Random thoughts on taxa. Written more for my non-biologist friends interested in semantics, but I’d be interested in critiques from taxonomists.
We classify in order to generate hypotheses by induction (i.e. prejudice). If members of a class C generally have property P, and an individual a is classed in C, then it might be a good bet that a has property P.
(I’m using ‘class’ in the sense of formal logic, not in the sense of the Linnaean rank.)
The problem of classification seems straightforward. You articulate a set of classes, each of which has a rule for determining membership. When a new individual comes along and you want to classify it, you test its properties using each class’s membership rule, and the result is a set of classes to which it belongs.
When classification is to become a community practice, names or phrases must be used to identify classes (so you and I can try to talk about the same class) and language has to be used to communicate membership rules. To come up with the same answers to a given membership question, or to be able to engage in evidence-based arguments, you and I have to have compatible interpretations of the tokens we use to communicate. By ‘compatible’ I mean in the sense of non-linguistic consequences, e.g. what observations or experiments we do in order to test some sentence under consideration (examining the properties of an individual to be classified, and so on).
A membership rule could be in terms of directly testable physical properties. But sometimes membership rules are not directly testable, such as rules involving descent. If the rule is that x is in class C if it descends from something having a given physical property, or that x is in C if x descends from the most recent common ancestor of y and z, then testing membership of a given x in C can be tricky. One has to evaluate potentially competing theories of descent, and the answer could be difficult to resolve.
The specification in language of membership rules can easily be incomplete, vague, or ambiguous, so that different people might judge membership differently depending on how they interpret the language of the rule. More likely, someone will detect the difficulty with a rule and refuse to interpret it one way or the other in those cases where it is unclear.
In biological taxonomy, a class’s membership rule is called a ‘circumscription’, and a class is called a ‘taxon concept‘. However, often people speak of a taxon rather than a class. It turns out that taxa and classes are very different from one another.
It is unclear what kind of thing a taxon is. It seems to be agreed that a taxon can change over time, i.e. a taxon can be connected with class c1 at time t2, and class c2 at time t2. Taxa are like houses, which change over time by being repaired, painted, and so on.
For example, the name Hyla denotes a taxon that formerly included the little frogs known as spring peepers, but no longer does.
It is not enough to say that a taxonomic name is interpreted to denote different classes at different times or in different contexts. This is because a taxon might be known by two names, or by different names at different times. The changing taxon really seems to be independent of its name, and is connected to sequence of classes over time.
So taxa are neither physical nor “abstract”, under the usual idea that abstract entities are eternal and don’t change.
There are similarly strange entities in other domains. Of digital documents we say that Alice changed ‘the document’ and sent it to Bob, who made another change. Alice’s ‘copy of the document’ no longer reflects the current state of the document after Bob’s change. A group of people editing a digital document somehow agree on what the contents of ‘the document’ are at any given time, because they are aware of how authority over the document is transferred, delegated, partitioned, and so on. But the document has no fixed physical location and may not even have a fixed name.
The bylaws of a corporation are similar. They are amended over time, and we don’t say that the corporation has new bylaws, we say that the bylaws have changed. We say that the bylaws used to say X, but now they say Y.
I suggest that taxa and digital documents, like promises, marriages, bylaws, contractual agreements, and so on, are the products of speech acts: the truth of their creation and their state changes are effected socially, through special kinds of communication. An unusual feature of taxa, compared to most speech-act products such as promises and marriages, is that they are referred to by name. The association of a name to a taxon is itself established by a speech act, similar to a christening. Promises and marriages can be created and changed and referred to, but they are not generally given names.
I’m sort of glossing over a complication, namely that the circumscription can stay the same, e.g. “conspecific with specimen X”, while theories about equivalent physical-property-based circumscriptions change over time, and in addition theories about what “conspecific” means change over time. We often see new publications for species that do not change the underlying definition “conspecific with specimen X” but do put forth some new theory of how to identify things that are. The pragmatic effect of this is that the taxon has changed, since the secondary or predictive circumscription has changed, although in some deeper sense it hasn’t.
What benefit does the community get through this level of indirection (name -> changing taxon -> class vs. name -> class)? Why not use different names for different classes?
In fact there are situations where taxonomists are careful to do so. References to classes take the form taxon-name sensu authority-reference, where the authority-reference (usually author + year) refers to a particular publication that lays out a particular circumscription. Taxa are therefore left out of the picture.
Taxonomists are responsible for taxon change – they perform “nomenclatural acts” which are similar in nature to the speech acts that create promises and marriages. Some acts create taxa, some change them for various reasons, and others reflect reclassification (the same taxon being placed in a different higher group). A taxon changes from association with c1 to c2 when an author judges c2 to be “better” than c1. Goodness might be judged, for example, according to whether c2 is thought more likely than c1 to be a clade (i.e. united by common descent), or considered better delimited than c1. Better delimitation could be either more precise description, or by being better biology, e.g. a better match to character discontinuities in natural populations.
At any given time a taxon has an associated class, and it also has a unique associated name. Nomenclatural speech acts change the name of a taxon. Some taxa are given a dozen or more names over their lifetime. (I don’t want to go into the rules of taxonomic nomenclature, as they’re explained elsewhere, e.g. here.)
I can only speculate on what’s going on here.
- Taxon names are shorter and easier to remember than class names. Most of the time the circumscription doesn’t change very much as the taxon changes, and people don’t get into much trouble by failing to specify which circumscription they mean. (Sometimes they do get into trouble, of course.) Most biologists consider the circumscription to be noise and leave it out, and they are often justified because many taxa have so far only had a single circumscription.
- Taxon names are good search keys when looking for biological information on an individual or population. What we know about individuals in class c1 may also apply to individuals in class c2, if they have both been associated with the same taxon.
- Replacing c1 with c2 for a taxon t with name n robs class c1 of a its association with n (via t), since c2 has taken over name n. This sends a strong message to the community not to use class c1 any more in classification.
These reasons may have made sense historically, but I don’t think they make for good science today. Now that we have an Internet, anyone ought to be able to look up a circumscription given a reference to it and figure out whether a given specimen satisfies it.
The advent of DOIs, and the increasing number of authority publications that have them, make them ideal as authority references.
The technical problems of using class references instead of taxon references are easy enough to solve. The hard part, obviously, is overcoming inertia and getting any sort of support for reform from biologists and publishers.
Membership in a class does change over time as a result of births and deaths. An individual that might belong to C at time t would probably not be said to belong to C before it was born or after it died. But because the class has a circumscription, we do not generally say that the class itself has changed, only that its membership has.
We can also speak of changing populations. Suppose that C is the class of members of population P. We might establish that at time t, an individual x is a member of C if and only if it is a member of a class D with a circumscription based on physical features. However, the population P can evolve, so that at some later time t’, an individual x’ might be in C but not in D.
Populations are physical entities, and like houses, they can change. The properties of members of C (in general) may have changed over time, but C itself (its membership rule) has not because it is based on a changeable physical entity.
These are completely different kinds of change than taxon change. What’s in common is that the truth of propositions that involve a class or taxon can change over time. But while class membership changes because the biological world changes, taxon membership changes because of speech acts.
I’m parodizing, a little bit, a world view in which we have lots of separate things and changes are localized to things in an orderly way. I’ve tried to explain taxa by following the logical consequences of the way biologists talk. I’m not satisfied that this is right or that there isn’t a better way to understand fictitious entities like taxa. One is tempted either to eliminate such entities, and thereby to remove speech acts from one’s understanding of the world (perhaps moving the locus of change inside ourselves), or to look for speech-act-nature in all ‘things’ and consider that there may be authority structures in all discourse.