PhyloCode is a formal set of rules governing phylogenetic nomenclature. Its current version is specifically designed to regulate the naming of clades, which do not have set ranks, unlike conventional Linnaean taxonomy. Later revisions will presumably also include new rules for the naming of species. Although some have argued that systems based on unranked clades should replace older systems, PhyloCode is designed so that it may be used alongside them. Phylocode is still a contentious issue and there are many who feel that the system is without merit. The number of supporters for official adoption of phylocode is still small, and it is uncertain, as of 2005, whether the system will be adopted or forgotten.

In PhyloCode, a clade name may be defined in one of the following manners (not exhaustive):

1. Node-based: "the least inclusive clade containing the species or organisms A, B, ..."
2. Stem-based: "the clade consisting of A and all organisms or species that share a more recent common ancestor with A than with Z"
3. Apomorphy -based: "the clade stemming from the first organism or species to possess the apomorphy (unique character) M as inherited by A"
4. Crown clade : "the clade stemming from the most recent common ancestor of A and all extant organisms or species that share a more recent common ancestor with A than with Z"

Although every organism belongs to the clade of living things at the very least, the ancestral organisms within clades are not included in any of the subclades.

If PhyloCode is officially adopted, a database will be established where authors can submit PhyloCode names and definitions of clades.

External links

• PhyloCode rules
• Michael Benton, Stems, nodes, crown clades, and rank-free lists: is Linnaeus dead?, Biological Reviews, 75:633–648 (2000). An article critical of PhyloCode.
• Christine Soares, What's in a Name?, Scientific American, (November 2004).
• Phylocode debate [1]