Molecular Phylogenetics and Evolution 40 (2006) 900–902 www.elsevier.com/locate/ympev Letter to the editor Sicyopterus lagocephalus: Widespread species, species for species recognition? With regard to the first question, complex, or neither? A critique on the use of molecular data we maintain that species diagnoses must rely on evidence for species identification that is in the form of character-based apomorphies (= character-state transformations). Pairwise divergences In a recent study, Keith et al. (2005) analyzed partial are necessarily distance-based, not character-based, so they cytochrome b sequences for 55 terminals belonging to the are incapable of identifying the character-state transforma- Indo-Pacific sicydiine (rock-climbing) goby genus Sicyopte- tions necessary for diagnosing any clade or terminal. With rus and concluded that the specimens they refer to as ‘‘Sicy- regard to the second question, we do not believe that there opterus lagocephalus’’ formed a single widespread species is a single immutable pairwise divergence value that is diag- that ‘‘occurs throughout 18,000 km from the west of the nostic for species recognition, particularly given wide vari- Indian Ocean... to the east of the Pacific Ocean.’’ They ation in the rates of molecular evolution among lineages based their ‘‘single widespread species’’ hypothesis on the (even those closely related), not to mention issues of taxon monophyly of an assemblage comprising the specimens sampling density. Nevertheless, the burden of proof falls they referred to as ‘‘Sicyopterus lagocephalus’’ and a curso- on the authors, who fail to provide an answer. ry discussion of pairwise sequence divergences. What Keith Species complex. Prior to Watson et al. (2000), Sicyopte- et al. fail to explain (or even address) is how they justify rus lagocephalus was restricted to islands of the western such a conclusion in light of the evidence presented and Indian Ocean (see below, and Sparks and Nelson, 2004 what hypothesis they have actually tested. The stated goal for additional commentary on the validity of this taxon). of their study was to analyze ‘‘representative taxa of the Watson et al. (2000) placed 12 species into the synonymy genus Sicyopterus, and specially [sic] the ubiquity of the of S. lagocephalus, greatly expanding the range of this spe- species S. lagocephalus, in order to determine whether its cies to include Pacific forms. It is clear that there is substan- huge range is really occupied by only one species, and to tial hierarchical structure evident for ‘‘S. lagocephalus’’ in describe its possible genetic subdivisions.’’ Unfortunately, the cladograms presented by Keith et al. (2005) (their Figs. they failed to test whether or not this particular clade of 2 and 3); such structure is suggestive of reproductive isola- Sicyopterus comprises a single widespread species, a species tion and a lack of gene flow. Furthermore, their own complex, or neither. We will briefly discuss each of these molecular clock estimates suggest that some ‘‘genetic sub- possibilities in light of the evidence presented by Keith divisions’’ of S. lagocephalus have been reproductively sep- et al. (2005) and previous studies of Sicyopterus, and con- arated longer than many other Sicyopterus species that they clude with a critique on the use of molecular methods for consider to be valid (their Fig. 3). From the evidence taxonomic purposes. (= topologies) presented, both the French Polynesian and Single widespread species. By the authors’ own account, Mascarene-Comoros clades of ‘‘S. lagocephalus’’ are rela- the conclusion that their ‘‘Sicyopterus lagocephalus clade’’ tively old (their Fig. 3). Indeed, the French Polynesian comprised but a single species was only justified by ‘‘the clade of ‘‘S. lagocephalus’’ is estimated to be older than tree topology together with the small genetic distances S. rapa and S. marquesensis, both of which they consider measured between haplotypes.’’ Given that species are his- to be valid species; however, this problematic and conflict- torical individuals, it is, therefore, unnecessary for species ing result is not discussed. Moreover, a maximal ‘‘intraspe- to be monophyletic (e.g., Skinner, 2004). As a result, the cific’’ divergence of 2.6% is reported for ‘‘S. lagocephalus’’ monophyly of this assemblage provides no evidence with between New Caledonian and Mauritian samples; whereas, regard to species recognition, although it may serve as a interspecific divergences of 3.7% are reported between both heuristic toward species delimitation. With regard to the S. marquesensis and ‘‘S. lagocephalus’’, and S. marquesensis ‘‘small’’ pairwise sequence divergences for partial cyto- and S. aiensis (Keith et al., 2005). Yet, there is no accom- chrome b