MITOCHONDRIAL-DNA VARIATION, POPULATION-STRUCTURE, AND EVOLUTION OF THE COMMON GRACKLE (QUISCALUS-QUISCULA)

The Common Grackle (Quiscalus quiscula) includes two phenotypically differentiated forms, the Purple Grackle (Q. q. quiscula), which breeds along the Atlantic and Gulf coasts of the United States, and the Bronzed Grackle (Q. q. versicolor), which occurs over the rest of eastern North America. These grackles meet and hybridize in a zone that stretches from northeastern to the extreme south central United States. We used 20 restriction endonucleases to survey the level and pattern of mitochondrial DNA (mtDNA) variation within and among 35 individuals taken from eight sites. To establish the phylogenetic position of Q. quiscula, we also studied the Greater Antillean Grackle (Q. niger), the supposed sister taxon of Q. quiscula, the Boat-tailed Grackle (Q. major), and the Great-tailed Grackle (Q. mexicanus). A relatively high number (29) of clones was detected in Q. quiscula, of which 23 occurred in single individuals. Using results of simulations by Avise et al. (1988), we found times to common ancestry of the 31 clones that are much lower than predicted by neutral theory; however, it is likely that long-term effective population sizes of grackles are much lower than current population size, which potentially explains the discrepancy. Sixty-six percent of the birds were genetically most similar to an individual in another population sample, suggesting that gene flow is high. Inspection of the distribution of individual fragment profiles, composite haplotypes (clones) and a maximum parsimony phylogenetic tree of clones reveals no evidence of geographic variation. Thus, the mtDNA gene tree is paraphyletic with respect to the "plumage" tree of Q. quiscula. The mtDNA data do not reflect the historical event that lead to the origin of either Purple or Bronzed grackles. The low diversity among clones (P = 0.28%) suggests a recent common ancestry of clones and recent colonization of North America, which we suggest explains the lack of geographic variation; plumage evolution must therefore have occurred rapidly, likely enhanced by sexual selection. The separation of quiscula and niger occurred 1-2 MY ago, but it is not clear that these species are sister taxa, whereas it is likely that major and mexicanus are.