Outgroup (cladistics)

In cladistics or phylogenetics, an outgroup 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. Character states present in the ingroup but absent in the outgroup are (often) synapomorphies that provide empirical support for the inferred monophyly of the ingroup; character states that are present in the outgroup and some members of the ingroup are symplesiomorphies, and their complementary synapomorphies shared among some members of the ingroup provide hypotheses of relationship within the ingroup clade. 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.