More precisely, it is shared by two or more taxa and their most recent common ancestor, whose own ancestor in turn does not possess the trait. Thus, it is not just an ancestral trait, but a recently evolved distinguishing trait. It helps to distinguish between two trees of descent which would otherwise be equally likely.
- Page, Roderic D.M. and Holmes, Edward C. 1998. Molecular evolution: a phylogenetic approach. Wiley-Blackwell.