Abstract
After having reconstructed a minimal biological characterisation of species, we endorse an “empirical approach” based on the idea that it is the peculiar evolutionary history of the species at issue—its peculiar origination process, its peculiar metapopulation structure and the peculiar mixture and strength of homeostatic processes vis à vis heterostatic ones—that determines species’ identity at a time and through time. We then explore the consequences of the acceptance of the empirical approach in settling the individuals versus kinds dispute. In particular, while conceptual arguments have been proposed to show that species can be equally treated as individuals and kinds because mereology’s and set-theory’s languages are inter-translatable, we advance instead a causal argument to sustain the claim that each species is both a kind (i.e., a class whose members share some properties included in a cluster) and an individual (i.e., a whole made of parts).
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Notes
Even though the general species concept is quite agreed upon when species of sexually reproducing organisms are at stake, its applicability to asexual species has been questioned (see for instance Doolittle and Zhaxybayeva 2009; and see Samadi and Barberousse 2006 for a possible way to overcome this problem).
It is important to emphasise that the concepts of gene pool and gene-phene pool could be defined by taking into consideration the extant organisms of the species—like Mayr does—or by taking into account all its past, present and future members. This latter characterization clearly shows the indefiniteness of the concepts of gene pool and gene-phene pool. Furthermore, it is also important to note that, unlike Mayr, we prefer to use the term “genetic property” in order to define the concept of gene-phene pool; the reason is that this latter term is more general and neutral as it might also refer to sequences of DNA with not yet known functional role in development.
de Queiroz (1995) admits the same possibility, on the basis of mainly semantic considerations.
The historical roots of the thesis are much older, going back at least to Buffon (Gayon 1996).
We characterise a cluster in terms of intrinsic and relational properties. Wilson et al. 2007 argue that a cluster consists of both kinds of properties for the reason that “… the features that promote cohesion within a species [i.e., that keep the cluster homeostatic] are typically relational properties of conspecifics….” Our argument does not depend on privileging relational over intrinsic properties.
It is interesting to note that no definite list of homeostatic mechanisms has been provided. In our opinion, the reason for this omission is simply that homeostasis is species-dependent: the homeostatic mechanisms at play depend on the peculiar speciation process and metapopulation structure and dynamics and thus may vary over time and/or space.
We use the terms “class” and “set” as synonymous and as intensionally defined.
This is called, by Kitcher (1984), “The fallacy of incomplete translation”. Kitcher reconstructs the argument in favour of SAI as follows: Sets cannot evolve; species evolve; hence species are not sets. In the argument, “species evolve” is, according to Kitcher, left untranslated. To complete the translation, we need to consider that a species, set-theoretically conceived, is a union of subsets—or stages. A stage is the set of organisms belonging to the species which are alive at a given time. Accordingly, the complete translation of “species evolve” would be something like: the frequency of the distribution of (genetic or genetic plus phenotypic) properties at one stage will differ from the frequency of the distribution of (genetic or genetic plus phenotypic) properties at a later stage. Kitcher argues that, proceeding in this way, claims about the evolutionary behaviour of species—such as for instance speciation or extinction—may be easily expressed in set-theoretical terms.
It may be objected that the kind of cohesiveness Elredge and Gould are talking about here is response cohesion. However, their reference to a homeostatic system implies the existence of causal interactions among the members of the population beyond gene flow.
We thank a reviewer for raising this objection.
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Acknowledgements
We thank Ingo Brigandt, Kevin de Queiroz, Matthew Slater, and Achille Varzi for incisive feedback and suggestions. We also thank the anonymous reviewers for their stimulating feedback, and the audience at the EPILOG seminar (in particular Cristina Amoretti and Marcello Frixione) of the University of Genoa, Italy, where the ideas proposed in this article starting to take shape. We acknowledge the financial support of the Fundação para a Ciência e a Tecnologia (BIODECON R&D Project Grant PTDC/IVC-HFC/1817/2014). Davide Vecchi also acknowledges the financial support of the Fundação para a Ciência e a Tecnologia (Grant No. SFRH/BPD/99879/2014; Grant No. UID/FIL/00678/2019).
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Casetta, E., Vecchi, D. Species are, at the same time, kinds and individuals: a causal argument based on an empirical approach to species identity. Synthese 198 (Suppl 12), 3007–3025 (2021). https://doi.org/10.1007/s11229-019-02199-5
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DOI: https://doi.org/10.1007/s11229-019-02199-5