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Yersinia pestis

■ Proteus vulgaris

■ Pseudomona aeruginosa (out-group organism)

FIGURE 20.22 Phylogenetic Tree for Enteric Bacteria

The phylogenetic relationship between bacteria can be deduced by comparing ribosomal RNA sequences. Here the sequences of the 16S rRNA genes are compared for the four enteric bacteria, E. coli, Erwinia herbicola, Yersinia pestis, and Proteus vulgaris. The relatively unrelated bacterium Pseudomona aeruginaosa, is used as an outgroup organism to provide the base or root of the tree. From these comparisons, it can be deduced that P. vulgaris was the first to branch from the primitive ancestor and that E. coli and E. herbicola were the latest.

to exactly restore the original length and sequence is vanishingly small. Consequently, if a subgroup of a family of related sequences all contain an indel of defined length and sequence at the same location, they must all have been derived from the same ancestral sequence.

Mitochondrial DNA changes fast enough to be used to classify subgroups within the same species.

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