‘Species’ without species

https://doi.org/10.1016/j.shpsa.2021.03.006Get rights and content

Highlights

  • Species concepts have a patchwork structure (structured polysemy).

  • This patchwork structure guides research into speciation processes.

  • Species concepts serve important functions even in the absence of species.

Abstract

Biological science uses multiple species concepts. Order can be brought to this diversity if we recognize two key features. First, any given species concept is likely to have a patchwork structure, generated by repeated application of the concept to new domains. We illustrate this by showing how two species concepts (biological and ecological) have been modified from their initial eukaryotic applications to apply to prokaryotes. Second, both within and between patches, distinct species concepts may interact and hybridize. We thus defend a semantic picture of the species concept as a collection of interacting patchwork structures. Thus, although not all uses of the term pick out the same kind of unit in nature, the diversity of uses reflects something more than mere polysemy. We suggest that the emphasis on the use of species to pick out natural units is itself problematic, because that is not the term’s sole function. In particular, species concepts are used to manage inquiry into processes of speciation, even when these processes do not produce clearly delimited species.

Introduction

Scientific concepts are tools designed to serve the purposes of scientific inquiry, shaped both by the nature of the world and by the aims of research. But the world is complex, and the aims of inquiry are myriad. Accordingly, central scientific concepts frequently develop complicated internal structures (Wilson, 2006).

The proliferation of species concepts in biology furnishes an excellent example. Biologists divide the living world into species for multiple reasons: as part of a general reference and classification system (Hennig, 1965), as units of analysis for investigating biological processes (Dobzhansky, 1935), for use in clinical settings (Suárez, 2016), and more. Beyond this diversity of aims, biologists also confront the diversity of the living world, which contains creatures that vary along every parameter (e.g., mode of reproduction, geographical dispersion, and ecological variability) relevant to delineating species. Even considering only the aim of picking out evolutionary units, biologists confront the fact that evolutionary processes are themselves shaped by evolved, contingent features of organisms (Beatty, 1995; McConwell, 2019). Thus, no single species concept can “reach across all domains of life” (O’Malley, 2014; cf. Wilkins, 2003), and none has gained universal acceptance. Despite this, biologists manage to use the term ‘species’ without undue confusion (Hey, 2006).

We aim to identify order within the bewildering array of species concepts, in a manner that does justice to the term’s communicative functions. Philosophical discussion often takes the form of a debate between monism and pluralism, with monists arguing that a single concept can handle the diversity of life and pluralists arguing that multiple concepts are required (Ereshefsky, 2017; sec. 3). Our interest, by contrast, lies in conceptual structure and application: how do different uses of the term hang together, and how does this structure assist biological inquiry?

We argue that species concepts form a set of interacting patchworks. This involves two claims. The first is that particular species concepts (e.g., the biological and ecological species concepts) are best viewed as patchworks (§4; patchworks are introduced in §3). The second claim is that these patchworks interact in theoretically fruitful ways (§5). Detailing the full set of interactions between all species concepts is beyond the scope of a single paper, so we will focus on the application of species concepts to a domain that has been the site of a great deal of controversy: prokaryotes.

Prokaryotes pose special challenges for thinking about species (Konstantinidis, Ramette, and Tiedje 2006; Franklin, 2007; O’Malley & Dupré, 2007; Morgan & Brad Pitts, 2008; Doolittle, 2009; Doolittle & Zhaxybayeva, 2009; Fraser et al., 2009; Ereshefsky, 2010). Many species concepts were developed primarily with large, multicellular (“macrobrial”) eukaryotes in mind, and are ill-equipped to handle the peculiarities of prokaryotes. For instance, the biological species concept’s emphasis on sexual reproduction is not applicable to asexual prokaryotes. Applying species concepts designed for sexually reproducing macrobes to prokaryotes requires adapting those concepts to local features of the new domain, and this generates patchwork structures.

