Editing Cladogram (section)

===Plesiomorphies and synapomorphies===

Researchers must decide which character states are "ancestral" (''[[plesiomorphy|plesiomorphies]]'') and which are derived (''[[synapomorphy|synapomorphies]]''), because only synapomorphic character states provide evidence of grouping.<ref>{{cite book |last=Hennig |first=Willi |title=Phylogenetic Systematics |year=1966 |publisher=University of Illinois Press |author-link=Willi Hennig}}</ref>  This determination is usually done by comparison to the character states of one or more ''outgroups''.  States shared between the outgroup and some members of the in-group are symplesiomorphies; states that are present only in a subset of the in-group are synapomorphies.  Note that character states unique to a single terminal (autapomorphies) do not provide evidence of grouping.  The choice of an outgroup is a crucial step in cladistic analysis because different outgroups can produce trees with profoundly different topologies.