Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic with respect to the excluded subgroups. In contrast, a Monophyly 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 symplesiomorphy. If many subgroups are missing from the named group, it is said to be polyparaphyletic.
The term received currency during the debates of the 1960s and 1970s accompanying the rise of cladistics, having been coined by zoologist Willi Hennig to apply to well-known taxa like Reptilia (), which is paraphyletic with respect to . Reptilia contains the last common ancestor of reptiles and all descendants of that ancestor except for birds. Other commonly recognized paraphyletic groups include fish, , and .
Conversely, the term monophyly, or monophyletic, builds on the Ancient Greek prefix (), 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 (), meaning "many, a lot of", and refers to the fact that a polyphyletic group includes organisms arising from multiple ancestral sources.
A group whose identifying features evolved convergently in two or more lineages is polyphyletic (Greek πολύς polys, "many"). More broadly, any taxon that is not paraphyletic or monophyletic can be called polyphyletic. Empirically, the distinction between polyphyletic groups and paraphyletic groups is rather arbitrary, since the character states of common ancestors are inferences, not observations.
These terms were developed during the debates of the 1960s and 1970s accompanying the rise of cladistics.
Paraphyletic groupings are considered problematic by many taxonomists, as it is not possible to talk precisely about their phylogenetic relationships, their characteristic traits and literal extinction. Related terms are stem group, chronospecies, budding cladogenesis, anagenesis, or 'grade' groupings. Paraphyletic groups are often relics from outdated hypotheses of phylogenic relationships from before the rise of cladistics.
Among plants, (in the traditional sense) are paraphyletic because the group excludes . "Dicotyledon" has not been used as a botanic classification for decades, but is allowed as a synonym of Magnoliopsida.The history of flowering plant classification can be found under History of the classification of flowering plants. Phylogenetic analysis indicates that the are a development from a dicot ancestor. Excluding monocots from the dicots makes the latter a paraphyletic group.. "It is now thought that the possession of two cotyledons is an ancestral feature for the taxa of the flowering plants and not an apomorphy for any group within. The 'dicots' ... are paraphyletic ...."
Among animals, several familiar groups are not, in fact, clades. The order Artiodactyla (even-toed ungulates) as traditionally defined is paraphyletic because it excludes (whales, dolphins, etc.). Under the ranks of the ICZN Code, the two taxa are separate orders. Molecular studies, however, have shown that the Cetacea descend from artiodactyl ancestors, although the precise phylogeny within the order remains uncertain. Without the Cetaceans the Artiodactyls are paraphyletic. The class reptile is paraphyletic because it excludes birds (class Aves). Under a traditional classification, these two taxa are separate classes. However birds are sister taxon to a group of dinosaurs (part of Diapsida), both of which are "reptiles".Alfred Romer & Parsons, T. S. (1985): The Vertebrate Body. (6th ed.) Saunders, Philadelphia.
Osteichthyes, bony fish, are paraphyletic when circumscribed to include only Actinopterygii (ray-finned fish) and Sarcopterygii (lungfish, etc.), and to exclude ; more recently, Osteichthyes is treated as a clade, including the tetrapods.
The "" are paraphyletic, consisting of the narrow-waisted Apocrita without the and . The sawflies (Symphyta) are similarly paraphyletic, forming all of the Hymenoptera except for the Apocrita, a clade deep within the sawfly tree. are not a clade because the Hexapoda (insects) are excluded. The modern clade that spans all of them is the Pancrustacea.
One of the goals of modern taxonomy over the past fifty years has been to eliminate paraphyletic taxa from formal classifications.Schuh, Randall T. "The Linnaean system and its 250-year persistence." The Botanical Review 69, no. 1 (2003): 59. Below is a partial list of obsolete taxa and informal groups that have been found to be paraphyletic.
Cladists advocate a phylogenetic species concept that does not consider species to exhibit the properties of monophyly or paraphyly, concepts under that perspective which apply only to groups of species.
Also, some systematists recognize paraphyletic groups as being involved in evolutionary transitions, the development of the first tetrapods from their ancestors for example. Any name given to these hypothetical ancestors to distinguish them from tetrapods—"fish", for example—necessarily picks out a paraphyletic group, because the descendant tetrapods are not included. Other systematists consider reification of paraphyletic groups to obscure inferred patterns of evolutionary history.
The term "evolutionary grade" is sometimes used for paraphyletic groups. Moreover, the concepts of monophyly, paraphyly, and polyphyly have been used in deducing key genes for DNA barcoding of diverse group of species.
|
|