On the subject of Charles Darwin and the Galapagos, most people inevitably picture the archipelago's famed giant tortoises or speciose finches. However, it is a little known fact, even among most biologists, that Darwin's initial evolutionary insights were inspired by neither of these. Rather, it was a much lesser known group of birds -- the Galapagos mockingbirds -- that first captured Darwin's attention and catalyzed his thoughts on the roles of geography and adaptive radiation in producing new species.
Despite their pivotal role in the development of evolutionary theory, the origin of the Galapagos mockingbirds and how the group diversified within the archipelago remain enigmatic more than a century-and-a-half after Darwin's visit.
Featured on the cover of this month's issue of the journal Evolution, a team of researchers, lead by HSU Assistant Professor Brian Arbogast, and David J. Anderson, Professor of Biology at Wake Forest University, provide the first DNA-based glimpse into the evolutionary history of the Galapagos mockingbirds. The researchers examined several mitochondrial genes from the four recognized species of mockingbirds from islands distributed throughout the archipelago. They also examined the same genes from most of the living species of mockingbirds found outside of the Galapagos (the group has a number of relatives in the Americas, including the Caribbean).
The team found a number of surprising results. As Darwin predicted, the mockingbirds appear to have radiated throughout the Galapagos following a single colonization of the archipelago from a non-Galapagos ancestor. But, that ancestor appears not to be closely related to the mockingbirds of the western South American mainland, as Darwin and many other researchers had previously believed. Rather, the closest living relatives to the Galapagos birds appears to be those currently found in the northern part of the Americas, including the familiar northern mockingbird, its close relative the tropical mockingbird, the Socorro mockingbird from the Revillagigedos archipelago off the coast of Baja Mexico, and the Bahama mockingbird of the Antilles in the Caribbean. The researchers concluded that the ancestor of the Galapagos mockingbirds arrived in the archipelago not long after the formation of the oldest of the emergent islands, about 2 to 3 million years ago.
There were some surprises within the archipelago as well. Currently, four different species of Galapagos mockingbirds are recognized based primarily on plumage differences. The DNA also suggests that there are four major evolutionary lineages of mockingbirds in Galapagos, but they don't correspond perfectly to the currently recognized species. Most notably, birds on three of the easternmost islands (Espanola, San Cristobol, and Genovesa) have remarkably similar mitochondrial sequences despite being phenotypically distinct, so much so that they are considered three different species. Similarly, the genetic data revealed that birds on the large western island of Isabela are more genetically distinct from populations on other islands than previously recognized. The genetic data did support the evolutionary uniqueness of the highly endangered Floreana mockingbird, which now numbers only approximately 200 birds and is restricted to a couple of small islets in the archipelago.