系統群

Also known as monophyletic group, monophyletic taxon, evolutionary clade

thumb|400px| Cladogram (a branching tree diagram) illustrating the relationships of organisms within groups of taxa known as clades. The vertical line (stem) at the base (bottom) represents the [[last common ancestor. The blue and red subgroups are clades, each defined by a common ancestor stem at the base of its respective subgroup (branch). The green subgroup alone, however, is not a clade; it is a paraphyletic group relative to the blue subgroup because it excludes the blue branch, which shares the same common ancestor. Together, the green and blue subgroups form a clade.]]