Winter Wren (Troglodytes troglodytes pacificus) in Vancouver (Source)
The Winter Wren (Troglodytes troglodytes*) has a Holarctic distribution, making it the only representative of the wren family (Troglodytidae) to leave the New World. It has long been recognized as variable, with as many as 44 subspecies diagnosed on variation in its extremely complex song as well as very subtle variations in barring, coloration, and morphometrics. Extensive work has already been done on the variation in this species, especially song variation. A recent phylogeny has also helped us understand the interrelationships of the various forms of this widespread species. Drovetski et al. (2004) found six distinct lineages using mitochondrial DNA, corresponding to different geographic populations:
Tree modified for clarity (I removed the sample names). Click to view full size.
Surprisingly, North American samples split into two different lineages, eastern and western, but these clades are not each other’s closest relatives. The western group is the basal group to all other winter wrens, whereas the eastern group is closer to all of the Eurasian populations. The Eurasian populations are split into four east-to-west lineages (Europe, Caucasus, Nepal, and East Asia). Analysis of song actually finds the same phylogeographic pattern – western North American wrens have a distinct song style, whereas eastern North American wren songs are more similar to those heard in Eurasia.
In North America there are three diagnosable subspecies groups – western birds (pacificus group), eastern birds (hiemalis group), and Aleutian birds (alascensis group). Eastern and western birds are divergent mainly in throat color, pacificus having a darker throat, and in song – pacificus has more higher frequency notes and more staccato trills. Aleutian birds are divergent from either in a larger size and paler coloration. In the Drovetski et al. phylogeny, the alascensis group is nearly identical to pacificus with almost no genetic divergence, despite the differences in coloration and size.
(Source: Dunn and Alderfer 2002)
Toews and Irwin, in their work examining contact zones between eastern and western divergent groups of birds (such as subspecies of Yellow-rumped Warbler, Audubon's and Myrtle), traveled to the eastern foothills of the Rockies in British Columbia and began looking for locations where the eastern and western groups of North American wrens came into contact. In Tumbler Ridge, they discovered locations where the eastern and western forms co-existed, and began their work using sound, morphometrics, and genetic analysis to answer the vexing species question.
Tackling this problem in avian taxa involves a specific set of questions. Where the two forms come into contact, is there a gradual change in traits (genetic, morphological, song, etc) as one would expect if there was gene flow between the two forms? Or, do the two forms overlap while maintaining their distinctive differences, indicating that they maintain reproductive isolation from one another? While people often think that the Biological Species Concept is all about whether the two forms will hybridize, the actual situation is more complicated. Even frequent hybridization can occur between species, yet reproductive isolation is maintained if strong selection against those hybrids occurs. In short, we have to apply as many different techniques as possible to assess whether two forms are reproductively isolated populations, and to what degree.
These same sets of data can also be applied to questions about why different forms are different. Are the differences and interactions between the forms consistent with ecological speciation, where morphological traits change as the two forms adapted in isolation from one another, before coming into contact again? Are the species divergent instead in behavioral or sexual traits, suggesting social or sexual selection played a role in the speciation event? We can use the answers to these questions to access other complicated bird groups and figure out the important mechanisms of speciation.
So what did Toews and Irwin find up on Tumbler Ridge? They found old growth forest where eastern and western Winter Wrens co-occurred, even sharing neighboring territories. They identified singing males based on song type, then captured many of the singing males and gathered data – does the singing type match the morphology? Does the singing type match the genetics? Is there any evidence of hybridization? They compared the birds at Tumbler Ridge with birds from control regions far away from the contact zone to see if they are more or less divergent.
Their analysis of morphometrics yielded no consistent results – no measurements were significant enough to consistently identify eastern versus western wrens although there are subtle differences in wing and tail length. Song differences are quite distinct between eastern and western birds with no overlap. The real kicker is the genetic analysis. They analyzed mitochondrial DNA as in the Drovetski et al. paper, as well as some nuclear DNA markers for an independent locus. All individuals analyzed matched perfectly in mtDNA to their previously identified song type – western songed birds had western DNA, and eastern birds had eastern DNA. The nuclear data also showed a perfect match, with one exception – one eastern mtDNA individual had some western nuclear DNA mixed in. This would be evidence of hybridization at the contact zone, except for the fact that this individual (a likely F1 hybrid with an eastern mother and western father) was found in one of the control regions sampled away from the contact zone in the range of the western form in British Columbia. This indicates hybrids can and do occur, but the fact that only one was found indicates reproductive isolation is still very strong.
