PhyloCode

Types of clade

PhyloCode is a formal set of rules governing phylogenetic nomenclature. It is specifically designed to regulate the naming of species and clades, which do not have set ranks, unlike conventional Linnaean taxonomy. 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.

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

1. Node-based: "the least inclusive clade containing 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"

For example, the amniotes are traditionally divided into three classes - Reptilia, Aves, and Mammalia. Under PhyloCode, they are instead divided into a tree of nested clades:

• Amniota
  • Synapsida
    • Mammalia
  • Reptilia
    • Anapsida
      • Testudines (turtles)
    • Diapsida
      • Lepidosauria
        • Squamata (lizards and snakes)
        • Sphenodontida (tuatara)
      • Archosauria
        • Crocodilia (crocodiles and alligators)
        • Aves (birds)

A particular clade may be identified by specifying a path through this tree from the root. Although every organism belongs to the clade of living things at the very least, but the ancestral organisms within clades are not included in any of the subclades.

External links

• PhyloCode rules (http://www.ohiou.edu/phylocode/index.html)
• Michael Benton, Stems, nodes, crown clades, and rank-free lists: is Linnaeus dead? (http://palaeo.gly.bris.ac.uk/Essays/phylocode/biolrev.html), Biological Reviews, 75:633–648 (2000).
• Christine Soares, What's in a Name? (http://www.sciam.com/article.cfm?articleID=000D7477-4199-1179-819983414B7FFE9F), Scientific American, (November 2004).
• Tree of Life Web (http://www.tolweb.org)
• [1] (http://www.users.bigpond.com/pmurray/Java/TolFrame.html) Tree of life applet (requires Sun java 1.4)