Eliminative Pluralism and Integrative Alternatives: The Case of SPECIES

Eliminative Pluralism and Integrative Alternatives: The Case of SPECIES Abstract Pluralisms of various sorts are popular in philosophy of science, including those that imply some scientific concept X should be eliminated from science in favour of a plurality of (typically more specific) concepts X1, X2, … Xn. This article focuses on influential and representative arguments for such eliminative pluralism about the concept species. The main conclusions are that these arguments fail, that all other extant arguments also fail, and that this reveals a quite general dilemma, one that poses a defeasible presumption against many eliminative pluralisms about various scientific concepts. The article ends by outlining a novel integrative alternative in defence of species. 1 Introduction 2 The species Concept, the Category ‘Species’, and the ‘Species’ Category Problem 3 What Are Eliminative Pluralism about species, and the Arguments for It? 4 Evaluation of Arguments   4.1 Splitting?   4.2 Lumping?   4.3 The eliminative pluralist’s dilemma 5 More General Lessons 6 Species Cohesion: An Integrative Alternative 7 Conclusion 1 Introduction You probably have first-hand experience of disputes about which scientific concepts are most legitimate, or best—or simply good, as it is often put—for helping predict or explain or control things. At her son’s recent medical appointment for symptoms he was experiencing, a mother asked ‘what about sensory processing disorder, should we test for that?’. The curt reply was that ‘in this country, we don’t believe in SPD’. Some medical professionals believe that sensory processing disorder is a good scientific concept, some do not. One of the most familiar threats to scientific concepts is a two-part view called eliminative pluralism. The threat can go unsuspected when it begins with just the pluralist part. Such pluralism about a concept X says that a plurality of concepts, X1, X2, … Xn, each have legitimate roles to play within some science or other. Kitcher’s ([1984]) pluralism about the concept species, for example, recognizes legitimate roles for both the concept structural similarity species and the concept phylogenetic species, and perhaps multiple types of each; yet Kitcher seems to still recognize legitimate roles for the more general concept species as well (see also Dupré [1993]). More generally, pluralisms about the concepts gene, organism, living agent, unit of selection, population, biological individual, clade, and species are all now dominant positions. A threat to a concept emerges or clarifies when the eliminative part is added. It says that the plurality X1, X2, … Xn should replace X—that there is no important explanatory, predictive, or similar role for X to play now that we have X1, X2, … Xn. This fits what some medical professionals have in mind about the initial example of sensory processing disorder, when they claim that the symptoms associated with it have disparate underlying bases—sometimes autism, other times attention-deficit disorder, other times anxiety disorder, and sometimes just slower than usual development (Zimmer and Desch [2012]). There is, they think, no explanatory, predictive, or similar gain made by relating these disparate phenomena within the concept sensory processing disorder. Whatever the concept in question, integrationism is one way of resisting such elimination, claiming there is some gain with X, in virtue of integrative relations between variables tracked by X1, X2, … Xn. This captures some uses of sensory processing disorder that oppose eliminative pluralism (Miller and Fuller [2014]), and Robert Wilson ([2005]) is an example of an integrationist about organism, species, and gene.1 When adjudicating between eliminative pluralists and their opposition, there can be much more at stake than ‘mere semantics’ or ‘logic chopping’ (also see Mishler [1999], p. 307). Eliminative pluralists about sensory processing disorder worry that that concept’s career has helped lead some parents to choose dubious treatments for their children (Heilbroner [2015]). The stakes in disputes about concepts that are more familiar in basic research can be less visible, but in some ways even more weighty. Just imagine where chemistry would be if, in the early 1800s, all engaged parties had ceased trying to justify their use of the concept fundamental system of chemical elements (which eventually took the name ‘Periodic Table’), and simply eliminated it in favour of a plurality of other concepts, such as equivalent weights system of elements derived from the work of Richter, hydrogen multiples system of elements from Prout, and triad system of elements from Döbereiner and Gmelin (Scerri [2007]). More generally, because eliminative pluralisms are about scientific concepts, and such concepts often play complex epistemic roles in science (Brigandt [2012]), we should predict that eliminative pluralisms often have important epistemic implications. For instance, with the clarity afforded by mathematics we can infer that eliminative pluralisms sometimes influence the degree to which data should be taken as evidence for a tested hypothesis (for example, see Barker [2017]). For these reasons, scrutiny of eliminative pluralisms should be more detailed, more fine-grained, than it usually is. But by choosing a target eliminative pluralism wisely, such depth of focus need not prevent arrival at conclusions of broad interest. Through focus on one concept, species (Section 2), and mostly on one target set of admirably clear, sophisticated, and influential arguments for eliminative pluralism about that concept, this article arrives at both specific and general conclusions. The target arguments are found in (Ereshefsky [1992]) (Section 3).2 My more specific conclusion is that those arguments fail to justify eliminative pluralism about species (Section 4). A more general conclusion is that all extant justifications of eliminative pluralism about species fail; more general still, I uncover a wide-ranging dilemma that confronts, at least provisionally, eliminative pluralisms about many other concepts (Section 5). The article ends by outlining an alternative view of species (Section 6), one that both respects the importance of a plurality of variables of species-hood while also suggesting these variables are integrated in ways that tell against eliminating species. 2 The Species Concept, the Category ‘Species’, and the ‘Species’ Category Problem If there is a category ‘species’ in nature, it is a different beast than any concept species. Any concept lives in our minds as a psychological entity, often developed to help us successfully interact with the world. A category ‘species’ in nature, on the other hand, is often believed to exist mind-independently. We can more precisely express this proposal in terms of stance-independence: the nature and characteristics of the category are believed to be independent of mere second-order mental attitudes about it (Shafer-Landau [2004]). The actions of mental agents like us can of course influence organisms within a species, cause whole species to go extinct, and create new species that previously were merely possible. But if the category ‘species’ is stance-independent, then without such action the second-order mental attitudes that agents have about it—their theories, desires, and feelings about the category—do not on their own influence the category’s nature or characteristics. Those who believe the concept species to be important usually believe or hope that it accurately, and stance-independently, corresponds with such a category. The infamous ‘species problem’ is, more exactly, a problem about the category ‘species’: what is the nature of this category? This can be put in other ways. What are species? What sets all species apart from all other things, including genera, sub-species, etcetera? Proposed answers typically have two parts. One is a definition of the concept species, the other is a name for this definition. For instance, according to one of Mayr’s ([2000], p. 17) definitions, a species is a group of interbreeding natural populations that is reproductively isolated from other such groups; this definition is named the biological species concept (BSC). Van Valen’s ([1976], p. 235) alternative definition, named the ecological species concept (ESC), states that a species is a ‘lineage (or a closely related set of lineages) which occupies an adaptive zone minimally different from that of any other lineage in its range and which evolves separately from all lineages outside its range’. Such definitions are not merely stipulative communication tools; often they are intended as a summary of a researcher’s theory about the very nature of the category ‘species’. It often takes careful interpretive work to formulate an accurate, fuller statement of the theory that a particular definition merely summarizes. Some interpreters do this by specifying the grouping criteria and ranking criteria that are implied by the definition they are interpreting (for example, Mishler and Donoghue [1982]; Barker [2010]). This helps clarify the differences between the different theories about the category ‘species’. Consider Figure 1a.3 The six images labelled A through F represent four actual populations of moth and two of liverworts. We can ask, do any of these populations belong to the same species group? This is a grouping question about populations. A set of species grouping criteria proposes an answer by stating the conditions that any pair of populations must meet in order to belong to the same species group. Figure 1. View largeDownload slide Depiction of how different definitions of species group and rank populations differently. (a) No definition applied. (b) BSC definition applied. Conspecific groups of populations in rectangles; the hashed rectangle encloses exactly one whole species. (c) GSC definition applied. Conspecific groups of populations in rectangles; the hashed rectangle encloses exactly one whole species. Figure 1. View largeDownload slide Depiction of how different definitions of species group and rank populations differently. (a) No definition applied. (b) BSC definition applied. Conspecific groups of populations in rectangles; the hashed rectangle encloses exactly one whole species. (c) GSC definition applied. Conspecific groups of populations in rectangles; the hashed rectangle encloses exactly one whole species. Once we believe that some populations belong to the same species group, we can ask whether those populations are constituting exactly one whole species group, a group ranked as a species rather than, say, a sub-species (which would lack some populations needed to be a whole species) or a genus (which would contain some extra populations that ensure the group is not just one species). This is a ranking question about groups of populations. A set of species ranking criteria proposes an answer by stating the conditions that any such group must meet in order to count as exactly one whole species. For instance, given various elaborations of the BSC, it is reasonable to infer that the BSC definition is a summary of a theory about the nature of the ‘species’ category that includes the following grouping and ranking criteria: BSC Grouping Criteria: any pair of populations belongs to the same species group if and only if genes can flow from one to the other either directly (for example, by sexual reproduction following immigration, or by other particular means) or indirectly (for example, via other populations). BSC Ranking Criteria: a group of populations is a species if and only if any pair of populations within it satisfies the BSC species grouping criteria, and genes cannot flow from any one of the populations in the group to any population outside the group. More simply, the theory is that species are closed gene pools. This theory’s grouping criteria imply that populations A and B in Figure 1a belong to the same species, and that C and D belong to one other species, and that E and F belong to a different other species. This is because genes can flow between A and B, and between C and D, and between E and F, but not between any other pairs of these. Figure 1b depicts this BSC grouping result by enclosing any populations that belong to the same species within a rectangle; hence, three rectangles. The theory’s ranking criteria, on the other hand, imply that only populations E and F form exactly one whole species. This is because E + F is the only depicted group of populations for which it is true that genes can flow between each population in the group, but not between these populations and any others on Earth. Figure 1b depicts this with hashed rectangle lines enclosing the group that makes up exactly one whole species. Many disagreements about the ‘species’ category stem from disagreeing grouping or ranking criteria, or both. The above BSC grouping criteria disagree with the grouping criteria summarized in many genealogy-based definitions. Those definitions deny the BSC view that genes must be able to flow between two populations in order for them to belong to the same species. Baum’s ([2009]) definition, for instance, would disagree with the BSC results in Figure 1b, and instead count all four of A, B, C, and D as belonging to the same species, because each is more closely genealogically related to each other than any of them are to any other populations, despite the fact that genes can no longer flow from A or B to C or D, nor can they flow from C or D to A or B (Brown et al. [1994]). Figure 1c shows how Baum’s definition of species, called the genealogical species concept (GSC), would draw the rectangles differently than the BSC. The final bit of context needed is a claim that Ereshefsky and many others endorse: despite disagreements between major species theorists, they typically agree on the minimal claim that species are basal lineages (Ereshefsky [1992], p. 674; de Queiroz [1998]; Padial et al. [2010], p. 2). Unfortunately, the terms ‘lineage’ and ‘basal lineage’ are ambiguous, vague, and deserve just as much scrutiny as ‘species’. For now I simply convey the rough (and perhaps, as we shall later see, no less vague) idea that a lineage is thought to be a group of populations that evolve separately from other groups of populations (Wiley [1978]; Padial et al. [2010]). One lineage can be nested inside another more inclusive one that contains additional populations that are evolving separately from still other populations (Haber [2012]). A basal lineage is a lineage with no smaller, less inclusive lineages nested inside it.4 3 What Are Eliminative Pluralism about species, and the Arguments for It? Within the foregoing context, Ereshefsky’s ([1992]) eliminative pluralism about species has two parts. The eliminativism part says we should eliminate the concept species from biological theorizing; the pluralism part says that instead of species we should use a plurality of basal lineage concepts. Here is his overall argument for this two-part view: If ‘species’ is very probably not a univocal and explanatorily or predictively significant category in nature, but in a plurality of basal lineage categories each basal lineage category has explanatory or predictive significance independently of the other basal lineage categories, then we should eliminate the concept species from biological theorizing, and instead use a plurality of basal lineage concepts. ‘Species’ is very probably not a univocal and explanatorily or predictively significant category in nature. In a plurality of basal lineage categories each basal lineage category has explanatory or predictive significance independently of the other basal lineage categories.