ABSTRACT
This chapter is restricted to the problem of how to define species under the current codes of nomenclature. The author and colleagues made attempts to forge a species concept that is compatible with Hennigian phylogenetic systematics or cladistics. Our phylogenetic species concept (PSC) was based on a generalized view of the meaning of phylogenetic criteria at any hierarchical level. The grouping criterion was monophyly, carefully understood to mean a group that contains all and only descendants of a common ancestor, originating in a single event (Mishler & Brandon 1987). However, since taxa at all levels are monophyletic, ranking criteria are needed to decide which monophyletic groups should be named at the rank of species. As argued in the previous chapter these ranking criteria must be pluralistic, with different criteria employed in different biological situations (Mishler & Theriot 2000).
