Friday, August 11, 2017
Warbler Guy, did the proposal to split Yellow-rumped Warbler into three species pass? Or is the Yellow-rumped Warbler still considered three subspecies by the American Ornithological Society (AOS)?
No, sorry, Kristi (in Middleton, CA): The American Ornithological Society did NOT pass this proposal in 2017.Here's the comments from the AOS Committee on Classification and Nomenclature (North & Middle America committee members as to why this proposal did not pass: (Regards, Daniel Edelstein, Avian Biologist & Birding Guide, warblerwatch.com)
Proposal: Split Yellow-rumped Warbler Setophaga coronata into three species
YES - 1 without comment.
YES. A side note – The Toews paper (published in The Auk) states in the introduction "….the complex is currently treated as a single species, Setophaga coronata, by the American Ornithologists' Union, but as 3 species by the International Ornithological Committee (IOC); we used the IOC taxonomy in this article." I think AOU publications should follow AOU taxonomy. This point should to be made to the editor, if it hasn't already.
YES to split of goldmani, given how well-differentiated genetically and morphologically it is. A weak YES to split of coronatus and auduboni (despite the hybrid zone), given the indirect evidence for post-mating isolating mechanisms (which are after all isolating mechanisms), their different calls, and their differing morphology.
YES. First, I think the level of plumage and genetic differentiation between goldmani and other coronata taxa is comparable to differences among species in other closely related species groups in Setophaga. Therefore goldmani should be treated as a species. In regards to coronata/auduboni, the lack of assortative mating is troublesome, but I think the proposal does an adequate job to show that selection against hybrids is operating at some scale, as is selection for traits that are differentiating these taxa. This selection appears to be stable or increasing, which will further barriers to introgression through time.
YES. A strong yes, in fact. I feel that the weight of the evidence is now strongly in favor of this re-split. The genomic evidence is very strong that (for example) auduboni and coronata are well-differentiated, and that some regions of their genomes are under differential selection. Within the hybrid zone there appears to be no assortative mating (= little prezygotic isolation), but there does appear to be only limited genetic introgression and some form of selection against hybrids (= substantial postzygotic isolation). This scenario is the one theoretically expected in the early stages of full speciation under the BSC, where the postzygotic fitness consequences of hybridization impose selection (aka reinforcement) that leads to the later evolution of pre-mating isolation. Moreover, the hybrid zone itself is narrow and apparently stable, despite the vast population sizes of these birds and their very high dispersal potential. And phenotypically they are very easily distinguished. On an aside: the evidence is even stronger for recognizing goldmani as a full species.
NO. The problem here as I see it is the hybrid zone between coronata and auduboni. There is no evidence that there is assortative mating where they come together, and they hybridize like crazy. The evidence of postzygotic selection against hybrids (Toews and Irwin 2009) is important, and I agree it is likely that "some form of selection impedes the fusion of the Myrtle and Audubon’s forms." (T & I 2009:3057). But this is a long way from species-level stuff. Evidence of adaptation is not evidence of speciation, and I think that may be being confused here: a lot of adaptation occurs between populations that are not species. Selection for particular alleles across geographic space and a shift in such a selection regime across subspecies boundaries and non-assortative (and extensive) mating at a contact zone screams "subspecies" to me.
Brelsford & Irwin (2009) said it well in their title "Incipient speciation despite little assortative mating," and in the abstract: "Pairing data indicate that assortative mating is either very weak or absent…" and "…there is moderate reproductive isolation between these populations…" Adaptation to different selection regimes (probably natural and sexual in this case) is great, and while reinforcement does appear to be occurring, it is only able to apparently stabilize what are actually pretty high rates of gene flow at least away from the contact zones." Toews et al. (2016) included multiple samples from each subspecies, but did not sample birds from the contact zone between S. c. coronata and S. c. auduboni." This leads to their uncertainty over whether more homogenized regions of the genome are due to hybridization or gene flow. Let me tell you from the specimens: gene flow will be responsible for a lot of it. I agree that "Clearly, performing additional genomic assays from birds across regions of sympatry will be beneficial." (Toews et al. 2016:708).
