Overall similarity among organisms is not the best indicator of phylogenetic relationships. Two species may be more similar to each other than to a third because they retain ancestral character states (whereas the third has diverged), because they independently evolved similar character states (homoplasy), or because they share derived character states that evolved in their common ancestor. Only unique shared derived character states are evidence of phylogenetic relationship. Thus a monophyletic group is marked by uniquely derived character states shared by the group’s members.