Outgroup (cladistics)

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In cladistics or phylogenetics, an outgroup is a 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:

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