These populations are a wonderful example of the speciation process, but the birds' behavior in contact shows that they fit the concept of subspecies rather than the closer-to-evolutionary-independence we expect from full biological species. In sum, I do not consider there to be enough reproductive isolation yet established to make these anything more than a subspecies pair headed perhaps at some distant date to become (nearly) fully reproductively isolated. Saying that they are so now (biological species) suggests that we’re guessing a long way into the future, and that the substantial levels of gene flow occurring now are somehow not consequential. They may be genomically diagnosable, but they are far from genomically independent, given levels of gene flow.
Genetic differentiation of goldmani is expected for an isolated population smaller than the others, simply through drift (as Toews et al. 2016:706 noted). So genetic distinctiveness and isolation are not good metrics of limits of biological species, in my view. Phenotype needs to be more rigorously assessed in relation to other closely related members, and coronata and auduboni suggest well-differentiated subspecies are still not reproductively isolated. So as to whether goldmani is a species, I think we will want to know a great deal more.
NO. Maintain as one species, for now. I'm sorry, but I just can't get past the issue "of little assortative mating in the hybrid zone." If you believe in the BSC as I do, that's pretty much it. And I'm not sure what the "indirect evidence for selection against hybrids in the contact zone" means. I can readily identify these two ssp. groups. They not only look different, both in alternate and basic plumage, their call notes sound different. I think songs differ too, but both have a variety of songs. Here in the West, "Myrtles" prefer more mesic habitats. The motion alludes to the NACC having merged various taxa in this complex, notably hooveri with nominate coronata and memorablis with auduboni. I don’t think the NACC has taken any position on ssp. since 1957. I believe memorablis is mainly a size ssp. (larger), but plumage differences have been described from hooveri, especially in basic plumage.
I look at lots of Yellow-rumped Warblers here in the West, and have only a few times been pretty sure of hybrids. On the other hand, I don't look that carefully at every bird, and identifying a basic plumaged bird would be pretty problematical. I did see several hybrids in summer along the Stikine River, southeast Alaska, and I believe there are pretty much all hybrids along one river, I think in northern BC.
One thing that interests me is the disgestive system of this species. "Myrtles" are well-studied and they are able to digest bayberries, wax myrtle berries, poison ivy and poison oak berries, something that other wood warblers can't do. That's why "Myrtles" winter so far north, as far north as the southern Great Lakes and Nova Scotia and they can be abundant in mid-winter on the Mid-Atlantic coast. Are Audubon's able to digest these berries as easily with comparable plants in the West?
Anyway, for now, I'm comfortable with a one species concept. As for nigrifrons I would be very surprised that they merit being treated as a separate species. I've seen them in Chihuahua and yes they did look blacker, but they sounded like "Audubon's" farther north. The distance isn't that far from the Sky Islands of the Southwest, where intergrades have been noted in southwest New Mexico and southeast Arizona. As for goldmani this is an interesting case that merits further study. I'm told that folks seeing them in Guatemala report them giving very different songs, but don't know if there are recordings (maybe some put on a web site?). I did not see them on the hike for the Horned Guans in southwest Chiapas, so guess they occur elsewhere in Chiapas. The BNA account (Hunt and Flaspohler 1998) state: "In this subspecies, Alternate and Basic plumages of adult males nearly identical; there may be no Prealternate molt (Hubbard 1970, 1980)." If that's true, it certainly is suggestive for separate species status.
Hunt, P.D., and D.J. Flaspohler. 1998. Yellow-rumped Warbler (Dendroica coronate). In The Birds of North America, No. 376 (A. Poole and F. Gill, eds.). The Birds of North America, Inc., Philadelphia, PA.
NO. At the outset, I would like to make it clear that I have always "disliked" the Myrtle-Audubon’s lump because in my tidy little worldview, if I can identify two taxa by call note as far away as I can see them, they "have to be" species. In fact, to this day, I use "Myrtle" and "Audubon’s" in my field notes etc. Also at the outset, I find these genetic data interesting and important to understanding the history and process of diversification, and I applaud the authors of the recent papers for making substantial progress. But application of my tidy little worldview to real world situations is, predictably, not always tidy, and application of genetic data to taxonomy is not always straightforward.
