Proposal (573) to South American Classification Committee
Elevate Knipolegus cabanisi to species rank
Effect on SACC: This would split Knipolegus cabanisi from K. signatus.
Background: The current SACC footnote summarizes the situation:
98. The history of Knipolegus signatus and cabanisi is complex and confusing. Cory & Hellmayr (1927) treated them as separate species in separate genera: signatus in Ochthodiaeta (now Myiotheretes) and cabanisi in Knipolegus. Meyer de Schauensee (1970) also treated them as separate species in separate genera, with signatus in Myiotheretes ("Jelski's Bush-Tyrant") and cabanisi in Knipolegus ("Plumbeous Tyrant"). Traylor (1979, 1982) identified signatus and cabanisi as sister taxa, transferred signatus to Knipolegus, and considered them conspecific, but noted that they might also be considered separate species, as also noted by Ridgely & Tudor (1994). Sibley & Monroe (1990) considered them conspecific and coined the name "Andean Tyrant" for the composite species, and this was followed by Ridgely & Tudor (1994) and Fitzpatrick (2004); FjeldsĆ & Krabbe (1990) also considered them conspecific but used "Plumbeous Tyrant," but see Ridgely & Tudor (1994) for reasons not to use that English name.
New information: Hosner and Moyle (2012) published a phylogeny of Knipolegus based on mitochondrial and nuclear DNA sequences. The phylogeny included two K. s. signatus and two K. s. cabanisi. The two subspecies were recovered as sister, but were strongly divergent: 5.7–5.9% uncorrected pairwise distance in mitochondrial ND2 and fixed differences in two introns. The genetic evidence from four individuals is not as convincing as a well-sampled phylogeographic analysis, but light of well-documented differences between male and female plumages, the weight of the total evidence supports two species level taxa.
Recommendation: Yes, genetic divergences support long-understood plumage differences. I don’t feel particularly strongly about English names, but Jelski’s Black-Tyrant for K. signatus and Plumbeous Tyrant for K. cabanisi would be appropriate.
Hosner, P. A. and R. G. Moyle. 2012. A molecular phylogeny of black-tyrants (Tyrannidae: Knipolegus) reveals strong geographic patterns and homoplasy in plumage and display behavior. Auk 129: 156-167.
Ridgely, R. S., and G. Tudor. 1994. The Birds of South America, vol. _. University of Texas Press, Austin.
Traylor, M. A., Jr. 1982. Notes on tyrant Ŗycatchers (Aves: Tyrannidae). Fieldiana Zoology New Series, no. __.
Peter A. Hosner, March 2013
Comments from Stiles: “YES. Strong genetic evidence clear plumage differences support this Split; Hosner’s proposed English names also seem good to me.”
Comments from Zimmer: “YES. Genetic evidence supports what plumage differences have been suggesting all along. I also would agree with the proposed English names.”
Comments from Pacheco: “YES. This proposal is well supported by genetic evidence presented by Hosner & Moyle (2012).”
Comments from Nores: “YES. Genetic evidence (and to some extent also plumage differences) leaves no doubt that treatment of cabanisi as a separate species is required.”
Comments from Remsen: “NO. The genetic evidence by itself is of questionable utility for assigning taxon rank with respect to its sister taxon. Would someone out there please write a paper on this! These two taxa differ roughly by about the same % sequence divergence as other taxa ranked at the species level in the same genus, and that is the reason that thee proposal and other committee members consider the genetic evidence as strong. However, these distance metrics are based on a tiny number of loci compared to the number of variable loci in the genus, and so the untested assumption is that other loci would show similar levels of divergence. Even if that were the case, what does it really mean? Small effective population sizes, quite likely to apply in a case like this, can underlie rapid differentiation at neutral loci. More generally, as Andrés Cuervo’s dissertation data for example have shown, % sequence divergence in Andean birds is all over the place at any taxonomic rank, and many taxa treated at the subspecies level or not accorded any taxonomic rank at all have % sequence divergence levels equivalent to or greater than this pair.
“That leaves differences in plumage. I cannot think of any other taxa treated as subspecies in the Tyrannidae that show geographic variation in female plumage, but with so few sexually dichromatic taxa, there is not much to compare to. Certainly for male plumage, no one treats geographic variation as a species-level character.
“In the Tyrannidae, vocal differences are the “gold standard” for recognizing taxa at the species level. Until those data are published for these two, I think we should be conservative on this one. The differences in female plumage suggest that these two should be ranked at the species level, but without vocal data, I think we should wait.”
Additional comments from Remsen: “Just in – an in-press paper by our postdoc Brian Smith shows that there is hope for using comparative genetic distances, but it’s not simple: http://www.genetics.org/content/early/2013/10/18/genetics.113.157776.full.pdf+html”
Comments from Jaramillo: “YES – This is not only a change here due to genetic distances, but the fact that the genetic distances support the original idea that species level taxa are involved. Males in Knipolegus are essentially black, some shiny, others less so, some with blue bills, others dark bills, white on wings or no white on wings. As far as males go, these two are as different as other males in the group which are clearly good species. They differ in size, how black the plumage is, no versus white edging on inner edge of flight feathers, and black vs. blue bill with black tip. The females differ as well as noted. Presumably plumage patterns in males and females aid in mate choice, as this is one of the few genera in the flycatchers where there are distinctive plumages in males and females! The problem is that most of this group is not all that vocal, and if we had abundance of vocal data we would likely find some more clarity on species level judgments in the group. Similarly from plumage and genetics there is a suggestion here that aterrimus may be a multi species group.”
Further comments from Remsen on English names: With the passage of this proposal, I tentatively used “Plumbeous Black-Tyrant” for K. cabanisi and “Jelski’s Black-Tyrant” for K. signatus. Use of just “Plumbeous Tyrant” is problematic because its sister species cabanisi is a “Black-Tyrant”. However, “Plumbeous Black-Tyrant” almost sounds contradictory. The English names in this genus need a review: some are hyphenated Black-Tyrants, others not, and our policy of use of hyphenated group names requires restriction to monophyletic groups. If the “Black-“ were removed from Amazonian Black-Tyrant, then “Black-Tyrant” would mark a monophyletic lineage within Knipolegus. The other option would be to drop the hyphens, and just use “Black Tyrant” for the ones that are truly black.” Proposal forthcoming.