Outgroup (cladistics)

A simple cladogram showing the evolutionary relationships between four species: A, B, C, and D. Here, Species A is the outgroup, and Species B, C, and D form the ingroup.

In cladistics or phylogenetics, an outgroup^[1] is a more distantly related group of organisms that serves as a reference group when determining the evolutionary relationships of the ingroup, the set of organisms under study, and is distinct from sociological outgroups. The outgroup is used as a point of comparison for the ingroup and specifically allows for the phylogeny to be rooted. Because the polarity (direction) of character change can be determined only on a rooted phylogeny, the choice of outgroup is essential for understanding the evolution of traits along a phylogeny.^[2]

^ Grimaldi, David; Engel, Michael S.; Engel, Michael S. (2005-05-16). Evolution of the Insects. Cambridge University Press. ISBN 9780521821490.
^ Farris, J. S. (1982). "Outgroups and Parsimony". Systematic Biology. 31 (3): 328–334. doi:10.1093/sysbio/31.3.328. ISSN 1063-5157.

[1] Grimaldi, David; Engel, Michael S.; Engel, Michael S. (2005-05-16). Evolution of the Insects. Cambridge University Press. ISBN 9780521821490.

[Farris1982-2] Farris, J. S. (1982). "Outgroups and Parsimony". Systematic Biology. 31 (3): 328–334. doi:10.1093/sysbio/31.3.328. ISSN 1063-5157.

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