CULTURAL-EVOLUTION IN THE EURASIAN TREE-SPARROW - DIVERGENCE BETWEEN INTRODUCED AND ANCESTRAL POPULATIONS

We investigated cultural evolution in the song of the introduced North American population of the Eurasian Tree Sparrow (Passer montanus), d erived from 12 pairs brought from Germany in 1870. These birds were li berated at St. Louis, Missouri, and spread into Illinois. Cultural evo lution is described here in terms of the processes of population diffe rentiation where the song meme was the unit of transmission. The distr ibution of song syllable memes in each meme pool fit a null hypothesis of a neutral model with an equilibrium between mutation, migration, a nd drift, indicating that the memes are functionally equivalent. The i ntroduced and ancestral (German) populations showed marked divergence in the level of meme sharing. The small size of the founding Noah Amer ican population, the loss of genetic diversity there, and the relative susceptibility of meme pools to founder effects suggest that much of the reduction in sharing of syllable types occurred during the foundin g event. Because memes also are susceptible to extinction due to drift , memes were probably lost in both populations as a result of random m emetic drift. Meme diversity in Illinois was comparable with that in G ermany, suggesting a large mutational input into the former population following its founding. Estimates of mutational divergence based on t he frequencies of song memes in meme pools showed more population stru cture in Illinois than in Germany. There also was less meme flow among meme pools in Illinois than in Germany. These results suggest that th ere were a series of founding events during the colonization of North America.