__________________________________________________________ We should eliminate the concept species from biological theorizing and instead use a plurality of basal lineage concepts. My evaluation will mostly focus on Premise 2, then later come to involve Premise 3 as well. To begin his defence of Premise 2, Ereshefsky organizes named definitions of species into three classes. One class contains so-called interbreeding definitions, including Mayr’s BSC; these emphasize the capacity for gene flow between conspecific populations. The second class contains ecological definitions, including Van Valen’s ESC; these recognize ecological relationships, such as niche sharing, among the definitive variables. The third class contains phylogenetic and other genealogical definitions, which would include Baum’s GSC; these emphasize genealogical relations, sometimes implying that a group of populations is a species only if it is monophyletic—it contains all and only populations descending from a shared ancestor. Ereshefsky ([1992], p. 676) then claims that each class of definition identifies different types of evolutionary forces that operate on and cause the formation of basal lineages. Reproductive forces do this, as emphasized by interbreeding definitions, but so do ecological ones, as proposed by ecological definitions, and so on. A next and key move is to imply that a basal lineage’s being caused by a particular type of force makes it a different type of basal lineage than one caused by another sort of force. For example, if it is primarily reproductive forces that cause the formation of a particular basal lineage, then the basal lineage is of the interbreeding type. If ecological forces are the main causes of basal lineage formation, the basal lineage is of the ecological type. This amounts to what I call Ereshefsky’s argument for basal lineage type-plurality, which is the first part of his defence for Premise 2 in his overall argument. This argument is simplified in the left column of Table 1, Premises 2(a)–(c). Table 1. The two arguments that together are intended to defend Premise 2 in Ereshefsky’s overall argument for eliminative pluralism about the concept species Argument for basal lineage type-plurality Argument for no significant univocal ‘species’ category 2(a) If there are at least three different types of evolutionary forces operating on and causally producing basal lineages, then there are at least three different types of basal lineage. 2(b) There are at least three different types of evolutionary forces operating on and causally producing basal lineages. ____________________________ 2(c) There are at least three different types of basal lineage. 2(d) Many populations each belong to two or more different types of basal lineage that do not wholly overlap. 2(e) There is no good reason to believe we will discover a parameter common to all types of basal lineage. 2(f) If 2(d) and 2(e), then ‘species’ is very probably not a univocal and explanatorily or predictively significant category in nature. ____________________________ (2) ‘Species’ is very probably not a univocal and explanatorily or predictively significant category in nature. Argument for basal lineage type-plurality Argument for no significant univocal ‘species’ category 2(a) If there are at least three different types of evolutionary forces operating on and causally producing basal lineages, then there are at least three different types of basal lineage. 2(b) There are at least three different types of evolutionary forces operating on and causally producing basal lineages. ____________________________ 2(c) There are at least three different types of basal lineage. 2(d) Many populations each belong to two or more different types of basal lineage that do not wholly overlap. 2(e) There is no good reason to believe we will discover a parameter common to all types of basal lineage. 2(f) If 2(d) and 2(e), then ‘species’ is very probably not a univocal and explanatorily or predictively significant category in nature. ____________________________ (2) ‘Species’ is very probably not a univocal and explanatorily or predictively significant category in nature. Table 1. The two arguments that together are intended to defend Premise 2 in Ereshefsky’s overall argument for eliminative pluralism about the concept species Argument for basal lineage type-plurality Argument for no significant univocal ‘species’ category 2(a) If there are at least three different types of evolutionary forces operating on and causally producing basal lineages, then there are at least three different types of basal lineage. 2(b) There are at least three different types of evolutionary forces operating on and causally producing basal lineages. ____________________________ 2(c) There are at least three different types of basal lineage. 2(d) Many populations each belong to two or more different types of basal lineage that do not wholly overlap. 2(e) There is no good reason to believe we will discover a parameter common to all types of basal lineage. 2(f) If 2(d) and 2(e), then ‘species’ is very probably not a univocal and explanatorily or predictively significant category in nature. ____________________________ (2) ‘Species’ is very probably not a univocal and explanatorily or predictively significant category in nature. Argument for basal lineage type-plurality Argument for no significant univocal ‘species’ category 2(a) If there are at least three different types of evolutionary forces operating on and causally producing basal lineages, then there are at least three different types of basal lineage. 2(b) There are at least three different types of evolutionary forces operating on and causally producing basal lineages. ____________________________ 2(c) There are at least three different types of basal lineage. 2(d) Many populations each belong to two or more different types of basal lineage that do not wholly overlap. 2(e) There is no good reason to believe we will discover a parameter common to all types of basal lineage. 2(f) If 2(d) and 2(e), then ‘species’ is very probably not a univocal and explanatorily or predictively significant category in nature. ____________________________ (2) ‘Species’ is very probably not a univocal and explanatorily or predictively significant category in nature. Now, suppose Ereshefsky is correct about there being different types of basal lineage. Why care? The answer comes in what I call the argument for no significant univocal ‘species’ category, which is simplified in Table 1’s right column. This begins with something we saw diagrammatically earlier. The GSC definition implied that populations A and B are conspecific with populations C and D (Figure 1c), while the BSC definition implied that this is false (Figure 1b). Now put the topic of species and conspecificity aside momentarily, and focus just on types of basal lineage. If Ereshefsky is correct in his claims about these, then there is a genealogical type of basal lineage that populations A, B, C, and D are part of (Figure 1c). But there is no interbreeding lineage to which all four populations belong; instead there are two distinct and smaller interbreeding lineages (Figure 1b). Thus some particular basal lineages of different types do not wholly overlap. This entails there are some populations, such as A, which belong to two or more different types of basal lineage that overlap only partially (Ereshefsky [1992], pp. 674–5, 77). Ereshefsky ventures the empirical assumption that this is true of many populations, as indicated in Premise 2(d) in Table 1. To that premise, Ereshefsky ([1992], p. 677) adds another, Premise 2(e): there is no good reason to believe we will discover a parameter that is common to all types of basal lineage. In other words, there is very probably no one theoretically significant thing that is common to all interbreeding basal lineages, ecological basal lineages, and genealogical basal lineages. Finally, Ereshefsky believes that the lack of a common feature, coupled with the frequent phenomena of populations belonging to basal lineages that do not wholly overlap, implies that among such basal lineages there is very probably no univocal ‘species’ category that has explanatory and predictive significance. This appears as Premise 2(f) in Table 1. It then follows deductively that ‘species’ is very probably not a univocal category in nature with explanatory or predictive significance. If Ereshefsky’s arguments are sound, then the definitions proposed by the BSC, ESC, GSC, and others are each defining separate and quite unrelated concepts. The BSC’s interbreeding criteria are perhaps defining the concept interbreeding group, while the ESC’s ecological criteria are defining ecological group, and genealogical criteria are defining genealogical group, etcetera, with no ‘species’-related considerations connecting these in any interesting way. For population A to belong simultaneously to one interbreeding basal lineage, as well as to an extensionally different genealogical basal lineage, would be no more puzzling than saying that I simultaneously belong to a faculty association, and to an extensional quite different hockey team. 4 Evaluation of Arguments Evaluation of Ereshefsky’s arguments first raises two problems, and appreciating these also allows formulation of a third problem that we can generalize to other cases. 4.1 Splitting? The first problem is a splitting problem, and centres on Premise 2(a). Why should we believe that basal lineages differ in type just because different causal forces act on and produce them? This could not be successfully derived from the following very general sort of principle: Ambitious Causal Splitting Principle: For any category x, if different forces act on and produce instances of x, then there are different types of x. That principle is false. Just let x = being a kind of braided rope. Adam makes braided ropes by hand. His friend Heather sees they are popular and so designs a machine to also make them. It’s a perfect machine, producing ropes indistinguishable from Adam’s. Adam’s handiwork is one sort of causal force, the machine’s operation is another, but this causal difference by itself does not entail that the ropes produced by each are of different types. True, other considerations could be added to try providing the missing justification for their counting as different types. We could add, for instance, that people prefer the handmade ropes to the machine-made ones, even though people cannot tell the difference between them in blind tests. But such additions would amount to a different, more comprehensive, proposed sufficient condition than the one that the ambitious causal splitting principle implies is sufficient for there being different types of braided rope. This suggests we might repair Premise 2(a) by replacing it with Premise 2(a)*: Premise 2(a)*: If there are at least three different types of evolutionary forces operating on and causally producing basal lineages, and because of how these forces are different the lineages they produce as effects are themselves, qua basal lineages, also different in the following explanatorily or predictively significant ways __________________, then there are at least three different types of basal lineage. However, for the repair to be useful, the added clause would then also have to be asserted as its own new premise elsewhere in the argument. This is problematic because it is unclear how to fill in the blank in that clause. What are the explanatorily or predictively significant differences between, say, interbreeding basal lineages and ecological basal lineages? For the argument to work, these must be differences other than the mere fact that the causal sources of the lineages differed, because we are asking why difference in causal source matters at all. It is surprising that pluralist philosophers about species do not address this in detail. To clarify this outstanding task for pluralists, let me elaborate what it would take for differences in causal source to matter in the way pluralists need, using the example of interbreeding groups. Each interbreeding group is a group of populations wherein genes can flow between each of the populations (at least serially) and yet not between any of them and any populations outside the group. This is just what we mean by ‘interbreeding group’. So by definition, all interbreeding groups share these gene flow relationships and any features crucial for realizing such relationships. Call exactly these relations and features GF, for short. For GF to matter in a way that warrants distinguishing interbreeding groups as a distinctive type of basal lineage, there must be justification for believing that the following four splitter commitments are true: Typically biologically significant claim: GF in interbreeding groups always or usually produces, or in some other manner usually explains or successfully predicts, certain other features or relationships. Distinctiveness claim: At least some of the other features or relationships produced or explained or successfully predicted by GF are distinctive from features or relationships typical of other types of groups, to at least an extent or in ways that justify then distinguishing interbreeding groups from those other groups. Lineage relevance claim: The distinctive other features or relationships produced or explained or successfully predicted by GF are relevant in some clear way to being a lineage, for example, they contribute to a group’s being a lineage. Basal lineage relevance claim: The distinctive other features or relationships produced or explained or successfully predicted by GF and which are relevant in some clear way to being a lineage, are also relevant to being a basal lineage in particular.5 An analogous set of four claims will need justification for any proposed type of basal lineage group other than an interbreeding group, in order to successfully fill the blank in Premise 2(a)*. I do not claim this cannot be successfully done, just that it will take new empirically informed philosophical work to do so. So far pluralists, not just Ereshefsky, are yet to appreciate the extent of the task. For example, Kitcher ([2001]) describes how malaria researchers in Africa benefited from switching their focus on morphologically individuated groups to a focus on individuation by interbreeding. The helpful difference was that populations of mosquito grouped together on the basis of morphological similarity were assigned to different groups when switching to interbreeding criteria, because genes could not flow between the morphologically similar groups. This helped explain the previously puzzling observation that malaria was contained to only some of the populations in the larger morphological group: malaria was contained because the infecting populations could not interbreed with non-infecting ones. That explanation is, of course, important. But it needn’t have anything to do with lineages, let alone basal lineages or the ‘species’ category. The fact that some mosquito populations were reproductively isolated from morphologically similar ones helps ground the prediction that malaria vectors will not spread from one of those groups to the other, but this would be so regardless of whether either of the two are lineages or not, or species or not. It would be false, for instance, to say that the malaria researchers benefited because they switched from one basal lineage concept to another. That concept was not necessary for their progress, because any successful explanations or predictions involved in their switch were epistemically independent of any basal lineage concept. The challenge is to find lineage differences, not other differences that we have other reasons to find important. Also, these differences must not reveal that only one of the types of group in question are basal lineages after all, since it is pluralism about lineage type that Premise 2(a)* is after. The premise (in this case) requires at least three different types of basal lineage, not just one. So the mosquito case leaves the implicated versions of the lineage relevance claim, and the basal lineage relevance claim, unjustified. Less obviously, justifying the other two pluralist commitments would also need further work. The lack of gene flow relationships between two groups of morphologically similar mosquitos may predict and explain aspects of malaria distribution. But that is very particular to those mosquito populations, or at most to mosquito populations of those sorts. The explanations and predictions that matter for splitting one concept into a plurality of others would be much more general. What is it that GF always or usually distinctively explains or successfully predicts, in all or most cases of interbreeding groups, not just in the mosquito case? This sort of issue has been overlooked in pluralisms about concepts other than species as well (Barker [2013], p. 621). 