What does it all mean?

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Most of the time up until now, we’re been working with sequences and trees in the abstract; sequence A, B, C and D. But what do you get if you use the ssu-rRNA molecular phylogenetic methods we've talked about with representatives from as wide a range of actual organisms as possible? You get this sort of unrooted dendrogram:

The Big 3-Domain tree
Representative "Universal" 3-Domain tree, redrawn to Norman Pace

Remember that the previously-common trees, such as the Whittaker 5-Kingdon tree, were subjective & qualitative. This molecular phylogenetic tree is quantitative and objective, based on statistical analysis of gene sequences. In this case, of course, the sequences used is the small subunit ribosomal RNA gene, for reasons we’ve already covered, but other appropriate molecules yield much the same result.

Three interesting topics that this tree brings up are discussed in the tlecture modules: