bio

Symplesiomorphy

An ancestral trait shared by many species - an old feature all of them inherited from a distant common ancestor. The crux of cladistics is that a symplesiomorphy CANNOT be used to group. It is the trap. Because every member carries it, it cannot single out any more-closely-related subgroup; grouping by it produces artificial paraphyletic groups. Only shared *derived* traits - synapomorphies - reveal close kinship.