Cryptic differentiation and geographic variation in genetic diversity of Hall's Babbler Pomatostomus halli

Sequence variation was examined in domain I of the mitochondrial control re gion in three Queensland populations of Hall's Babbler Pomatostomus halli. a geographically restricted, monotypic songbird in eastern Australia. Surpr isingly, we found that domain I sequences were strongly differentiated into two major clades differing by 3.29%. These two clades exhibited nearly com plete geographic concordance with northern and southern populations, except for two haplotypes which were sampled in the north of the range but were p hylogenetically allied to the southern clade. We also round a seven-fold hi gher level of genetic diversity in the northern than in the southern popula tions. Neutrality and molecular clock tests suggested that selection or dif ferences in substitution rates were not responsible for this difference in diversity. However. a maximum likelihood analysis of ene flow between the n orth and south suggested that the difference in diversity could be due to b oth greater population size in the north and asymmetric gene flow dominated by south to north dispersal events. A likelihood ratio test rejected a mod el in which population sizes were equal and rates of gene flow symmetric, a nd came close to rejecting a model in which only population sizes were cons trained to be equal. These results suggest that different population sizes and asymmetric gene flow could be a major source of differences in genetic variation between populations of Hall's Babbler, although ecological and bi ogeographic causes for these differences are obscure.