Yellow-rumped Warbler (Setophaga coronata)
Photo: Alan D. Wilson (Wikimedia Commons)
This is the final part of a three part series on hybridization in warblers (family Parulidae) called Infus'd Betwixt.
Earlier in this series of essays, I looked at two situations where hybridization is widespread and leading to a diminution of biodiversity; one of those cases is a human-induced process and has just begun in the last century or so, while the other appears to be a millennia-old natural process. It's a fact of life, as it were, that hybridization can cause the loss of species; it can also lead to the creation of species. Hybrid speciation has long been recognized in plants, but not until recently has it been shown to play a similar role in the production of animal diversity. The hybrid origins of a number of bird species has been proposed but molecular evidence to corroborate such suspicions has been equivocal or altogether lacking. The Yellow-rumped Warbler (Setophaga coronata) is the first example of hybrid speciation in birds.
Before we get too far along I’ve got to tell you that Yellow-rumped Warbler taxonomy is, like so much in the biological sciences, far from simple. Most North American ornithologists recognize four distinct subspecies, not full species, of Yellow-rumped Warbler. Each of these subspecies differs from one another mainly in their head and face plumage. The protean subspecies are the nominate coronata which occurs throughout the boreal forest; auduboni, mainly west of the Rocky Mountains; nigrifrons in Mexico’s western Sierra Nevada; and goldmani in Guatemala. That’s the view of the American Ornithologists’ Union (AOU), the big boss man of North American bird taxonomy. By contrast, the International Ornithological Congress (IOC), which has limited influence over the goings on of North American ornithology, considers coronata, auduboni, nigrifrons and goldmani to be four distinct species.
This taxonomic controversy is nothing new. Each of the four distinctive subspecies (I’ll stick with the AOU definition), were originally described as a separate species: coronata by Linnaeus in 1766, auduboni by Townsend in 1837, nigrifrons by Brewster in 1889 and goldmani by Nelson in 1897. Eventually, owing to the similarities in their plumages nigrifrons and goldmani were lumped as subspecies of auduboni. That meant that by the 1920’s only two species remained: coronata and auduboni. However when it was discovered that the two remaining species hybridized, they too were lumped to form one species with four rather distinctive subspecies. Even with the solid molecular data available today, the current taxonomic disagreement between the AOU and the IOC has not had a final resolution.
Let's not get bogged down in the Yellow-rumped Warbler species/subspecies debate. Just bear in mind that the process of by which subspecies arise is more or less the same as the process by which full species arise, it's just a matter of degrees. Speciation can lead to fully formed species, with enough genetic distance from their congeners to be considered something onto themselves, or it can mean the evolution of incipient, but not yet full species. Incipient species are ones that aren’t quite there yet, they’re not genetically or phenotypically distinct enough to stand alone, to avoid hybridization, to count as fully-formed, honest-to-goodness species. Full or incipient species status for the Yellow-rumped Warblers doesn’t make too much difference in terms of our discussion. The process of hybrid speciation is the same, as I've said, it’s just a matter of degrees. Because evolution is such a gradual process all species begin as incipient species, before becoming a full species species. Regardless of whether you take the AOU or the IOC view, the Yellow-rumped Warblers still present us with a fascinating and important biological story and, unlike all the other putative instances of avian hybrid speciation, this one is supported by genetic data.
So just what does that molecular data say? It shows that coronata and auduboni are closely related, their mtDNA differing by only by about 0.2% (a minuscule difference), and that they hybridize often when they occur in sympatry. These are the reasons that the AOU considers the Yellow-rumped Warblers to be subspecies and not full species. The data also shows that coronata and auduboni DNA differs greatly from nigrifrons and goldmani and that the later two subspecies split from the former two about 1.7 million years ago. The molecular data also suggests that auduboni, had a rather remarkable origin.
Auduboni has within its DNA some genes of both coronata and nigrifrons. There are some visual clues too: auduboni resembles nigrifrons, both posses a yellow throat, a bold white patch on the wing, an indistinct eyeline and a fairly dark face. Morphometrically, northern populations of auduboni are intermediate between coronata and nigrifrons. In the southern part of the Audubon’s range, birds are morphometrically most similar to nigrifrons.
Perhaps the most telling clue to auduboni’s hybrid origins are something called private alleles. Private alleles are forms of genes found only within one species. They’re found solely there and nowhere else because they arose through evolution in that species alone. A species with an evolutionary past that involves hybridization would share some alleles, or forms of DNA, with its parent species. It may evolve some of its own private alleles but it will certainly have genetic material that is shared by its parent species. In the case of auduboni, almost all of its DNA and mtDNA is shared with coronata and nigrifrons. There are only a small number of auduboni-specific alleles, which are recently evolved and unique to that subspecies. By contrast, almost no alleles are shared by coronata and nigrifrons except for one, which is probably an ancestral allele left over from a deeper evolutionary time when all the subspecies had yet to split from a common ancestor. The abundance of shared alleles suggests that auduboni may be have a hybrid origin, with coronata and nigrifrons being the parent subspecies.
Is it not possible that nigrifrons could have arisen through regular old speciation - that is to say, without hybridization? When we look closely at the DNA it appears that simply cannot be the case. If all three of these subspecies came from a common ancestor in the recent past it stands to reason that they should all carry some similar alleles. So if the origin of auduboni was through the normal speciation process, then private alleles should be equally common among all three subspecies. As I’ve said, that’s not the case.
It’s difficult to say where or when this hybrid origin occurred. Presently there are two known Yellow-rumped Warbler hybrid zones. One is an active coronata × auduboni hybrid zone spanning a 150 km wide swath of western Alberta and British Columbia (which incidentally includes the aforementioned Peace Country). Further south in Arizona, auduboni × nigrifrons hybrids have been discovered. But there is currently no hybrid zone between coronata and nigrifrons pumping out auduboni. When this might have occurred remains uncertain, but it was certainly sometime in the very recent past, maybe even as recently as 16 000 years ago. The scenario that some ornithologists propose is that auduboni may have arisen at the end of one of the last glacial maxima, when the previously allopatric coronata and nigrifrons came into contact and began interbreeding. After it had been isolated from its parent subspecies in another glacial refugium during a subsequent ice age, auduboni spread outward across western North America when post-glacial conditions allowed. It’s been proposed that the initial hybridization between coronata and nigrifrons took place in the Canadian Rockies, just as with so many other species pairs; indeed auduboni and coronata form hybrid zones there today. Perhaps genetic technology and prehistoric climate modelling will eventually give us a better picture of the way in which all the warbler species and subspecies arose.
