Dr. John Klicka on Timberline Sparrows

There is a long-standing debate in the birding world about whether or not the Timberline Sparrow (Spizella taverneri) should be recognized as a separate species from the Brewer's Sparrow (Spizella breweri). The Timberline was originally described as a distinct species due to details of appearance and differences in behavior and range, but the bird has long been officially recognized as a Brewer's subspecies.

In 1999, Dr. John Klicka (along with Robert M. Zink, Jon C. Barlow, W. Bruce McGillivray and Terry J. Doyle) published a paper in the COS Condor which recommended that the Timberline Sparrow be once again recognized as a distinct species. The paper, titled Evidence Supporting the Recent Origin and Species Status of the Timberline Sparrow (.pdf), argues for separate recognition based on morphologic (structural) and molecular research done by the authors.

I talked with Dr. Klicka, now the Curator of Birds at UNLV's Barrick Museum of Natural History, about the Timberline Sparrow and the controversy surrounding it.

The Timberline Sparrow was originally described as a separate species from Brewer's Sparrow...why was it subsequently classified as a subspecies?

JK: Under the prevailing species concept (Biological species concept), taxa were being lumped if it was thought that they could potentially interbreed; i.e. evidence of reproductive isolation is lacking. Those taxonomists in charge of such things at the time (The AOU Committee on Classification) obviously thought that Brewer’s and Timberline sparrows had the strong potential to hybridize.

Was that 'strong potential' backed with evidence or was it just presumed? How much interbreeding is there between Brewer's and Timberline sparrows?

JK: It was presumed. At the time the range of the Timberline sparrow was poorly understood and its natural history was unknown. Its still a taxon that we know very little about. The main point in the Condor paper was that we had some genetic evidence indicating that these two sparrows were genetically distinct, with no evidence of hybridization. Of course sampling was sparse and more data are needed.

For those who don't know much about the specifics of taxonomy (read: me), how much genetic difference is needed to separate species from subspecies? With the understanding that more research needs to be done, how much genetic difference has been found between Brewer's and Timberline sparrows?

JK: You put your finger on one of the problems with the current taxonomic practices. Many species are more closely related than are some subspecies. For example, Blue-winged and Golden-winged Warblers are considered distinct species, yet are only slightly differentiated genetically. Whereas, the several races of Fox Sparrow are as genetically distinct as are many pairs of species but remain lumped. Genetic distance has only recently begun to play a role in considering whether or not taxa warrant species status. To complicate matters, many birds that are really different genetically still maintain the ability to hybridize so genetic distance is really not a good measure of reproductive isolation.

Timberline and Brewer’s sparrows are nearly identical genetically. We identified a single genetic mutation that separated the two. We consider them incipient species. They probably diverged since the last major glacial advance.

I'm going to venture a guess that the genetic difference between Golden-Winged and Blue-Winged Warblers has something to do with "wing color." How is the genetic mutation you found manifested in Brewer's and Timberline Sparrows?

JK: Undoubtedly the morphological differences between Blue-winged and Golden-winged warblers are under some genetic control and these differences almost certainly reside somewhere within the vast nuclear genome. There are some scientists that work towards find the linkage between specific traits, and specific genetic differences, but that is not what systematists are doing.

We tend to use mostly DNA from the mitochondria of cells. While this plasmid contains coding genes, none are known to code for specific morphological features and most of the genetic variation expressed in "mtDNA" is thought to be selectively neutral, or nearly so. When we report the "genetic distance" between two organisms it is simply the relative number of genetic differences between them. In general we have no idea how this genotypic changes affect the phenotypes, although since we presume they are neutral we also presume a negligible affect on phenotype. A simple answer to your question; in most cases, we have no idea how (or if) the genetic differences we uncover in mtDNA might manifest themselves in the taxa being studied.

Your paper concludes with a recommendation that the Timberline Sparrow should be recognized as a separate species from the Brewer's Sparrow. Let's say that more research comes out in support of your conclusion. What would have to happen in order for biology textbooks and birding field guides to show two species instead of one? In other words, who is your recommendation aimed at?

JK: The decision on whether not a taxon is officially to be given species status is ultimately made by members of the AOU (American Ornithologists’ Union) Committee on Classification and Nomenclature. Their decisions are reflected in the AOU check-lists that are published periodically.

Do you have any indications whether or not your recommendations will be adopted by the AOU? How long do you think it will be before the Timberline and Brewer's Sparrows are officially recognized as separate species?

JK: I don't expect the Timberline Sparrow to be formally recognized any time soon, or perhaps ever. It all comes down to how one defines a species, and my view is very different than that of most members of the AOU Classification Committee. ø