A
sister group or
sister taxon is a
systematic term from
cladistics denoting the closest relatives of a given unit in a
phylogenetic tree. The expression is most easily illustrated by the
cladogram, where A, B, and C each represents a group:
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
last common ancestor, constitute a
clade, here clade AB; the sister group to clade AB is C. The whole clade ABC will itself be rooted in (branched from) the larger tree, which offers yet more sister-group branches that are related but further removed from the leaf nodes, such as A and B. As per cladistic standards, A, B, and C may represent specimens,
species,
taxon-groups, etc. In cases where they represent species, the term
sister species is sometimes used.