Local adaptation drives population isolation in a tropical seabird

Aim: Understanding the mechanisms of population isolation in marine top predators is challenging owing to their high mobility and the inherent difficulty in quantifying oceanographic variables. In this study, the worldwide distributed brown booby Sula leucogaster was used to test the potential role of isolation by distance (IBD) and isolation by environment (IBE) in promoting intraspecific diversity. Location: A heterogeneous seascape in the south-western Atlantic Ocean, along a latitudinal gradient from 0 degrees to 27 degrees S. Methods: Population structure was assessed using nine microsatellite loci. Between-colony geographical distances were used to test IBD, while air temperature, sea surface temperature, chlorophyll concentration, colony density and isotopic niche width were used to test IBE. Results: Genetic isolation of a remote small colony was associated with local selective pressures on land and in foraging areas. Clustering of the remaining colonies was explained by seascape differences between neritic and oceanic environments. Main conclusions: Seabirds can easily overcome large geographical distances, but their dispersal ability seems to be lower than their mobility. In this context, gene flow can be disrupted even between relatively close colonies if there are strong selective pressures. Local adaptation and IBE seems to be most plausible explanation for patterns found in brown boobies; this is particularly noticeable for birds at a small offshore archipelago, for which the identification of the key selective forces shaping genetic and phenotypic differences is the main issue.