Avian species described on the basis of DNA only.
In addition, even in divergent genealogies (phylogenies), the inheritance of polymorphisms with subsequent sorting in different lineages can obscure relationshipsa. Thus, species monophyly is a difficult criterion to apply using currently available cladistic methodologies, and even if monophyly could be accurately assessed this would still leave the problem of establishing exclusivity of descent. It remains to be seen whether protocols can be developed for studying species monophyly and exclusivity (approaches based on genetic coalescence seem likely candidates) and whether these will be sufficiently practical to gain acceptance in the systematic community. As well as the operational problems summarized above, acceptance of the monophyletic species definition would involve a significant shift in taxonomic practice. For example, when a small group of related founders leaves a monophyletic ‘species’ and forms a descendant ‘species’, the ancestral ‘species’ will cease to be monophyletics,15. There are three ways to accommodate such a pattern in a hierarchical taxonomic nomenclature: (I) recognize both as species, thereby discarding the monophyly criterion; (2) treat the descendant as a distinct species only when the ancestor becomes monophyletic through interbreeding and extinction (as will happen eventuallys); or (3) recognize the ancestor as some entity other than a species. This last alternative has gained the most support among proponents of the monophyly criterion, and the term ‘metaspecies’ has been proposed for nonmonophyletic, ancestral groups existing at the same hierarchical level as species4fsf1* (but note that authors differ slightly in their definitions of metaspecies). The debate over phylogenetic species concepts is certain to continue to be lively. The two main approaches advocated to date diagnosabilityand monophyly-each seem to provide internally consistent definitions of the species category. Thus, despite claims to the contrary, the outcome of the discussion will not rest on demonstrating errors in the opposing formulation but in the systematic community deciding whether the advantages of making species monophyletic justify the requisite changes in taxonomic practice. The outcome will also be affected by ongoing conflicts between alternative philosophies of systematics, in particular the question of the primacy of pattern or processs,14. The discussion cannot be restricted solely to paradigmatic, sexual species, but will need to deal with asexual organisms, fossils and hybrids, as well as the question of how the choice of species definition affects the concept of ‘speciation’. Whatever the outcome, there can be little doubt that the phylogenetic species debate will spark novel insights and enhance our understanding of genealogy, systematics and the origin of biological diversity.