A sister group or sister taxon is a phylogenetic term denoting the closest relatives of another given unit in an evolutionary tree. The expression is most easily illustrated by a cladogram: A, B, and C each represent a taxon:
The sister group to A is B; conversely, the sister group to B is A. Groups A and B, together with all other descendants of their most recent common ancestor, form the clade AB. The sister group to clade AB is C.
The whole clade ABC is itself a subtree of a larger tree, which offers yet more sister group branches that are related but farther removed from the leaf nodes, such as A, B, and C.
The term "sister group" is used in phylogenetic analysis, and only groups identified in the analysis are labeled as sister groups. An example is in birds, whose sister group is commonly cited as the crocodiles, but that is true only when dealing with extant taxa. The bird family tree is rooted in the dinosaurs, making for a number of extinct groups branching off before coming to the last common ancestor of birds and crocodiles. Thus, the term sister group must be seen as a relative term, with the caveat that the sister group is the closest relative only among the groups/species/specimens that are included in the analysis.
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