In cladistics or phylogenetics, an outgroup is a (monophyletic) group of organisms that serves as a reference group for determination of the evolutionary relationship among three or more monophyletic groups of organisms.
The chosen outgroup is hypothesized to be rather closely related to the other groups, but less closely than any single one of the other groups is to each other. The evolutionary conclusion from this is that the outgroup branched from the parent group before the other two groups branched from each other.
Some examples, with outgroup on the right:
- Humans, chimpanzees — gorillas
- Placental mammals, Marsupials — Monotremes
- Tetrapoda, Actinopterygii — Elasmobranchii
- Chordates, Echinoderms — Mollusks