Genomic landscapes of divergence among island bird populations: evidence of parallel adaptation but at different loci?

When populations colonise new environments they may be exposed to novel selection pressures but also suffer from extensive genetic drift due to founder effects, small population sizes, and limited interpopulation gene flow. Genomic approaches enable us to study how these factors drive divergence, and disentangle neutral effects from differentiation at specific loci due to selection. Here, we investigate patterns of genetic diversity and divergence using whole-genome resequencing (> 22X coverage) in Berthelot’s pipit (Anthus berthelotii), a passerine endemic to the islands of three north Atlantic archipelagos. Strong environmental gradients, including in pathogen pressure, across populations in the species range, make it an excellent system in which to explore traits important in adaptation and/or incipient speciation. Firstly, we quantify how genomic divergence accumulates across the speciation continuum, i.e., among Berthelot’s pipit populations, between subspecies across archipelagos, and between Berthelot’s pipit and its mainland ancestor, the tawny pipit (Anthus campestris). Across these colonisation timeframes (2.1 million – ca. 8,000 years ago), we identify highly differentiated loci within genomic islands of divergence and conclude that the observed distributions align with expectations for non-neutral divergence. Characteristic signatures of selection are identified in loci associated with craniofacial/bone and eye development, metabolism, and immune response between population comparisons. Interestingly, we find limited evidence for repeated divergence of the same loci across the colonisation range but do identify different loci putatively associated with the same biological traits in different populations, likely due to parallel adaptation. Incipient speciation across these island populations, in which founder effects and selective pressures are strong, may therefore be repeatedly associated with morphology, metabolism, and immune defence.

Methods
Whole genome data from six populations of Berthelot's pipit across the three archipelagos of their range in the North Atlantic. Resequencing data was generated by mapping Illumina HiSeq reads to a reference genome and calling variants using GATK HaplotypeCaller. 

Genomic landscapes of divergence where assessed using pairwise FST across different population comparisons across the geographic range of the species'. Peaks of divergence were then charaterised using a range of population genomic statistics to elucidate the evolutionary drivers of divergence in these regions. 

Individual-based simulations of genomic divergence, under neutral and selective processes, were then used to further understand the patterns detected in the empirical genomic datasets.