XX.3 Theoretical concepts of species are reflected in the way in which individual species are defined in taxonomic practice
Basically, the question “what is a species” is ambiguous.On the one hand, we can ask what is the taxonomic category of a species, in what does it differ from other types of taxons, i.e. from a subspecies, the genus, family, etc.So far, we have been concerned with just this question.However, we can pose the same question as to how to define, i.e. delimit a certain species taxon in nature, i.e. which individuals to include in it and which not (Mayr 1982).The questions related to the concept of a species as taxonomic category and the conception of a species as specific taxons occurring in nature are, of course, related and, moreover, the answers to them will overlap to a certain degree.Theoretical understanding of the phenomenon of the existence of species determines, amongst other things, which criteria are considered to be decisive for delimiting the boundaries of species in real nature.It is apparent that a nominalist will attempt to define species on the basis of different criteria than an essentialist, which will again be different from those favored by the proponents of a historical concept of a species or of some variants of the model of species cohesion.However, it must also be realized that a theoretically based selection of criteria decisive for the definition of a species need certainly not agree with the selection of criteria according to which a specific taxonomist would actually define species in his professional work.Here, the situation is surprisingly much simpler than in the area of theory – for purely pragmatic reasons the typological definition of a species clearly prevails here.