paraphyly

Paraphyly is a term in phylogenetics used to describe a group that includes the most recent common ancestor of its members but excludes one or more of that ancestor's descendants. By contrast, a monophyletic group (a clade) includes the MRCA and all of its descendants, while a polyphyletic grouping gathers members that do not share the most recent common ancestor within the group. Paraphyly thus reflects a grouping defined by shared ancestral traits while omitting some lineages that also descend from the same ancestor.

Etymology-wise, paraphyly comes from Greek para- meaning beside or near, and phyle meaning race or tribe. The concept is central to discussions of evolutionary relationships and classification.

Classic examples are traditional vertebrate groups such as Reptilia in its historic sense. Because birds evolved from within the reptile lineage, excluding birds makes traditional reptiles paraphyletic. Another commonly cited case is Pisces, the historical grouping of fishes; tetrapods emerged from within the fish lineage, rendering Pisces paraphyletic with respect to tetrapods. In modern taxonomy, there is a preference for monophyletic groups, which can lead to redefinitions that incorporate descendants previously excluded.

Paraphyly is detected in phylogenetic analyses that combine morphological and molecular data to infer evolutionary trees. It remains a topic of discussion because, while monophyletic groups are often favored for clarity and coherence in classification, paraphyletic groups can reflect historical, functional, or practical considerations in particular contexts.