary/article/phylogenetics_02) orBaldauf  \cite{Baldauf_2003} Here we provide a brief introduction to phylogenetic trees. A phylogenetic tree is a diagram representing a model of evolutionary relationships. Phylogenetic trees have three main components: taxa, branches, and nodes. These are defined below: