Diversity of organisms, especially of non-prokaryotic organisms is measured in terms of form, shape, structure, color etc. differences. For microorganisms these criteria are not particularly relevant. Because of their small size, physical traits are of limited use. The diversity of microorganisms is expressed mostly in terms of physiological capabilities and biochemical and molecular differences. For a long time bacterial classification was mostly based on such characteristics. In recent decades diversity of bacteria and archaea has been more and more expressed in the light of molecular and genetic diversity.