Remarkably, some cladists
have understood the union of numerical
, on the other hand, distinguish between two types of similarity - those that are derived through descent from a common ancestor (synapomorphies), and those representing retained ancestral characters (symplesiomorphies) - and argue that only the former count as evidence when assessing degrees of relatedness.
radiating into new taxa, rather than becoming extinct, as cladists
Polyphyletic species are abhorrent to cladists
- this was set up as a rule to eliminate polyphyly caused by improper classification and taxonomy, but polyphyly is also caused by hybridization and introgression of traits from one species to others, including sister clades.
, by contrast, in opting for phylogenetic criteria, assume that any adequate response to the species problem must make patterns of descent explicit.
Whereas there is a substantial body of literature investigating the ability of hierarchical systems to transmit taxonomic information--much of it embodied in the debates among cladists
, pheneticists, and evolutionary taxonomists--there seems to be little in the way of what I would view as objective analysis of the attributes of systems of nomenclature.