In phylogenetics , a sister group or sister taxon , also called an adelphotaxon , comprises the closest relative(s) of another given unit in an evolutionary tree .
6-644: Actinopteri ( / æ k t ɪ ˈ n ɒ p t ə r aɪ / ) is the sister group of Cladistia ( bichirs ) in the class Actinopterygii (ray-finned fish). Dating back to the Permian period, the Actinopteri comprise the Chondrostei ( sturgeons and paddlefish ), the Holostei ( bowfins and gars ), and the teleosts ; in other words, all extant ray-finned fish other than the bichirs. In this clade
12-417: Is most easily illustrated by a cladogram : Taxon A Taxon B Taxon C More tree branches Taxon A and taxon B are sister groups to each other. Taxa A and B, together with any other extant or extinct descendants of their most recent common ancestor (MRCA), form a monophyletic group,
18-564: The clade AB. Clade AB and taxon C are also sister groups. Taxa A, B, and C, together with all other descendants of their MRCA form the clade ABC. The whole clade ABC is itself a subtree of a larger tree which offers yet more sister group relationships, both among the leaves and among larger, more deeply rooted clades. The tree structure shown connects through its root to the rest of the universal tree of life . In cladistic standards, taxa A, B, and C may represent specimens, species , genera , or any other taxonomic units. If A and B are at
24-643: The lungs have evolved into a swim bladder . Characters of Actinopteri include paired fin structure, a single dorsal fin, fulcra, a large median gular and numerous branchiostegals, ganoid scales, and a valvula cerebelli in the brain. The following cladogram summarizes the evolutionary relationships of extant Actinopteri. Divergence time for each clade in mya are based on: Acipenseriformes ( sturgeons , paddlefish ) [REDACTED] Teleostei 310 mya [REDACTED] Amiiformes ( bowfin ) [REDACTED] Lepisosteiformes ( gars , alligator gars ) [REDACTED] Sister group The expression
30-410: The relationship between birds and crocodiles appears distant. Although the bird family tree is rooted in the dinosaurs , there were a number of other, earlier groups, such as the pterosaurs , that branched off of the line leading to the dinosaurs after the last common ancestor of birds and crocodiles . The term sister group must thus be seen as a relative term, with the caveat that the sister group
36-428: The same taxonomic level, terminology such as sister species or sister genera can be used. The term sister group is used in phylogenetic analysis , however, only groups identified in the analysis are labeled as "sister groups". An example is birds , whose commonly cited living sister group is the crocodiles , but that is true only when discussing extant organisms ; when other, extinct groups are considered,
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