4.2 Lumping? Now for the second problem. This is a lumping problem for believing that the more general category ‘basal lineage’ exists as supposed at all. The argument’s Premise 2(b) implies that this category exists. It says there are at least three different types of evolutionary forces operating on and causally producing basal lineages. In the context of the arguments, that implies that there exists an explanatorily or predictively significant ‘basal lineage’ category, whether it in fact admits of several types or not. Such a category may exist. But look at Table 1 to see that Premise 2(b)’s commitment to this, in the left-hand argument, appears in tension with Premise 2(e) plus Premise 2(f) in the right-hand argument. Premise 2(e) asserts that the different types into which this category divides have no parameter in common, which, when combined with Premise 2(f), seems to cast doubt on there being a significant and general ‘basal lineage’ category at all. Why think there is such a category if all the different types lack a common identifying feature? It is not just that the right-hand argument ironically poses this question to the left-hand argument. It is that the right-hand argument’s Premise 2(f) suggests an answer that the left-hand argument cannot live with, as Premise 2(f) implies that without a common identifying feature, the types of basal lineage that only partially overlap do not form the sort of general and scientifically significant ‘basal lineage’ category that Premise 2(b) presupposes. Notice it seems an essentialist criterion is lurking. Premise 2(f) seems to say that if the partially overlapping types of basal lineage do not share in a ‘basal lineage’ essence then no putative ‘basal lineage’ category nor ‘species’ category is explanatorily or predictively significant. And Premise 2(e) is saying that, indeed, we have no reason to believe there is such an essence, while Premise 2(b) instead seems to suggest in this context that there is such an essence. My own view is that an essentialist criterion should not be used here at all. David Hull ([1965]) made this point over fifty years ago. Moreover, it is now widely known that the prodigious variation between organisms and between populations is trouble for individuating groups essentialistically (Sober [1980]; Okasha [2002]; Barker and Velasco [2013]; cf. Devitt [2008a]), and alternative sorts of individuation are well developed to respect this empirical reality (Boyd [1999]; Wilson et al. [2007]; Slater [2015]). These alternatives vary in interesting ways, but they share in the claim that some groups are individuated by clusters of features (properties, relations, states, etcetera), with no single feature in an individuative cluster being essential or necessary for group membership, and with multiple sub-sets of the features each being sufficient for group membership. On such a view, group members tend to share many features and relationships, and this helps make the group explanatorily and predictively significant even though the feature-sharing and relation-sharing don’t live up to the standards of an essentialism on which sharing is exceptionless between group members. Can such cluster views help dissolve the tension between the left- and right-hand arguments? To answer, we need to see how this would change the right-hand argument, so consider Table 2. Table 2. Revised statements indicated by * Argument for basal lineage type-plurality Argument for no significant univocal species category* 2(a) If there are at least three different types of evolutionary forces operating on and causally producing basal lineages, then there are at least three different types of basal lineage. 2(b) There are at least three different types of evolutionary forces operating on and causally producing basal lineages. _____________________________ 2(c) There are at least three different types of basal lineage. 2(d) Many populations each belong to two or more different types of basal lineage that do not wholly overlap. 2(e)* There is no good reason to believe we will discover a parameter common to all types of basal lineage, and that rather these types will tend, that is, with some exceptions, to share many features and relationships. 2(f)* If 2(d) and 2(e)*, then ‘species’ is very probably not a univocal and explanatorily or predictively significant category in nature. _____________________________ 2 ‘Species’ is very probably not a univocal and explanatorily or predictively significant category in nature. Argument for basal lineage type-plurality Argument for no significant univocal species category* 2(a) If there are at least three different types of evolutionary forces operating on and causally producing basal lineages, then there are at least three different types of basal lineage. 2(b) There are at least three different types of evolutionary forces operating on and causally producing basal lineages. _____________________________ 2(c) There are at least three different types of basal lineage. 2(d) Many populations each belong to two or more different types of basal lineage that do not wholly overlap. 2(e)* There is no good reason to believe we will discover a parameter common to all types of basal lineage, and that rather these types will tend, that is, with some exceptions, to share many features and relationships. 2(f)* If 2(d) and 2(e)*, then ‘species’ is very probably not a univocal and explanatorily or predictively significant category in nature. _____________________________ 2 ‘Species’ is very probably not a univocal and explanatorily or predictively significant category in nature. Table 2. Revised statements indicated by * Argument for basal lineage type-plurality Argument for no significant univocal species category* 2(a) If there are at least three different types of evolutionary forces operating on and causally producing basal lineages, then there are at least three different types of basal lineage. 2(b) There are at least three different types of evolutionary forces operating on and causally producing basal lineages. _____________________________ 2(c) There are at least three different types of basal lineage. 2(d) Many populations each belong to two or more different types of basal lineage that do not wholly overlap. 2(e)* There is no good reason to believe we will discover a parameter common to all types of basal lineage, and that rather these types will tend, that is, with some exceptions, to share many features and relationships. 2(f)* If 2(d) and 2(e)*, then ‘species’ is very probably not a univocal and explanatorily or predictively significant category in nature. _____________________________ 2 ‘Species’ is very probably not a univocal and explanatorily or predictively significant category in nature. Argument for basal lineage type-plurality Argument for no significant univocal species category* 2(a) If there are at least three different types of evolutionary forces operating on and causally producing basal lineages, then there are at least three different types of basal lineage. 2(b) There are at least three different types of evolutionary forces operating on and causally producing basal lineages. _____________________________ 2(c) There are at least three different types of basal lineage. 2(d) Many populations each belong to two or more different types of basal lineage that do not wholly overlap. 2(e)* There is no good reason to believe we will discover a parameter common to all types of basal lineage, and that rather these types will tend, that is, with some exceptions, to share many features and relationships. 2(f)* If 2(d) and 2(e)*, then ‘species’ is very probably not a univocal and explanatorily or predictively significant category in nature. _____________________________ 2 ‘Species’ is very probably not a univocal and explanatorily or predictively significant category in nature. The turn away from a problematic essentialist criterion to a cluster criterion is foreshadowed in new Premise 2(e)*. But if the right-hand argument is to retain its valid form, Premise 2(f) must then become Premise 2(f)*: it must be a conditional whose antecedent states the new Premise 2(e)* rather than the old Premise 2(e). With this change, Premise 2(f)* will imply that partially overlapping types of basal lineage will not be explanatorily and predictively significant even if they can be grouped together as a cluster view suggests. This is to deny the significance of such cluster-based groups. Two objections greet the argument modifications. First, a heavy burden now falls on the proponent of the modified argument to support the denial of significance for cluster-based groups. This is because the very purpose of the development of the cluster views was to explicate the explanatory and predictive significance of groups, by paying much closer attention to work in biology than did traditional essentialists. The arguments for those cluster views show how such groups retain the significance that biologists imply they have, despite lack of essences.6 So those arguments have a long head-start against any defence of Premise 2(f)*’s denial of such significance. The second objection is more internal to the left- and right-hand arguments. Even if the first objection is met, and the significance of the cluster-based groups refuted, the tension that afflicted the left- and right-hand arguments arises in new garb. This is because Premise 2(f)* plus Premise 2(e)* continues to deny there is the general sort of ‘basal lineage’ category (whether cluster-based or essentialistic) that Premise 2(b) presupposes. 4.3 The eliminative pluralist’s dilemma The splitting problem from Section 4.2 and the lumping problem from Section 4.1 each stand on their own. But when combining considerations from them, an additional problem arises. This problem leaves fewer escape routes than either the splitting problem or lumping problem do on their own, because it takes dilemma form. The dilemma starts with two mutually exhaustive possibilities: either the different types of basal lineage that Ereshefsky or others identify do tend to share some features and relationships that are of interest to biological theorizing, or they do not. Certainly a presumption among many species theorists has been that the first of these possibilities is the case. An example of this idea is that even though an interbreeding criterion for individuating groups does not sort organisms exactly as, say, a genealogical criterion does, there are features and relationships most groups of both sorts tend to share, and at least some of these shared features and relationships are of great interest to biological theorizing—enough that we recognize most groups of both sorts as sharing in a particular property: being a species. Of course, Ereshefsky doubts that this popular presumption is correct. That reflects his eliminativism. But his view is not merely eliminativism. It is eliminative pluralism. And if he is correct to doubt the common presumption about ‘species’ and eliminate the concept species, then the other horn of the dilemma awaits the pluralism part of his view. That part commits to multiple different types of basal lineage that are each of independent biological interest. But the same doubt he raises about such interest in the case of the general categories ‘species’ and ‘basal lineage’ then arises for each of the different types of lineage he recognizes—for ‘interbreeding group’, and ‘ecological group’, and ‘genealogical group’—even if we stop subsuming them under a general basal lineage or species concept. In Section 4.1 we saw the challenges that this horn of the dilemma involves, namely, the challenges of justifying all four splitter commitments. Since there are only two possibilities—either the different types of basal lineage tend to share some features and relationships of biological interest, or they do not—and both possibilities would result in one part of Ereshefsky’s eliminative pluralism being so far unjustified, this eliminative pluralism must be so far unjustified. 5 More General Lessons Ereshefsky’s view is of quite general interest partly because the dilemma raised for it also arises, at least provisionally, for eliminative pluralisms about various scientific concepts. This clarifies upon generalizing the dilemma’s premises by using X to stand for the concept whose elimination is proposed, and X1, X2, … Xn for the plurality of concepts to be recognized instead of X. The eliminative pluralist’s dilemma, generalized: Either proposed categories X1, X2, … Xn do tend to share some features and relationships that are of general interest to scientific theorizing, or not. If X1, X2, … Xn do tend to share some features and relationships that are of general interest to scientific theorizing, then these features and relationships are a promising basis for recognizing the general superordinate concept X, and therefore the eliminative part of eliminative pluralism about concept X is not yet justified. If X1, X2, … Xn do not tend to share some features and relationships that are of general interest to scientific theorizing, raising doubt about the superordinate concept X, then it becomes strikingly unclear what warrants recognizing each putative category (X1, and also X2, … and also Xn) in the plurality of X1, X2, … Xn as having scientific interest distinct from every other category in that plurality, since the same type of doubt about the scientific interest of superordinate concept X also afflicts each category in the proposed plurality, and therefore the pluralism part of eliminative pluralism about concept X is not yet justified._______________________________________________________________ Eliminative pluralism about concept X is not yet justified. This dilemma is provisional in the sense that it poses a defeasible presumption against eliminative pluralism about this or that concept, one that can probably be defeated most readily by making a case that Premise (3) is false for the concept in question. I suspect that this is, at least implicitly, the way that successful eliminative pluralisms typically gain acceptance. Many concepts of formerly recognized diseases come to mind, such as Kitcher’s ([2001]) neurasthenia example. My arguments in Section 4.1 illustrate what is involved in defeating Premise (3) of the dilemma in this way, and suggest that Ereshefsky has yet to make such a defeat work in the case of species. What about other eliminative pluralists about species? Aside from Ereshefsky, it is curiously difficult to determine whether well-known pluralists about species are also eliminative pluralists about species. Kitcher (especially in his [1984]) is an influential pluralist, though he does not explicitly argue for elimination of species as Ereshefsky does; accordingly, Devitt ([2008b]) struggles to interpret Kitcher’s view on elimination, and decides that if Kitcher’s pluralism is eliminativist, it is the same as Ereshefsky’s, and thus their views stand or fall together. Stanford ([1995]) is sometimes considered an eliminative pluralist about species, but here it is commitment to the pluralism part that is unclear. Ereshefsky ([1998]) shows that Stanford’s anti-realist view is independent of pluralism; and in any case, Ereshefsky ([1998]) and Devitt ([2008b]) argue convincingly that Stanford’s argument fails. I read other leading species pluralists (Mishler and Donoghue [1982]; Mishler and Brandon [1987]; Dupré [1993]; Wilkins [2003]) as clearly not also eliminativists, with the interesting possible exception of Brent Mishler in his more recent work (for example, Mishler [1999]), and others who similarly call for eliminating species along with the entire Linnaean Hierarchy and all its other ranks (for example, kingdom, phylum, class, order, family, genus).7 This is interesting because although Mishler is, like Ereshefsky, willing to recognize some notion of basal lineage (‘basal taxon’; Mishler [1999], p. 307), Mishler’s argument for elimination does not depend on this notion, while Ereshefsky’s does. This helps show that the notion is inessential to the failure of eliminative pluralism about species. If Ereshefsky abandons the concept basal lineage because it seems suspiciously like the concept species, he will, like Mishler, retain the core concept lineage. The idea will then be that there are a plurality of lineage types—for example, interbreeding lineages, ecological lineages, genealogical lineages—but no basal lineages or species. But then the problem of avoiding Premise (3) in the eliminative pluralist’s dilemma remains, where now we demand to know exactly which ‘lineage features and relations’ would often be produced or explained or predicted in biologically interesting and distinct ways by interbreeding, and in other interesting and distinct ways by niche sharing, and so on. The reliance on lineage rather than basal lineage does nothing to remove this problem. With no successful version of eliminative pluralism about species so far established, I infer that all extant versions of this view are, like Ereshefsky’s, so far unjustified. 