Species as Explanatory Hypotheses: Refinements and Implications
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Species as Explanatory Hypotheses: Refinements and Implications Kirk Fitzhugh
Received: 25 November 2008 / Accepted: 26 January 2009 / Published online: 18 February 2009 Springer Science+Business Media B.V. 2009
Abstract 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. Keywords Abduction Inference Ontogeny Species Species concepts Taxa Tokogeny Phylogeny Systematics
1 Introduction In a recent paper, Fitzhugh (2005a, p. 164, emphasis original; see also Fitzhugh 2006a, 2008a, b, c) presented the following definition of the term species: ‘‘an explanatory hypothesis of tokogenetic-based causal relationships, a ‘lineage,’ derived from tokogenetic theory, and applied to a group of individuals.’’ The line of K. Fitzhugh (&) Research & Collections Branch, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA 90007, USA e-mail: [email protected]
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reasoning upon which this point of view was developed can be summarized as follows. First, the goal of phylogenetic systematics is to causally account for the presence of shared similarities among groups of individuals to which two or more species hypotheses refer,1 and ‘cladograms’ are minimal, graphic representations of the explanatory hypotheses that provide some degree of understanding of those similarities (Fitzhugh 2005a, b, 2006a, c, d, 2008a, b, c). Given that cladograms represent a set of explanatory hypotheses, then ‘supraspecific’ biological systematics names serve to formally summarize any number of those hypotheses (Fitzhugh 2008b). Just as individual organisms are the focus of phylogenetic hypotheses, those organisms are also the focus of species. A consequence is that the same goal of providing explanatory hypotheses should also apply to species names. In fact, the line of reasoning presented by Fitzhugh (2005a, 2006d, 2008a, b, c) is consistent with Hennig’s (1966, Fig. 6) cogent recognition of three major classes of causal relationships in biological systematics
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