More on Drosophila and Sophophora

Before I start this post, a public service announcement. This month, a group of bloggers have joined together to present an initiative called Silence is the Enemy. The aim of this initiative is to raise awareness about the epidemic of sexual assualt faced by women in war-torn parts of Africa, and also to raise awareness about the issues of sexual assault in general. As part of this campaign, all those websites connected to it will be donating their proceeds from the month of June to Médecins Sans Frontières, which is working as we speak to bring aid and comfort to women affected by sexual assault (as well as bringing aid in countless other ways to people around the world who would otherwise be unable to obtain proper medical treatment). Contributing sites include The Intersection, On Becoming a Domestic and Laboratory Goddess, Bioephemera, Adventures in Ethics and Science, Aetiology, Neurotopia, The Questionable Authority and Drugmonkey. Every time you visit one of those sites in the coming month, you help to raise revenue for an extremely important cause. So visit early, visit often.

Now, on to today's post:

Some of you may remember this post from over a year ago, when the proposal was put before the ICZN to make Drosophila melanogaster the type species of the genus Drosophila. For this post, I'm going to take that other post as read. At the present point in time, the ICZN has not yet voted on that application. But this does not mean that things have been sitting unremarked - quite to the contrary.

As well as publishing applications to the ICZN and their results, the Bulletin of Zoological Nomenclature also publishes comments from the general taxonomic public on cases being considered, allowing other workers to put forward their arguments for whether the commission should accept or deny an application. To be quite honest, this is usually the dullest part of the Bulletin. The majority of applications are fairly straightforward, and it is fairly uncommon for comments to say anything much more extensive than either "sounds good to me!" or "No sir, I don't like it"*. The Drosophila case, however, has inspired a barrage of commentary in the pages of the Bulletin to a level that has probably never been seen there before. The June 2008 issue of the Bulletin included nearly fourteen pages of commentary on the application. Some of these "comments" bordered on being full articles - Prigent (in the June 2008 issue), for instance, filled up three full pages ("no sir, I don't like it"), while McEvey et al. (also June) put their names on two and a half pages ("no sir, I don't like it, but it should probably happen anyway"). [Disclaimer: I haven't yet seen the March 2009 issue of the Bulletin, but apparently it's got even more comments to read.] So what have been the main points raised for and against the proposal?

*This is not to say that the comments are completely pointless - for a start, they're probably the primary means for the commissioners to gauge the popularity or otherwise of an application. Still, the vast majority of them are not particularly likely to become citation classics.

A common complaint has been that supporting the decision would be to support a particular classification or method of classification (or, to put it another way, "Oh noes! They be taking my paraphylum!") As stated by Thompson et al. (June 2008):

The proposal declares that the current concept of Drosophila is 'paraphyletic' and thus 'violates modern systematic practice'. That practice is cladistics or Hennigian systematics. For followers of 'evolutionary' systematics or phenetics, paraphyletic taxa are acceptable. Then there are the issues of the utility of large and small taxa (i.e. lumping vs splitting). We feel strongly that the Commission should not be endorsing one classification paradigm over another.

Some of you may have been struck by the gratuitous conflations of phylogenetic and taxonomic methodologies in the second and third sentences there*, but let's ignore those for now and move on, shall we? It would indeed be a strong violation of the ICZN's principles to judge between paradigms (though I've previously questioned the possibility of a truly paradigm-free nomenclatorial system). In this case, however, the Commission is being asked to do no such thing. Those authors who wished to retain Drosophila melanogaster and D. funebris (the current type species) within a single genus, whether paraphyletic or not, would still be perfectly free to do so - from their perspective, the case is largely irrelevant. Even if an author wised to divide up Drosophila by non-cladistic means, then odds are that they would still end up wanting to place D. melanogaster and D. funebris, because the two species are about as different from each other as any taxa within Drosophila could be - that's why they've been placed in separate subgenera in the first place - and then we'd be facing the exact same question. Indeed, it could be argued (whether validly or not) that the current situation is the one impeding taxonomic freedom, because no-one has been willing to take the step of removing D. melanogaster from Drosophila. So overall, the "no paradigm endorsement" argument is dead in the water from the outset.

