Neutral and selective drivers of colour evolution in a widespread Australian passerine

Aim: Rump plumage coloration of the Eastern Yellow Robin (Eopsaltria australis), a widespread Australian songbird, varies from bright yellow in the tropical north to olive-green in the temperate south. Here, we test whether colour variation: (1) correlates most strongly with neutral genetic variation and so is best explained by historical processes, (2) reflects selection associated with different visual environments (dense versus open habitats) and/or (3) reflects selection associated with climatic variation. 

Location: Eastern Australia. 

Methods: We quantified colour variation using reflectance spectrometry and visual models. We performed geographical cline analysis of colour and neutral genetic variation (genome-wide single nucleotide polymorphisms). We tested for correlations of colour variation with climate, vegetation density, geographical location and genetic variation. We accounted for covariation and spatial autocorrelation, and conducted analyses at continental and regional spatial scales. 

Results: Clinal variation of colour traits and neutral genetic markers were largely concordant. At the continental scale, colour variation was strongly associated with neutral genetic structure and geography, and to a lesser extent with environment. At the regional scale, environmental variation was a better predictor of colour variation than it was at the larger scale. 

Main conclusion: At the continental scale, colour variation is strongly associated with large-scale population history. In contrast, at the regional scale, where the influence of history and geography is weaker, environmental variation has a role in facilitating the maintenance of colour variation. Our results highlight the need to assess selective and neutral alternatives at multiple spatial scales when studying geographical variation.