The taxa that appear in biological classifications are commonly seen as representing information about the traits of their member organisms. This paper examines in what way taxa feature in the storage and retrieval of such information. I will argue that taxa do not actually store much information about the traits of their member organisms. Rather, I want to suggest, taxa should be understood as functioning to localize organisms in the genealogical network of life on Earth. Taxa store information about where organisms are localized in the network, which is important background information when it comes to establishing knowledge about organismal traits, but it is not itself information about these traits. The view of species and higher taxa that is proposed here follows from examining three problems that occur in contemporary biological systematics and are discussed here: the problem of generalization over taxa, the problem of phylogenetic inference, and the problematic nature of the Tree of Life.
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The decline of inferential reliability with increase in taxon rank is due to the inclusivity of taxa: higher taxa simply encompass larger parts of biodiversity than lower taxa. There is no strict correlation between the degree to which reliable inferences are possible over a taxon and the taxon’s rank, though.
Recently, Leonelli (2013) also argued that some classifications in biology can be a form of theory, highlighting the explanatory role of some types of classification. However, Leonelli focused on bio-ontologies and classifications of developmental stages and not on classifications involving taxa. In this paper, I will not address the general question whether classifications can be thought of as theories but I will shed doubt on the explanatory role of the classification of organisms into taxa and thus on its status as theory.
Even though I am critical of these accounts and of their counting biological taxa as natural kinds, I will not pursue the topic of natural kinds in this paper.
This means that the inference bases are the groups of male and female platypuses, respectively, rather than the group denoted by the name Ornithorhynchus anatinus.
The counterfactual is “Had the animal that I’m looking at now (in my local zoo) been born in the wild, it would have lived in eastern Australia or Tasmania.” I’m not sure how this claim could be true or false for the animal under consideration. While Ornithorhynchus anatinus is an indigenous species of eastern Australia or Tasmania, there is no reason why organisms of the species cannot be born in the wild elsewhere, for example as offspring of a pair that was shipped to a different continent and let loose in the wild.
For examples and discussion, see Reydon (2006).
Common characters, for instance, are worse indicators of relatedness than rare characters (Sober 1988: 213).
Note that for illustrative purposes I am oversimplifying in my description of methodological principles. While commitment to the principle of parsimony implies commitment to a particular school of thought in phylogenetic systematics (namely, Cladistics), within this commitment there still are multiple methods to choose from, such as Camin-Sokal parsimony or Dollo parsimony. Thus, even within one broad school of thought, there are different approaches that yield different results on the basis of the same data (e.g., Felsenstein 1979, 1988, 2004: 73–86).
As Richards put it: “Which hypothesis we accept as the best phylogenetic hypothesis depends on how we individuate characters. But if we have no satisfactory grounds for preferring one character individuation scheme over another, it is unclear why we should regard our evaluation of phylogenetic hypotheses as anything more than a reflection of our predispositions or biases. The outcome of phylogenetic inference therefore seems as much a consequence of illegitimate nonscientific factors as it is a consequence of legitimate scientific factors” (Richards 2003: 277).
E.g.: “The erection of (at least approximately) monophyletic higher taxa does, as cladists insist, make a significant contribution to the accommodation of inferential practices in evolutionary biology to relevant causal structures” (Boyd 1999: 183).
For eukaryotes, however, bifurcating trees often are adequate (Bapteste et al. 2009).
Note that it is a particular kind of distribution. On Waters’ account, distributions come in various forms, and the distributions that I am considering here are just one such form.
While the content of the location information changes in taxonomic revisions, taxa retain location information as such. When groups are moved in taxonomic revisions, old location information is replaced by new location information.
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I am indebted to two anonymous reviewers and to the guest editors of this special issue for helpful comments on an earlier version of this paper.
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Reydon, T.A.C. Taxa hold little information about organisms: Some inferential problems in biological systematics. HPLS 41, 40 (2019). https://doi.org/10.1007/s40656-019-0281-y
- Phylogenetic inference
- Systematic biology
- Tree of Life