6 Species Cohesion: An Integrative Alternative None of this is to say that all species theorists should presently resist eliminativism. Rather, as a community of researchers we would probably benefit most from some researchers trying to further develop eliminativism that eludes my criticisms, others developing non-eliminativisms. This would amount to a methodological meta-pluralism, at the community level. Toward that end, and to show that we can at least conceive of plausible non-eliminativist alternatives, I want to end the article by outlining one option for species that has the potential to accommodate pluralist insights without advising eliminativism. The option is an integrative version of non-eliminativism because it involves two integrative steps. The first step is to highlight the well-known distinction between epistemic and ontic approaches to species (Wheeler and Meier [2000]; Baum [2009]), and then make the less appreciated point that species concepts from each approach can be compatible and complementary. Definitions of epistemic species concepts attempt to tell us that certain groups are species and certain others are not, without also proposing a theory of the very nature of species-hood that would help explain what makes this group a species, that group not, and so on. Definitions of ontic species concepts, on the other hand, do venture the more ambitious theory of ‘species’ nature. Epistemic concepts are popular with strict empiricists, and with taxonomists and others attempting to delimit species via field research. Definitions of these concepts typically appeal to similarity or genealogical relations, as seen in the morphological species concepts (for example, Cronquist [1978]), concepts based on diagnosable characters (for example, Cracraft [1983]; Nixon and Wheeler [1990]), some GSCs (for example, Baum and Shaw [1995]), and some concepts designed specifically for prokaryotes (for example, Stackebrandt and Goebel [1994]; Rosselló-Mora and Amann [2001]). Certainly, some of these concepts fail to be co-extensive with each other and with ontic concepts. But it is quite possible that biologists will eventually be best served by having one ontic concept, and then a variety of epistemic species concepts that help them to determine, in a variety of different cases, which of the groups they investigate count as species according to the ontic concept (de Queiroz [1998]). This is already being done by researchers who deploy several different ‘species delimitation’ software programs for detecting species boundaries, while claiming to be guided by a single ontic species concept, all under the banner ‘integrative taxonomy’ (for example, Carstens et al. [2013], p. 4378; Satler, Carstens, and Hedin [2013]). The second integrative step concerns just ontic concepts. Phylogenetic botanist David Baum ([2009], p. 74) characterizes these as functionalist concepts that seize on the putative cohesive aspect of species qua lineages, and how, accordingly, species are believed to function within evolutionary processes. He also clarifies that although phylogeneticists often demand that only monophyletic groups be recognized as taxa, some phylogeneticists withhold this demand from species groups in particular, because they believe not all types of lineages—and especially species lineages that do things within evolutionary processes—are monophyletic. Thus Baum notes that in addition to species concepts that are obviously of the ontic, functionalist sort, including the BSC and the ESC, many species concepts favoured by phylogeneticists are also of this sort, including Ridley’s ([1989]) internodal species concept (ISC), the evolutionary species concepts (EvSC) of Simpson ([1961]) and Wiley ([1978]) and Wiley and Mayden ([2000]), de Queiroz’s ([1998]) general lineage concept (GLC), and Templeton’s ([1989]) cohesion concept (CSC). We can interpret definitions of such concepts as implying that certain patterns of cause-and-effect are what set species groups apart from others. The definitions then differ about the details of the patterns, and in their emphasis either on causes, or instead effects, in the patterns. Most of the concepts are cause-focused, as nicely brought out by Ereshefsky’s discussion of different causes of basal lineage formation. Definitions of cause-focused species concepts single out what their advocates believe to be the most important causes of the effects in question. A variable’s being a most important cause is thus implied to be enough to recognize the variable as more than just a cause—the variable is implied to be part of what constitutes species-hood as well. For example, the BSC singles out certain gene flow relations as the most important causes and as constitutive of being a species. The ESC points to niche sharing relations. And so on (see Ereshefsky [1991]; Barker and Wilson [2010]). Many disputes about species concepts can be understood as disagreements about which causes are most important. But upon appreciating this, we also see that the focus on causes has come at the expense of very little attention on effects (Barker and Wilson [2010]; Ereshefsky [2001], p. 114). I have suggested the effect is the formation of basal lineages, or, as it is also common to say, the formation of evolutionary unity or cohesion across populations that make up a proposed species (for example, Simpson [1961]; Hull [1976]; Wiley [1978]; Mayr [1963]; Templeton [1989]; Ereshefsky [1991]; Brooks and McLennan [2002]; Morjan and Rieseberg [2004]; Barker and Wilson [2010]; Palma-Silva et al. [2011]). But beyond that, remarkably little is said about the nature of such cohesion, especially in comparison with the very sophisticated and quantitative analyses that are used for studying its putative causes. Moreover, what an author says about cohesion is often vague, ambiguous, and not clearly compatible with what other authors say (Mishler and Brandon [1987]; Ereshefsky [1991]; Ereshefsky [2001]; Barker and Wilson [2010]). This surely bodes poorly for cause-focused species concepts. Imagine where skin cancer research would be if there were ample investigation of its causes, but virtually no analysis of the nature of skin cancer itself as an effect—if, for example, we had stopped focusing on the nature of cancer cells before ever distinguishing between melanoma versus basal cell carcinoma, or these and freckles for that matter. We would almost certainly be making wildly mistaken claims about causes of skin cancer, evidence for its frequency, and so on. We would be epistemically adrift, not just linguistically vague. Leaving species cohesion and similar concepts vague and underdeveloped is even more obviously a problem upon turning to effect-based ontic functionalist concepts—such as the ISC, EvSC, GLC, and CSC that I listed above—since these are explicitly defined in terms of cohesion, unity, etcetera. A species is said, for instance, to have cohesion insofar as it is a lineage with ‘its own independent evolutionary fate and historical tendencies’ (Wiley and Mayden [2000], p. 73). Defenders of these concepts sometimes claim such vagueness as a strength that allows for flexible and widespread application, noting that their cause-focused counterparts are too exclusive in appealing to just this or that small set of causes. I agree that such exclusiveness is a problem for cause-focused concepts, as empirical research suggests many causes work in interactive concert to hold populations of a species together (Morjan and Rieseberg [2004]; Barker and Wilson [2010]). But vagueness is no way for the alternative effect-focused concepts to answer, if the aim is for a concept whose definition summarizes an accurate theory of the nature of species. Being too inclusive is just as problematic as being too exclusive (Richards [2010], p. 132). Think, again, of the skin cancer example. In sum, ontic functionalist concepts divide roughly into those that are cause-focused and those that are effect-focused. Cause-focused concepts are too exclusive. Effect-focused sorts are too vague, and very probably too inclusive. And both sorts suffer from not being more comprehensive, that is, being both cause- and effect-focused. To remedy this, the second of the integrative steps that I am proposing would develop an integrative definition of species that attempts three things. First, it would refer to both causes and effects. Second, its reference to causes would be more inclusive, beginning to trace the complex interactions of several causes important to species cohesion. Third, its reference to effects such as species cohesion would be far more developed and specific. One way to develop such a concept is to explore the idea that the concept species cohesion has remained elusive thus far because it is not an effect at all.8 In other words, relations of gene flow picked out by the BSC, niche sharing picked out by the ESC, and other causes picked out by other concepts, are not producing some different type of effect (so far vaguely described). Instead, instances of these variables at one time are causing or increasing the chance of further instances of these same variables at a later time. Gene flow between populations now increases the probability that there will be gene flow between them in the future. Likewise for niche sharing. And so on. Moreover, each of these things increases the future chance of the other; gene flow increases the chance of future niche sharing, and vice versa. Rather than species cohesion then being some different type of thing at the end of such causal networks, it simply is the repeated instantiation or cycling of these networks. A species would have evolutionary cohesion to the extent that it is a positive feedback system. Then, to be a species would be to have a degree or grade of evolutionary cohesion greater than that had by higher taxa and more inclusive lineages, and less than sub-species, populations, and less inclusive lineages. Having such cohesion would implement a feedback system of diverse variables—gene flow, niche sharing, developmental homeostasis, common exposures to selection regimes, etcetera—that cycles with more frequency or greater magnitude (or both) than higher taxa and more inclusive lineages, but less than sub-species, populations, and less inclusive lineages. Were that so, a long-term pursuit would be formulating mathematical models that accurately describe the species grade of evolutionary cohesion and define species. You might worry that all of this would amount to a kind of paradigm shift in the way we conceptualize species. Even if that is so in some respects, my discussion suggests its seeds have been long sewn and waiting. 7 Conclusion Regardless of whether the two-step integrative approach I have sketched proves worthy of extensive development and competing with alternatives, it serves a useful purpose here. It shows that we are far from exhausting the possibilities for species that are plausible enough to compete with the eliminative pluralism I have shown to be so far unjustified. More generally, at the heart of the argument for that pluralism was the claim that ‘species’ is probably not an explanatorily or predictively significant category in nature. Although my criticisms show how one line of influential reasoning for that claim fails, sceptics about ‘species’ may call on other reasons. They may speculate about how the history of ‘species’ theorizing differs from the history of theorizing about scientific categories that currently enjoy more consensus, or invoke some criterion about how long to develop concepts about a category before giving up. But when new and plausible views about a category emerge, such bases for sceptical outlooks should be suspended in favour of the opportunity to test and evaluative the new view against existing ones. That opportunity has emerged here. Footnotes 1 I set aside the taxonomic issue of whether to count some integrationisms as types of non-eliminative pluralism. 2 In Ereshefsky’s ([1998]) his eliminative pluralism about species remains the same while he more explicitly adds an anti-realist interpretation of it. In his ([2010]), Ereshefsky still implies that we should eliminate species from our list of explanatorily (or otherwise epistemically) significant concepts, but adds that we should retain the term ‘species’ for practical reasons. 3 The image used for A and B, and the image used for C and D, were each created by Olle Pelmyr. I located them at the online ‘Tree of Life Web Project’ (see <bit.ly/2GKiNJW> and <bit.ly/2ICCpjC>, respectively). They are used here under Creative Commons Non-commercial terms, Version 3.0 (see <creativecommons.org/licenses/by-nc/3.0/>). The image used for E and F is public domain and can be found at <bit.ly/2GO2PhM>. 4 Any less inclusive group of populations inside it would not count as a lineage because it would exclude at least one population that shares the same evolutionary trajectory. 5 One reason to distinguish ‘basal lineage’ relevance from just ‘lineage’ relevance clarifies in Section 4.3. 6 See, for example, Boyd’s ([1999]) discussion of his ‘accommodation thesis’, and the discussion of ‘explanatory integrity’ in (Wilson et al. [2007]). 7 These pluralists cannot help the eliminative pluralist’s attempt to justify her splitting commitments from Section 4.1 because these pluralists are, in effect, leaning on the presupposed significance of a general ‘species’ category or analogous taxonomic category, in order to justify the biological significance of the plurality of species types they recognize. For example, interbreeding groups and ecological groups are said to both be biologically significant because the features and relations associated with each are both important ways of forming one and the same thing: species. 8 The ideas in this and the next paragraph are spelled out in more detail in (Barker [forthcoming]). Acknowledgments For constructive feedback I thank Marc Ereshefsky, Matthew Slater, Elliott Sober, Laura Gallivan, and those in the audiences for presentation of some material from this article at the Department of Philosophy, Université du Québec à Montréal (UQÀM), and at the August 2014 Inland Northwest Philosophy Conference (INPC). The Fonds de recherche du Québec—Société et culture (FRQSC) generously provided research funding (grant 2014-NP-176774) in support of this project, and Michèle Martin, Josine Lafontaine, John Nenniger, and Laura Gallivan provided research assistance. References Barker M. J. 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[ 2000 ]: Species Concepts and Phylogenetic Theory , New York : Columbia University Press . Wiley E. O. [ 1978 ]: ‘The Evolutionary Species Concept Reconsidered’, in Erefshefsky M. (ed.), The Units of Evolution: Essays on the Nature of Species , Cambridge, MA : MIT Press , pp. 79 – 92 . Wiley E. O. , Mayden R. [ 2000 ]: ‘The Evolutionary Species Concept’, in Wheeler Q. , Meier R. (eds), Species Concepts and Phylogenetic Theory: A Debate , New York : Columbia University Press , pp. 70 – 89 . Wilkins J. S. [ 2003 ]: ‘ How to Be a Chaste Species Pluralist-Realist: The Origins of Species Modes and the Synapomorphic Species Concept ’, Biology and Philosophy , 18 , pp. 621 – 38 . Google Scholar CrossRef Search ADS Wilson R. A. [ 2005 ]: Genes and the Agents of Life: The Individual in the Fragile Sciences Biology , Cambridge : Cambridge University Press . Wilson R. A. , Barker M. J. , Brigandt I. [ 2007 ]: ‘ When Traditional Essentialism Fails: Biological Natural Kinds ’, Philosophical Topics , 35 , pp. 189 – 215 . Google Scholar CrossRef Search ADS Zimmer M. , Desch L. [ 2012 ]: ‘ Sensory Integration Therapies for Children with Developmental and Behavioral Disorders ’, Pediatrics , 129 , pp. 1186 – 9 . Google Scholar CrossRef Search ADS PubMed © The Author(s) 2017. Published by Oxford University Press on behalf of British Society for the Philosophy of Science. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png The British Journal for the Philosophy of Science Oxford University Press