Songs of eastern and western Winter Wrens, compared at and away from the contact zone
(click to view full size)
(click to view full size)
Analysis of songs, showing discrete differences between eastern and western birds but no significant differences across the range of either at and away from the contact zone
(click to view full size)
Toews and Irwin use their data to argue that eastern and western Winter Wrens are in fact reproductively isolated species based on several arguments:
The strong differences in song type are as distinct in allopatry as in sympatry, indicating they are good for diagnosing the different forms of Winter Wren and there is no pattern of gradual change between them.
Strong concordance between the mitochondrial DNA, the nuclear DNA, and song patterns, indicating there is little to no gene flow and no cultural learning of song crossing between the two forms.
The nuclear DNA divergence is just as strong in sympatry as in allopatry. If gene flow was occurring in any measureable level, then the two forms would be less genetically distinct closer to the contact zone.
The co-occurence of the Eastern and Western Winter Wrens could just be a fluke occurrence of recent generations, without allowing for hybrization and gene flow to fully kick in. This is not the likely case, however, as the two forms have been known from the general area for 50+ years.
These arguments present a very good case for a ‘new’ species in North America, a rare occurrence. There is some concern over low sample size in the contact zone, but if these two species were not reproductively isolated then the genetic signature should turn up even in the samples collected. The authors suggest calling the western group the Pacific Wren (Troglodytes pacificus) including the pacificus, salebrosus, and the Aleutian subspecies, while retaining eastern Winter Wrens and Eurasian birds as the Winter Wren (Troglodytes troglodytes) with subspecies hiemalis, pullus, and all of the old world groups.
Toews and Irwin’s work also provides some insight into why there are two very similar cryptic species of Winter Wren. Western and eastern birds are not divergent in morphometrics at the contact zone or in distant control locations. Where they do come into contact, there is no differentiation in habitat use – singing males of both species occupy territories right next to each other, even replacing one another from year to year. This suggests they inhabit essentially the same ecological niche, and ecological speciation in allopatry is not a likely hypothesis.
Instead, the most obvious difference between the taxa, consistent throughout their range, is song. Song is used in mate assessment, and can both lead to reproductive isolation via sexual selection and maintain isolation through assortative mating. This is seems to be the case in Winter Wrens, and adds more data to the growing recognition of the importance of song and sexual selection in bird speciation.
Song and mate recognition is a prezygotic barrier to gene flow – it prevents birds from mating in the first place, rather than preventing fertile offspring. Toews and Irwin hypothesize a potential postzygotic barrier to gene flow – the migratory divide between eastern and western wrens, with the Pacific Wren wintering along the Pacific coast, and the Winter Wren wintering in the southeast US. Migratory divides such as these have been shown to drive divergence in other birds, and this could help create and maintain reproductive isolation by selecting against hybrids who, because migration has a genetic component, would probably end up lost somewhere in Mexico.
This discovery is significant because it proves that strongly reproductively isolated species can still exist undiscovered in North America. I certainly don’t think we are going to find any completely unknown species, as are still being discovered in South America, Africa, and Asia, but we are not done with taxonomic surprises in North America. Given the previous phylogenetic results, I expected some differentiation between eastern and western Winter Wrens, but I envisioned a contact zone with hybridization, as seen in many other east-west divergences (Yellow-rumped Warbler subspecies, Baltimore and Bullock’s Orioles, Indigo and Lazuli Buntings, Rose-breasted and Black-headed Grosbeaks, Northern Flicker subspecies, etc).
One thing to take notice of is the degree of mitochondrial divergence between the winter wren species – about 8.8%, while the average divergence between North American sister species of birds is 1.8% (Johnson and Cicero 2004). I don’t think we can use percent divergence alone as a criterion for species, as it doesn’t necessarily correlate with the evolution of barriers to gene flow (it does indicate the independent evolution of lineages for some amount of time in the past). What we can do is look at it as a strong indication that something is going on, and investigate further. We already have some good test cases to identify more cryptic species in North American birds.
