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18-Apr-2020 04:25

As shown in the diagram above, the archaea and eucarya seem to have arisen from a common line more recently than the divergence of these two groups from the bacteria.

In other words, there does not seem to be an ordered, linear sequence of genetic change during the early evolution of the major groups of eukaryotes.

There will be many changes before we arrive at a universally acceptable tree, if ever! Perhaps all the terminal branches of the tree represent kingdoms or taxa of similar major rank, and more will be added as the sequences of other organisms become available.

Clearly, some of these kingdoms (or potential kingdoms) have advanced and expanded much more than others - the plants, animals and fungi are major groups of organisms with distinctive lifestyles, whereas the slime moulds and ciliates, for example, have not expanded to the same degree.

The Five Kingdom approach is attractive in its simplicity, but has significant problems.

One of these concerns the protists - a wide range of disparate organisms such as amoebae, slime moulds, ciliates, algae, etc.Some regions of this SSU r RNA (also termed 16S r RNA) are highly conserved in all organisms, whereas other regions are more variable.Proposed universal phylogenetic tree, based on nucleotide sequence comparisons of the DNA coding for the RNA of the small ribosomal subunit of different organisms. [Based on a diagram in R Woese (1994) Microbiological Reviews 58, 1-9.] We should note a technical point about this tree: the comparison of ribosomal RNA gene sequences can show the possible relatedness of organisms, but other information is needed to provide the root of a tree.Eukaryotes must have arisen from prokaryotic ancestors.