Discordance between genomic divergence and phenotypic variation in a rapidly evolving avian genus (Motacilla)

TL;DR

This study reveals a complex relationship between genetic divergence and phenotypic variation in wagtails, showing that genetic patterns often do not align with physical traits or traditional taxonomy.

Contribution

It provides the first time-calibrated species tree for Motacilla and demonstrates discordance between genotype and phenotype across Eurasian wagtail species.

Findings

Genetic differentiation supports species-level distinctions.

Phenotypic traits do not always match genetic groups.

Pleistocene events influenced wagtail diversification.

Abstract

Generally, genotypes and phenotypes are expected to be spatially congruent, however, in widespread species complexes with few barriers to dispersal, multiple contact zones, and limited reproductive isolation, discordance between phenotypes and phylogeographic groups is more probable. Wagtails (Aves: Motacilla) are a genus of birds with striking plumage pattern variation across Eurasia. Up to 13 subspecies are recognized within a single species, yet previous studies using mitochondrial DNA have supported phylogeographic groups that are inconsistent with subspecies plumage characteristics. In this study, we investigate the link between phenotypes and genotype by comparing populations thought to be at different stages along the speciation continuum. We take a phylogeographic approach by estimating population structure, testing for isolation by distance, conducting demographic modeling, and estimating the first time-calibrated species tree for the genus. Our study provides strong evidence for species-level patterns of differentiation in wagtails, however population-level differentiation is less pronounced. We find evidence that three of four widespread Eurasian species exhibit an east-west divide that contradicts both subspecies taxonomy and phenotypic variation. Both the geographic location of this divide and time estimates from demographic models are overlapping in two sympatric species, indicating that coincident Pleistocene events shaped their histories.