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, where 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 last common ancestor, constitute a clade, here 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 further removed from the leaf nodes, such as A, B and C. In cladistic standards, A, B, and C may represent specimens, species, taxon-groups, etc. Where they represent species, the term sister species is sometimes used.
The term "sister group" is used in phylogenetic analysis, where only those groups identified in the analysis are labelled as sister groups. An example is in birds, whose sister group is commonly cited as the crocodiles; but this 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 included in the analysis.
- Eernisse, D.J. "Introduction to Phylogeny: What is a Sister Taxon?". Biology 404 - Evolution. Department of Biological Science, California State University, Fullerton. Retrieved 4 January 2012.
- Padian, Kevin; Lindberg, David R.; Polly, Paul David (1 May 1994). "Cladistics and the Fossil Record: The Uses of History". Annual Review of Earth and Planetary Sciences. 22 (1): 63–89. doi:10.1146/annurev.ea.22.050194.000431.
- Kemp, T.S. (1 January 1988). "Haemothermia or Archosauria? The interrelationships of mammals, birds and crocodiles". Zoological Journal of the Linnean Society. 92 (1): 67–104. doi:10.1111/j.1096-3642.1988.tb01527.x.
- Hughes, J.M. "Ancient bird-crocdilian ancestor uncovered". AVES VITAE - The lives of birds. Retrieved 4 January 2012.
- Podani, J. (2010). "Taxonomy in Evolutionary Perspective - An essay on the relationships between taxonomy and evolutionary theory". Synbiologia Hungarica. 5: 1–42.