Ranking

A diversion in the natural classification series.

In natural classification, we typically do not find that patterns due to the process of historical causation come arrayed neatly in boxes within boxes, and yet one of the most common temptations is for classifiers to set up fixed ranks. The Linnaean scheme of classification for animals, plants and minerals famously set up a number (initially five) of ranks, with the “Empire of Nature”, divided into “kingdoms”, “orders”, “classes”, “genera”, and “species”. Later, “families” were added, and then “phyla” (among animals) and “divisions (for plants). By the end of the nineteenth century, however, there had been a constant division of ranks into “super-” and “infra-” ranks, and new ranks such as “tribes” were added. Ranked classifications for minerals, however, were abandoned fairly soon after Linnaeus, and was replaced by the scheme of “metamorphic, igneous and sedimentary” rocks (a process-based classification) and the later scheme for minerals based on chemical composition (which will be discussed later).

However, Linnaeus himself did not take his ranks seriously, or not as seriously as those who followed him, including Darwin and many of the post-Darwin evolutionists, right up to the Modern Synthesis in the period 1940-1970. “The species and the genus”, he wrote, “are always the work of nature; the variety mostly of cultivation; the class and the order [are the work] of nature and art.”* In contrast, even Darwin tried to come up with laws of the speciation rates of higher taxa on the understanding they were real things in themselves.

When people assert blithely that Darwin did not believe in the reality of species, they appeal to the following text (Origin of Species, 5th edition used):

Certainly no clear line of demarcation has as yet been drawn between species and sub-species – that is, the forms which in the opinion of some naturalists come very near to, but do not quite arrive at, the rank of species: or, again, between sub-species and well-marked varieties, or between lesser varieties and individual differences. These differences blend into each other by an insensible series; and a series impresses the mind with the idea of an actual passage. [p44f]

From these remarks it will be seen that I look at the term species as one arbitrarily given, for the sake of convenience, to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms. The term variety, again, in comparison with mere individual differences, is also applied arbitrarily, for convenience’ sake. [p46]

But here we see Darwin denying not the reality of the species, but of the rank of species (which Ernst Mayr incorrectly called the “species category”). Darwin holds that organisms fade from varieties to species and above. This sort of rank nominalism is very different from species “nominalism” (or better, species denialism; since there is no sense in which the species are nominal essences).

Ranking is one of those classificatory activities we feel obliged to undertake, and once we have done so, we also feel obliged to defend as natural, but this cannot be so. There are no reasons to think higher taxa are real ranks, although the things we rank can be. For example, the rank of “phylum” is arbitrary and depends solely on what we, as observers, take to be salient differences. But the phyla we name, such as “Chordata”, are real entities if they are formed by monophyletic relations. It’s just that you cannot compare one with another (bacterial phyla are a case in point: you cannot compare Chrysogenetes with one species, with Xenobacteria, with 26, let alone with, say, Annelida (worms), with 16 thousand and more. What makes a phylum (or, in plants, a Division)? Basically it’s that some systematist with authority has declared that it doesn’t resemble some other group “enough”. The declaration of a phylum is a joint fact: it depends on facts about the groups themselves, but also on facts about what impresses systematists.

In work done by Scott Atran and others (Atran 1995, 1998, 1999; Sousa et al 2002; Berlin 1973, 1976; Berlin et al 1973), it appears that we have a tendency to rank things at or around 5 to 7 levels (see figure). This melds nicely with Linnaeus’s initial ranking scheme of five, which was later extended to include family and phylum (division) before the rank explosion of the mid-19th century. This is a psychologistic fact, a fact about us. It is not the foundation for inferences about nature, but for inferences about the psychology, politics and social structure of science.

From Berlin et al, 1973: 215.

* “NATURAE opus semper est Species Genus: CULTURAE saepius Varietas; NATURAE & ARTIS Classis & Ordo.”, Carolus Linnaeus, Philosophia Botanica (1751), aphorism 162. Trans. Franz A. Stafleu. 1971. Linnaeus and the Linnaeans. The spreading of their ideas in systematic botany, 1735-1789, Regnum vegetabile, v. 79. Utrecht: Oosthoek, 67, modified by me.

References
Atran, Scott. 1995. Causal constraints on categories and categorical constraints on biological reasoning across cultures. In Causal cognition: a multidisciplinary debate, edited by D. Sperber, D. Premack and A. J. Premack. Oxford, UK: New York: Clarendon Press; Oxford University Press:205–223.
———. 1998. Folk biology and the anthropology of science: cognitive universals and the cultural particulars. Behavioral and Brain Sciences 21 (4):547–609.
———. 1999. The universal primacy of generic species in folkbiological taxonomy: Implications for human biological, cultural and scientific evolution. In Species, New interdisciplinary essays, edited by R. A. Wilson. Cambridge, MA: Bradford/MIT Press:231-261.

Berlin, Brent. 1973. Folk Systematics in Relation to Biological Classification and Nomenclature. Annual Review of Ecology and Systematics 4 (1):259-271.
–––. 1976. The concept of rank in ethnobiological classification: Some evidence from Aguaruna folk botany. American Ethnologist 3 (3):381-399.

Berlin, Brent, Dennis E. Breedlove, and Peter H. Raven. 1973. General Principles of Classification and Nomenclature in Folk Biology. American Anthropologist 75 (1):214-242.
Sousa, Paulo, Scott Atran, and Douglas Medin. 2002. Essentialism and Folkbiology: Evidence from Brazil. Journal of Cognition and Culture 2 (3):195.