Controversies over taxonomic and nomenclatural instability: an empirical approach

Abstract: Taxonomic names serve two important functions: they reflect hypotheses about the existence of taxa, and they serve as the primary means to communicate about biodiversity. The dual purpose of names is a source of conflict and misunderstanding among taxonomists and end-users of taxonomy. In recent years, proposals have been made to curb nomenclatural change both at the species level and at higher taxonomic levels. However, the causes and extent of taxonomic and nomenclatural instability are poorly known. In this thesis, I focus on two major controversies in systematics. The first of these deals with the question whether taxonomic changes at the species level are real or merely caused by a shift of the species concept. The second deals with the question whether the current rank-based (Linnaean) nomenclatural system should be replaced by an alternative nomenclatural system. Both debates have been dominated by conflicting theoretical arguments in high-profile journals, but with very little input from quantitative empirical studies.            In manuscript 1, I address the highly influential claim that recent increases of the number of vertebrate species are not real but are due to reinterpretations of previous data under a Phylogenetic Species Concept (PSC). To this end, I examine 747 proposals to change the taxonomic rank of birds in the period 1950–2007. The trend to recognize more species of birds started at least two decades before the introduction of PSCs. Most (85%) newly recognized species were supported by new taxonomic data. Proposals to recognize more species resulted from application of all six major taxonomic criteria. Many newly recognized species (63%) were not based exclusively on PSC-based criteria (diagnosability, monophyly and exclusive coalescence of gene trees). Therefore, this study finds no empirical support for the idea that the increase in species is primarily epistemological rather than data-driven. This study shows that previous claims about the causes and effects of taxonomic inflation lack empirical support. I argue that a more appropriate term for the increase in species is ‘taxonomic progress’.            In manuscript 2, I present a quantitative analysis of nomenclatural instability in birds. The dataset included 826 name-taxon associations in seven major classifications of birds published between 1934 and 2007. High levels of synonymy (38% of taxa, affecting 68% of names) and homonymy (18% of names, affecting 46% of taxa) were found. On average, supra-generic taxa accepted in all seven classifications are known by 3.3 different names, and very few (2%) of these taxa are known by a single name. A significant inverse relationship between taxonomic stability and nomenclatural stability was found. Furthermore, each new classification introduced additional names for previously recognized taxa and re-applied previous names to other taxa. Overall, 94% of synonyms and 69% of homonyms were caused by differences in opinion among taxonomists about the rank of taxa. In addition, variation in the taxonomic contents of names did not become less with increased recognition of names. These findings argue against recent claims that taxonomists using rank-based nomenclature spontaneously settle on a consensus about the choice of taxon names. These results further indicate that rank-based nomenclature so far has failed to accomplish a reasonably stable association of taxonomic names and clades.            These studies show that at species rank, there is a strong empirical basis for taxonomic (and hence nomenclatural) change. As a consequence, pleas for stability at the species level are unrealistic. However, at higher ranks, the empirical basis for nomenclatural change is weak, and thus attempts should be made to curb unnecessary nomenclatural instability (e.g. by adopting phylogenetic nomenclature). These results underscore that empirical studies of taxonomic practise may usefully inform theoretical debates.