A Review of Current Classification Methods with Particular Reference to African Passerids (Aves)

Passerid birds are cosmopolitan, with an African origin that dates back to the Early Tertiary. Whereas primitive representatives were insectivores and frugivores, the Miocene rise and spread of savanna grassland and adaptation to granivory resulted in an explosive radiation that is not reflected in the systematics of the group. Since evolution is complex, a classification that accurately replicates genealogy must be complex also, and far more taxa are required to depict this passerine diversification than are currently admitted. Although most of these names are available, a failure to understand the true purpose of taxonomy has led to their obfuscation by taxonomic lumping and subjective synonymy. So as to determine genealogy, Darwinian (phylogenetic) taxonomy requires identification of primitive and derived characters for correct taxonomic placement, since weighting of these two character-sets greatly influences classification. Moreover it demands cognisance of the phenotypic discontinuities created by extinct and unknown taxa that represent the majority sample (~90%). These form the twigs, stems and branches of the evolutionary tree, and are of paramount importance in replicating ancestry and determining taxonomic rank. Although cladograms are the prevailing method of depicting inferred evolutionary relationships, their use for classification produces incongruous associations. This is because the gaps separating adjacent branches of a cladogram represent phenotypic discontinuities of variable magnitude, that are different for every branch of every cladogram and range from species to families. Equalisation of these internodal gaps draws taxa far closer phylogenetically than they are biologically, so that the use of cladograms and PAUP analysis for classification produces erroneous taxonomic associations. Cladograms provide only an indication of affiliation among the sample under consideration, not close taxonomic bond. Linnaean and Darwinian taxonomy are not different classification systems, they merely draw the cut-off line at different levels in the evolutionary tree; Linnaean taxonomy trims the outer growth whereas Darwinian taxonomy insists on its retention. Since all evolution is a continuum, it is suggested that quantum evolution and punctuated equilibrium are artificial constructs resulting from taxonomic lumping, phenotypic discontinuities and the alternating environmentally-stimulated processes of cladogenesis and anagenesis. The biological species concept is shown to have been fallacious since the day of inception, and the subjectivity of taxa construction is emphasized. Molecular classifications have produced some of the most incongruous taxonomic associations ever to have afflicted avian systematics and, to date, have failed to produce a natural classification. Phenotypic analysis remains the only method capable of tracking close evolutionary relationships and, with due attention to “trifling characters”, accurately replicating the finest intricacies of the evolutionary process necessary to achieve a “natural classification”.