Cladistic principles
Groups must be monophyletic — include common ancestor and all descendants. Paraphyletic groups (excluding some descendants) are rejected in modern systematics.
Homoplasy vs synapomorphy
Synapomorphies are shared derived characters supporting clades. Homoplasy is similarity from convergence or reversal — confuses classification if not analyzed carefully.
Practical impact
Field guides and floras update names as phylogenies improve — students must check current accepted names in databases like Plants of the World Online.