Monotypicity

Monotypicity is the condition of a taxon being monotypic, that is, containing only a single subordinate taxon at the next lower rank, or, in common usage, consisting of a single species. The term is used across biology to describe monotypic genera (one species in the genus), monotypic families (one genus in the family), and, less commonly, higher ranks with only one subordinate taxon. Monotypicity can arise through evolutionary or taxonomic processes such as extinction of related taxa, limited diversification, or taxonomic revisions that merge or split groups.

A standard example is the genus Ginkgo, which is monotypic with the sole living species Ginkgo biloba; the family Ginkgoaceae contains only that genus, making both the genus and the family monotypic. Another example is the red panda, Ailurus fulgens, the sole living species in the genus Ailurus, with the family Ailuridae consisting of that genus. The platypus, Ornithorhynchus anatinus, is the only living species in the genus Ornithorhynchus, the sole extant genus in the family Ornithorhynchidae. Extinct relatives are often known, but they do not alter the current monotypic status of living taxa.

Monotypicity has implications for conservation and evolutionary studies because it signals limited diversification within a lineage and can affect phylogenetic interpretation. The concept is widely used in botany and zoology and contrasts with polytypic taxa, which contain two or more subordinate taxa at the next rank.