A sister group or sister taxon is a systematic term from cladistics denoting the closest relatives of a group in a phylogenetic tree. The expression is most easily illustrated by a cladogram:
The sister group to A is B. Likewise, the sister group to B is A. These two groups, together with all other descendants of their last common ancestor, constitute a clade; its sister group is C. The whole cladogram will again be rooted in a larger tree, offering yet more further removed sister groups. As per cladistic standards, A, B, and C may here represent specimens, species or groups. In cases where they represent species, the term sister species is sometimes used.
The term "sister group" is always used in relationship to a phylogenetic analysis. Only groups considered in the analysis will be labelled as sister groups. A commonly cited example is birds, whose sister group is crocodiles. This is however only true when dealing with extant taxa. The bird family tree is rooted in the dinosaurs, making for a number of 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 included in the analysis.
References