Specifically, as confirmed by these recent papers, there is no evidence for assortative mating at the Audubon’s-Myrtle contact zone. Hubbard's original results based on phenotype are confirmed by genetic analyses. Despite the call note differences and fairly conspicuous plumage differences, they are not isolating mechanisms – the two taxa treat each other as "same" where given the chance to show us the relevance of these characters --- these differences evidently make no difference when it comes to mate choice. So, if these two taxa don’t treat each other as different species (my version of BSC), why should we? Perhaps rescuing my tidy worldview is that as far as is known songs of Audubon's and Myrtle are not readily distinguishable, although I don't think this has been adequately studied. Thus, perhaps it isn't a surprise that the two are not reproductively isolated. Toews et al. (2016) used song playback to capture individuals, and I wish they had reported on the details of this, i.e. whether responses were equivalent, etc.
The authors found evidence for selection against hybrids away from the contact zone and interpret that as evidence for reproductive isolation. This is the same interpretation of the BSC that, unfortunately, allowed the Scrub Jay split to pass, a result with which I strongly disagree. Neglected in this interpretation of the BSC is that there is presumably selection against intergrades away from the contact zone in EVERY parapatric subspecies pair. Otherwise, there would be a smooth cline between subspecies taxa and thus …. they would not be treated as valid subspecies. Presumably natural selection favors the phenotype-genotype combination that characterizes the populations of diagnosable subspecies away from zones of intergradation. The only "escape clause" from this conclusion is that the situation is not in equilibrium. However, because most subspecies follow biogeographic patterns, selection favoring those phenotype-genotype plateaus is implied. Therefore, the phenotypic and genetic results of the recent studies favor ranking them as subspecies, not species, opposite of their published recommendations and those of the proposal. In other words, if Myrtle and Audubon’s are ranked as species under this interpretation of the BSC, then all parapatric, diagnosable subspecies with zones of introgression should also be elevated to species rank.
As for ranking the allotaxa goldmani and nigrifrons as separate species, this would require showing that these taxa have diverged from coronata and each other in song more so than coronata and auduboni have, preferably also with playback trials. Such trials and comparative analyses of vocalizations have obvious problems in interpretation … but so does every set of criteria for species delimitation. What song and playback trials have going for them is that they consistently predict absence or presence of free gene flow between parapatric and sympatric taxa --- thus, they work, empirically, for predicting the all-important process of gene flow (as in the Toews-Irwin study of Winter vs. Pacific wrens). As for the problems with use of song in oscines because it is "learned," this oversimplification should not be perpetuated. First, oscines are genetically hard-wired to learn there "own" songs --- otherwise, for example, in areas where multiple wood-warblers are syntopic, they would all just sing the same Esperanto song. Second, experiments show that some components of song are genetically determined: genetics constrains which components of song are learned vs. inherited. I look forward to seeing published results on song differences (as hinted in the proposal).
I end with a string of minor comments on the evidence. (1) Because as far as I can tell, no specimens were collected, all interpretations of phenotype are evidently based on in-hand inspection in the field; perhaps digging into some of the background studies would reveal that there are archived photos for each individual? (2) As for the finding that nigrifrons and goldmani are reciprocally monophyletic, although this has an awesome ring to it, keep in mind that it is based on a limited N of individuals and loci, and so in any such statements, one additional sample might erase such proclamations, i.e. all such proclamations should be accompanied with the qualifier "based on N individuals and loci." (3) As for the general finding that all four taxa are genetically defined, we "already know this" assuming that the plumage characters that define the four taxa have a genetic basis. That neutral loci also reflect the genetic differences underlying the phenotypic differences is interesting but taxonomically not particularly relevant. (4) That there is no gene flow between auduboni and nigrifrons despite nonbreeding sympatry is expected; the only wintering audubonithat might remain in the range of nigrifrons during the breeding season are probably defective individuals that would be unlikely to breed, and even if they did, the rarity of those events might make them difficult to detect without much larger N. (5) I really look forward to studies of the potential contact zones between auduboni and nigrifrons.
NO. After reading the commentary from both the pro-split and pro-lump camps, I think that leaving these taxa together best represents the admittedly complex and somewhat intermediate situation that these taxa represent. I am open to being persuaded otherwise, however.
NO. Like the other "No" votes here, I think that the lack of assortative mating in the contact zone between coronata and auduboni argues against recognizing them as biological species - along with the lack of known song differences. It's unfortunate that the genomic data didn't include sampling in/near the contact zone. I wonder why those samples were not included? I could possibly be convinced to split goldmani given genetic and morphologic differences, but would like to see the unpublished data on vocal differences.