Blue-winged Warbler (Vermivora cyanoptera).
Photo: Mark Conboy
I like hybrids. It's an atypical interest, I know, but maybe it has something to do with the fact that hybrids are for all intents and purposes, freaks of nature, and we all, in our own strange and twisted ways can’t get enough of freaks.
Nature has gone to great lengths to ensure that hybrids are rare, to ensure that like breeds with like and that like produces like. That Yellow Warblers (Setophaga petechia) breed with Yellow Warblers and give rise to more Yellow Warblers, that Pacific Giant Octopi (Enteroctopus dofleini) breed with Pacific Giant Octopi and give rise to more Pacific Giant Octopi, that Elm Sawflies (Cimbex americana) breed with only with Elm Sawflies and...you get the idea. That’s the normal course of things: when two individuals hook up, they normally chose members of the same species with which to do the dirty deed. Sometimes though, things don’t go as they should, and from time to time, members of two distinct species will mate and produce offspring – they hybridize.
Over time, differentiation between species has resulted in the development of barriers to hybridization, and it’s those barriers which keep each species on an independent evolutionary trajectory. These barriers can take various forms, but in essence each serves to make it impossible or at the very least difficult, to hybridize.
The most obvious kinds of barriers to hybridization are the phenotypic ones: to look, sound, smell or taste different from other species. Then there are genotypic barriers such as genetic incompatibility. This means that even if two heterospecifics (two individuals of different species) mate, their gametes are rendered useless because their genetic material is simply incompatible. Even though two species may be physically able to have sex, the real crucial step in sexual reproduction, the combining of egg and sperm and their genetic payloads, cannot take place for any number of physiological reasons (e.g., different numbers of chromosomes). But there are plenty of cases, particularly among birds, where two species mate, their genetic material combines, and they do produce offspring. In those cases another type of barrier might come into play. The hybrid offspring might itself be rendered infertile by complications caused by having a genome made up of heterospecific DNA, and is itself incapable of reproducing. The freak show ends there. These kinds of barriers, prezygotal (the ones that stop species from mating in the first place) and postzygotal (the ones that stop hybrids themselves from passing on their own mixed genes), tend to be strong between species that have experienced long periods of evolutionary divergence. But in species with less evolutionary history separating them, hybridization does occur, and at least among some species, with astonishing frequency.
Hybrids are a problematic lot. They are problematic for biologists who want a neat and tidy way of defining species. For the past two centuries, naturalists and biologists have argued over exactly what a species is. It’s not an unimportant, arcane thing; the concept of a species is fundamental to our understanding of biology. We use the word to describe diversity, to distinguish one type of organism from another. Birders keep lists of species. We have legislation to protect rare and disappearing species. But what exactly is a species? There are a myriad of definitions. The most widely used has been and continues to be the biological species concept. By this definition, a species is defined as any group of organisms that can produce fertile offspring. It’s a difficult definition to apply widely in biology without running into problems, not the least of which is how do you treat species that hybridize? The biological species concept would suggest that any organisms which can hybridize and produce fertile offspring (offspring that can themselves go on to produce young of their own) are the same species. Period. No ifs, ands or buts. If you stuck firmly to this definition, you’d have to concede that no matter how different two organisms look, sound, smell or taste, no matter how ecologically dissimilar they were, if they could produce fertile kids, they’d be the same species. That’s not exactly a satisfactory definition, I think you’ll agree. Biology is never that simple. For example, what if the frequency of hybridization is rare? What if species never interact in nature because of geographic barriers, but in captive situations they readily interbreed? But I digress. I don't want to get into an endless discussion of species concepts here. Let's talk more about hybridization itself.
A strange bird was netted in Wethersfield, New York in June 2006. The ornithologist who pulled it from the mist net must have said something like "what the fuck is this?!" It may not be a direct quote, but I’m sure it was an approximation of their thoughts. The bird was unlike anything in any bird book, anywhere. It was warbler (family Parulidae) to be sure, and it would eventually be given the name Junkin’s Warbler. The bird was photographed and released, as per the researchers’ protocol. It was clearly a hybrid, but of what two species? The photos and descriptions of the bird were shown around. Everyone who saw it took their best guess. But best guesses just don’t cut it among bird people. The bird had to be identified! So an effort was made to track it down and catch it again.
Success! The mystery warbler was recaptured, looked at by a few more ornithologists, and had a few feathers plucked. DNA was obtained and sequenced from those feathers. The lab at Cornell University already had a genetic sequence database of almost all the warbler species and it was just a matter of comparing the genetic sequence of the Junkin’s Warbler to the others in the database. Mitochondrial DNA (the mtDNA), which is only maternially inherited was used to determine that the mother was a Kentucky Warbler (Geothlypis formosa). Nuclear DNA comes from both parents, one copy is passed on by the mother, the other is passed on by the father. One nuclear DNA sequence obviously matched Kentucky Warbler; that came from the mother. The other sequence matched Mourning Warbler (Geothlypis philadelphia).