sequences that the authors present as evidence panying discussion as to why 3.7% divergence in partial for a single species, Keith et al. fail to address the critical cytochrome b sequences implies distinct species, whereas questions: can pairwise sequence divergences be used to 2.6% does not. In short, the authors’ decision to recognize diagnose species and, if they can diagnose species, what distinct species in the former (3.7% divergence) and a single amount of cytochrome b sequence divergence is required species in the latter (2.6% divergence) is entirely arbitrary 1055-7903/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.ympev.2006.05.016 Letter to the editor / Molecular Phylogenetics and Evolution 40 (2006) 900–902 901 and unsupported by the divergence time estimates argument that S. lagocephalus, identified by Watson presented. et al.’s (2000) morphometric and meristic determination, Neither. Although largely ignored by Keith et al. (2005), is S. lagocephalus sensu Watson et al. (2000). An equally the availability of the name Sicyopterus lagocephalus has tenable result of their study could have been that all Sicy- been refuted by Sparks and Nelson (2004), who argued that opterus form a single species because they too are mono- no Indo-Pacific sicydiine goby could properly be referred phyletic; a line of reasoning that could be extrapolated to as Sicyopterus lagocephalus. Briefly, the type specimen out as far as one would like to take it. This example illus- of Gobius lagocephalus Pallas, 1770 is lost, the original trates why a rational and defensible justification for species description and illustration lack diagnostic features, and limits cannot be based on a topology alone (see also Davis the type was collected from ‘‘American Seas.’’ Given that and Nixon, 1992; Brower, 1999; DeSalle et al., 2005). Sicyopterus is restricted to the Indo-Pacific and does not The use of molecular methods for taxonomic purposes. occur in American seas, and that Pallas’ description and Most taxonomic studies still employ traditional compara- illustration can only be accurately classified to the subfam- tive anatomical approaches, but there is a growing body ilial level, Sparks and Nelson (2004) contended that Gobius of literature focusing on the integration of molecular data lagocephalus must be treated as a nomen dubium. Therefore, into taxonomy. A charge for the use of sequence data in despite Watson et al.’s (2000) claim to the contrary, the species identification is being spearheaded by the Barcode name is not available for any Indo-Pacific Sicyopterus of Life initiative (see October 2005 Philosophical Transac- species. tions of the Royal Society, London: Biological Sciences We believe that the primary reason that Keith et al. 360:1462 for the proceedings of the First International Bar- failed to achieve their goal of testing the ubiquity of their code Conference). It is obvious from this symposium vol- ‘‘Sicyopterus lagocephalus clade’’ is because they uncritical- ume and the studies cited within, that the methods for ly accepted Watson et al.’s (2000) ‘‘determination’’ of the DNA taxonomy alone, or its integration into taxonomy species limits of S. lagocephalus a priori as fact or back- as a whole, are still a topic of considerable debate. Most ground knowledge (Keith et al., 2005, 722). We stress that studies focusing on barcoding as a means of species recog- this acceptance was solely an appeal to authority or faith, nition have relied on distance-based methods (e.g., Hebert in that Watson et al. (2000) failed to provide any apomor- et al., 2003); however, such an approach is at odds with tra- phic features (genotypic or phenotypic) or a unique combi- ditional character-based phenotypic taxonomy (DeSalle nation of apomorphies that, in conjunction with a et al., 2005). Although we agree with DeSalle et al. cladogram, would allow either Watson et al. (2000) or (2005) that character-based phylogenetic methods are more Keith et al. (2005) to conclude that what they had in their appropriate than distance-based methods for establishing hand could be assigned to S. lagocephalus (see Sparks and so-called DNA barcodes or addressing alpha-taxonomic Nelson, 2004). Perhaps this might explain why one of the problems, we believe that a combination of character- putative S. lagocephalus specimens examined in the Keith based phylogenies and non-tree based concepts or methods et al. study (SIC840, AY940724) is instead identified on (e.g., population aggregation analysis [Davis and Nixon, GenBank as a Cotylopus. We maintain that since Keith 1992], cladistic haplotype aggregation [Brower, 1999]) et al. assumed a priori that Watson et al.’s (2000) determi- should be employed to examine all available evidence for nation for species