Regular Article

Acta Biotheoretica

, Volume 57, Issue 1, pp 201-248

First online:

Species as Explanatory Hypotheses: Refinements and Implications

  • Kirk FitzhughAffiliated withResearch & Collections Branch, Natural History Museum of Los Angeles County Email author 

Rent the article at a discount

Rent now

* Final gross prices may vary according to local VAT.

Get Access


The formal definition of species as explanatory hypotheses presented by Fitzhugh (Marine Biol 26:155–165, 2005a, b) is emended. A species is an explanatory account of the occurrences of the same character(s) among gonochoristic or cross-fertilizing hermaphroditic individuals by way of character origin and subsequent fixation during tokogeny. In addition to species, biological systematics also employs hypotheses that are ontogenetic, tokogenetic, intraspecific, and phylogenetic, each of which provides explanatory hypotheses for distinctly different classes of causal questions. It is suggested that species hypotheses can not be applied to organisms with obligate asexual, parthenogenetic, and self-fertilizing modes of reproduction. Hypotheses explaining shared characters among such organisms are, instead, strictly phylogenetic. Several implications of this emended definition are examined, especially the relations between species, intraspecific, and phylogenetic hypotheses, as well as the limitations of species names to be applied to temporally different characters within populations.


Abduction Inference Ontogeny Species Species concepts Taxa Tokogeny Phylogeny Systematics