The breeding ranges of Kentucky and Mourning Warblers overlap only slightly in Pennsylvania so their chance to hybridize under normal circumstances is rather limited. But if one of those species somehow got itself settled out of its usual range during the breeding season, it may have had no choice but to mate with a member of another species or mate not all. As we all know, the drive for sex is a strong one, and sometimes you just have to make due. Male warblers often return very near their place of hatching, while females often disperse more widely. The Junkin’s Warbler was thought to be a male, but apparently that was never fully confirmed. That could mean that its place of capture, in western New York, was close to its place of birth, and this location is one at which Kentucky Warbler would normally not breed, but where Mourning Warblers normally do. If a female Kentucky Warbler decided to settle in western New York, for some reason, where she had no chance of finding a conspecific male, she’d have no choice but to mate with a male from another species.
The Junkin's Warbler is just one of many examples of hybridization among warblers (family Parulidae). The warblers are a favourite group of mine: they're pretty, they're vocal, they're widespread and ubiquitous. But for me, the real fascination with warblers stems from their natural history, which by virtue of their diversity and their wide geographic distribution, covering a huge swath of the Americas from the Alaskan tree line to the Argentine Paraná wetlands, warblers have evolved a wide array of strategies for tackling all of life's challenges. A kind of beautiful intrigue also lies in their evolutionary history, one in which climate and biogeography, as well as natural and sexual selection, have conspired to shape each species into the forms we recognize today. When it comes to hybridization, they are no less intriguing. This is the first essay in a three part series on hybridization in warblers called Infus'd Betwixt.
Decades ago Kenneth Parkes, for whom the waterthrush genus Parkesia is named, thought he saw some patterns in the way warblers hybridize. Hybrids, he said, could essentially be grouped in two types: those formed between closely related species and those formed between more distantly related species. When all known hybrid types were considered, it was apparent that there were more examples of occasional hybrid pairings between distantly related species than between closely related species. Certainly, some hybrids were the result of two very closely related species pairing, but such cases were more uncommon except among a few select groups of sister species. Sister species are those that share a very recent common ancestor. Examples include Golden-winged (Vermivora chrysoptera) and Blue-winged Warblers (Vermivora cyanoptera), Northern (Setophaga americana) and Tropical Parulas (Setophaga pitiayumi), and Golden-fronted (Myioborus ornatus) and Spectacled Whitestarts (Myioborus melanocephalus).
In Parkes’ reckoning, most of the hybrid pairings were between members of different genera. The going theory of the day was that isolating signals – things like song or plumage characteristics – worked most strongly among closely related species (the exception was among a few sister species that hybridized with some regularity probably because they were still just a little too similar), but had become relaxed between more distantly related species. This pattern of hybridization came to be known as Parkes’ Paradox.
It was a paradox because normally we think of closely related species as being the most similar and therefore more likely to hybridize than distantly related, more differentiated species. Distantly related species are more different from one another, generally speaking, because the long periods they’ve had to evolve differences. True, two closely related species can look and sound as different from each other as two distantly related species. But one thing that closely related species usually do share, that distantly related species usually do not are their ecologies. Species that occupy the same habitats, place their nests in the same locations or forage for the same foods, should in theory, be more suitable candidates for successful hybridization than species that differ in all these traits. That makes sense, no? For Kenneth Parkes and others interested in warbler hybridization the pattern was clear. But did it have any evolutionary resonance? Was Parkes’ Paradox telling us something important about speciation and species recognition?
Thirty four years after Parkes’ paper another look at patterns of hybridization in warblers reveals something entirely different. I did a quick search of the literature and came up with 39 hybrid combinations, though there are probably more. Of these, 27 of the hybrid combinations were between members of the same genus (intrageneric) and only 12 were between members of different genera (intergeneric). The makes 69% intrageneric and 31% intergeneric. Why the disparity between Parkes’ Paradox and the numbers I’ve just presented? The reason is twofold. First, since Parkes’ time many new hybrid combinations have been discovered. Second, the classification of warblers today is markedly different from that Parkes knew in 1978. The most recent phylogenetic hypothesis for the Parulidae represents a vast rearrangement of species within and between genera compared to the classification Parkes was working with in the 1970's. In effect, many of Parkes’ intergeneric hybrids were actually intrageneric. Without the benefit of modern genetics to peer inside the genomes of all the warblers Parkes had no way to know this. Interestingly, all of Parkes’ intergeneric hybrid pairs are still classified as such. A number of the intergeneric hybrid pairings that Parkes cited are now considered to be intragenetic pairs. For example, Northern parula (formerly Parula americana) is now classificed in the genus Setophaga, thus making a formerly intrageneric hybrid pairing with American Redstart (Setophaga ruticilla) an intrageneric union. But for the most part, the intrageneric pairings outnumber the intergeneric pairings in the sheer number of different combinations reported in the literature. So Parkes' Paradox has been rendered a little less paradoxical.
