Biological classification, or scientific classification in biology, is a method of scientific taxonomy used to group and categorize organisms hierarchically. Rank-based systems use a fixed number of levels in the hierarchy, such as kingdom, family, genus or species. Rankless systems use an arbitrary number of levels. The groups in the classification are known as taxa (singular: taxon). Modern biological classification has its root in the work of Carolus Linnaeus, who grouped species according to shared physical characteristics. These groupings have since been revised to improve consistency with the Darwinian principle of common descent. With the introduction of the cladistic method in the early 20th century, formalized by Willi Hennig in the mid-20th century, phylogenetic taxonomy in which organisms are grouped purely on inferred evolutionary relatedness (based either on classical evidence of morphology, chemistry, physiology, ecology or molecular evidence or both) has become common in biology. Molecular phylogenetics, which uses DNA sequences as data, has driven many recent revisions and is likely to continue doing so. Biological classification belongs to the science of systematics.