Genomic consequences of range expansion and colonisation in the reed warbler (Acrocephalus scirpaceus)
Range expansion is a common natural phenomenon, which may be intensified by human-induced drivers such as climate change and alterations of habitat. The genetic consequences of range expansion are potentially major, and it is important to study known cases of range expansion to understand how human activities affect contemporary evolution, and to learn more about the genetic adaptive potential of species. The reed warbler (Acrocephalus scirpaceus) is a long-distance migratory bird breeding in Eurasia and wintering south of Sahara. The reed warbler has expanded its range both northwards and southwards in Europe, due to different anthropogenic factors, namely climate change and habitat restoration, respectively. In this study, we investigate the genetic consequences of these two-directional range expansions with RAD-seq, looking at 10 populations from north to south in Europe. We investigate population structure and genome diversity, and assess the role of selection in divergence between populations across the species range. We do not find evidence of strong genetic structure in the reed warbler populations, and the youngest edge populations do not exhibit any substantial loss in genetic diversity, suggesting ongoing gene flow. However, the edge populations are the most genetically distinct of the populations studied, and we identify environmental disparity, especially in precipitation variability, as the main barrier of gene flow, to a greater extent than geographic distance. We find no evidence that the loci involved in population divergence and adaptation in the core populations are the same that are involved in adaptation at the range edges. Using three genome scan methods to identify selection, we found 49 SNPs putatively under selection, of which 33 were in introns of 28 genes. Most of these are correlated with differences in climatic variables of temperature and precipitation. Some genome scan outliers show signs of being part of nascent selective sweeps, especially one which is distinct in the northern range edge. We find no evidence for a role of genome wide chromatin interactions in adaptive population divergence during range expansion. Our results suggest that in the reed warbler, contemporary range expansion has had little effect on molecular diversity and has been rapidly followed by local adaptation to climatic conditions, which could further corroborate the rapid pace at which colonisation of novel environments has occurred both northwards and southwards.