The inferential basis of species hypotheses: the solution to defining the term ‘species’

A formal definition of the term ‘species’ is presented that is logically consistent with the inferential structures that lead to other taxonomic categories in biological systematics. A species name denotes an explanatory hypothesis that accounts for specified intrinsic or relational properties of organisms that can be accounted for by way of past tokogenetic, i.e. , reproductive events. The inference of species hypotheses involves a form of non‐deductive reasoning known as abduction. Hypotheses in other taxonomic categories, such as ‘semaphoronts’ ( sensu W. Hennig, Phylogenetic Systematics , University of Illinois Press, Urbana, 1966), genera, families, etc. , exhibit the same abductive form. The differences between semaphoront, species, and supraspecific hypotheses is in the causal theories applied to infer each – these theories being ontogenetic, tokogenetic, and phylogenetic, respectively. By formally defining the term species as an explanatory hypothesis accounting for properties of organisms by way of tokogeny, there are several distinct consequences: (1) species cannot have the ontological status of individuals, and it would be inaccurate to reduce them to class constructs; (2) acknowledging species as representative of explanatory hypotheses leaves available a variety of tokogenetic theories, such as asexual, sexual, and metagenetic reproductive processes to account for relevant organismal properties; (3) removal of species hypotheses from consideration, as in the ‘least inclusive taxonomic unit’ concept of Pleijel & Rouse [ Proceedings of the Royal Society of London, Series B 267 (2000) 627; Zoologica Scripta 29 (2000) 157] is nihilistic; and (4) advocating the reduction of inferences of species hypotheses to one class of characters, e.g. , ‘DNA barcoding,’ would be irrational.