Eliminative Pluralism and Integrative Alternatives: The Case of SPECIES

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Abstract

Abstract Pluralisms of various sorts are popular in philosophy of science, including those that imply some scientific concept X should be eliminated from science in favour of a plurality of (typically more specific) concepts X1, X2, … Xn. This article focuses on influential and representative arguments for such eliminative pluralism about the concept species. The main conclusions are that these arguments fail, that all other extant arguments also fail, and that this reveals a quite general dilemma, one that poses a defeasible presumption against many eliminative pluralisms about various scientific concepts. The article ends by outlining a novel integrative alternative in defence of species. 1 Introduction 2 The species Concept, the Category ‘Species’, and the ‘Species’ Category Problem 3 What Are Eliminative Pluralism about species, and the Arguments for It? 4 Evaluation of Arguments   4.1 Splitting?   4.2 Lumping?   4.3 The eliminative pluralist’s dilemma 5 More General Lessons 6 Species Cohesion: An Integrative Alternative 7 Conclusion 1 Introduction You probably have first-hand experience of disputes about which scientific concepts are most legitimate, or best—or simply good, as it is often put—for helping predict or explain or control things. At her son’s recent medical appointment for symptoms he was experiencing, a mother asked ‘what about sensory processing disorder, should we test for that?’. The curt reply was that ‘in this country, we don’t believe in SPD’. Some medical professionals believe that sensory processing disorder is a good scientific concept, some do not. One of the most familiar threats to scientific concepts is a two-part view called eliminative pluralism. The threat can go unsuspected when it begins with just the pluralist part. Such pluralism about a concept X says that a plurality of concepts, X1, X2, … Xn, each have legitimate roles to play within some science or other. Kitcher’s ([1984]) pluralism about the concept species, for example, recognizes legitimate roles for both the concept structural similarity species and the concept phylogenetic species, and perhaps multiple types of each; yet Kitcher seems to still recognize legitimate roles for the more general concept species as well (see also Dupré [1993]). More generally, pluralisms about the concepts gene, organism, living agent, unit of selection, population, biological individual, clade, and species are all now dominant positions. A threat to a concept emerges or clarifies when the eliminative part is added. It says that the plurality X1, X2, … Xn should replace X—that there is no important explanatory, predictive, or similar role for X to play now that we have X1, X2, … Xn. This fits what some medical professionals have in mind about the initial example of sensory processing disorder, when they claim that the symptoms associated with it have disparate underlying bases—sometimes autism, other times attention-deficit disorder, other times anxiety disorder, and sometimes just slower than usual development (Zimmer and Desch [2012]). There is, they think, no explanatory, predictive, or similar gain made by relating these disparate phenomena within the concept sensory processing disorder. Whatever the concept in question, integrationism is one way of resisting such elimination, claiming there is some gain with X, in virtue of integrative relations between variables tracked by X1, X2, … Xn. This captures some uses of sensory processing disorder that oppose eliminative pluralism (Miller and Fuller [2014]), and Robert Wilson ([2005]) is an example of an integrationist about organism, species, and gene.1 When adjudicating between eliminative pluralists and their opposition, there can be much more at stake than ‘mere semantics’ or ‘logic chopping’ (also see Mishler [1999], p. 307). Eliminative pluralists about sensory processing disorder worry that that concept’s career has helped lead some parents to choose dubious treatments for their children (Heilbroner [2015]). The stakes in disputes about concepts that are more familiar in basic research can be less visible, but in some ways even more weighty. Just imagine where chemistry would be if, in the early 1800s, all engaged parties had ceased trying to justify their use of the concept fundamental system of chemical elements (which eventually took the name ‘Periodic Table’), and simply eliminated it in favour of a plurality of other concepts, such as equivalent weights system of elements derived from the work of Richter, hydrogen multiples system of elements from Prout, and triad system of elements from Döbereiner and Gmelin (Scerri [2007]). More generally, because eliminative pluralisms are about scientific concepts, and such concepts often play complex epistemic roles in science (Brigandt [2012]), we should predict that eliminative pluralisms often have important epistemic implications. For instance, with the clarity afforded by mathematics we can infer that eliminative pluralisms sometimes influence the degree to which data should be taken as evidence for a tested hypothesis (for example, see Barker [2017]). For these reasons, scrutiny of eliminative pluralisms should be more detailed, more fine-grained, than it usually is. But by choosing a target eliminative pluralism wisely, such depth of focus need not prevent arrival at conclusions of broad interest. Through focus on one concept, species (Section 2), and mostly on one target set of admirably clear, sophisticated, and influential arguments for eliminative pluralism about that concept, this article arrives at both specific and general conclusions. The target arguments are found in (Ereshefsky [1992]) (Section 3).2 My more specific conclusion is that those arguments fail to justify eliminative pluralism about species (Section 4). A more general conclusion is that all extant justifications of eliminative pluralism about species fail; more general still, I uncover a wide-ranging dilemma that confronts, at least provisionally, eliminative pluralisms about many other concepts (Section 5). The article ends by outlining an alternative view of species (Section 6), one that both respects the importance of a plurality of variables of species-hood while also suggesting these variables are integrated in ways that tell against eliminating species. 2 The Species Concept, the Category ‘Species’, and the ‘Species’ Category Problem If there is a category ‘species’ in nature, it is a different beast than any concept species. Any concept lives in our minds as a psychological entity, often developed to help us successfully interact with the world. A category ‘species’ in nature, on the other hand, is often believed to exist mind-independently. We can more precisely express this proposal in terms of stance-independence: the nature and characteristics of the category are believed to be independent of mere second-order mental attitudes about it (Shafer-Landau [2004]). The actions of mental agents like us can of course influence organisms within a species, cause whole species to go extinct, and create new species that previously were merely possible. But if the category ‘species’ is stance-independent, then without such action the second-order mental attitudes that agents have about it—their theories, desires, and feelings about the category—do not on their own influence the category’s nature or characteristics. Those who believe the concept species to be important usually believe or hope that it accurately, and stance-independently, corresponds with such a category. The infamous ‘species problem’ is, more exactly, a problem about the category ‘species’: what is the nature of this category? This can be put in other ways. What are species? What sets all species apart from all other things, including genera, sub-species, etcetera? Proposed answers typically have two parts. One is a definition of the concept species, the other is a name for this definition. For instance, according to one of Mayr’s ([2000], p. 17) definitions, a species is a group of interbreeding natural populations that is reproductively isolated from other such groups; this definition is named the biological species concept (BSC). Van Valen’s ([1976], p. 235) alternative definition, named the ecological species concept (ESC), states that a species is a ‘lineage (or a closely related set of lineages) which occupies an adaptive zone minimally different from that of any other lineage in its range and which evolves separately from all lineages outside its range’. Such definitions are not merely stipulative communication tools; often they are intended as a summary of a researcher’s theory about the very nature of the category ‘species’. It often takes careful interpretive work to formulate an accurate, fuller statement of the theory that a particular definition merely summarizes. Some interpreters do this by specifying the grouping criteria and ranking criteria that are implied by the definition they are interpreting (for example, Mishler and Donoghue [1982]; Barker [2010]). This helps clarify the differences between the different theories about the category ‘species’. Consider Figure 1a.3 The six images labelled A through F represent four actual populations of moth and two of liverworts. We can ask, do any of these populations belong to the same species group? This is a grouping question about populations. A set of species grouping criteria proposes an answer by stating the conditions that any pair of populations must meet in order to belong to the same species group. Figure 1. View largeDownload slide Depiction of how different definitions of species group and rank populations differently. (a) No definition applied. (b) BSC definition applied. Conspecific groups of populations in rectangles; the hashed rectangle encloses exactly one whole species. (c) GSC definition applied. Conspecific groups of populations in rectangles; the hashed rectangle encloses exactly one whole species. Figure 1. View largeDownload slide Depiction of how different definitions of species group and rank populations differently. (a) No definition applied. (b) BSC definition applied. Conspecific groups of populations in rectangles; the hashed rectangle encloses exactly one whole species. (c) GSC definition applied. Conspecific groups of populations in rectangles; the hashed rectangle encloses exactly one whole species. Once we believe that some populations belong to the same species group, we can ask whether those populations are constituting exactly one whole species group, a group ranked as a species rather than, say, a sub-species (which would lack some populations needed to be a whole species) or a genus (which would contain some extra populations that ensure the group is not just one species). This is a ranking question about groups of populations. A set of species ranking criteria proposes an answer by stating the conditions that any such group must meet in order to count as exactly one whole species. For instance, given various elaborations of the BSC, it is reasonable to infer that the BSC definition is a summary of a theory about the nature of the ‘species’ category that includes the following grouping and ranking criteria: BSC Grouping Criteria: any pair of populations belongs to the same species group if and only if genes can flow from one to the other either directly (for example, by sexual reproduction following immigration, or by other particular means) or indirectly (for example, via other populations). BSC Ranking Criteria: a group of populations is a species if and only if any pair of populations within it satisfies the BSC species grouping criteria, and genes cannot flow from any one of the populations in the group to any population outside the group. More simply, the theory is that species are closed gene pools. This theory’s grouping criteria imply that populations A and B in Figure 1a belong to the same species, and that C and D belong to one other species, and that E and F belong to a different other species. This is because genes can flow between A and B, and between C and D, and between E and F, but not between any other pairs of these. Figure 1b depicts this BSC grouping result by enclosing any populations that belong to the same species within a rectangle; hence, three rectangles. The theory’s ranking criteria, on the other hand, imply that only populations E and F form exactly one whole species. This is because E + F is the only depicted group of populations for which it is true that genes can flow between each population in the group, but not between these populations and any others on Earth. Figure 1b depicts this with hashed rectangle lines enclosing the group that makes up exactly one whole species. Many disagreements about the ‘species’ category stem from disagreeing grouping or ranking criteria, or both. The above BSC grouping criteria disagree with the grouping criteria summarized in many genealogy-based definitions. Those definitions deny the BSC view that genes must be able to flow between two populations in order for them to belong to the same species. Baum’s ([2009]) definition, for instance, would disagree with the BSC results in Figure 1b, and instead count all four of A, B, C, and D as belonging to the same species, because each is more closely genealogically related to each other than any of them are to any other populations, despite the fact that genes can no longer flow from A or B to C or D, nor can they flow from C or D to A or B (Brown et al. [1994]). Figure 1c shows how Baum’s definition of species, called the genealogical species concept (GSC), would draw the rectangles differently than the BSC. The final bit of context needed is a claim that Ereshefsky and many others endorse: despite disagreements between major species theorists, they typically agree on the minimal claim that species are basal lineages (Ereshefsky [1992], p. 674; de Queiroz [1998]; Padial et al. [2010], p. 2). Unfortunately, the terms ‘lineage’ and ‘basal lineage’ are ambiguous, vague, and deserve just as much scrutiny as ‘species’. For now I simply convey the rough (and perhaps, as we shall later see, no less vague) idea that a lineage is thought to be a group of populations that evolve separately from other groups of populations (Wiley [1978]; Padial et al. [2010]). One lineage can be nested inside another more inclusive one that contains additional populations that are evolving separately from still other populations (Haber [2012]). A basal lineage is a lineage with no smaller, less inclusive lineages nested inside it.4 3 What Are Eliminative Pluralism about species, and the Arguments for It? Within the foregoing context, Ereshefsky’s ([1992]) eliminative pluralism about species has two parts. The eliminativism part says we should eliminate the concept species from biological theorizing; the pluralism part says that instead of species we should use a plurality of basal lineage concepts. Here is his overall argument for this two-part view: If ‘species’ is very probably not a univocal and explanatorily or predictively significant category in nature, but in a plurality of basal lineage categories each basal lineage category has explanatory or predictive significance independently of the other basal lineage categories, then we should eliminate the concept species from biological theorizing, and instead use a plurality of basal lineage concepts. ‘Species’ is very probably not a univocal and explanatorily or predictively significant category in nature. In a plurality of basal lineage categories each basal lineage category has explanatory or predictive significance independently of the other basal lineage categories.