The measurable (!) phylogenetic diversity is essentially based on the evolution of microbes. It is now clear that from the beginning of evolution to the present day, genes have been freely exchanged by horizontal gene transfer (lateral gene exchange) between different prokaryotes.
Thus, microbes cannot be easily identified by the taxonomic differentiation features used so far, such as morphology (colony shape and colour, cell shape, size, possibly cell assemblies, motility, type of flagellation -electron microscope-, cell wall type -Gram test and/or electron microscope-, cell inclusions, extensions, capsules, sporulation, etc.) and physiology (utilization of genes).) and physiology (utilization of substrates as C, N or S source, ability to grow aerobically and anaerobically, analysis of fermentation products -gas-, acid formation, chemical analysis-, temperature limits of growth, tolerance to acid, salt, etc.).
By comparing the SSU-rRNA gene sequences of different microorganisms in pairs, a phylogenetic tree can be constructed, corresponding to an evolutionary map, on which previously unknown organisms can be assigned a position. This approach, also called molecular phylogenetics, allows the characterization of each organism based on its evolutionary distance to other organisms. The different phylogenetic types (phylotypes) can be seen as branches of an evolutionary tree.