Paraphyly

Paraphyly is a condition in which a taxonomic group includes a common ancestor but excludes some of its descendants. Such a group is paraphyletic rather than a complete clade.

Clade: A group consisting of a common ancestor and all its descendants. A clade includes every descendant, unlike a paraphyletic group.
Reptile: An amniote vertebrate in the traditional sense, excluding birds and mammals. Traditional reptiles exclude birds, although birds descend from within the reptile lineage.
Phylogenetic nomenclature: A system of naming taxa by reference to evolutionary relationships and ancestry. It favors definitions that include all descendants of a designated ancestor.
Taxonomic rank: A formal level in a classification hierarchy, such as species, genus, or family. Traditional ranks can preserve familiar paraphyletic groups despite clade-based standards.
Monophyly: The condition of a group containing a common ancestor and all its descendants. Monophyly is the condition paraphyly fails to meet.
Fish: Aquatic vertebrates and related chordates traditionally grouped by aquatic life and anatomy. Traditional fish groups exclude tetrapods, which descend from within their evolutionary lineages.
Linnaean taxonomy: A hierarchical classification system that assigns organisms ranked names such as genus and family. Its traditional ranks often retain groups that phylogenetic analysis finds paraphyletic.
Taxonomic revision: A scientific reassessment that changes the circumscription or names of taxa. Recognizing paraphyly can prompt revisions that either split or expand familiar groups.
Polyphyly: The condition of a group assembled from organisms without including their most recent common ancestor. It also falls outside clade-based grouping, but excludes the shared ancestor itself.
Prokaryote: An organism whose cell lacks a membrane-enclosed nucleus, including bacteria and archaea. The grouping excludes eukaryotes despite their ancestry within the broader tree of life.

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