__________________________________________________________ We should eliminate the concept species from biological theorizing and instead use a plurality of basal lineage concepts. My evaluation will mostly focus on Premise 2, then later come to involve Premise 3 as well. To begin his defence of Premise 2, Ereshefsky organizes named definitions of species into three classes. One class contains so-called interbreeding definitions, including Mayr’s BSC; these emphasize the capacity for gene flow between conspecific populations. The second class contains ecological definitions, including Van Valen’s ESC; these recognize ecological relationships, such as niche sharing, among the definitive variables. The third class contains phylogenetic and other genealogical definitions, which would include Baum’s GSC; these emphasize genealogical relations, sometimes implying that a group of populations is a species only if it is monophyletic—it contains all and only populations descending from a shared ancestor. Ereshefsky ([1992], p. 676) then claims that each class of definition identifies different types of evolutionary forces that operate on and cause the formation of basal lineages. Reproductive forces do this, as emphasized by interbreeding definitions, but so do ecological ones, as proposed by ecological definitions, and so on. A next and key move is to imply that a basal lineage’s being caused by a particular type of force makes it a different type of basal lineage than one caused by another sort of force. For example, if it is primarily reproductive forces that cause the formation of a particular basal lineage, then the basal lineage is of the interbreeding type. If ecological forces are the main causes of basal lineage formation, the basal lineage is of the ecological type. This amounts to what I call Ereshefsky’s argument for basal lineage type-plurality, which is the first part of his defence for Premise 2 in his overall argument. This argument is simplified in the left column of Table 1, Premises 2(a)–(c). Table 1. The two arguments that together are intended to defend Premise 2 in Ereshefsky’s overall argument for eliminative pluralism about the concept species Argument for basal lineage type-plurality Argument for no significant univocal ‘species’ category 2(a) If there are at least three different types of evolutionary forces operating on and causally producing basal lineages, then there are at least three different types of basal lineage. 2(b) There are at least three different types of evolutionary forces operating on and causally producing basal lineages. ____________________________ 2(c) There are at least three different types of basal lineage. 2(d) Many populations each belong to two or more different types of basal lineage that do not wholly overlap. 2(e) There is no good reason to believe we will discover a parameter common to all types of basal lineage. 2(f) If 2(d) and 2(e), then ‘species’ is very probably not a univocal and explanatorily or predictively significant category in nature. ____________________________ (2) ‘Species’ is very probably not a univocal and explanatorily or predictively significant category in nature. Argument for basal lineage type-plurality Argument for no significant univocal ‘species’ category 2(a) If there are at least three different types of evolutionary forces operating on and causally producing basal lineages, then there are at least three different types of basal lineage. 2(b) There are at least three different types of evolutionary forces operating on and causally producing basal lineages. ____________________________ 2(c) There are at least three different types of basal lineage. 2(d) Many populations each belong to two or more different types of basal lineage that do not wholly overlap. 2(e) There is no good reason to believe we will discover a parameter common to all types of basal lineage. 2(f) If 2(d) and 2(e), then ‘species’ is very probably not a univocal and explanatorily or predictively significant category in nature. ____________________________ (2) ‘Species’ is very probably not a univocal and explanatorily or predictively significant category in nature. Table 1. The two arguments that together are intended to defend Premise 2 in Ereshefsky’s overall argument for eliminative pluralism about the concept species Argument for basal lineage type-plurality Argument for no significant univocal ‘species’ category 2(a) If there are at least three different types of evolutionary forces operating on and causally producing basal lineages, then there are at least three different types of basal lineage. 2(b) There are at least three different types of evolutionary forces operating on and causally producing basal lineages. ____________________________ 2(c) There are at least three different types of basal lineage. 2(d) Many populations each belong to two or more different types of basal lineage that do not wholly overlap. 2(e) There is no good reason to believe we will discover a parameter common to all types of basal lineage. 2(f) If 2(d) and 2(e), then ‘species’ is very probably not a univocal and explanatorily or predictively significant category in nature. ____________________________ (2) ‘Species’ is very probably not a univocal and explanatorily or predictively significant category in nature. Argument for basal lineage type-plurality Argument for no significant univocal ‘species’ category 2(a) If there are at least three different types of evolutionary forces operating on and causally producing basal lineages, then there are at least three different types of basal lineage. 2(b) There are at least three different types of evolutionary forces operating on and causally producing basal lineages. ____________________________ 2(c) There are at least three different types of basal lineage. 2(d) Many populations each belong to two or more different types of basal lineage that do not wholly overlap. 2(e) There is no good reason to believe we will discover a parameter common to all types of basal lineage. 2(f) If 2(d) and 2(e), then ‘species’ is very probably not a univocal and explanatorily or predictively significant category in nature. ____________________________ (2) ‘Species’ is very probably not a univocal and explanatorily or predictively significant category in nature. Now, suppose Ereshefsky is correct about there being different types of basal lineage. Why care? The answer comes in what I call the argument for no significant univocal ‘species’ category, which is simplified in Table 1’s right column. This begins with something we saw diagrammatically earlier. The GSC definition implied that populations A and B are conspecific with populations C and D (Figure 1c), while the BSC definition implied that this is false (Figure 1b). Now put the topic of species and conspecificity aside momentarily, and focus just on types of basal lineage. If Ereshefsky is correct in his claims about these, then there is a genealogical type of basal lineage that populations A, B, C, and D are part of (Figure 1c). But there is no interbreeding lineage to which all four populations belong; instead there are two distinct and smaller interbreeding lineages (Figure 1b). Thus some particular basal lineages of different types do not wholly overlap. This entails there are some populations, such as A, which belong to two or more different types of basal lineage that overlap only partially (Ereshefsky [1992], pp. 674–5, 77). Ereshefsky ventures the empirical assumption that this is true of many populations, as indicated in Premise 2(d) in Table 1. To that premise, Ereshefsky ([1992], p. 677) adds another, Premise 2(e): there is no good reason to believe we will discover a parameter that is common to all types of basal lineage. In other words, there is very probably no one theoretically significant thing that is common to all interbreeding basal lineages, ecological basal lineages, and genealogical basal lineages. Finally, Ereshefsky believes that the lack of a common feature, coupled with the frequent phenomena of populations belonging to basal lineages that do not wholly overlap, implies that among such basal lineages there is very probably no univocal ‘species’ category that has explanatory and predictive significance. This appears as Premise 2(f) in Table 1. It then follows deductively that ‘species’ is very probably not a univocal category in nature with explanatory or predictive significance. If Ereshefsky’s arguments are sound, then the definitions proposed by the BSC, ESC, GSC, and others are each defining separate and quite unrelated concepts. The BSC’s interbreeding criteria are perhaps defining the concept interbreeding group, while the ESC’s ecological criteria are defining ecological group, and genealogical criteria are defining genealogical group, etcetera, with no ‘species’-related considerations connecting these in any interesting way. For population A to belong simultaneously to one interbreeding basal lineage, as well as to an extensionally different genealogical basal lineage, would be no more puzzling than saying that I simultaneously belong to a faculty association, and to an extensional quite different hockey team. 4 Evaluation of Arguments Evaluation of Ereshefsky’s arguments first raises two problems, and appreciating these also allows formulation of a third problem that we can generalize to other cases. 4.1 Splitting? The first problem is a splitting problem, and centres on Premise 2(a). Why should we believe that basal lineages differ in type just because different causal forces act on and produce them? This could not be successfully derived from the following very general sort of principle: Ambitious Causal Splitting Principle: For any category x, if different forces act on and produce instances of x, then there are different types of x. That principle is false. Just let x = being a kind of braided rope. Adam makes braided ropes by hand. His friend Heather sees they are popular and so designs a machine to also make them. It’s a perfect machine, producing ropes indistinguishable from Adam’s. Adam’s handiwork is one sort of causal force, the machine’s operation is another, but this causal difference by itself does not entail that the ropes produced by each are of different types. True, other considerations could be added to try providing the missing justification for their counting as different types. We could add, for instance, that people prefer the handmade ropes to the machine-made ones, even though people cannot tell the difference between them in blind tests. But such additions would amount to a different, more comprehensive, proposed sufficient condition than the one that the ambitious causal splitting principle implies is sufficient for there being different types of braided rope. This suggests we might repair Premise 2(a) by replacing it with Premise 2(a)*: Premise 2(a)*: If there are at least three different types of evolutionary forces operating on and causally producing basal lineages, and because of how these forces are different the lineages they produce as effects are themselves, qua basal lineages, also different in the following explanatorily or predictively significant ways __________________, then there are at least three different types of basal lineage. However, for the repair to be useful, the added clause would then also have to be asserted as its own new premise elsewhere in the argument. This is problematic because it is unclear how to fill in the blank in that clause. What are the explanatorily or predictively significant differences between, say, interbreeding basal lineages and ecological basal lineages? For the argument to work, these must be differences other than the mere fact that the causal sources of the lineages differed, because we are asking why difference in causal source matters at all. It is surprising that pluralist philosophers about species do not address this in detail. To clarify this outstanding task for pluralists, let me elaborate what it would take for differences in causal source to matter in the way pluralists need, using the example of interbreeding groups. Each interbreeding group is a group of populations wherein genes can flow between each of the populations (at least serially) and yet not between any of them and any populations outside the group. This is just what we mean by ‘interbreeding group’. So by definition, all interbreeding groups share these gene flow relationships and any features crucial for realizing such relationships. Call exactly these relations and features GF, for short. For GF to matter in a way that warrants distinguishing interbreeding groups as a distinctive type of basal lineage, there must be justification for believing that the following four splitter commitments are true: Typically biologically significant claim: GF in interbreeding groups always or usually produces, or in some other manner usually explains or successfully predicts, certain other features or relationships. Distinctiveness claim: At least some of the other features or relationships produced or explained or successfully predicted by GF are distinctive from features or relationships typical of other types of groups, to at least an extent or in ways that justify then distinguishing interbreeding groups from those other groups. Lineage relevance claim: The distinctive other features or relationships produced or explained or successfully predicted by GF are relevant in some clear way to being a lineage, for example, they contribute to a group’s being a lineage. Basal lineage relevance claim: The distinctive other features or relationships produced or explained or successfully predicted by GF and which are relevant in some clear way to being a lineage, are also relevant to being a basal lineage in particular.5 An analogous set of four claims will need justification for any proposed type of basal lineage group other than an interbreeding group, in order to successfully fill the blank in Premise 2(a)*. I do not claim this cannot be successfully done, just that it will take new empirically informed philosophical work to do so. So far pluralists, not just Ereshefsky, are yet to appreciate the extent of the task. For example, Kitcher ([2001]) describes how malaria researchers in Africa benefited from switching their focus on morphologically individuated groups to a focus on individuation by interbreeding. The helpful difference was that populations of mosquito grouped together on the basis of morphological similarity were assigned to different groups when switching to interbreeding criteria, because genes could not flow between the morphologically similar groups. This helped explain the previously puzzling observation that malaria was contained to only some of the populations in the larger morphological group: malaria was contained because the infecting populations could not interbreed with non-infecting ones. That explanation is, of course, important. But it needn’t have anything to do with lineages, let alone basal lineages or the ‘species’ category. The fact that some mosquito populations were reproductively isolated from morphologically similar ones helps ground the prediction that malaria vectors will not spread from one of those groups to the other, but this would be so regardless of whether either of the two are lineages or not, or species or not. It would be false, for instance, to say that the malaria researchers benefited because they switched from one basal lineage concept to another. That concept was not necessary for their progress, because any successful explanations or predictions involved in their switch were epistemically independent of any basal lineage concept. The challenge is to find lineage differences, not other differences that we have other reasons to find important. Also, these differences must not reveal that only one of the types of group in question are basal lineages after all, since it is pluralism about lineage type that Premise 2(a)* is after. The premise (in this case) requires at least three different types of basal lineage, not just one. So the mosquito case leaves the implicated versions of the lineage relevance claim, and the basal lineage relevance claim, unjustified. Less obviously, justifying the other two pluralist commitments would also need further work. The lack of gene flow relationships between two groups of morphologically similar mosquitos may predict and explain aspects of malaria distribution. But that is very particular to those mosquito populations, or at most to mosquito populations of those sorts. The explanations and predictions that matter for splitting one concept into a plurality of others would be much more general. What is it that GF always or usually distinctively explains or successfully predicts, in all or most cases of interbreeding groups, not just in the mosquito case? This sort of issue has been overlooked in pluralisms about concepts other than species as well (Barker [2013], p. 621). 