Data from: 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 previously ruled out the actions of isolation-by-distance or secondary contact. Using ABC simulations and genetic data on 390 individuals from 20 locations genotyped at 22 microsatellites loci, we first determined how barn owls colonized Europe after the last glaciation. Using these results in new simulations on the evolution of the colour phenotype, and assuming various genetic architectures for the colour trait, we demonstrate that the observed colour cline cannot be due to the surfing of a neutral mutation. Taking advantage of spatially explicit ABC, which proved to be a powerful method to disentangle the respective roles of selection and drift in range expansions, we conclude that the formation of the colour cline observed in the barn owl must be due to natural selection.变异梯度——或称渐变群（cline）——历来令生物学家深感着迷。传统上，这类渐变群被解读为选择与基因流之间拮抗互作的产物。此外，渐变群也可通过中性过程形成，例如距离隔离（isolation-by-distance）或是先前隔离种群间的二次接触（secondary contact）。可通过比较中性遗传标记（neutral genetic marker）与目标性状位点的遗传分化水平，检测自然选择与这两种中性过程在渐变变异形成中的相对重要性。第三种中性过程——新突变在新栖息地定殖过程中的突变冲浪（surfing）效应——则更难开展检测。本研究构建了一套空间显式近似贝叶斯计算（approximate Bayesian computation，ABC）模拟框架，用以评估欧洲仓鸮（Tyto alba）中观测到的遗传调控红羽色强渐变群，究竟是物种分布范围扩张的副产物，还是需要引入选择作用方能解释——此前本研究已排除了距离隔离与二次接触对该羽色渐变群的影响。本研究利用ABC模拟与来自20个采样点的390份个体的遗传数据（这些个体已在22个微卫星（microsatellite）位点完成基因分型），首先明确了末次冰期后仓鸮定殖欧洲的具体过程。基于上述结果开展羽色表型演化的新模拟，并针对该羽色性状设定多种遗传架构（genetic architecture），本研究证实观测到的羽色渐变群并非源于中性突变的冲浪效应。借助经证实可有效解析分布范围扩张过程中选择与遗传漂变（genetic drift）各自作用的空间显式ABC方法，本研究最终得出结论：欧洲仓鸮观测到的羽色渐变群的形成，必然归因于自然选择。