Abstract
Despite the traditional focus on metaphysical issues in discussions of natural kinds in biology, epistemological considerations are at least as important. By revisiting the debate as to whether taxa are kinds or individuals, I argue that both accounts are metaphysically compatible, but that one or the other approach can be pragmatically preferable depending on the epistemic context. Recent objections against construing species as homeostatic property cluster kinds are also addressed. The second part of the paper broadens the perspective by considering homologues as another example of natural kinds, comparing them with analogues as functionally defined kinds. Given that there are various types of natural kinds, I discuss the different theoretical purposes served by diverse kind concepts, suggesting that there is no clear-cut distinction between natural kinds and other kinds, such as functional kinds. Rather than attempting to offer a unique metaphysical account of ‘natural’ kind, a more fruitful approach consists in the epistemological study of how different natural kind concepts are employed in scientific reasoning.
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Notes
Ereshefsky (2007) argues against construing taxa as kinds, claiming that HPC kinds unlike individuals can be paraphyletic and even non-historical. However, this conflates the empirical question as to whether all taxa are monophyletic with the metaphysical question as to whether such taxa are kinds or individuals. Ereshefsky fallaciously concludes from the fact that some “HPC kinds need not be historical entities” that “HPC theory allows taxa to be nonhistorical entities” (p. 297; my emphasis). ‘HPC kind’ being a basic metaphysical category, some HPC kinds are historical and others are non-historical. In the special case of taxa, for empirical reasons taxa must be historical and probably exclusively monophyletic—also when viewed as HPC kinds. Similarly, being monophyletic is not part of the metaphysical category of ‘individual’, and some individuals (non-taxa) are not ‘monophyletic’.
Keller et al. (2003) and Rieppel (2006) argue that SAI is committed to a version of essentialism (features that determine a taxon’s identity), which Rieppel calls ‘origin essentialism’ (taxon’s identity is determined by its origin). Ereshefsky (2007) objects that these authors conflate origin essentialism and (outdated) qualitative essentialism. However, pointing out that origin essentialism is a variety of essentialism does not conflate origin with qualitative essentialism. Ereshefsky is obviously inconsistent when both arguing that origin essentialism and qualitative essentialism are two distinct types of essentialism and that endorsing origin essentialism (as SAI does) is not endorsing essentialism. As proponents of the view that taxa are kinds do not endorse traditional qualitative essentialism but permit relational-historical essences, they endorse the same kind of essentialism as the SAI account.
If the different persons living in a neighborhood communicate and interact to exert a social or political influence, the neighborhood manifests integrative cohesion. In contrast, in the case of a power outage, different persons in this neighborhood could respond in the same way (e.g., lighting candles, calling the electricity company). This can have an overall effect (e.g., the restoration of power), in which case the neighborhood displays response cohesion without any interaction among the various individuals (Barker and Wilson unpubl.).
Stemming from Ghiselin (1974) and Hull (1978), a common objection against viewing taxa as kinds is the idea that unlike individuals, kinds are spatiotemporally unrestricted (Ereshefsky 2007). On my account, ‘species-individual A is spatially unrestricted’ has to be translated by a kind approach as ‘the space occupied by the members of species-kind A is unrestricted’. If the putative objection is that the actual space taken up by a species-individual is restricted, so is the space taken up by the members of the corresponding species-kind. If instead the objection is construed as the idea that it is possible for members of a species-kind to occupy spatial locations far beyond the ones they actually occupy, the same applies for the species-individual. E.g., if the boundary of a taxon-individual/taxon-kind is determined by monophyly, taxon parts/members can possibly occupy any position of the universe insofar as it is empirically possible to reach that position without breaking monophyly.
While the individualism versus kinds debate is an issue about species taxa, species pluralism is a question about species categories or concepts. I endorse a plurality of species concepts (combined with a generic species concept encompassing all particular species concepts similar to de Queiroz 1999), but in addition to metaphysical arguments I emphasize that there are also epistemological reasons (Brigandt 2003). The metaphysical basis of pluralism is that there are several mechanisms generating species cohesion which sometimes do not align and delineate different species taxa, so that different species concepts can focus on one or the other mechanism. But there are also different legitimate scientific tasks or epistemic aims pursued (taxonomic, evolutionary, ecological tasks; explanatory, operational aims); and a particular species concept may be better suited than others for such an epistemic task. Some species concepts (used in some epistemic contexts) focus on species as evolutionary units, while others (used in other contexts) focus on species as taxonomic units, which in line with the above means that species are better viewed as individuals or kinds depending on the epistemic context and species concept employed.
A law that is applied to one range of systems can be applied to many other kinds of systems. The flipside of the universality of laws is that for a particular system, several laws have to be combined in a system-specific manner.
For example, a gene is a particular stretch of DNA, yet what makes this DNA segment a gene is its causal ability to code for a functional product, which also depends on DNA elements external to this gene and on non-genetic factors (a gene can turn into a pseudogene because of changes external to it). Being a gene is a property of a DNA segment, but an extrinsic one (holding in part due to features external to the bearer of the property).
The HPC view acknowledges that such an account/concept of the natural kind, in particular its homeostatic mechanisms basis, may become available not until substantial empirical discoveries are made, long after the introduction of the term denoting the putative natural kind.
Okasha (2002) recognizes that on the traditional picture, a natural kind’s essence has two roles: (1) determining the kind’s identity, (2) causally explaining the kind’s characteristic properties. While essences have often been construed as intrinsic properties, Okasha argues (in line with my account) that the traditional account can be revised by permitting relational essences. However, he maintains that in this case essences cannot play the putative second role, based on the assumption that only intrinsic properties can underwrite generalizations and explanations. My discussion shows that he is wrong on that count. The causal effect of many biological kinds (e.g., genes producing RNAs and polypeptides) depends on their context and relations to other entities. Successful explanations in ecology, physiology, economics and other disciplines routinely invoke extrinsic factors and stable relations among entities.
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Acknowledgments
I thank Francisco Vergara-Silva and Rasmus Winther for the invitation to contribute to this special issue. I am indebted to Matt Barker, Joseph LaPorte, Alan Love, Rob Wilson, and Rasmus Winther for helpful comments on earlier versions of this paper. The work on this essay was funded with an Izaak Walton Killam Memorial Postdoctoral Fellowship by the Killam Trusts of Canada, and with a Standard Research Grant by the Social Sciences and Humanities Research Council of Canada.
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Brigandt, I. Natural Kinds in Evolution and Systematics: Metaphysical and Epistemological Considerations. Acta Biotheor 57, 77–97 (2009). https://doi.org/10.1007/s10441-008-9056-7
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DOI: https://doi.org/10.1007/s10441-008-9056-7