Extensive phenotypic diversification coexists with little genetic divergence and a lack of population structure in the White Wagtail subspecies complex ( Motacilla alba )

Geographically clustered phenotypes often demonstrate consistent patterns in molecular markers, particularly mitochondrial DNA (mt DNA ) traditionally used in phylogeographic studies. However, distinct evolutionary trajectories among traits and markers can lead to their discordance. First, geographic structure in phenotypic traits and nuclear molecular markers can be co‐aligned but inconsistent with mt DNA (mito‐nuclear discordance). Alternatively, phenotypic variation can have little to do with patterns in neither mt DNA nor nuclear markers. Disentangling between these distinct patterns can provide insight into the role of selection, demography and gene flow in population divergence. Here, we examined a previously reported case of strong inconsistency between geographic structure in mt DNA and plumage traits in a widespread polytypic bird species, the White Wagtail ( Motacilla alba ). We tested whether this pattern is due to mito‐nuclear discordance or discrepancy between morphological evolution and both nuclear and mt DNA markers. We analysed population differentiation and structure across six out of nine commonly recognized subspecies using 17 microsatellite loci and a combination of microsatellites and plumage indices in a comprehensively sampled region of a contact between two subspecies. We did not find support for the mito‐nuclear discordance hypothesis: nuclear markers indicated a subtle signal of genetic clustering only partially consistent with plumage groups, similar to previous findings that relied on mt DNA . We discuss evolutionary factors that could have shaped the intricate patterns of phenotypic diversification in the White wagtail and the role that repeated selection on plumage ‘hotspots’ and hybridization may have played.