*I'm not sure that a phenetically-derived classification really can be said to "permit" paraphyly - it's more that for phenetics, questions of monophyly vs. paraphyly vs. polyphyly become irrelevant.

The second major argument is that changing the type species increases the amount of taxonomic instability rather than decreasing it. Unlike the first argument, this one actually has some legs. As I mentioned in the earlier post, the current subgenus Drosophila is considerably larger than the subgenus Sophophora to which D. melanogaster belongs. Therefore, making D. melanogaster the type species of Drosophila potentially means that more individual species will end up undergoing name changes than if the type species remained D. funebris. Also, while D. melanogaster is the most commonly used Drosophila species in research, it is not the only species used in research. A significant number of other species have also come under the microscope, and some of these other model species (such as D. virilis and D. mojavensis) belong to subgenus Drosophila rather than Sophophora. On the other hand, many more model species (such as D. simulans and D. yakuba) are also members of Sophophora, so changing the type species preserves their names in their current combinations as well.

From a purely taxonomic viewpoint (as many commenters have stated), the answer is a simple one - the current situation is clearly valid under the rules, nomenclatural changes are a perfectly valid part of an developing taxonomic system, there ain't nothing wrong with Sophophora melanogaster, whaddya complaining about? Unfortunately, the reason why this case was proposed in the first place was that such a change does not only affect taxonomists. The case is most elegantly summarised, I think, by McEvey et al.:

The binomen Sophophora melanogaster would continue to convey a precise meaning, and in this sense there would be no confusion. [However] 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.

Thompson et al. claim that such fears of confusion are overblown, and specifically cite the case of the mosquito Aedes aegypti, widely studied as a major disease vector, which has been renamed and almost universally accepted as Stegomyia aegypti in recent taxonomic revisions. This was perhaps the most comic moment in the affair to date, because as pointed out by van der Linde et al. in the December 2008 Bulletin, Aedes-errr-Stegomyia aegypti provides a very strong argument in favour of the application. Taxonomists have mostly accepted the name Stegomyia aegypti, but almost everyone else working on the beast - ecologists, parasitologists, epidemiologists - has rejected it, and continues to use the name Aedes aegypti. A wide rift has developed between the two sides, making communication between disciplines increasingly difficult. One should never underestimate the holding power a name can have if it somehow catches the public's imagination. After all, we still witness the occasional trotting out of Brontosaurus, a name that was in proper use for less than twenty-five years and was sunk into synonymy more than a hundred years ago.

So ultimately the question is whether preserving the names of a smaller number of economically significant taxa is more important than preserving those of a much larger number of taxa of less direct significance to humans. And I have to admit, I'm glad I'm not one of the people having to make an actual decision on this one.

REFERENCES

Linde, K. van der, G. Bächli, M. J. Toda, W.-X. Zhang, T. Katoh, Y.-G. Hu & G. S. Spicer. 2008. Comment on the proposed conservation of usage of Drosophila Fallén, 1823 (Insecta, Diptera). Bulletin of Zoological Nomenclature 65 (4): 304-307.

McEvey, S. F., M. Schiffer, J.-L. Da Lage, J. R. David, F. Lemeunier, D. Joly, P. Capy & M.-L. Cariou. 2008. Comment on the proposed conservation of usage of Drosophila Fallén, 1823 (Insecta, Diptera). Bulletin of Zoological Nomenclature 65 (2): 147-150.

Prigent, S. R. 2008. Comment on the proposed conservation of usage of Drosophila Fallén, 1823 (Insecta, Diptera). Bulletin of Zoological Nomenclature 65 (2): 137-140.

Thompson, F. C., N. L. Evenhuis, T. Pape & A. C. Pont. 2008. Comment on the proposed conservation of usage of Drosophila Fallén, 1823 (Insecta, Diptera). Bulletin of Zoological Nomenclature 65 (2): 140-141.