Observant scientists have discovered a rare example of animal hybrid speciation, in the forests of the Appalachian Mountains in the eastern United States. The creature in question, the Appalachian tiger swallowtail butterfly (Papilio appalachiensis), is the genetically unique result of the union of two related butterflies, the Canadian tiger swallowtail (P. canadensis), and the Eastern tiger swallowtail (P. glaucus). Although the casual observer might easily mistake one of these colorful creatures for another, biologists have noted distinct differences in the genetic makeups and physical characteristics of these divergent species.
In a recent report in the journal, PLoS Genetics, researchers detailed the implications of this finding, which offers a rare glimpse into one of the mechanisms by which new species form. Understanding how new species occur is one of the primary goals of evolutionary biology. Lead author, Krushnamegh Kunte of Harvard University notes that hybrid speciation is fairly common among plants, "...But there are very few cases in animals," says Kunte. "This study may create the fullest picture we have to date of hybrid speciation occurring in an animal."Co-author, Larry Gilbert, a biologist at the University of Texas at Austin, notes that hybridization is one of the methods by which species can adapt to new environments. "...Hybridization can create populations with a new combination of life history and morphological traits," says Gilbert, "allowing colonization of novel environments by a 'mosaic genome.'" In the case of Appalachian tiger swallowtails, traits related to avoiding predation and adapting to colder climates has enabled these colorful insects to fill an ecological niche in certain parts of the ancient North American mountain range that is less likely to be occupied by either of its progenitor species.
Although the various species routinely come into contact with one another, their genomes have remained distinct. Scientists believe that speciation ordinarily occurs when a species splits into two over time. According to this model, the two new species eventually become increasingly reproductively isolated from each other. But hybrid speciation, which, is exceedingly rare in the animal kingdom, occurs when two species interbreed to create viable hybrids, which subsequently evolve independently.
Scientists believe that the Eastern and Canadian tiger swallowtails, characterized by black and yellow stripes, and small "tails" on their hind wings, diverged from one another about 600,000 years ago. The Appalachian, however, appeared as recently as 100,000 years ago, according to genetic analysis. "That's not a very long time," says Kunte, "but still we found that the Appalachian tiger has been isolated long enough to have a different appearance and genetic makeup than its parent species."
Top Image Credit: © Willie F. M.