Plesiomorphy

Definition

An ancestral character state that is shared by multiple taxa but does not indicate exclusive common ancestry among those taxa because it is also present in outgroups; the trait is primitive relative to a more derived (apomorphic) condition in the clade of interest. In cladistic analysis, plesiomorphies are phylogenetically uninformative for grouping the taxa that share them.

Etymology

Greek plēsios (near) + morphē (form), contrasting with apomorphy (derived form).

Example

The presence of six legs is a plesiomorphy for Hexapoda: while all insects share this trait, it also occurs in springtails and other non-insect hexapods, so it cannot distinguish Insecta from its sister groups. Similarly, chelicerate mouthparts (chelicerae) are a plesiomorphy for Arachnida, shared with horseshoe crabs and sea spiders, and thus do not unite spiders, scorpions, and mites to the exclusion of other chelicerates.

Synonyms

• symplesiomorphy (when shared by multiple taxa)
• ancestral character state

Related Terms

• Apomorphy
• synapomorphy
• autapomorphy
• symplesiomorphy
• Cladistics
• outgroup comparison
• character polarity

Usage Notes

Specialists distinguish plesiomorphy (the character state itself) from symplesiomorphy (the shared possession of that state by multiple taxa). A trait is plesiomorphic only relative to a specified ingroup; the same character may be apomorphic at a deeper phylogenetic level. Contrast with synapomorphy, which is shared and derived, thus grouping taxa. Misidentifying plesiomorphies as synapomorphies leads to erroneous cladograms.