The Condor 93:318-329 0 The Cooper Ornithological Society 1991 MITOCHONDRIAL DNA VARIATION, POPULATION STRUCTURE, AND EVOLUTION OF THE COMMON GRACKLE (QUISCALUS QUISCULA)’ ROBERT M. ZINK, WILLIAM L. ROOTES, AND DONNA L. DITTMANN Museum of Natural Science, Louisiana State University, Baton Rouge, LA 70803 Abstract. The Common Grackle (Quiscalus quiscula) includes two phenotypically dif- ferentiated forms, the Purple Grackle (Q. q. quiscula), which breedsalong 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 southcentralUnited 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 alsostudiedthe Greater Antillean Grackle(Q. niger), the supposed sistertaxon 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 occurredin singleindividuals. Using results of simulationsby Avise et al. (1988) we found times to common ancestryof the 31 clones that are much lower than predicted by neutral theory; however, it is likely that long-term effective population sizesof 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 in- dividual fragment profiles, composite haplotypes(clones)and a maximum parsimony phy- logenetic tree of clonesrevealsno evidence of geographic variation. Thus, the mtDNA gene tree is paraphyleticwith respectto 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 ancestryof clones and recent colonization of North America, which we suggest explainsthe lack of geographic variation; plumage evolution must thereforehave occurred rapidly, likely enhanced by sexual selection. The separation of quiscula and niger occurred l-2 MY ago,but it is not clear that these species are sistertaxa, whereasit is likely that major and mexicanus are. Key words: Common Grackle; Quiscalus quiscula; mitochondrial DNA; population struc- ture; gene flow. INTRODUCTION The advent and widespread application of mo- lecular methods of detecting genetic variation have greatly facilitated empirical estimatesof the degreeof population subdivision and its relation (if any) to geographic barriers, and the nature and extent of gene flow (Avise et al. 1987). New methods of data analysis further encourage the molecular approach to genetic analysis of pop- ulation structure and geographicvariation (e.g., Slatkin and Maddison 1989). Geographic vari- ation, in effecta measure of population structure, was previously studied by comparisons of exter- nal morphology. Studies of geographic variation in birds, especially, figured prominently in the I Received 6 November 1990. Final acceptance 24 January 1991. development of ideas concerning the evolution of populations (Mayr 1963). However, the degree to which morphological patterns of avian geo- graphic variation are genetically based is rarely tested. It is appropriate therefore, that modem molecular approaches be usedto describegenetic variation within and among avian populations to provide a new perspectiveon geographic vari- ation. Although ornithologistslagged behind workers in other fields in applying techniques of protein (allozyme) electrophoresis to problems in geo- graphic variation, there is a moderate data base now available for birds from the New World temperate zone (Evans 1987, Zink 1988). In gen- eral, most allozyme studies have revealed little or no intraspecific genetic differentiation (Bar- rowclough 1983). In contrast, analysis of the ge- ography of mitochondrial DNA (mtDNA) dif- ferentiation has yielded mixed results in avian [3181