It is much easier to find an image to illustrate a story about sin than it is to find one to illustrate synonymy and those sinful images are more likely to tempt you to read the article too! I’ve chosen this painting by Rubens as it definitely contains the most biodiversity of the images available and, perhaps, the original sin. Your religious views may vary.

This is the fourth post in a series about the Taxon Name Linking Service project. In this post I’ll try and cover the issues surrounding synonymy. Synonyms are by their nature abstract things and hard to visualise. This is partly because we use the term to mean subtly different things.

The core principle of the International Code of Nomenclature for Algae, Fungi and Plants (ICNAFP) is the separation of nomenclature and taxonomy. Names are created via nomenclatural events (being validly published either for the first time or in a new combination) and bound to a type specimen (possibly via lectotypification). Taxa are created by authors using combinations of descriptions (characters) and example specimens. Names are bound to taxa via the rule of priority, the first published name who’s type specimen falls within the taxon is the accepted name of that taxon (barring special instances of conservation). Names thus follow the placement of their type specimens and are subservient to taxonomic opinion.

The classic analogy used is to imagine that each name is written on an index card. The card contains all the information of when and where that name was published and what the type specimen is. There are around 1.5 million of these virtual index cards stored alphabetically in the World Flora Online Plant List. To build a taxonomy the name cards are placed into a hierarchical set of folders, one folder for each taxon. There are about four hundred thousand such folders in the WFO Plant List. Each folder has a key card which is the accepted name for that taxon. Under the rules of priority this is usually the oldest (first published) name. The other cards in the folder are the synonyms. Placing the cards in the folders cannot affect what is written on them (they are immutable) but what is written on them may govern which folders they can be placed in. e.g. If the genus part of the name does not match the genus folder in which it is placed it can’t be the accepted name of a species.

My preferred analogy is that each name is a Christmas tree decoration in a big box of decorations waiting to be hung on the tree and the classification is a Christmas tree with each branch representing a taxon. We are all collaborating in decorating the tree.

There we have our basic definition of what a synonym is. It is a name that is considered to be in the same taxon as the accepted or preferred name for that taxon. This is similar to the everyday use of synonym – two words that mean exactly or nearly the same. The etymology of the synonym is with-name or together-name which doesn’t imply they necessarily have the same meaning but that they occur together. This reflects more closely the biological meaning. The fact that an accepted name of a taxon X is now a synonym in taxon Y does not mean that X is exactly equal to Y. In fact they are different by definition. For practical purpose we often treat them as being nearly equal until issues arise.

But how do those names end up in the same taxon? There are several answers to that question depending on what type of synonym we are talking about.

Homotypic synonyms

If a name has the same type specimen as the accepted name of a taxon then it should become a synonym within that taxon automatically – because name placement is governed by type specimen placement. This most commonly happens when species are moved between genera. If the same species is placed in another genus then it must have a new binomial name incorporating the genus name. The species that has been known as Apium repens (Jacq.) Lag. has recently been moved to the genus Helosciadium and become Helosciadium repens (Jacq.) W.D.J.Koch but it was originally published as Sium repens Jacq. All three of these names share the same type specimen and so must all be in the taxon that that specimen is identified to. Some taxonomists consider homotypic synonyms to be the only true synonyms. They occur purely as a matter of procedure. The person who defines the taxon doesn’t need to mention them we know they are synonyms because of the type of the accepted name they use.

Heterotypic synonyms

What if the taxonomist identifies the type specimen of another name (not the accepted name) to be part of their species? In that case the synonym is considered to be a heterotypic synonym. Its inclusion is a matter of judgment by the person who creates he classification. Both homotypic and heterotypic synonyms belong in a taxon because their type specimens are considered to belong in the taxon by the person who defines that taxon.

Automatic synonyms

If the person who defines the taxon includes the type of a name and creates a heterotypic synonym by doing so but doesn’t mention the other names that are based on that type specimen then we can automatically assume these are synonyms in the taxon as well. I refer to these as automatic synonyms but it isn’t an official term. It is just shorter than saying homotypic names of heterotypic synonyms of the taxon not mentioned by the author. They are worth identifying in a database because if they are placed anywhere else in the classification that would be a data integrity error.

Informal synonyms

It is common for taxonomic experts to want to express the relationship between a name that is invalid or illegitimate in some way and an accepted taxon. This might be because it is commonly used for this taxon or because the available description would place it within this taxon. Typically the name may be listed as a note in a flora or monograph if it isn’t included as a regular synonym. In the WFO Plant List the name is simply flagged with the appropriate nomenclatural status and placed as a synonym to indicate this and it is common to just refer to such names as synonyms.

Consequences for name matching

The Taxon Name Linking Service is mainly concerned with name matching. A user has the name of a taxon, typically a species, and they want to look it up in a database. But what are they looking up? That depends or who they are. A taxonomist may only be interested in nomenclatural information and will just want to be returned the information on the virtual index card for that name. From this they may deduce the homotypic synonyms but the taxonomy is less relevant. A regular person is far more likely to be looking up a taxon. If they enter a name that matches a synonym of taxon they will want to see details about the taxon perhaps with a warning that they have used one of its “alternative” names and that they should use the preferred/accepted name in the future. The owner of the data may or may not what the name matching to expand to automatic synonyms that have been calculated rather than explicitly declared by the author.

All these things need to be considered as part of a name matching specification that will apply across services.

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This project has received funding from the European Union’s Horizon Europe Research and Innovation programme within the framework of the TETTRIs Project funded under Grant Agreement Nr 101081903.

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