The use of amino-acid or nucleotide-sequence data in determining the evolutionary relationships of different organisms. Essentially it involves comparing the sequences of functionally homologous molecules (e.g. ribosomal RNA subunits) from each organism being studied, and determining the number of differences between them. The greater the number of differences, the more distantly related the organisms are likely to be. Moreover, since the number of nucleotide substitutions, and hence substitutions of corresponding amino acids, is generally proportional to time, some indication of the time scale involved can be obtained (see molecular clock). This information has proved particularly useful where there are gaps in the fossil record and can be combined with other evidence from morphology, physiology, and embryology to produce more accurate phylogenetic trees. In microbiology molecular systematics has transformed bacterial phylogeny, in particular establishing the view that there are two quite distinct lineages of prokaryotes, the bacteria and the archaea. There has been an equally radical reassessment of the classification of eukaryotes, which on current molecular evidence form an unrooted phylogenetic tree consisting of four supergroups plus other assemblages. The availability of rapid fully automated DNA sequencing has led to the growth of comparative genomics, which enables comprehensive genomic comparisons between different organisms, with computerized analysis of the data lending much greater reliability to inferences about evolutionary relationships.