4.2 Lumping? Now for the second problem. This is a lumping problem for believing that the more general category ‘basal lineage’ exists as supposed at all. The argument’s Premise 2(b) implies that this category exists. It says there are at least three different types of evolutionary forces operating on and causally producing basal lineages. In the context of the arguments, that implies that there exists an explanatorily or predictively significant ‘basal lineage’ category, whether it in fact admits of several types or not. Such a category may exist. But look at Table 1 to see that Premise 2(b)’s commitment to this, in the left-hand argument, appears in tension with Premise 2(e) plus Premise 2(f) in the right-hand argument. Premise 2(e) asserts that the different types into which this category divides have no parameter in common, which, when combined with Premise 2(f), seems to cast doubt on there being a significant and general ‘basal lineage’ category at all. Why think there is such a category if all the different types lack a common identifying feature? It is not just that the right-hand argument ironically poses this question to the left-hand argument. It is that the right-hand argument’s Premise 2(f) suggests an answer that the left-hand argument cannot live with, as Premise 2(f) implies that without a common identifying feature, the types of basal lineage that only partially overlap do not form the sort of general and scientifically significant ‘basal lineage’ category that Premise 2(b) presupposes. Notice it seems an essentialist criterion is lurking. Premise 2(f) seems to say that if the partially overlapping types of basal lineage do not share in a ‘basal lineage’ essence then no putative ‘basal lineage’ category nor ‘species’ category is explanatorily or predictively significant. And Premise 2(e) is saying that, indeed, we have no reason to believe there is such an essence, while Premise 2(b) instead seems to suggest in this context that there is such an essence. My own view is that an essentialist criterion should not be used here at all. David Hull ([1965]) made this point over fifty years ago. Moreover, it is now widely known that the prodigious variation between organisms and between populations is trouble for individuating groups essentialistically (Sober [1980]; Okasha [2002]; Barker and Velasco [2013]; cf. Devitt [2008a]), and alternative sorts of individuation are well developed to respect this empirical reality (Boyd [1999]; Wilson et al. [2007]; Slater [2015]). These alternatives vary in interesting ways, but they share in the claim that some groups are individuated by clusters of features (properties, relations, states, etcetera), with no single feature in an individuative cluster being essential or necessary for group membership, and with multiple sub-sets of the features each being sufficient for group membership. On such a view, group members tend to share many features and relationships, and this helps make the group explanatorily and predictively significant even though the feature-sharing and relation-sharing don’t live up to the standards of an essentialism on which sharing is exceptionless between group members. Can such cluster views help dissolve the tension between the left- and right-hand arguments? To answer, we need to see how this would change the right-hand argument, so consider Table 2. Table 2. Revised statements indicated by * Argument for basal lineage type-plurality Argument for no significant univocal species category* 2(a) If there are at least three different types of evolutionary forces operating on and causally producing basal lineages, then there are at least three different types of basal lineage. 2(b) There are at least three different types of evolutionary forces operating on and causally producing basal lineages. _____________________________ 2(c) There are at least three different types of basal lineage. 2(d) Many populations each belong to two or more different types of basal lineage that do not wholly overlap. 2(e)* There is no good reason to believe we will discover a parameter common to all types of basal lineage, and that rather these types will tend, that is, with some exceptions, to share many features and relationships. 2(f)* If 2(d) and 2(e)*, then ‘species’ is very probably not a univocal and explanatorily or predictively significant category in nature. _____________________________ 2 ‘Species’ is very probably not a univocal and explanatorily or predictively significant category in nature. Argument for basal lineage type-plurality Argument for no significant univocal species category* 2(a) If there are at least three different types of evolutionary forces operating on and causally producing basal lineages, then there are at least three different types of basal lineage. 2(b) There are at least three different types of evolutionary forces operating on and causally producing basal lineages. _____________________________ 2(c) There are at least three different types of basal lineage. 2(d) Many populations each belong to two or more different types of basal lineage that do not wholly overlap. 2(e)* There is no good reason to believe we will discover a parameter common to all types of basal lineage, and that rather these types will tend, that is, with some exceptions, to share many features and relationships. 2(f)* If 2(d) and 2(e)*, then ‘species’ is very probably not a univocal and explanatorily or predictively significant category in nature. _____________________________ 2 ‘Species’ is very probably not a univocal and explanatorily or predictively significant category in nature. Table 2. Revised statements indicated by * Argument for basal lineage type-plurality Argument for no significant univocal species category* 2(a) If there are at least three different types of evolutionary forces operating on and causally producing basal lineages, then there are at least three different types of basal lineage. 2(b) There are at least three different types of evolutionary forces operating on and causally producing basal lineages. _____________________________ 2(c) There are at least three different types of basal lineage. 2(d) Many populations each belong to two or more different types of basal lineage that do not wholly overlap. 2(e)* There is no good reason to believe we will discover a parameter common to all types of basal lineage, and that rather these types will tend, that is, with some exceptions, to share many features and relationships. 2(f)* If 2(d) and 2(e)*, then ‘species’ is very probably not a univocal and explanatorily or predictively significant category in nature. _____________________________ 2 ‘Species’ is very probably not a univocal and explanatorily or predictively significant category in nature. Argument for basal lineage type-plurality Argument for no significant univocal species category* 2(a) If there are at least three different types of evolutionary forces operating on and causally producing basal lineages, then there are at least three different types of basal lineage. 2(b) There are at least three different types of evolutionary forces operating on and causally producing basal lineages. _____________________________ 2(c) There are at least three different types of basal lineage. 2(d) Many populations each belong to two or more different types of basal lineage that do not wholly overlap. 2(e)* There is no good reason to believe we will discover a parameter common to all types of basal lineage, and that rather these types will tend, that is, with some exceptions, to share many features and relationships. 2(f)* If 2(d) and 2(e)*, then ‘species’ is very probably not a univocal and explanatorily or predictively significant category in nature. _____________________________ 2 ‘Species’ is very probably not a univocal and explanatorily or predictively significant category in nature. The turn away from a problematic essentialist criterion to a cluster criterion is foreshadowed in new Premise 2(e)*. But if the right-hand argument is to retain its valid form, Premise 2(f) must then become Premise 2(f)*: it must be a conditional whose antecedent states the new Premise 2(e)* rather than the old Premise 2(e). With this change, Premise 2(f)* will imply that partially overlapping types of basal lineage will not be explanatorily and predictively significant even if they can be grouped together as a cluster view suggests. This is to deny the significance of such cluster-based groups. Two objections greet the argument modifications. First, a heavy burden now falls on the proponent of the modified argument to support the denial of significance for cluster-based groups. This is because the very purpose of the development of the cluster views was to explicate the explanatory and predictive significance of groups, by paying much closer attention to work in biology than did traditional essentialists. The arguments for those cluster views show how such groups retain the significance that biologists imply they have, despite lack of essences.6 So those arguments have a long head-start against any defence of Premise 2(f)*’s denial of such significance. The second objection is more internal to the left- and right-hand arguments. Even if the first objection is met, and the significance of the cluster-based groups refuted, the tension that afflicted the left- and right-hand arguments arises in new garb. This is because Premise 2(f)* plus Premise 2(e)* continues to deny there is the general sort of ‘basal lineage’ category (whether cluster-based or essentialistic) that Premise 2(b) presupposes. 4.3 The eliminative pluralist’s dilemma The splitting problem from Section 4.2 and the lumping problem from Section 4.1 each stand on their own. But when combining considerations from them, an additional problem arises. This problem leaves fewer escape routes than either the splitting problem or lumping problem do on their own, because it takes dilemma form. The dilemma starts with two mutually exhaustive possibilities: either the different types of basal lineage that Ereshefsky or others identify do tend to share some features and relationships that are of interest to biological theorizing, or they do not. Certainly a presumption among many species theorists has been that the first of these possibilities is the case. An example of this idea is that even though an interbreeding criterion for individuating groups does not sort organisms exactly as, say, a genealogical criterion does, there are features and relationships most groups of both sorts tend to share, and at least some of these shared features and relationships are of great interest to biological theorizing—enough that we recognize most groups of both sorts as sharing in a particular property: being a species. Of course, Ereshefsky doubts that this popular presumption is correct. That reflects his eliminativism. But his view is not merely eliminativism. It is eliminative pluralism. And if he is correct to doubt the common presumption about ‘species’ and eliminate the concept species, then the other horn of the dilemma awaits the pluralism part of his view. That part commits to multiple different types of basal lineage that are each of independent biological interest. But the same doubt he raises about such interest in the case of the general categories ‘species’ and ‘basal lineage’ then arises for each of the different types of lineage he recognizes—for ‘interbreeding group’, and ‘ecological group’, and ‘genealogical group’—even if we stop subsuming them under a general basal lineage or species concept. In Section 4.1 we saw the challenges that this horn of the dilemma involves, namely, the challenges of justifying all four splitter commitments. Since there are only two possibilities—either the different types of basal lineage tend to share some features and relationships of biological interest, or they do not—and both possibilities would result in one part of Ereshefsky’s eliminative pluralism being so far unjustified, this eliminative pluralism must be so far unjustified. 5 More General Lessons Ereshefsky’s view is of quite general interest partly because the dilemma raised for it also arises, at least provisionally, for eliminative pluralisms about various scientific concepts. This clarifies upon generalizing the dilemma’s premises by using X to stand for the concept whose elimination is proposed, and X1, X2, … Xn for the plurality of concepts to be recognized instead of X. The eliminative pluralist’s dilemma, generalized: Either proposed categories X1, X2, … Xn do tend to share some features and relationships that are of general interest to scientific theorizing, or not. If X1, X2, … Xn do tend to share some features and relationships that are of general interest to scientific theorizing, then these features and relationships are a promising basis for recognizing the general superordinate concept X, and therefore the eliminative part of eliminative pluralism about concept X is not yet justified. If X1, X2, … Xn do not tend to share some features and relationships that are of general interest to scientific theorizing, raising doubt about the superordinate concept X, then it becomes strikingly unclear what warrants recognizing each putative category (X1, and also X2, … and also Xn) in the plurality of X1, X2, … Xn as having scientific interest distinct from every other category in that plurality, since the same type of doubt about the scientific interest of superordinate concept X also afflicts each category in the proposed plurality, and therefore the pluralism part of eliminative pluralism about concept X is not yet justified._______________________________________________________________ Eliminative pluralism about concept X is not yet justified. This dilemma is provisional in the sense that it poses a defeasible presumption against eliminative pluralism about this or that concept, one that can probably be defeated most readily by making a case that Premise (3) is false for the concept in question. I suspect that this is, at least implicitly, the way that successful eliminative pluralisms typically gain acceptance. Many concepts of formerly recognized diseases come to mind, such as Kitcher’s ([2001]) neurasthenia example. My arguments in Section 4.1 illustrate what is involved in defeating Premise (3) of the dilemma in this way, and suggest that Ereshefsky has yet to make such a defeat work in the case of species. What about other eliminative pluralists about species? Aside from Ereshefsky, it is curiously difficult to determine whether well-known pluralists about species are also eliminative pluralists about species. Kitcher (especially in his [1984]) is an influential pluralist, though he does not explicitly argue for elimination of species as Ereshefsky does; accordingly, Devitt ([2008b]) struggles to interpret Kitcher’s view on elimination, and decides that if Kitcher’s pluralism is eliminativist, it is the same as Ereshefsky’s, and thus their views stand or fall together. Stanford ([1995]) is sometimes considered an eliminative pluralist about species, but here it is commitment to the pluralism part that is unclear. Ereshefsky ([1998]) shows that Stanford’s anti-realist view is independent of pluralism; and in any case, Ereshefsky ([1998]) and Devitt ([2008b]) argue convincingly that Stanford’s argument fails. I read other leading species pluralists (Mishler and Donoghue [1982]; Mishler and Brandon [1987]; Dupré [1993]; Wilkins [2003]) as clearly not also eliminativists, with the interesting possible exception of Brent Mishler in his more recent work (for example, Mishler [1999]), and others who similarly call for eliminating species along with the entire Linnaean Hierarchy and all its other ranks (for example, kingdom, phylum, class, order, family, genus).7 This is interesting because although Mishler is, like Ereshefsky, willing to recognize some notion of basal lineage (‘basal taxon’; Mishler [1999], p. 307), Mishler’s argument for elimination does not depend on this notion, while Ereshefsky’s does. This helps show that the notion is inessential to the failure of eliminative pluralism about species. If Ereshefsky abandons the concept basal lineage because it seems suspiciously like the concept species, he will, like Mishler, retain the core concept lineage. The idea will then be that there are a plurality of lineage types—for example, interbreeding lineages, ecological lineages, genealogical lineages—but no basal lineages or species. But then the problem of avoiding Premise (3) in the eliminative pluralist’s dilemma remains, where now we demand to know exactly which ‘lineage features and relations’ would often be produced or explained or predicted in biologically interesting and distinct ways by interbreeding, and in other interesting and distinct ways by niche sharing, and so on. The reliance on lineage rather than basal lineage does nothing to remove this problem. With no successful version of eliminative pluralism about species so far established, I infer that all extant versions of this view are, like Ereshefsky’s, so far unjustified. 