Taxonomists identify, name, and classify taxa. Taxonomic systems provide classificatory guidelines and rules of nomenclature, but no guidance on identification. Taxonomists have traditionally used a Linnaean system. Though modified to accommodate modern biological theory, this system retains vestiges of its eighteenth-century origin. Kingdoms, phyla, orders, etc., are likely familiar to readers of this journal—despite the fact that most biologists no longer consider these ranks ‘real’. Critics argue that these and other conflicts with modern biological theory should be eliminated. One remedy is to replace the Linnaean system with a system that is conceptually grounded in the principles of phylogenetics.
Marc Ereshefsky's The Poverty of Linnaean Hierarchy recommends just such a remedy. Structured into three parts, the book offers a new philosophy of classification, detailing a conceptual underpinning for a post-Linnaean system of taxonomy. Part 1 provides an overview of competing philosophies of classification, laying the groundwork for a rejection of essentialism and acceptance of taxa as historical entities; Part 2 is a description and defense of pluralism; Part 3 details the offensive parts of the Linnaean system, while recommending rules and conventions for a post-Linnaean system.
In Part 1, Ereshefsky analyzes essentialism, individuality, and the nature of biological taxa, rejecting essentialist approaches to biological classification and instead arguing that biological taxa are best thought of as historical entities. To be a part of a species, then, is just to be located in the appropriate spatiotemporal region (which may involve standing in the relevant causal relations to other appropriate parts of the species).
Though I am sympathetic to this position, I was still left with some questions. Consider the three main tenets of essentialism, as defined by Ereshefsky (95):
1. All and only the members of a kind share a common set of traits that is the real essence of that kind.
2. That essence makes an entity a particular kind of thing.
3. Knowing an entity's essence is crucial for explaining the other properties typically associated with an entity of that kind.
This raises a question for Ereshefsky. His main argument against essentialism is that we have strong theoretical and empirical evidence to suggest that organisms simply do not possess the kinds of intrinsic properties that advocates of essentialism typically have in mind. Fair enough, and to my mind, correct. Notice, though, that the spatiotemporal location of an organism seems capable of qualifying as its essence, as all three of the tenets will be satisfied. The point is that Ereshefsky has rejected essentialism for a thesis about which some essentialist story could be told (rightly or wrongly). The question for Ereshefsky is why (or whether) we should reject such a story. There may be good reasons for rejecting this stance (e.g., an appeal to external versus internal properties, the tenets should not be construed as sufficient conditions for essentialism, etc.), or Ereshefsky could bite the bullet and simply accept this as an innocuous, yet trivial, brand of essentialism. Unfortunately, we are left with little guidance of how to answer this question.
Ereshefsky might have avoided this by questioning whether essentialism is really at the crux of the debate. He appears to accept, without question, Mayr's equating (and demonizing) typological thinking in biological classification with essentialism. I think Ereshefsky misses an opportunity to challenge this assumption. I suspect that pulling apart typology and essentialism would have facilitated Ereshefsky's positive and critical exposition. For example, in an earlier discussion, Ereshefsky explicitly separates the ‘species as individuals’ thesis from the ‘species as historical entities’ thesis. I think this is exactly the right approach, and a similar treatment of essentialism may prove similarly insightful. It may be that essentialism, like individuality, may sometimes be a red herring in debates over the nature of species.
Part 2 of the book describes and defends species pluralism. The question addressed here is not whether a particular species has an essence, but whether there is an essence of the species category. On this, Ereshefsky is both a metaphysical and a taxonomic pluralist. The former position entails that the world can be divided into multiple, overlapping, natural kinds. If true, then we should expect taxonomic pluralism, i.e., a multiplicity of (overlapping) classifications of the natural world. In fact, this is what we do see. Part 2 is revealing of Ereshefsky's methodology and strategy. He clearly believes that both the practice and philosophy of classification should be driven by metaphysical commitments, and he proves adept at clearly articulating his metaphysical assumptions and then pursuing the avenues these commitments compel him to follow. Less clear is how pluralism fits into the larger argument of the book. After all, both Linnaean and phylogenetic systems of taxonomy ought to be capable of accommodating pluralism.
Part 3, “Hierarchies and Nomenclature,” describes the development of the modern Linnaean system (Chapter 6), proposes rules and conventions for a post-Linnaean taxonomy (Chapter 7), and offers a summary and practical assessment of post-Linnaean taxonomy (Chapter 8). Here is found the strongest attack on Linnaean taxonomy. Most prominent and convincing is the rejection of Linnaean ranks. Ranks are informative within classifications (i.e., as relative hierarchy), but not across classifications (i.e., as absolute hierarchy) because, as Ereshefsky forcefully reminds us, Linnaean ranks are ‘ontologically empty categories’. Linnaean taxonomists are well aware of the ontological poverty of ranks, and warn against comparing ranks across classifications, e.g., ranks of flowering plants with ranks of placental mammals. Nonetheless, Ereshefsky is right that this feature of Linnaean taxonomy is at best misleading, and at worst apt to run afoul of evolutionary theory. As a remedy, Ereshefsky recommends the outright elimination of ranks.
In a post-Linnaean system the rank of a taxon would no longer be incorporated into its name. This would go far toward eliminating artificial distinctions made at different levels of inclusiveness. This extends to species names as well, for “if Linnaean ranks are dropped, then so too should the nomenclatural distinction between species and higher taxa” (250). Here, though, Ereshefsky may have pushed too far. How artificial is the distinction between species and other higher taxa? Surely it is possible that the kind of cohesion that constitutes being a species is of enough biological interest that it warrants distinction, even in a post-Linnaean taxonomy. This is not to say that we ought to retain ranks or binomial species names, only that it might prove useful to (somehow) designate species in a post-Linnaean system. Currently, advocates of phylogenetic taxonomy are in disagreement over the treatment of species (Mishler Reference Mishler and Wilson1999; Dayrat, Schander, and Angielczyk Reference Dayrat, Schander and Angielczyk2004).
Less convincing is Ereshefsky's argument that a phylogenetic system would lead to greater stability in taxonomy. The problem is that stability arguments can work both ways. Ereshefsky is right that names would remain more stable in a phylogenetic system; however, the content of taxon names may become less stable. Moving a branch on a phylogenetic tree may have radical implications for taxa that incorporate the branch as a specifier in a phylogenetic definition (Benton Reference Benton2000). If name and content stability trade off, the stability argument is not as decisive as Ereshefsky suggests. It may be that instability of content is preferable to instability of names (e.g., as a direct reflection of the instability of our phylogenetic hypotheses), but this is not a possibility Ereshefsky discusses.
The Poverty of the Linnaean Hierarchy reads as though it were written with an eye to accessibility to both the professional philosopher and the biologist. This is at one time a strength and weakness of the book. While this approach reaches a broader audience, the discussion of debates can leave one wanting a bit more depth. This is easy to remedy by supplementing the book with additional readings (not a bad idea, anyway, given the recent development of and debate surrounding PhyloCode). Ereshefsky makes the debates in taxonomy accessible to the outsider (no small feat) by presenting his arguments carefully and slowly, in a clear and transparent fashion, while nicely balancing the philosophical and biological concerns underlying these complex debates. This makes the book a good introduction to taxonomic debates, and I recommend it to anyone with a philosophical interest in the nature, naming, or classifying of biological taxa.
