bio

Outgroup

A species or related group known to have branched off *before* the group under study, used as an anchor to decide which character states are ancestral and which are derived. Without an outgroup, a family tree is a rootless floating diagram - you see where species differ but not which direction is "old" and which is "new." A state present in the outgroup is taken as ancestral; a state found only inside the ingroup is derived. The outgroup is the compass that orients the whole tree.