Paraphyly

In taxonomy, a grouping is paraphyletic if it consists of the grouping's last common ancestor and most of its descendants, but excludes a few monophyletic subgroups. The grouping is said to be paraphyletic with respect to the excluded subgroups. In contrast, a monophyletic grouping (a clade) includes a common ancestor and all of its descendants. The terms are commonly used in phylogenetics (a subfield of biology) and in the tree model of historical linguistics. Paraphyletic groups are identified by a combination of synapomorphies and symplesiomorphies. If many subgroups are missing from the named group, it is said to be polyparaphyletic. The term was coined by Willi Hennig to apply to well-known taxa like Reptilia (reptiles) which is paraphyletic with respect to birds. Reptilia contains the last common ancestor of reptiles and all descendants of that ancestor except for birds. Other commonly recognized paraphyletic groups include fish, monkeys, and lizards. The term paraphyly, or paraphyletic, derives from the two Ancient Greek words παρά (), meaning "beside, near", and φῦλον (), meaning "genus, species", and refers to the situation in which one or several monophyletic subgroups of organisms (e.g., genera, species) are left apart from all other descendants of a unique common ancestor. Conversely, the term monophyly, or monophyletic, builds on the Ancient Greek prefix μόνος (), meaning "alone, only, unique", and refers to the fact that a monophyletic group includes organisms consisting of all the descendants of a unique common ancestor. By comparison, the term polyphyly, or polyphyletic, uses the Ancient Greek prefix πολύς (), meaning "many, a lot of", and refers to the fact that a polyphyletic group includes organisms arising from multiple ancestral sources. Groups that include all the descendants of a common ancestor are said to be monophyletic. A paraphyletic group is a monophyletic group from which one or more subsidiary clades (monophyletic groups) are excluded to form a separate group.