Читать книгу Principles of Microbial Diversity - James W. Brown - Страница 59

Whenever possible, an outgroup sequence should be included in the analysis; an outgroup is a sequence that is known to be outside of the group you are interested in treeing. Only by including an outgroup can the root of a tree be located. For example, when building trees from mammalian sequences, the sequence from a reptile might be included as an outgroup. Outgroups provide the root to the rest of the tree; although no tree generated by these methods has a real root, if other information shows that one (or a set) of the sequences is unrelated to the rest, wherever that branch connects to the rest of the tree defines the root (common ancestor) of that portion of the tree. In our example tree above, sequences A to D might be mammalian whereas E and F might be reptilian. If the tree included only mammalian sequences, it would be impossible to know where the root is, but the inclusion of an outgroup provides that information.