Recent spread of blue tits into the Barcelona urban environment: morphological differences and the role of balanced dispersal

Rates of phenotypic change are greater in cities than in any other habitat. Consequently, urban habitats are an ideal experimental area to study contemporary evolution and adaptation. A key question related to phenotypic changes in urban animals relates to the patterns of gene flow between the city and the natural habitats surrounding cities, but the results obtained thus far have been contradictory. The aim of this study was to analyze genetic differentiation and patterns of gene flow and size variation in urban blue tits Cyanistes caeruleus in Barcelona city, using a panel of 26 microsatellites. We compared birds from one locality at the city center, one at the suburban area, and one in a nearby forest. We found a significant genetic differentiation between the populations, which has evolved over a period of about 35 generations (50 years), based on historical data. There was significant gene flow between all populations. There was no significant asymmetric gene flow between the forest and the other two populations, which is consistent with balanced gene flow and dispersal. City birds were smaller than individuals from the forest. Simulations showed that the selection required to achieve these phenotypic differences would have to be high, which in turn suggests that phenotype-dependent dispersal may be involved.