DNA sequencing has provided a new approach for studying evolutionary relationships, since:
1. All organisms have a genome
2. The genes that code for vital cellular functions are conserved to a remarkable degree through evolutionary time
3. Even these genes accumulate random changes with time (usually in the regions that are not vital for function). In this respect the gene changes are rather like the scars on a boxer’s face- a record of the accumulated impact of time.
So, by comparing the genes that code for vital functions of all living organisms, it was possible to assess the relatedness of different organisms. The gene most commonly used for this codes for the RNA in the small subunit (SSU) of the ribosome. [Ribosomes are the structures on which proteins are synthesised].
Some regions of this SSU rRNA (also termed 16S rRNA) are highly conserved in all organisms, whereas other regions are more variable.
By comparing the DNA sequences for 16S rRNA, Woese and his colleagues constructed a proposed universal phylogenetic tree, shown in simplified form below. (Fig. 8.1)
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. One of the principal modes of evolution is thought to involve gene duplication followed by divergence. The original gene retains its vital function, while the copy can change and ultimately can encode a new function.
If these paralogous gene pairs can be identified by sequence similarity, then the original gene should be present in all organisms whereas the new version will be present only in the more recently derived organisms. The root for the tree in the diagram was determined by using paralogous genes for translation elongation factors involved in synthesis of protein chains on the ribosomes.