Some warbler hybrids occur frequently enough, to have common names of their own: Brewster’s and Lawrence’s (Golden-winged × Blue-winged Warbler and various backcross combinations), Sutton’s (Yellow-throated Warbler [Setophaga dominica] × Northern Parula) and Cincinnati (Blue-winged × Kentucky Warbler). Early ornithologists seemed unwilling or unable to accept the occurrence of hybrids; Brewster’s, Lawrence’s and Cincinnati warblers were all considered new species when they were first collected in the 1870’s. They were all anointed with the unwieldy genus name Helminthophaga and the species leucobronchialis, lawrencii and cincinnatiensis, respectively. It wasn't until a decade or two later that they were finally recognized as hybrids.
It’s not as though the concept of hybridization was a new one in biology. Darwin himself thought long and hard about it. He wrote about it too. While composing his thoughts for the Origin of Species he wrestled with hybridization, trying to decide if it was possibly an important source of variation and whether it could lead to the advent of new species. Later, in Variation of Animals and Plants under Domestication, he addressed hybridization again, asserting that it could in fact be a cause of speciation. Among ornithologists working during the late 1800’s and early 1900’s, hybridization was not yet well understood. There was no helpful genetic testing at the time, just plumage, song and behavioural clues to go on. In addition to being called new species, hybrids were called "spontaneous mutants" and colour morphs of already known species. It wasn’t until 1908 that Mendelian genetics were used to illustrate how Golden-winged × Blue-winged Warbler hybrids inherited traits from their respective parents that ornithologists began to readily accept the reality of hybridization among the warblers.
Among the "named" hybrids, Cincinnati Warbler is one of the rarest. It doesn’t occur as frequently in nature as the Brewster’s or even Lawrence’s Warblers. When it was first collected in 1880 it was one of a kind. But in 1948 another suspicious-looking warbler appeared; it was thought to be a Blue-winged × Mourning Warbler hybrid. The specimens were compared to each other, to their putative parent species and to all other warblers. Based on the exhaustive comparison it looked as though both birds were indeed of Blue-winged Warbler parentage on one side. However, the original specimen from 1880 bird from Ohio appeared to be a backcross between a pure Blue-winged and a hybrid Blue-winged × Kentucky Warbler. The 1948 bird from Michigan was considered to be the product of a pure Blue-winged and pure Kentucky.
Backcrossing makes the already challenging problem of diagnosing a hybrid bird’s parentage more difficult. To understand the concept of backcrossing you need to think about generations. When a Blue-winged Warbler and a Kentucky Warbler mate and produce a Cincinnati Warbler the Cincinnati Warbler gets labeled as F1 – filial generation one. Provided this hybrid is fertile, and F1's often are among warblers, they can go on to mate and produce offspring with one of their parent species – that’s a backcross. The resultant offspring of a backcross would be called F2. If hybrids were frequent enough in a population, and they sometimes are among certain warblers, two F1's may find each other and mate. They will produce what are called double-cross hybrids (they are also F2's). Backcrossing can create a plethora of confusing-looking and -sounding hybrids. Perhaps the most important example of backcrossing mania can be found among the Vermivora.
To fully understand the Golden-winged × Blue-winged Warbler backcross phenomenon, its worth looking at the concept of allopatric speciation. No, I'm not kidding, its actually a rather important piece step in the history of these hybrids. One of the primary ways new bird species arise is through the isolation of an ancestral species in two or more pockets of habitat. When no gene flow occurs between those insular populations, the birds differentiate from one another and eventually become new species. This is the process of allopatric speciation. Over time, species which arose in allopatry come into contact with each other. This happens naturally all the time: mountain ranges erode, ocean basins close, forests colonize grasslands and species meet each other as a result. If contact happens before those species are fully formed, hybridization can occur. It can occur so extensively that where there were once two species, given enough time, they can become one. When two species rejoin in this way naturally, and it has probably happened countless times over the millennia, it’s a natural curiosity. It’s a real world demonstration of evolutionary principles, the kind of thing that keeps biologists awake at night giddy with excitement. But when it is caused by humans, and it definitely has been in latter centuries, it’s an entirely separate thing. It’s the sort of thing that keeps biologists awake at night for another reason all together. I'll address that issue further in Part 2 of Infus'd Betwixt.
