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 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. 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/specimen included in the analysis.
Famous quotes containing the words sister and/or group:
“I should fear the infinite power and inflexible justice of the almighty mortal hardly as yet apotheosized, so wholly masculine, with no sister Juno, no Apollo, no Venus, nor Minerva, to intercede for me, thumoi phileousa te, kedomene te.”
—Henry David Thoreau (18171862)
“We often overestimate the influence of a peer group on our teenager. While the peer group is most influential in matters of taste and preference, we parents are most influential in more abiding matters of standards, beliefs, and values.”
—David Elkind (20th century)