Demography and linked selection interact to shape the genomic landscape of codistributed woodpeckers during the Ice Age

Abstract

The glacial cycles of the Pleistocene had a global impact on the evolution of species. Although the influence of genetic drift on population genetic dynamics is well understood, the role of selection in shaping patterns of genomic variation during these dramatic climatic changes is less clear. We used whole genome resequencing data to investigate the interplay between demography and natural selection and their influence on the genomic landscape of Downy and Hairy Woodpecker, species co-distributed in previously glaciated North America. Our results revealed a dynamic population history with repeated cycles of bottleneck and expansion, and genetic structure associated with glacial refugia. Levels of nucleotide diversity varied substantially along the genomes of Downy and Hairy Woodpecker, but this variation was highly correlated between the two species, suggesting the presence of conserved genomic features. Nucleotide diversity in both species was positively correlated with recombination rate and negatively correlated with gene density, suggesting that linked selection played a role in reducing diversity in regions of low recombination and high density of targets of selection. Despite strong temporal fluctuations in N_e, our demographic analyses indicate that Downy and Hairy Woodpecker were able to maintain relatively large effective population sizes during glaciations, which might have favored natural selection. The magnitude of the effect of linked selection seems to have been modulated by the individual demographic trajectory of populations and species, such that purifying selection has been more efficient in removing deleterious alleles in Hairy Woodpecker owing to its larger long-term N_e. These results highlight that while drift captures the expected signature of contracting and expanding populations during climatic perturbations, the interaction of multiple processes produces a predictable and highly heterogeneous genomic landscape.