The hierarchy of biological classification's eight major taxonomic ranks. Intermediate minor rankings are not shown. This diagram uses a 3Domains / 6 Kingdoms format
Main article: Systematics
Multiple speciation events create a tree structured system of relationships between species. The role of systematics is to study these relationships and thus the differences and similarities between species and groups of species. However, systematics was an active field of research long before evolutionary thinking was common.
Traditionally, living things have been divided into five kingdoms:Monera; Protista; Fungi; Plantae; Animalia. However, many scientists now consider this five-kingdom system outdated. Modern alternative classification systems generally begin with the three-domain system: Archaea (originally Archaebacteria);Bacteria (originally Eubacteria) and Eukaryota (includingprotists, fungi, plants, and animals) These domains reflect whether the cells have nuclei or not, as well as differences in the chemical composition of key biomolecules such asribosomes.
Further, each kingdom is broken down recursively until each species is separately classified. The order is: Domain; Kingdom;Phylum; Class; Order; Family; Genus; Species. Outside of these categories, there are obligate intracellular parasites that are "on the edge of life"in terms of metabolic activity, meaning that many scientists do not actually classify these structures as alive, due to their lack of at least one or more of the fundamental functions or characteristics that define life. They are classified as viruses, viroids, prions, or satellites.