Natural History Photographs


Short Note on Butterfly Names

In this short note I briefly mention some issues regarding butterfly names. Because the note is so short, little is explained. In the much longer Notes on the Naming Game I also explain why names keep changing. These notes are written for a non-specialist audience.

The anatomy of scientific names

Biologists classify all living creatures in a hierarchical scheme. The things to be classified are species. Resembling species are grouped into genera (singular: genus), which in turn are grouped into families. Ranks above the family level are order, class and phylum. If these levels do not suffice, one may prefix them with sub- or super- to indicate a level just below or above the prefixed level. So subfamily is a level just below family rank, but above genus.

A species is referred to by a combination of the genus and species name; scientific names are thus binomial. For example, the Swallowtail is referred to as Papilio machaon. A complete reference also includes the name of the author that first described the species, as well as the year in which the description was published. For the Swallowtail the author was Linnaeus, who described it in 1758. So you might read Papilio machaon Linnaeus 1758. If the author's name is in parentheses, a later author changed the genus name; the species name of a valid description never changes. The validity of a name is formally regulated by strict rules, the International Code of Zoological Nomenclature.

A subspecies is a geographically isolated recognizably different form within a species. Reference to a subspecies is made by three names: those of genus, species and subspecies. For instance, the Swallowtaild around Norfolk are referred to as Papilio machaon ssp. britannicus. When confusion is unlikely to arise, genus and species name are normally abbreviated. Scientific names ar customarly italicized, or set in bold face type.

Below subspecies level there are the semi-formal ranks form, variety and aberration. They always refer to individual specimens, not to populations. In this way they differ from subspecies level, which always applies to entire (groups of) populations. They are semi-formal in that the above mentioned code does not regulate their use. Form is often used to designate seasonal variation in butterflies, but is also used for different colour forms that fly at the same time. Variety is used for all kinds of individual variations. Aberrations are mostly very rare colour forms that differ greatly from the normal pattern.

Why scientific names keep changing

The scientific names of butterflies appear to change frequently. For instance, the list of species typically found in butterfly field guides changes with every new edition. This is rarely because a new species is found, but because of instabilities in the naming game.

There are many ways in which butterflies can be classified, depending on what features you select, and what relationships you want your classification to express. Biologists aim to express evolutionary relatedness in their classification. To date, evolutionary relatedness is mostly expressed in terms of common ancestry. Now any grouping of species has a common ancestor, and any common ancestor in fact represents a node in the evolutionary tree. In view of the large number of butterfly species, there are many more levels in their evolutionary tree than can be represented in the traditional classification scheme. One therefore has to make more or less arbitrary choices which nodes of the evolutionary tree to express in the traditional classification scheme.

There are two reasons why butterfly classifications might change. First, insights in evolutionary relationships may change. This happens quite often, but mostly affects only the levels above the genus. Since species are referred to by genus plus species name, the every day user is often not even aware of these changes. However, the second reason that makes classifications volatile definitely affects every day use, because it especially concern genus and species level.

I just mentioned that one has to choose which nodes in the evolutionary tree one wants to express in the classification for every day use. Because different authors give different weights to the levels of the classification scheme, they pick different nodes to be expressed at the genus level. So the same node may be expressed by one author at the subgenus level, while another author expressed exactly the same node at genus level. For instance, in the past 25 years or so, you could encounter two names for the Painted Lady, Vanessa cardui or Cynthia cardui. Nowadays, Cynthia is relegated to the level of subgenus by most, but not all, authors. There are no facts that impinge on this choice; it is merely a subjective and ultimately arbitrary decision.

Not only do different authors make different choices for what level in the evolutionary tree is represented by a particular level in the classification scheme, the weights of the levels also change in time. Linnaeus put all butterflies in the genus Papilio. Given the state of knowledge (or lack thereof) in his days, that was a reasonable thing to do. But when more and more species were described, a single genus for all butterflies became quite inconvenient. Later authors thus started to create new genus names, like Iphiclides for the Scarce Swallowtail. This is an example of what I call rank inflation.

Rank inflation means that ranks in the hierarchy of classification tend to be upgraded in time. So a subspecies gets species-rank, species tend to get the rank of subgenus, and genera get family-rank. A clear example is the original genus Papilio as defined by Linnaeus now has the rank of superfamily, the Papilionoidea.

Rank inflation at the subspecies-species boundary is probably the most important reason why a new edition of a field guide might contain new 'species'. There is no spectacular new find of a hitherto unknown species; much more mundanely, some author has upgraded subspecies to species rank. A tell-tale sign of this is the occurrence of two closely related 'species' in adjacent areas, like south and north of the Pyrenean Mountains.