Apolyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor.[1] The term is often applied to groups that share similar features known ashomoplasies, which are explained as a result ofconvergent evolution. The arrangement of the members of a polyphyletic group is called apolyphyly/ˈpɒlɪˌfaɪli/.[2] It is contrasted withmonophyly andparaphyly.
For example, the biological characteristic ofwarm-bloodedness evolved separately in the ancestors of mammals and the ancestors of birds; "warm-blooded animals" is therefore a polyphyletic grouping.[3] Other examples of polyphyletic groups arealgae,C4 photosynthetic plants,[4] andedentates.[5]
Manytaxonomists aim to avoid homoplasies in grouping taxa together, with a goal to identify and eliminate groups that are found to be polyphyletic. This is often the stimulus for major revisions of the classification schemes. Researchers concerned more with ecology than with systematics may take polyphyletic groups as legitimate subject matter; the similarities in activity within the fungus groupAlternaria, for example, can lead researchers to regard the group as a validgenus while acknowledging its polyphyly.[6] In recent research, the concepts of monophyly, paraphyly, and polyphyly have been used in deducing key genes forbarcoding of diverse groups of species.[7]
The termpolyphyly, orpolyphyletic, derives from the twoAncient Greek wordsπολύς (polús) 'many, a lot of', andφῦλον (phûlon) 'genus, species',[8][9] and refers to the fact that a polyphyletic group includes organisms (e.g., genera, species) arising frommultiple ancestral sources.
Conversely, the termmonophyly, ormonophyletic, employs the ancient Greek adjectiveμόνος (mónos) 'alone, only, unique',[8][9] and refers to the fact that a monophyletic group includes organisms consisting of all the descendants of aunique common ancestor.
By comparison, the termparaphyly, orparaphyletic, uses the ancient Greek prepositionπαρά (pará) 'beside, near',[8][9] and refers to the situation in which one or several monophyletic subgroups areleft apart from all other descendants of a unique common ancestor.
In many schools oftaxonomy, the recognition of polyphyletic groups in a classification is discouraged.Monophyletic groups (that is,clades) are considered by these schools of thought to be the only valid groupings oforganisms because they are diagnosed ("defined", in common parlance) on the basis ofsynapomorphies, while paraphyletic or polyphyletic groups are not. From the perspective of ancestry, clades are simple to define in purelyphylogenetic terms without reference to clades previously introduced: anode-based clade definition, for example, could be "All descendants of the last common ancestor of species X and Y". On the other hand, polyphyletic groups can be delimited as a conjunction of several clades, for example "the flying vertebrates consist of the bat, bird, and pterosaur clades".
From a practical perspective, grouping species monophyletically facilitates prediction far more than does polyphyletic grouping. For example, classifying a newly discovered grass in the monophyletic familyPoaceae, the true grasses, immediately results in numerous predictions about its structure and its developmental and reproductive characteristics, that are synapomorphies of this family. In contrast, Linnaeus' assignment of plants with twostamens to the polyphyletic class Diandria, while practical for identification, turns out to be useless for prediction, since the presence of exactly two stamens has developed convergently in many groups.[10]
Species have a special status in systematics as being an observable feature of nature itself and as the basic unit of classification.[11] It is usually implicitly assumed that species are monophyletic (or at leastparaphyletic). However,hybrid speciation arguably leads to polyphyletic species.[12] Hybrid species are a common phenomenon in nature, particularly in plants wherepolyploidy allows for rapid speciation.[13] Somecladist authors do not consider species to possess the property of "-phyly", which they assert applies only to groups of species.[14][15]
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