BIODIVERSITY AT THE MOLECULAR-LEVEL - THE DOMAINS, KINGDOMS AND PHYLAOF LIFE

The results of comparative sequence analysis, mainly of small subunit (SSU) ribosomal (r)RNA sequences, have suggested that all of cellular life can be placed in one of three domains: the Archaea, Bacteria or E ucarya. There is some evidence that the Archaea may not be a monophyle tic assemblage, but as yet this issue has not been resolved. Most of t he lineages, and all of the deepest ones, in the tree based upon SSU r RNA sequences, are microbial. Traditional ideas of classification such as Whittaker's five kingdom scheme do not adequately describe life's diversity as revealed by sequence comparisons. There are many microbia l groups that demonstrate much greater amounts of SSU rRNA sequence di vergence than do members of the classical kingdoms, Animalia, Plantae and Fungi. The old microbial kingdoms Monera and Protista are clearly paraphyletic but as yet there is no consensus as to how they should be reorganized in taxonomic terms. New data from environmental analysis suggests that much of the microbial world is unknown. Every environmen t which has been analysed by molecular methods has revealed many previ ously unrecorded lineages. Some of these show great divergence from th e sequences of cultured microorganisms suggesting that fundamentally n ew microbial groups remain to be isolated. The relationships of some o f these new lineages may be expected to affect how the tree of life is organized into higher taxa, and to also influence which features will be recognized as synapomorphies. There is currently no objective meas ure whereby microbial diversity can be quantified and compared to the figures which are widely quoted for arthropods and other Metazoa.