6 Species Cohesion: An Integrative Alternative None of this is to say that all species theorists should presently resist eliminativism. Rather, as a community of researchers we would probably benefit most from some researchers trying to further develop eliminativism that eludes my criticisms, others developing non-eliminativisms. This would amount to a methodological meta-pluralism, at the community level. Toward that end, and to show that we can at least conceive of plausible non-eliminativist alternatives, I want to end the article by outlining one option for species that has the potential to accommodate pluralist insights without advising eliminativism. The option is an integrative version of non-eliminativism because it involves two integrative steps. The first step is to highlight the well-known distinction between epistemic and ontic approaches to species (Wheeler and Meier [2000]; Baum [2009]), and then make the less appreciated point that species concepts from each approach can be compatible and complementary. Definitions of epistemic species concepts attempt to tell us that certain groups are species and certain others are not, without also proposing a theory of the very nature of species-hood that would help explain what makes this group a species, that group not, and so on. Definitions of ontic species concepts, on the other hand, do venture the more ambitious theory of ‘species’ nature. Epistemic concepts are popular with strict empiricists, and with taxonomists and others attempting to delimit species via field research. Definitions of these concepts typically appeal to similarity or genealogical relations, as seen in the morphological species concepts (for example, Cronquist [1978]), concepts based on diagnosable characters (for example, Cracraft [1983]; Nixon and Wheeler [1990]), some GSCs (for example, Baum and Shaw [1995]), and some concepts designed specifically for prokaryotes (for example, Stackebrandt and Goebel [1994]; Rosselló-Mora and Amann [2001]). Certainly, some of these concepts fail to be co-extensive with each other and with ontic concepts. But it is quite possible that biologists will eventually be best served by having one ontic concept, and then a variety of epistemic species concepts that help them to determine, in a variety of different cases, which of the groups they investigate count as species according to the ontic concept (de Queiroz [1998]). This is already being done by researchers who deploy several different ‘species delimitation’ software programs for detecting species boundaries, while claiming to be guided by a single ontic species concept, all under the banner ‘integrative taxonomy’ (for example, Carstens et al. [2013], p. 4378; Satler, Carstens, and Hedin [2013]). The second integrative step concerns just ontic concepts. Phylogenetic botanist David Baum ([2009], p. 74) characterizes these as functionalist concepts that seize on the putative cohesive aspect of species qua lineages, and how, accordingly, species are believed to function within evolutionary processes. He also clarifies that although phylogeneticists often demand that only monophyletic groups be recognized as taxa, some phylogeneticists withhold this demand from species groups in particular, because they believe not all types of lineages—and especially species lineages that do things within evolutionary processes—are monophyletic. Thus Baum notes that in addition to species concepts that are obviously of the ontic, functionalist sort, including the BSC and the ESC, many species concepts favoured by phylogeneticists are also of this sort, including Ridley’s ([1989]) internodal species concept (ISC), the evolutionary species concepts (EvSC) of Simpson ([1961]) and Wiley ([1978]) and Wiley and Mayden ([2000]), de Queiroz’s ([1998]) general lineage concept (GLC), and Templeton’s ([1989]) cohesion concept (CSC). We can interpret definitions of such concepts as implying that certain patterns of cause-and-effect are what set species groups apart from others. The definitions then differ about the details of the patterns, and in their emphasis either on causes, or instead effects, in the patterns. Most of the concepts are cause-focused, as nicely brought out by Ereshefsky’s discussion of different causes of basal lineage formation. Definitions of cause-focused species concepts single out what their advocates believe to be the most important causes of the effects in question. A variable’s being a most important cause is thus implied to be enough to recognize the variable as more than just a cause—the variable is implied to be part of what constitutes species-hood as well. For example, the BSC singles out certain gene flow relations as the most important causes and as constitutive of being a species. The ESC points to niche sharing relations. And so on (see Ereshefsky [1991]; Barker and Wilson [2010]). Many disputes about species concepts can be understood as disagreements about which causes are most important. But upon appreciating this, we also see that the focus on causes has come at the expense of very little attention on effects (Barker and Wilson [2010]; Ereshefsky [2001], p. 114). I have suggested the effect is the formation of basal lineages, or, as it is also common to say, the formation of evolutionary unity or cohesion across populations that make up a proposed species (for example, Simpson [1961]; Hull [1976]; Wiley [1978]; Mayr [1963]; Templeton [1989]; Ereshefsky [1991]; Brooks and McLennan [2002]; Morjan and Rieseberg [2004]; Barker and Wilson [2010]; Palma-Silva et al. [2011]). But beyond that, remarkably little is said about the nature of such cohesion, especially in comparison with the very sophisticated and quantitative analyses that are used for studying its putative causes. Moreover, what an author says about cohesion is often vague, ambiguous, and not clearly compatible with what other authors say (Mishler and Brandon [1987]; Ereshefsky [1991]; Ereshefsky [2001]; Barker and Wilson [2010]). This surely bodes poorly for cause-focused species concepts. Imagine where skin cancer research would be if there were ample investigation of its causes, but virtually no analysis of the nature of skin cancer itself as an effect—if, for example, we had stopped focusing on the nature of cancer cells before ever distinguishing between melanoma versus basal cell carcinoma, or these and freckles for that matter. We would almost certainly be making wildly mistaken claims about causes of skin cancer, evidence for its frequency, and so on. We would be epistemically adrift, not just linguistically vague. Leaving species cohesion and similar concepts vague and underdeveloped is even more obviously a problem upon turning to effect-based ontic functionalist concepts—such as the ISC, EvSC, GLC, and CSC that I listed above—since these are explicitly defined in terms of cohesion, unity, etcetera. A species is said, for instance, to have cohesion insofar as it is a lineage with ‘its own independent evolutionary fate and historical tendencies’ (Wiley and Mayden [2000], p. 73). Defenders of these concepts sometimes claim such vagueness as a strength that allows for flexible and widespread application, noting that their cause-focused counterparts are too exclusive in appealing to just this or that small set of causes. I agree that such exclusiveness is a problem for cause-focused concepts, as empirical research suggests many causes work in interactive concert to hold populations of a species together (Morjan and Rieseberg [2004]; Barker and Wilson [2010]). But vagueness is no way for the alternative effect-focused concepts to answer, if the aim is for a concept whose definition summarizes an accurate theory of the nature of species. Being too inclusive is just as problematic as being too exclusive (Richards [2010], p. 132). Think, again, of the skin cancer example. In sum, ontic functionalist concepts divide roughly into those that are cause-focused and those that are effect-focused. Cause-focused concepts are too exclusive. Effect-focused sorts are too vague, and very probably too inclusive. And both sorts suffer from not being more comprehensive, that is, being both cause- and effect-focused. To remedy this, the second of the integrative steps that I am proposing would develop an integrative definition of species that attempts three things. First, it would refer to both causes and effects. Second, its reference to causes would be more inclusive, beginning to trace the complex interactions of several causes important to species cohesion. Third, its reference to effects such as species cohesion would be far more developed and specific. One way to develop such a concept is to explore the idea that the concept species cohesion has remained elusive thus far because it is not an effect at all.8 In other words, relations of gene flow picked out by the BSC, niche sharing picked out by the ESC, and other causes picked out by other concepts, are not producing some different type of effect (so far vaguely described). Instead, instances of these variables at one time are causing or increasing the chance of further instances of these same variables at a later time. Gene flow between populations now increases the probability that there will be gene flow between them in the future. Likewise for niche sharing. And so on. Moreover, each of these things increases the future chance of the other; gene flow increases the chance of future niche sharing, and vice versa. Rather than species cohesion then being some different type of thing at the end of such causal networks, it simply is the repeated instantiation or cycling of these networks. A species would have evolutionary cohesion to the extent that it is a positive feedback system. Then, to be a species would be to have a degree or grade of evolutionary cohesion greater than that had by higher taxa and more inclusive lineages, and less than sub-species, populations, and less inclusive lineages. Having such cohesion would implement a feedback system of diverse variables—gene flow, niche sharing, developmental homeostasis, common exposures to selection regimes, etcetera—that cycles with more frequency or greater magnitude (or both) than higher taxa and more inclusive lineages, but less than sub-species, populations, and less inclusive lineages. Were that so, a long-term pursuit would be formulating mathematical models that accurately describe the species grade of evolutionary cohesion and define species. You might worry that all of this would amount to a kind of paradigm shift in the way we conceptualize species. Even if that is so in some respects, my discussion suggests its seeds have been long sewn and waiting. 7 Conclusion Regardless of whether the two-step integrative approach I have sketched proves worthy of extensive development and competing with alternatives, it serves a useful purpose here. It shows that we are far from exhausting the possibilities for species that are plausible enough to compete with the eliminative pluralism I have shown to be so far unjustified. More generally, at the heart of the argument for that pluralism was the claim that ‘species’ is probably not an explanatorily or predictively significant category in nature. Although my criticisms show how one line of influential reasoning for that claim fails, sceptics about ‘species’ may call on other reasons. They may speculate about how the history of ‘species’ theorizing differs from the history of theorizing about scientific categories that currently enjoy more consensus, or invoke some criterion about how long to develop concepts about a category before giving up. But when new and plausible views about a category emerge, such bases for sceptical outlooks should be suspended in favour of the opportunity to test and evaluative the new view against existing ones. That opportunity has emerged here. Footnotes 1 I set aside the taxonomic issue of whether to count some integrationisms as types of non-eliminative pluralism. 2 In Ereshefsky’s ([1998]) his eliminative pluralism about species remains the same while he more explicitly adds an anti-realist interpretation of it. In his ([2010]), Ereshefsky still implies that we should eliminate species from our list of explanatorily (or otherwise epistemically) significant concepts, but adds that we should retain the term ‘species’ for practical reasons. 3 The image used for A and B, and the image used for C and D, were each created by Olle Pelmyr. I located them at the online ‘Tree of Life Web Project’ (see <bit.ly/2GKiNJW> and <bit.ly/2ICCpjC>, respectively). They are used here under Creative Commons Non-commercial terms, Version 3.0 (see <creativecommons.org/licenses/by-nc/3.0/>). The image used for E and F is public domain and can be found at <bit.ly/2GO2PhM>. 4 Any less inclusive group of populations inside it would not count as a lineage because it would exclude at least one population that shares the same evolutionary trajectory. 5 One reason to distinguish ‘basal lineage’ relevance from just ‘lineage’ relevance clarifies in Section 4.3. 6 See, for example, Boyd’s ([1999]) discussion of his ‘accommodation thesis’, and the discussion of ‘explanatory integrity’ in (Wilson et al. [2007]). 7 These pluralists cannot help the eliminative pluralist’s attempt to justify her splitting commitments from Section 4.1 because these pluralists are, in effect, leaning on the presupposed significance of a general ‘species’ category or analogous taxonomic category, in order to justify the biological significance of the plurality of species types they recognize. For example, interbreeding groups and ecological groups are said to both be biologically significant because the features and relations associated with each are both important ways of forming one and the same thing: species. 8 The ideas in this and the next paragraph are spelled out in more detail in (Barker [forthcoming]). Acknowledgments For constructive feedback I thank Marc Ereshefsky, Matthew Slater, Elliott Sober, Laura Gallivan, and those in the audiences for presentation of some material from this article at the Department of Philosophy, Université du Québec à Montréal (UQÀM), and at the August 2014 Inland Northwest Philosophy Conference (INPC). The Fonds de recherche du Québec—Société et culture (FRQSC) generously provided research funding (grant 2014-NP-176774) in support of this project, and Michèle Martin, Josine Lafontaine, John Nenniger, and Laura Gallivan provided research assistance. References Barker M. J. 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