Based on the extent of sequence conservation through rRNA comparisons. Each nucleotide in a genome mutates at more or less the same rate, depending upon the base. However, the inheritable rate of change can vary from site to site on the genome due to selective pressure. During the course of evolution the inheritable rate of change at any one site might vary due to changes in selective pressure caused by changes in the environment. Their environment is extreme and unchanging - so little need to evolve.
Eukaryotes are more related to archaebacteria than are eubacteria are to archaebacteria.
Therefore eubacteria represent the most evolved at least with respect to core cellular components. Eukaryotes have evolved genome complexity. Gene families that can rapidly evolve . Gives rise to the diversity of cell types in multi-cellular organisms. Allows them to deal more effectively with changing environment .