Inphylogenetics, asister group orsister taxon, also called anadelphotaxon,[1] comprises the closest relative(s) of another given unit in anevolutionary tree.[2]
The expression is most easily illustrated by acladogram:
| Sister group relationships |
Taxon A and taxon B are sister groups to each other. Taxa A and B, together with any otherextant orextinct descendants of theirmost recent common ancestor (MRCA),[Note 1] form amonophyletic group, theclade 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 theleaves and among larger, more deeplyrooted clades. The tree structure shown connects through its root to the rest of theuniversal tree of life.
Incladistic standards, taxa A, B, and C may represent specimens,species,genera, or any other taxonomic units. If A and B are at the same taxonomic level, terminology such assister species orsister genera can be used.
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The termsister group is used inphylogenetic analysis, however, only groups identified in the analysis are labeled as "sister groups".
An example isbirds, whose commonly citedliving sister group is thecrocodiles, but that is true only when discussingextant organisms;[3][4] when other, extinct groups are considered, the relationship between birds and crocodiles appears distant.Although the bird family tree is rooted in thedinosaurs, there were a number of other, earlier groups, such as thepterosaurs, that branched off the line leading to the dinosaurs after the last common ancestor ofbirds and crocodiles.[5]
The termsister group must thus be seen as a relative term, with the caveat that the sister group is only the closest relative among the groups/species/specimens that are included in the analysis.[6]
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