Recently a presentation was made to the Fly Board regarding maintaining Drosophila melanogaster as a valid name by redesignating it as type of the genus Drosophila. The presentation reflects a case submitted to the Bulletin of Zoological Nomenclature (Case 3407, BZN 64(4): 238-242). The purpose of this memo is to clarify the issues raised by that submission and to provide the Fly Board with the perspective of the Drosophila taxonomy and systematics community
Taxonomy, Nomenclature and Systematics
Taxonomy is the field of describing species and placing them in higher-level groups (genera and families). Rules of nomenclature, codified by the International Commission of Zoological Nomenclature (http://www.iczn.org/iczn/index.jsp), provide a stable framework with which taxonomists can name and place species. For example, Meigen described the species “melanogaster” in 1830. He placed it in the genus Drosophila; a group defined by Fallen in 1823, based on similarities between melanogaster and the type of the genus, Drosophila funebris Fabricius, 1787. Since D. funebris was described first, it is designated as the type of the genus Drosophila.
Systematics, while related to taxonomy and nomenclature, is a field with distinctly different goals. Modern systematics uses a rigorous analytical framework to reconstruct the evolutionary history of a group using either morphological or molecular characters. This is often done to “test” the existing taxonomic framework. Are traditional groups monophyletic (i.e., descended from a single common ancestor) or are they the result of multiple evolutionary events? Monophyly or non-monophyly of a specific group in a systematic analysis can have important implications to subsequent evolutionary analyses but has little or no bearing on taxonomy or nomenclature. This means that, even if the genus Drosophila is not monophyletic, there is no real impediment to describing or naming new species or to placing them in existing or new taxonomic groups. The fact that the type of the genus historically has been in a clade (monophyletic group) separate from D. melanogaster does not mean that the genus name of the species melanogaster must change.
Congruence between Taxonomy and Phylogenetic Systematics
Many researchers attempt to revise existing taxonomic groupings so they correspond to monophyletic groups. This is useful for downstream evolutionary analysis, but is not a prerequisite for either systematic or taxonomic progress, nor does the Code of Zoological Nomenclature require such action. If one were to attempt such a revision, the following steps would be considered best practice:
1. Phylogenies should be based on repeatable analyses of primary data, not literature reviews or summaries of other work that may (or may not) be independent of one another.
2. Statistical support for relationships slated for revision should be strong. Changes based on weakly supported conclusions will lead to taxonomic instability and confusion in the literature.
3. The division of a paraphyletic genus into smaller, monophyletic units should be accomplished by a thorough examination of all species within that genus. Incomplete or poorly executed taxonomy will lead to groups that, while monophyletic, are not well defined in terms of morphological characters, meaning that recognizing this group will be difficult or impossible without phylogenetic analyses. A cursory (or absent) examination of material will lead to taxonomic instability – species will need to be removed from or added to genera in the future.
With a group as large as Drosophila, it is clear that a number of taxonomists and systematists, working in concert, will be required to bring about this restructuring.
The van der Linde Proposal
In the case of van der Linde’s work on Drosophila, none of the three criteria above are met. No new data are brought to bear on the question of Drosophila phylogeny; all the supporting analyses are based on studies existing in the literature. The support for many of these groups is weak due to poor taxon and/or gene sampling. Furthermore, many studies overlap in terms of gene sampling, meaning that they are not truly independent lines of evidence. Finally, there is no synthetic revision accompanying this work – or even being proposed. This means that, while the genus names of about 300 species in the melanogaster and obscura species groups will remain Drosophila, the names of over 2500 other species will need to be changed, including four of the recently sequenced species (D. willistoni, D. virilis, D. mojavensis, D. grimshawi). There is no proposal for what groups these species will now belong to or how this Herculean task will be accomplished. Comments in opposition have been published by nearly all Drosophila taxonomists not signatory to the van der Linde proposal (Grimaldi, 2008; McEvey et al. 2008; O’Grady et al. 2008; Thompson et al. 2008). Based on these arguments, Dr. Toda, one of van der Linde’s coauthors and the preeminent Drosophila taxonomist in Asia, has withdrawn his support for her proposal.
The Drosophila community has several options at this point and has yet to meet to discuss them fully. A workshop this November will address these issues and attempt to come to some sort of consensus. The entire Drosophila community will be invited to attend. Potential options include:
• Divide the genus Drosophila, as Dr. van der Linde suggests. If this course were taken, we would preserve the name Drosophila melanogaster through redesignation of the type. This action, however, would necessitate revision of over 3000 species and the erection of new genera for the over 2500 taxa not placed in the subgenus Sophophora.
• Redefine the genus Drosophila more broadly and include several previously defined genera (e.g., Zaprionus, Scaptomyza) as subgenera of Drosophila. This would require no redesignation of the type (D. melanogaster would still be Drosophila) and only localized revision of the groups sank to subgeneric status.
• Do nothing. A growing number of taxonomists are discarding the Linnean system altogether in favor of a tree-based (phylogenetic) system of nomenclature. Such a system would require no taxonomic revision and no redesignation of types. All species and their taxonomic status could easily be tracked and updated in Flybase as additional data are generated.
A decision not to support of Dr. van der Linde’s proposal at this time is NOT a decision against preserving the name Drosophila melanogaster. It is merely a decision to wait until additional data accumulate and is analyzed in a comprehensive, repeatable fashion. In essence, it is a decision in favor of good scientific practice. No geneticist would describe a gene without a phenotype (or a sequence). This is basically what Dr. van der Linde is proposing to do.
There is no compelling taxonomic, nomenclatural, or phylogenetic justification to change the genus name of Drosophila melanogaster. In fact, no Drosophila taxonomist or systematist (e.g., anyone who has ever described a species or published a phylogeny) is proposing to do so. There is no consistent statistical support for new generic groups, and no morphological characters to define them. The Drosophila taxonomy community is opposed to van der Linde’s proposal. To date, three comments have been published in the Bulletin of Zoological Nomenclature on this issue and six more are in press for the June issue. Of these, the only one supporting van der Linde’s position was submitted by a wasp systematist.