Analyses of the nucleotide sequences of ribosomal RNA (rRNA) genes provided the first comprehensive evidence of evolutionary relationships among prokaryotes. Comparisons of rRNA genes are often used to identify microbes. For several reasons, rRNA is particularly useful for phylogenetic studies and identification purposes:
532
rRNA was present in the common ancestor of all life and is therefore evolutionarily ancient.
No free-
rRNA plays a critical role in translation in all organisms, so lateral transfer of rRNA genes among distantly related species is unlikely.
rRNA has evolved slowly enough that gene sequences from even distantly related species can be aligned and analyzed.
Although studies of rRNA genes reveal much about the evolutionary relationships of prokaryotes, they don’t always reveal the entire evolutionary history of these organisms. In some groups of prokaryotes, analyses of multiple gene sequences have suggested several different phylogenetic patterns. How could such differences among different gene sequences arise? Studies of whole prokaryotic genomes have revealed that even distantly related prokaryotes sometimes exchange genetic material.