Basal

'Basal' is a term in biology for 'primitive' or 'ancestral'. Basal is preferred because it is neutral and non-judgmental. This terminology came into use with cladistics. The term is used in evolution and classification to mean the group which gave rise to later forms. It is a term which works at any level: a basal species gives rise to derived species, and a basal group gives rise to derived groups.

'Derived' is the correct term in biology for specialised or advanced. It is used for groups which have evolved from a more basal group. A derived trait is a trait that is present in an organism, but was not in the last common ancestor of the group being considered. Simplicity is often secondarily derived. For example, no mitochondria in the anaerobic protist Entamoeba histolytica is a result of their secondary loss.

General view –

• Basal group
  • Derived group
  • Derived group
  • Derived group

Example –

• Early primates
  • Monkeys
    • Apes
      • Homo

In the last example, note that apes are derived from monkeys but basal with respect to our own genus. The status of being basal or derived is relative, though living species can only be derived. In any basal-derived pair, the basal member must have evolved before the derived member, with evidence that some of its species gave rise to (evolved into) the derived member.

Other terms

See also Wiley et al. 1991.

• Ancestral taxon = basal group
• Characters or attributes:
  • Plesiomorphy = ancestral trait inherited from earlier forms.
  • Apomorphy = trait or innovation derived from a plesiomorphic character. It may define a clade.
  • Synapomorphy = a trait or character of a whole clade.