Low genetic differentiation and symmetric migration between urban and forest populations of great tits

Gene flow may be limited between urban and non-urban populations of wild animals that can influence their landscape-level genetic structure and potential to adapt to new ecological conditions. To test this idea, we genetically characterized great tit (Parus major) populations breeding in an urban and a forest area 3.5 km apart, differing in several phenotypic traits some of which may contribute to adaptation to urban living. We used 16 microsatellite markers to genotype 189 breeding adult individuals (119 urban and 70 forest birds) and (1) tested whether the two populations are genetically differentiated, and (2) estimated the rate and direction of migration between the sites. Heterozygosity tended to be lower in the urban than in the forest habitat. Genetic population structure analyses did not show a consistent clustering of breeding birds between the urban and forest sites, and this conclusion was not affected by the inclusion of phenotypic data in the analyses. The pairwise fixation index (F_st) was low (0.009) and only 1% of the total genetic variance was explained by variation between populations. Finally, there was detectable gene flow between the two areas, and its estimated values did not suggest asymmetry in the direction of migration. We conclude that great tits living in the city are genetically connected to the nearby forest population by reciprocal migration, which may explain the low level of genetic differentiation.