Data from: A tight balance between natural selection and gene flow in a southern African arid-zone endemic bird

Gene flow is traditionally thought to be antagonistic to population differentiation and local adaptation. However, recent studies have demonstrated that local adaptation can proceed provided that selection is greater than the homogenising effects of gene flow. We extend these initial studies by combining ecology (climate), phenotype (body size), physiological genetics (oxidative phosphorylation genes) and neutral loci (nuclear microsatellites and introns) to test whether selection can counter-balance gene flow and hence promote local adaptation in a bird whose distribution spans an aridity gradient. Our results show that the Karoo scrub-robin’s climatic niche is spatially structured, providing the potential for local adaptation to develop. We found remarkably discordant patterns of divergence among mtDNA, morphology and neutral loci. For the mitochondrial genes, two amino acid replacements, strong population structure and reduced gene flow were associated with the environmental gradient separating western coastal sites from the interior of southern Africa. In contrast, morphology and the neutral loci exhibited variation independent of environmental variables, and revealed extensive levels of gene flow across the aridity gradient, 50 times larger than the estimates for mitochondrial genes. Together, our results suggest that selective pressures on physiology, mediated by the mitochondrial genome, may well be a common mechanism for facilitating local adaptation to new climatic conditions.

传统观点认为，基因流（gene flow）会拮抗种群分化（population differentiation）与本地适应（local adaptation）。然而近期研究表明，只要自然选择（natural selection）强于基因流的同质化效应，本地适应即可发生。本研究结合生态学（气候）、表型（phenotype，如体型）、生理遗传学（physiological genetics）、氧化磷酸化基因（oxidative phosphorylation genes）与中性位点（neutral loci，包括核微卫星（nuclear microsatellites）与内含子（introns）），对上述早期研究进行拓展，以检验在分布跨越干旱梯度的鸟类中，自然选择能否抵消基因流并进而促进本地适应。研究结果显示，卡鲁灌丛鸲（Karoo scrub-robin）的气候生态位具有空间结构，这为本地适应的形成提供了可能。我们发现线粒体DNA（mtDNA）、形态学（morphology）与中性位点之间的分化模式存在显著不一致性。对于线粒体基因而言，两处氨基酸替换（amino acid replacements）、显著的种群结构以及减弱的基因流，均与分隔南非西部沿海采样点与内陆的环境梯度（environmental gradient）相关联。与之相反，形态学性状与中性位点的变异与环境变量无关，且显示出该干旱梯度上存在广泛的基因流，其强度是线粒体基因流估计值的50倍。综合来看，本研究结果表明，由线粒体基因组（mitochondrial genome）介导的生理选择压力（selective pressures），或许是促进物种在新气候条件下形成本地适应的普遍机制。