Evolutionary distance tree of the bacterial domain showing currently recognized divisions and putative (candidate) divisions. Division-level groupings of two or more sequences are depicted as wedges. The depth of the wedge reflects the branching depth of the representatives selected for a particular division. Divisions which have cultivated representatives are shown in black; divisions represented only by environmental sequences are shown in outline. Numbers in parentheses are the number of representatives of the division for which genome sequence analysis has been or is being conducted. The scale bar indicates 0.1 changes per nucleotide. Inset: Bacterial divisions recognized in 1987 (Woese, 1987) (from Hugenholtz et al., J. Bacteriol. 180:4765-4774, 1998)