By Christopher Taylor at 1/29/2008 10:20:00 am
As I've commented before on this blog, taxonomy holds an unusual position in the biological sciences in that it fills two equally significant roles. On the one hand, it is a science in its own right, investigating the best way to describe and express the relationships between organisms. On the other hand, it supplies the means for communication between biologists in all fields. For the most part, these two aims compliment each other, but sometimes they can clash. The first aim implies continual change, as our understanding of the relationships between organisms changes and (hopefully) improves. Wheeler (2007) commented in a recent editorial that "Doing taxonomy as an independent science advances simultaneously both the aims
of taxonomy and its users", a sentiment that I agree with fully (be warned, though, that the general tone of Wheeler's editorial is fairly incendiary). To fulfil the second aim, however, a certain amount of stability is usually desired, as researchers who are not working in taxonomy may have trouble keeping up with the changes (or, for that matter, appreciating their necessity).
All of the codes of nomenclature have a central commission to regulate taxonomy - zoology has the International Commission on Zoological Nomenclature, botany has the International Association for Plant Taxonomy. One of the main roles of these commissions is to allow suspension of the usual rules in cases where their strict application would cause more trouble for communication than otherwise. In the case of zoology, applications for rulings on such cases that are submitted to the ICZN are published in the journal Bulletin on Zoological Nomenclature, allowing researchers the opportunity to comment on submissions before the Commission decides on them. One submission that appeared in the December 2007 issue of the BZN involves a case that could affect a large number of researchers in many fields - the impending revision of the fly genus Drosophila.
Drosophila is a very large genus, containing about 1500 species. However, phylogenetic studies (e. g. Robe et al., 2005) have found that Drosophila as currently defined is significantly paraphyletic with regard to a number of other genera in the family Drosophilidae. There are two options to resolve this situation. One is to sink all the smaller genera arising from Drosophila into the larger genus. However, this is not regarded as a suitable solution - not only would it leave Drosophila with over 2000 species, but it would result in over a hundred secondary homonyms (two or more species ending up with the same name as a result of change in genus assignment) that would require correction. The other option, that seems much more likely to be used, is to divide Drosophila into a number of smaller genera. The name Drosophila would then be restricted to a smaller group of species closely related to the type species.
All this would be fairly routine, except that one of the species affected happens to be one of the most widely used model organisms in genetics - the "fruit fly" Drosophila melanogaster (the inverted commas are because Drosophila isn't really a fruit fly proper, but a vinegar fly). So familiar is this species that many people simply refer to it as Drosophila without invoking the species name. One might be forgiven for expecting D. melanogaster to be the type species of Drosophila, but it's not. That honour goes to Drosophila funebris (shown at the top of the post in a photo from here). And as it happens, the two species are not that closely related. If Drosophila is divided up, the Drosophila melanogaster everyone knows and loves becomes a far less familiar Sophophora melanogaster. How will geneticists respond to the loss of their favourite organism?
To avert an apocalypse in evolutionary biology, van der Linde et al. (2007) have made a submission to the ICZN to redefine the type species of Drosophila. They suggest that that honour be given to D. melanogaster rather than D. funebris, meaning that D. melanogaster would remain forever more Drosophila. But if this is accepted, what will become of D. funebris and its close friends and relatives? Will the ICZN exalt D. melanogaster to the position of type species? Or will the geneticists just have to learn to refer to Sophophora, and like it?
Linde, K. van der, G. Bächli, M. J. Toda, W.-X. Zhang, Y.-G. Hu & G. S. Spicer. 2007. Case 3407: Drosophila Fallén, 1832 (Insecta, Diptera): proposed conservation of usage. Bulletin of Zoological Nomenclature 64 (4).
Robe, L. J., V. L. S. Valente, M. Budnik & E. L. S. Loreto. 2005. Molecular phylogeny of the subgenus Drosophila (Diptera, Drosophilidae) with an emphasis on Neotropical species and groups: a nuclear versus mitochondrial gene approach. Molecular Phylogenetics and Evolution 36: 623-640.
Wheeler, Q. D. 2007. Invertebrate systematics or spineless taxonomy? Zootaxa 1668: 11-18.