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In cladistics or phylogenetics, an outgroup is a (monophyletic) group of organisms that serve as a reference group when determining the evolutionary relationship among three or more monophyletic groups of organisms.
The chosen outgroup is hypothesized to be closely related to the other groups but less closely related than any single one of the other groups is to each other. The evolutionary conclusion from these relationships is that the outgroup branched from the parent group before the other groups branched from each other. An outgroup may be a sister group to the groups in question or may be more distantly related.
Some examples, with outgroup on the right:
- Humans, chimpanzees — gorillas
- Placental mammals, Marsupials — Monotremes
- Tetrapoda, Actinopterygii — Elasmobranchii
- Chordates, Echinoderms — Mollusks
- Sister group, a group that may be closely related to an outgroup
- Basal (phylogenetics), lineages that branch early within a group, and may be outgroups to the rest of the group
- Plesiomorphy, an ancestral trait of an organism
- Primitive (phylogenetics), an outdated term for ancestral traits
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