Conceptual issues in phylogeny, taxonomy, and nomenclature

Phylogenetic hypotheses are designed and tested (usually in implicit form) on the basis of a set of presumptions, that is, of statements describing a certain order of things in nature. These statements are to be accepted as such, no matter whatever evidence for them exists, but only in the absence of reasonably sound evidence pleading against them. A set of the most current phylogenetic presumptions is discussed, and a factual example of a practical realization of the approach is presented. A comparison is made of the three main taxonomic approaches hitherto developed, viz., phenetics, cladistics, and phylistics (= evolutionary systematics). The latter term denotes an approach that tries explicitly to represent the basic features of traditional taxonomy and particularly its use of evidence derived from both a similarity and the relatedness of the taxa involved. The phylistic approach has certain advantages in the answering of the basic aims of taxonomy. Taxonomic nomenclature is found to rely ultimately on a few basic principles. Nine of these principles are formulated explicitly: six of them are taxonomically independent, and three are taxonomically dependent, that is, they are only compatible with particular taxonomic concepts. Judging from current taxonomic practice, a taxon is neither a class nor an individual, but a continuum (a notion combining some features of both the class and the individual) of subtaxa that is delimited by a gap separating it from other such continua. The type concept is found to be the best available tool to operate within the concept of the taxon-continuum.