The mule is in many ways an admirable creature. It’s tough and adaptable. It has a mind of its own, but it’s open to negotiation. The US Army has rediscovered the virtues of mules as pack animal in inhospitable terrain like most of Afghanistan. The one thing a mule can’t do, of course, is reproduce its own kind. The offspring of a male horse and a female donkey, it’s the archetypal sterile hybrid.
Not all hybrids share the mule’s fate, however. Sometimes they are not only reproductively viable, but have a competitive edge over either of the parent species. That’s how a hybrid can become a founder of a whole new species. Natural selection has the last word on that; but hybridization provides another source of genetic variation to work on, along with mutation and random drift.
Hybrid speciation is old hat to botanists. It can happen in one of two ways in plants: polyploidy speciation, in which the hybrid offspring has a different number of chromosomes from either parent, and homoploid speciation, in which chromosome number remains constant but parental genes are recombined. The polyploidy version happens more often, accounting for 2 to 4 percent of speciation events in flowering plants. Some twenty homoploid plant species of hybrid origin are known, including a trio of desert sunflowers that outcompete their moisture-loving parent species in dry habitats.
For animals, homoploid or recombination speciation is the dominant mode. Scientists are just beginning to appreciate the potential of hybridization in the creation of new animal species. It was first recognized in invertebrates, mainly butterflies. Matthew Forister of the University of Nevada, Reno identified a newly discovered species of butterfly in the High Sierra as a natural hybrid of the northern blue and the Melissa blue. The Appalachian tiger swallowtail was found by Harvard biologists Marcus Kronfost and Krushnamegh Kunte to be a hybrid of the Canadian tiger and eastern tiger swallowtails. The ranks of known hybrid species also include a couple of fish, notably a southwestern minnow called the Virgin (as in Virgin River) chub.
Now it’s the birds’ turn. Earlier this year a group of European biologists reported that the Italian sparrow originated through hybridization between the ubiquitous house, or English, sparrow and the more narrowly distributed Spanish sparrow. Closer to home, an article in Molecular Ecology by Alan Brelsford and Darren Irwin of the University of British Columbia and Borja Mila of UCLA make a persuasive case that the bird formerly known as the Audubon’s warbler is a hybrid species (abstract at www.mendeley.com/research/hybrid-origin-audubons-warbler.)
There is a taxonomic thicket to hack through at this point, so bear with me. Between 1766 and 1897, ornithologists described four similar species of wood warbler: the myrtle warbler of eastern North America, the Audubon’s warbler of western North America, the black-fronted warbler of northern Mexico, and the Goldman’s warbler of Chiapas and Guatemala. The four could be distinguished by plumage. Among more subtle differences, adult male myrtles have white throats; the other three have yellow throats, and black-fronted and Goldman’s have more extensive black in their plumage than Audubon’s. Audubon’s breeds in the California mountains and coastal forest and is a common winter visitor in the Bay Area; myrtle winters here in lower numbers.
The two Mexican forms were demoted to subspecies of Audubon’s in 1921. After myrtle and Audubon’s warblers were caught hybridizing in western Canada, the American Ornithological Union merged the two in a new taxon christened the yellow-rumped warbler. Birders continued to use the old nomenclature, and some biologists have argued that the two were lumped on insufficient evidence. Last year a proposal to split the yellow-rumped complex into two, three, or four separate species was voted down by the AOU.
That might have to be reconsidered in light of the new data from Brelsford, Mila, and Irwin. Analyzing blood and tissue samples taken within the breeding ranges of all four types, they focused on mitochondrial DNA, a signal for maternal inheritance, and nuclear DNA sequences identified by amplified fragment length polymorphism markers, which are used to test hypotheses of hybrid speciation. Mitochondrial patterns grouped most Audubon’s warblers with myrtle warblers, except for Arizona samples that aligned with the black-fronted warbler. The AFLP picture was different: myrtle and black-fronted samples formed well-separated clusters, with Audubon’s intermediate between the two and partially overlapping with black-fronted. Three of the forms—myrtle, black-fronted, and Goldman’s—had unique AFLP variants. Audubon’s had none.
Their conclusion: “the Audubon’s warbler…as defined by previous taxonomy clearly represents an admixture between two long-divergent lineages, coronata [myrtle] and nigrifrons [black-fronted], which differ substantially in plumage, morphology, migratory behavior and both mitochondrial and nuclear DNA.” While allowing the possibility of other origin scenarios, they regard hybridization between myrtle and black-fronted as the most likely explanation of their findings. When and where the two parent species met remains to be determined; it may have happened in the Canadian Rockies about 16,000 years ago.
So Audubon’s warbler, if we can call it that, may be a genetically distinct organism, bounded by stable hybrid zones in the north (with myrtle) and south (with black-fronted.) In between, natural selection has favored the perpetuation of the mix.
The cases of hybrid speciation described so far may be just the tip of the iceberg. With this, and the emerging patterns of lateral gene transfer among bacteria and more complex organisms, it may be time to retire the venerable image of the evolutionary tree: the only image in Darwin’s Origin of Species. The tree is beginning to look more like a web.