Recurrent selection explains genomic regions of high relative but low absolute differentiation in the greenish warbler ring species
Recent technological developments allow investigation of the repeatability of evolution at the genomic level. Such investigation is particularly powerful when applied to a ring species, in which spatial variation can be used to represent the evolutionary changes that occurred during the evolution of two species from one. We examined patterns of genomic variation among three populations of the greenish warbler ring species, using genotypes at 13,013,950 nucleotide sites along a new greenish warbler consensus genome assembly. Genomic regions of low within-group variation are remarkably consistent between the three populations. These regions show high relative differentiation but surprisingly low absolute differentiation between populations. We propose that these regions underwent selective sweeps over a broad geographic area followed by within-population selection-induced reductions in variation. A surprising implication of this “sweep-before-differentiation” model is that genomic regions of high relative differentiation may have moved among populations more recently than regions elsewhere in the genome.