DNA barcoding papers have identified unusually large divergences in mtDNA in several species, including Solitary Sandpiper (Tringa solitaria), Warbling Vireo (Vireo gilvus), and Eastern Meadowlark (Sturnella magna). Have a look at the graph below from Hebert et al. (2004), which shows the maximum within-species divergence on the x-axis, against minimum between-species divergence within a given genus on the y-axis. As you can see, these three species are well out of place! The high divergence within Eastern Meadowlark is from the southwestern desert form Lilian’s Meadowlark, for which I believe a split is in the works. The Warbling Vireo has several distinct populations and may turn out to be two or even three distinct species. Solitary Sandpiper is the most intriguing. It has two described subspecies with some subtle coloration differences, and I’m not sure if any work has been done on a contact zone between the two. Someone needs to go look!
Finally, we are not be done with Winter Wrens! Within North America, the Aleutian island populations are divergent in morphology and color, and may be distinct from Pacific Wren even though they are only very subtly different in DNA. In Europe, there are four distinct mtDNA lineages. While I don’t think we can ever test reproductive isolation between eastern North American Winter Wrens and Eurasian birds, we can find areas to test in Eurasia between their clades. Who knows what we might find? If those lineages are shown to be reproductively isolated as well, then we would end up carving Winter Wren into as many as six cryptic but valid species.
People think systematists just keep revising species to keep themselves busy. The discovery of cryptic species of bird in North America proves we have a lot of meaningful work left to do.
* - Winter Wren, Troglodytes troglodytes of the family Troglodytidae, is probably not a Troglodytes! A phylogeny of the genus (which includes the House Wren complex, and several neotropical species), places Winter Wren outside of Troglodytes with respect to the neotropical Timberline Wren (Thryorchilus) and the Marsh and Sedge Wrens (Cistothorus) (Rice et al. 1999). Other studies find similar relationships (Gomez et al 2005, Drovetski et al. 2004 which places Winter Wren as sister to Cistothorus rather than Cistothorus + Thryorchilus + Troglodytes), but so far many people (including Toews and Irwin) have deferred taxonomic change to further study. Rice et al. 1999 say that Troglodytes aedon is the type species for the genus, meaning Winter Wren rather than the other species of Troglodytes needs to be renamed, and they suggest Nannus as the next available genus name for Winter Wren. I haven’t been able to look into their taxonomic references on the subject, but uBio says Troglodytes troglodytes was described by Linnaeaus 1758 and Troglodytes aedon by Vieillot 1809, so I’m not entirely certain why Winter Wren is the one that needs to be changed. I think the evidence is plenty sufficient to justify placing Winter Wren and Pacific Wren in the genus Nannus, and that name appears to be in use already in Europe - see this birdforum discussion on the subject.
Dunn JL, Alderfer J (2002) Field Guide to the Birds of North America 4th Ed. National Geographic Society, Washington.
Drovetski SV, Zink RM, Rohwer S, Fadeev IV, Nesterov EV, Karagodin I, Koblik EA, Red’kin YA (2004) Complex biogeographic history of a Holarctic passerine. Proceedings of the Royal Society B 271:545-551
Gomez JEM, Barber BR, Peterson AT (2005) Phylogenetic position and generic placement of the Socorro Wren (Thryomanes sissonii). Auk 122:50-56
Hebert PDN, Stoeckle MY, Zemlak TS, Francis CM (2004) Identification of birds through DNA barcodes. PLoS Biology 2:e310 Available online here.
Johnson NK, Cicero C (2004) New mitochondrial DNA data confirm the importance of Pleistocene speciation in North American birds. Evolution 58:1122-1130
Rice NH, Peterson AT, Escalona-Segura G (1999) Phylogenetic patterns in montane Troglodytes wrens. Condor 101:446-451
TOEWS, D.P., IRWIN, D.E. (2008). Cryptic speciation in a Holarctic passerine revealed by genetic and bioacoustic analyses. Molecular Ecology, 17(11), 2691-2705. DOI: 10.1111/j.1365-294X.2008.03769.x