Relationship of Phylogeny to "Natural Classification"
& Principles of Nomenclature

    A taxon (plural, taxa) is a group of organisms that is assigned a particular name at some categorical rank. The categories used in zoological taxonomy are Kingdom, Phylum, Class, Order, Family, Genus, & Species, ranked in order from most to least inclusive. [A time-honored mnemonic for KPCOFGS is 'King Philip Came Over For Good Sex']. Any of the categories may be modified by the prefixes Super- or Sub-, which indicate more or less inclusive groups, respectively. Familial names typically include a suffix that indicates that rank within Orders: these are -oidea for a superfamily, -idae for a family, and -inae for a subfamily. Other category names sometimes used include Tribe (between Family and Genus), for which the suffix is -ini. Taxon-specific naming conventions includes the suffix -a for Mammalian orders, -iformes for Bird families, and substitution of Division for Phylum in botanical taxonomy.

    Assignment of names for newly-discovered taxa, and resolution of conflicting names, is done according to quasi-legal, formal rules in nomenclature, the science of naming. For example, the genus names of harp seals and ivory gulls are Pagophilus and Pagophila, respectively, -us being a Latin masculine ending and -a a feminine ending. Under the rules, taxa differing in gender endings are permitted. The rules of nomenclature further require that species names follow the genus names in Latin gender: seals are Pagophilus groenlandicus and gulls Pagophila eburnea. For the seals, this is true even though they were originally given the masculine name of Phoca groenlandica: the grammatical rule takes precedence over the stability rule.

    The network shows the phylogenetic relationships (evolutionary history) of some taxa of terrestrial animals in the Class Mammalia. The animals shown are part of a single evolutionary lineage that has been assigned the taxon name Carnivora at the category rank of Order. Within this order, various groups of animals have evolved, including the cat-like superfamily Feloidea, including small cats in the family Felidae, and the superfamily of bear-, dog-, & weasel-like Canoidea, which are assigned the family names Ursidae, Canidae, &Mustelidae, respectively. The categorical rank of each of these groups as a family is indicated by the -idae suffix. Within Mustelidae, two lineages corresponding to weasels and martens are recognized among others, which have been assigned the genus names Mustela and Martes, respectively: names in capitalized italics indicate the genus category (plural, genera). Particular forms of weasels and martens are the species (sing., species) Mustela erminea and Martes americana. With no exceptions, the invariant rule is that genus & species names are always written in italics, genus names are always capitalized, and species names are always in lower-case. After the first use in scientific writing, and (or) where it will not be ambiguous, the genus name may be abbreviated, in this case as M. erminea and M. americana. The family Ursidae includes the extinct Cave Bear and Giant Panda.

    Other taxonomic arrangements are possible. Dog- & cat-like carnivores may be placed either in separate superfamilies as above. Taxonomists who wish to emphasize differences or degree of relationship may place them in different suborders: Caniformia and Feliformia. [Note the suffix -iformia, similar to that used for bird orders, ending here in -a].

    Like previous classificatory systems, such as Linnaeus's Systema Naturae (1758), the modern system is hierarchical, because each level of category names is contained entirely within the next more inclusive category. Unlike older systems, the system is natural because the hierarchy of categorical names unambiguously indicates the degree of evolutionary relationship among organisms. For example, organisms placed in the same genus are always more closely related to each other than those in different genera in the same family, and organisms in the same family are more closely related than those in different orders. Ranking within more inclusive taxa reflects degree of common ancestry. A natural classification allows instant recognition of the current hypothesis of evolutionary relationships among organisms.

    It is important to realize that in phylogenetic taxonomy, the classification of creatures in any category is a hypothesis as to how its component taxa are related: new data or analysis may revise that hypothesis. For example, Linnaeus (1758) placed all true (eared) seals of the North Atlantic in the single genus Phoca. Subsequent molecular investigation divides these into several genera, in particular separating Harp Seals (Pagophilus) as only distantly related to Harbor Seals (Phoca).