Natural selection in a post-glacial range expansion: the case of the colour cline in the European barn owl

Gradients of variation – or clines – have always intrigued biologists. Classically, they have been interpreted as the outcomes of antagonistic interactions between selection and gene flow. Alternatively, clines may also establish neutrally with isolation-by-distance or secondary contact between previously isolated populations. The relative importance of natural selection and these two neutral processes in the establishment of clinal variation can be tested by comparing genetic differentiation at neutral genetic markers and at the studied trait. A third neutral process, surfing of a newly arisen mutation during the colonisation of a new habitat, is more difficult to test. Here, we designed a spatially-explicit ABC simulation framework to evaluate whether the strong cline in the genetically-based reddish coloration observed in the European barn owl (Tyto alba) arose as a by-product of a range expansion or whether selection has to be invoked to explain this colour cline, for which we have ...