subkingdoms

Subkingdom is a taxonomic rank used in some biological classification schemes to subdivide a kingdom. Positioned below the kingdom and above the phylum (or division), a subkingdom groups together a broad set of organisms that share fundamental traits or evolutionary history.

The use of subkingdom is not standardized; different authors place different groups into subkingdoms, and some kingdoms have more than one subkingdom while others have none. Over time, many taxonomists have shifted toward using clades or other ranks that reflect evolutionary relationships, making subkingdoms less common in modern classifications. Because the concept depends on the particular taxonomic system, subkingdoms are best regarded as historical or optional rankings rather than universal standards.

Examples have been used historically in various kingdoms. In some older botany schemes, non-vascular and vascular plants were treated as separate subkingdoms. In zoological systems, proposals to separate unicellular from multicellular animals, or to group major multicellular lineages, have appeared as subkingdoms in certain classifications. The exact content of a subkingdom thus varies by author and era, and many modern treatments do not recognize subkingdoms at all.

In contemporary taxonomy, there is a trend to describe relationships through clades and other phylogenetic groupings rather than fixed ranks. Subkingdoms, when used, are one of several historical tools for organizing large and diverse groups, and their applicability depends on the taxonomy being applied.