Case 3407 (van der Linde et al., 2007) is a proposal to conserve the binomen Drosophila melanogaster. The combination is threatened by revision of the genus Drosophila. We agree that the binomen should be preserved and we agree that the genus is large and paraphyletic but we disagree with some of the qualifying arguments. We would prefer that the taxonomy of DROSOPHILIDAE be guided, not driven, by molecular biological data.
We have the view that the Commission should accept this application to conserve the binomen Drosophila melanogaster but we strongly believe that the splitting up of the genus, once redefined, should only occur after comprehensive taxonomic investigation, not driven by the latest molecular phylogenetic data. We acknowledge that there is a range of views about how to deal with the various groups of species in Drosophila but we feel that there is still much work to be done before the numerous species can be correctly reassigned. We feel that this can proceed more freely, with less constraint, if melanogaster is the type of Drosophila. We see no reason why Drosophila should automatically be subdivided into different genera if the nominal species becomes melanogaster.
The genus Drosophila is the largest in the family DROSOPHILIDAE. It is divided into a number of subgenera and it also holds many species incertae sedis. Musca funebris is the type species of the genus Drosophila, and Drosophila melanogaster is the type species of the subgenus Sophophora. There is widespread agreement that the species currently accommodated in Sophophora represent a natural group that could well be removed from Drosophila and treated as a genus. Controversy surrounds this action and its consequences because the binomen Drosophila melanogaster is by far the most frequently used in all of biological science, and few people want to change it. An alternative is to make Sophophora an objective junior synonym of Drosophila (by replacing funebris with melanogaster as type species), thus preserving the binomen Drosophila melanogaster. The underlying concept of Drosophila, and the family DROSOPHILIDAE, would shift as a result of changing the nominal species of the nominal genus – put simply, the concept of the typical Drosophila (and therefore typical drosophilid) would change from D. funebris to D. melanogaster.
An application (Case 3407) is currently before the Commission to use its plenary powers to set aside all previous type fixations for Drosophila Fallén, 1823 and designate Drosophila melanogaster Meigen, 1830, as the type species of Drosophila Fallén, 1823; etc.
Six qualifications underpin the application; our comments on each are as follows:
1. ‘Phylogenetic and molecular studies show that Drosophila is paraphyletic and that an assortment of drosophilids lie within it.’ We agree that Drosophila is paraphyletic and we are confident that with further taxonomic work the genus will decrease in size and become a more natural group. This will require detailed and, unfortunately, prolonged taxonomic work. See, for example, the taxonomic work that led to the widely accepted proposals to remove the subgenera Scaptodrosophila Duda, 1923, Hirtodrosophila Duda, 1923, Lordiphosa Basden, 1961 and Dichaetophora Duda, 1940 (Grimaldi, 1990; Hu & Toda, 2002).
2. ‘The genus consists of four major clades.’ Taxonomically we see a single genus with a wide range of species, we see some clustering of species and we predict that those clusters will eventually be assigned to genera. There are 78 species incertae sedis and there are many species represented only by poorly preserved specimens. The task is not as simple as dividing the genus in four and elevating the remaining subgenera to the rank of genus.
3. ‘The paraphyletic nature of the genus Drosophila is unacceptable as it violates modern systematic practice.’ The statement may be true for systematics but paraphyly is permitted within our traditional nomenclature. That is not to say that it is desirable. Paraphyly is not a violation of taxonomic practice, it is regarded as suboptimal but it is tolerated until evidence allows better classification.
4. ‘The binomen Sophophora melanogaster would cause instability and confusion.’ We agree, but our discussion on this point led us to appreciate that many of our colleagues do not foresee a problem, which surprised us. If the binomen S. melanogaster was introduced, we think that in evolutionary biology and molecular phylogenetics – perhaps yes, there would be instability. In zoological taxonomy – no. In the normal course of taxonomic work, name-changes (new combinations) are commonplace, they are seen for what they are: a response to new and more compelling data.
The binomen Sophophora melanogaster would continue to convey a precise meaning, and in this sense there would be no confusion. With respect to nomenclatural instability, there may be considerable reluctance to adopt the unfamiliar binomen Sophophora melanogaster and many would, no doubt, continue using Drosophila melanogaster, Drosophila or just melanogaster. And this would be confusing. Information retrieval would be hampered.
Van der Linde et al. argue that many use Drosophila as a shorthand or vernacular name for Drosophila melanogaster and instability and confusion would result if melanogaster was combined with Sophophora. The use of a generic name for a species is already an undesirable situation that is not likely to improve or change whatever decision the Commission makes. Drosophila, not in italics, is also used as a common name for Drosophila melanogaster and other species, but when written in this way – in normal font – it has the same diminished significance as ‘fruit fly’, ‘vinegar fly’, ‘ferment fly’, or ‘pumice fly’. Application of these names is unrestricted, haphazard and entirely outside the Code. In the French language, researchers are accustomed to using ‘‘les Drosophiles’’ as a vernacular and this is (and could still be) used for flies in any drosophilid genus.
The question asked above could be reframed ‘would changing the type of Drosophila cause instability and confusion?’ To which we would answer: no. It is a layer of complexity for taxonomists but not for the rest of the scientific community who work with these flies.
For the drosophilid taxonomist the concept of Drosophila is based on the original type species D. funebris (Fabricius, 1787). Over the last 115 years new species have been placed in Drosophila either because they conform to its underlying generic concept based on D. funebris, or because no other genus in the DROSOPHILIDAE was available and the cautious or conservative taxonomist chose not to describe a new one. Lamb (1914), for example, when working on a collection from the Seychelles described 20 new drosophilid species placing 18 of them in the genus Drosophila; those 18 are now in seven different genera. Unfortunately and inevitably many large families have such ‘trash can’ genera, so-called because of the untidy assemblage of species they accommodate. Drosophila currently accommodates the flotsam and jetsam of the DROSOPHILIDAE and it will take a very long time for each species to be examined and correctly re-assigned. It is not simply a matter of examining a type specimen and assigning it to a genus.
The removal from Drosophila of the large subgenera Scaptodrosophila, Hirtodrosophila and Lordiphosa and the treatment of them as genera (Grimaldi, 1990) was widely accepted. However, his treatment of the Hawaiian drosophilids (many of which were originally placed in Drosophila) has met with disagreement and alternative classifications have subsequently been proposed (O’Grady, 2002). We see the process of offering alternate hypotheses and classification schemes as entirely appropriate and we have a strong sense that gradually we are moving towards a more natural classification scheme overall.
In summary, we agree that it is useful to replace D. funebris as the type of Drosophila in order to preserve the binomen Drosophila melanogaster. But we do not think it is appropriate, at this stage, to automatically and without additional comprehensive taxonomic work, remove all major groups of the newly defined genus.
Grimaldi, D.A. 1990. A phylogenetic, revised classification of genera in the Drosophilidae. (Diptera). Bulletin of the American Museum of Natural History, 197: 1–139.
Hu, Y.-G. & Toda, M.J. 2002. Cladistic analysis of the genus Dichaetophora Duda (Diptera: Drosophilidae) and a revised classification. Insect Systematics & Evolution, 33: 91–102.
Lamb, C.G. 1914. The Percy Sladen Trust Expedition to the Indian Ocean in 1905 etc. Transactions of the Linnean Society of London, Series 2, Zoology, 16: 307–372.
O’Grady, P. 2002. Notes on the nomenclature of the endemic Hawaiian Drosophilidae. Bishop Museum Occasional Papers, 69: 36–40.