Sister group
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:
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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]
[edit] References
- ^ Eernisse, D.J.. "Introduction to Phylogeny: What is a Sister Taxon?". Biology 404 - Evolution. Department of Biological Science, California State University, Fullerton. http://biology.fullerton.edu/biol404/phylo/sisters.html. 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 2008). "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. http://onlinelibrary.wiley.com/doi/10.1111/j.1096-3642.1988.tb01527.x/abstract.
- ^ Hughes, J.M.. "Ancient bird-crocdilian ancestor uncovered". AVES VITAE - The lives of birds. http://www.avesvitae.org/www.avesvitae.org/News/Entries/2011/6/13_Ancient_Bird-Crocodilian_Ancestor_Uncovered.html. 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. http://www.scribd.com/doc/32972918/Podani-Taxonomy-in-evolutionary-perspective-Synbiol-Hung-6-1-42-2010.
