Discerning the relative roles of adaptive and non-adaptive processes in generating differences among populations and species, as well as how these processes interact, are fundamental aims in biology. Both genetic and phenotypic divergence across populations can be the product of limited dispersal and gradual genetic drift across populations (isolation by distance), of colonisation history and founder effects (isolation by colonisation) or of adaptation to different environments preventing migration between populations (isolation by adaptation). Here we attempt to differentiate between these processes using island populations of Berthelot's pipit (Anthus berthelotii), a passerine bird endemic to three Atlantic archipelagos. Using microsatellite markers and approximate Bayesian computation, we reveal that the northwards colonisation of this species ca 8,500 years ago resulted in genetic bottlenecks in the colonised archipelagos. We then show that high levels of genetic structure exist across archipelagos, and that these are consistent with a pattern of isolation by colonisation, but not with isolation by distance or adaptation. Finally, we show that substantial morphological divergence also exists and that this is strongly concordant with patterns of genetic structure and bottleneck history, but not with environmental differences or geographic distance. Overall our data suggest that founder effects are responsible for both genetic and phenotypic changes across archipelagos. Our findings provide a rare example of how founder effects can persist over evolutionary timescales, and suggest that they may play an important role in the early stages of speciation.

Microsatellite dataMicrosatellite genotypes for 371 individuals and 27 loci. Island abbreviations as in the manuscript.Morphometric dataMorphometric traits for all Berthelot's pipits from 2005/2006. Individual codes as in the microsatellite file. Island codes as in the manuscript.Climate dataClimate data for 13 Berthelot's pipits populations, used to make geographical and ecological distance matrices.