According to the species-as-individuals thesis (hereafter S-A-I), species are cohesive entities. Barker and Wilson recently pointed out that the type of cohesion exhibited by species is fundamentally different from that of organisms (paradigmatic individuals), suggesting that species are homeostatic property cluster kinds. In this article, I propose a shift in how to approach cohesion in the context of S-A-I: instead of analyzing the different types of cohesion and questioning whether species have them, I focus on the role played by cohesion in the identity of individuals. This shift allows us to recognize why cohesion matters to S-A-I, as well as to reconceive the analogy between species and organisms (paradigmatic individuals), and also allows us to highlight the context sensitivity of both “cohesion” and “individuals.” From this perspective, I identify two problems in Barker and Wilson’s argumentation. Firstly, the authors fail to recognize that species are individuals even if they do not have the same type of cohesion that organisms have. Secondly, their argument relies on a misinterpretation of S-A-I. I conclude that species cohesion is still best framed as a feature of species individuality rather than a feature of species as homeostatic property cluster kinds. The arguments presented here contribute to the re-articulation and reevaluation of S-A-I in the face of contemporary discussions.
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Barker and Wilson’s (2010) paper focuses mainly on the role of gene flow on species cohesion, arguing against the idea that gene flow is the main cause of cohesion. Here I am interested only in their criticism of Hull, which appears at the end of their paper.
An obvious way to block Barker and Wilson's criticism is to claim that cohesion is not a necessary feature of individuals. This view is adopted by Ghiselin (1997), and I will briefly discuss it later in the article.
This distinction is inspired by—but also slightly different from—Dawkins (1976).
This distinction is not entirely new, but explicitly based on Mishler and Brandon (1987).
Sometimes Hull seems to associate the term “cohesion” only with synchronic identity (1976, p. 183). But it is necessary to be cautious here. First, Hull takes genealogical relations as providing species identity over time (see Ereshefsky 2014). In this sense, genealogy promotes cohesion. Second, the other aforementioned processes also occur over time. For these reasons, I attach “cohesion” to identity without relating it particularly to synchronic or diachronic identity.
Hamilton et al.’s commitment to spatiotemporal relations as identity criteria for individuals is not explicit, but rather follows from another commitment they have: part-whole relations in individuals are empirical in nature and, therefore, should be understood by science. This point is also made by Haber (2015), who says that individual's part-whole relations are not in agreement with classic mereology (the logical study of part-whole relations). In this article, however, I do not intend to discuss the nature of part-whole relations. The reference to such authors here is relevant only insofar as they conceive an individual's identity as a matter of spatiotemporal relations and, as we shall see, treat “cohesion” as a context-sensitive term.
The same type of context sensitivity in regard to “individuals” has been highlighted by Ereshefsky and Pedroso (2015). The authors point out that the sort of relation that matters for individuality depends on the particular type of individual being discussed (e.g., natural selection, systematics etc).
Such properties are considered to be intrinsic, existing inside each member of a class (Wilson et al. 2007). Hence, in this traditional account of classes, relational properties are not allowed.
I do not claim that cohesion establishes all the necessary and sufficient identity criteria for individuals, but rather that it establishes a necessary sort of identity criterion for them, namely, spatiotemporal part-whole relationships.
Systematists rely on properties in order to diagnose species and to construct classifications. Notwithstanding, they usually imply that species are organized in terms of genealogical and reproductive relations (e.g., Simpson 1961; Hennig 1966). Such relations are examples of spatiotemporal part-whole relationships (see also Haber 2015).
Such differences can lead to further differentiation and speciation. Nonetheless, they are not enough to distinguish populations into different species.
Two observations give more clarity to this claim. Firstly, Barker and Wilson assume that spatial isolation prevents the biologically relevant causal interaction (physical and biochemical) among conspecific organisms and populations. Secondly, Barker and Wilson admit that some species can have biologically relevant integrative cohesion (2010, p. 77). They argue that, even in these cases, integrative cohesion is not typical of those species.
Contra Barker and Wilson, another line of reasoning could also be pursued here. It could be argued that many multicellular organisms are individuals even though they lack integrative cohesion. Interestingly, examples of such organisms—such as slime molds—were provided by Hull (1978). I thank one of the reviewers for pointing this out.
As one reviewer has pointed out, the appeal to “local integrative cohesion” in species cohesion can blur the very distinction between integrative cohesion and responsive cohesion. This observation has important consequences, since it can suggest that the difference between integrative and responsive cohesion is a matter of degree not kind. I am sympathetic to this suggestion. Unfortunately, I cannot develop it here. At the same time, this suggestion does not affect my argument. Barker and Wilson claim that, to be individuals, a species needs to have integrative cohesion connecting all its organisms/populations (global integrative cohesion). I think this claim is wrong, but I remain neutral about the importance of “local integrative cohesion” to species cohesion. In this paper, whenever I use both “integrative cohesion” and “responsive cohesion” I am referring to the “global” versions of these theses.
It must be recognized how limited is the example of the power cut. For instance, neighbors have to share some cultural similarity to be able to respond in a similar way to the cut. But this sharing seems also to be required for many causal interactions between neighbors as well (e.g., organizing a protest). Additionally, it is not clear how to individuate the “stimuli-response” chain. Is the power cut the only external cause leading to neighbors responding similarly on the occasion of a power cut? These problems make it hard to appreciate the complex relation between responsive and integrative cohesion, as well as their subtle differences and variations.
Spatiotemporal relations have priority over similarity in species cohesion, but it is still an open question whether species cohesion requires some degree of similarity. I remain neutral about the necessity of similarity to species cohesion. Nonetheless, my argumentative strategy here consists in showing that even if species have responsive cohesion in the precise way Barker and Wilson define, they are still individuals.
It could be replied by Barker and Wilson that Hull indeed overlooks that homeostasis and common selective pressure do not promote integrative cohesion. But this reply is based on a very uncharitable reading of the philosopher. Furthermore, it would ignore that Hull cites the work of biologists such as Ehrlich and Raven (1969), which shows the cohesive effects in isolated conspecific populations being exposed to similar selective regimes. Citations such as this rather suggest that Hull is aware of the fact that species cohesion need not imply causal interactions among cospecific organisms and populations.
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For providing helpful comments on early versions I thank Thomas Reydon, Matthew Haber, and Matthew Barker. I also thank two anonymous reviewers for their valuable comments. For providing financial support I thank the Brazilian National Council for Scientific and Technological Development (Conselho Nacional de Desenvolvimento Científico e Tecnológico—CNPq), grant no. 200188/2014-3.
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Neto, C. Rethinking Cohesion and Species Individuality. Biol Theory 11, 138–149 (2016). https://doi.org/10.1007/s13752-016-0243-5