Paraphyly

Paraphyly describes a group of organisms that includes an ancestor and some, but not all, of its descendants. In other words, a paraphyletic group shares a common ancestor, but excludes one or more clades (monophyletic groups) that descended from that ancestor. This exclusion makes the group not a natural evolutionary grouping.

Paraphyletic groups are often identified when classification is based on shared ancestral characters (symplesiomorphies) rather than shared derived characters (synapomorphies). This means that the traits defining the group are present in the ancestor and retained by some, but not all, of its descendants.

The identification and avoidance of paraphyletic groups are important in modern phylogenetic systematics, as the goal is to define classifications that accurately reflect evolutionary relationships. While paraphyletic groupings can be useful for certain practical purposes (e.g., discussing organisms with similar characteristics), they are generally avoided in formal taxonomic classifications because they do not represent complete evolutionary lineages.

The use of molecular data and cladistic analysis has led to the re-evaluation of many historically accepted taxonomic groups, resulting in the recognition of paraphyly and subsequent revisions to create monophyletic groups.