A sister group or sister taxon is a systematic term from cladistics denoting the closest relatives of a group in a phylogenetic tree.^[1] 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 represents species, 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 labelled as sister groups. A commonly cited example is birds, whose sister group is crocodiles.^[2][3] This is however only true when dealing with extant taxons. 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.^[4] 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/specimen included in the analysis.^[5]

References[link]