Faced with complex concepts, which inherently involve some degree of polysemy, it is natural to wonder whether we might not be better off replacing them with a suite of disambiguated, monosemous concepts (Taylor & Vickers, 2017). Accordingly, we argue, not only that ‘species’ has a particular complex structure, but that this structure is beneficial and worth preserving (§§6–7). Our argument for this latter point is two-fold: we show that there are several ways in which the various uses of ‘species’ are tied together, and we argue that the complex structure generated by these ties serves useful communicative functions. Arguments for eliminating the species concept (notably, those of Ereshefsky, 1992, Ereshefsky, 1998, Ereshefsky, 2010) rest, in part, on a more limited view of what can serve to unify the uses of a term. In light of this, we take Ereshefsky’s arguments, and in particular his emphasis on the role of species concepts in picking out natural units, as a running foil for our approach.

A quick terminological note: on our view, there is an overarching but patchy species concept, under which fall the biological species concept, the ecological species concept, and so on. Where we can do so without generating confusion, we refer to both as “concepts”. Where necessary, however, we refer to the latter as “subconcepts”.

Section snippets

Species problems

The diversity of extant species concepts generates an ontological and an epistemological task. The ontological task is to determine how the term ‘species’ links up to the world. Do all species share some property or set of properties, or do different species concepts pick out fundamentally different types of units? The epistemological task is to bring order to the diversity of species concepts. Is one of the extant concepts superior to the others, or else can the diversity be subsumed under a

Conceptual patchworks

Conceptual patchworks are formed by extending concepts to new domains—when “a given domain of linguistic application enlarges into a neighboring territory” (Wilson, 2006, p. 58). The issue of temporal order here is not especially crucial: a concept might be used across a wide variety of domains in non-scientific contexts before it is ever made precise (e.g., ‘hard’; see below), or it might develop a specialized use in one domain and then be extended to a new domain (as in the examples below).

Applying species concepts to prokaryotes

Below, we illustrate patchwork formation in the case of the biological and ecological species concepts. Our argument in this paper is twofold: a descriptive analysis of the structure of ‘species’ and its subconcepts, and a normative defense of the value of this complex structure. Here, we are concerned only with the former. The descriptive analysis itself has two parts: (1) an analysis of particular ‘species’ subconcepts as individually possessing patchwork structures, and (2) an analysis of

Interacting patchworks

Thus far, we’ve shown that particular species concepts can be understood as having a patchwork structure, completing the first part of our descriptive analysis. This establishes that BSC and ESC, while each in an important sense polysemous, are polysemous in a structured way. It does not, however, establish the unity of ‘species’ in general, understood as subsuming BSC and ESC (inter alia). We now zoom out a level, turning to ask whether there’s any sense in which the various subconcepts are

‘Species’ without species

One perplexing feature of Darwin’s On the Origin of Species is that “early on and throughout the Origin Darwin denies the reality of species” (Beatty, 1982, p. 215; but see Wilkins, 2009, pp. 129–63; Zachos, 2016, sec. 2.3 for different interpretations). On Beatty’s interpretation, Darwin seems to claim both that “species originate and evolve naturally” and that “species are not real” (Beatty, 1982, p. 215). One way to reconcile this is to recognize that while Darwin had much to say about

Semantic patchworks and linguistic management

But perhaps this prima facie justification of the complexity of ‘species’ is, in fact, defeated. Why tolerate conceptual patchworks? Why rely on concepts with complex internal structures? Precisely because distinct patches of various species concepts can be identified, one could in principle devote a separate concept to each patch, rather than deal with a single, complex concept that applies in multiple patches (Taylor & Vickers, 2017). This would allow one to dispense with any overarching

Conclusion

Doolittle (2013, 374) has suggested that considerations of the nature of prokaryotic evolution should lead to a state in which “species remains a term of use in practice but is seen as problematic as a category or ‘natural kind’.” The eliminative pluralist takes such a view to indicate the poverty of the term. We have developed an alternative semantic picture of ‘species’ that shows how it can be theoretically contentful even if it does not pick out a natural kind. We have shown that the

Acknowledgments

We thank Joyce Havstad, Jeremy Wideman, Eden McQueen, Adrian Currie, and four anonymous reviewers for comments on earlier drafts of this work. We are grateful to Fred Cohan for discussing with us the development of his thinking on prokaryotic species, and for permission to quote from his correspondence. This work was supported by the Natural Sciences and Engineering Research Council of Canada, grant number GLDSU/447989.

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