Determining species status is logically a two-step procedure involving first grouping and then ranking. It follows, I think, that there must be two parts to a complete concept or definition of species (or of any other taxonomic category)—a group-forming criterion and a criterion for assigning rank. […] I can now state the grouping criterion of the phylogenetic species concept. According to the phylogenetic concept only monophyletic groups are acceptable; “positively” para- or polyphyletic groups are not.