Evidence supporting the recent origin and species status of the TimberlineSparrow

The Timberline Sparrow (Spizella taverneri), although originally described as a species, is currently classified as a subspecies of the more widesprea d Brewer's Sparrow (S. breweri). We investigated the taxonomic status and r ecent evolutionary history of these species by comparison of both morpholog ical and molecular characters. Morphometric comparisons using 6 external an d 18 skeletal measurements show that S. taverneri specimens from two widely separated populations (Yukon and southwestern Alberta, Canada) are indisti nguishable with respect to size yet are significantly larger (by 3%) than r epresentatives of several breweri populations. Analysis of 1,413 base pairs of mitochondrial DNA (mtDNA) for 10 breweri and 5 taverneri samples repres enting widely scattered breeding populations revealed a maximum divergence among any breweri-taverneri pair of 0.21% and an overall average of 0.13%. In contrast, the average (+/- SE) pairwise distance among the other Spizell a species is 5.7 +/-: 0.5%. We discovered that breweri and taverneri could be distinguished on the basis of a single, fixed nucleotide difference. Of an additional 11 taverneri and 8 breweri surveyed for this diagnostic site, a single bird(morphologically a taverneri) from northwest British Columbia did not sort to "type." Overall, 18 of 18 breweri and 15 of 16 taverneri w ere diagnosable. We interpret these results to suggest that gene flow does not currently occur between these two forms and that each is on an independ ent, albeit recently derived, evolutionary course. The molecular data are c onsistent with theoretical expectations of a Late Pleistocene speciation ev ent. We believe that for passerine birds, this is the first empirical valid ation of this widely accepted evolutionary model. The data presented corrob orate plumage, vocal, and ecological evidence suggesting that these taxa ar e distinct. As such, we suggest that Spizella taverneri be recognized as a species.