Assessing genetic connectivity among populations in high gene flow species is sometimes insufficient to evaluate demographic connectivity. Genetic differentiation quickly becomes zero as soon as a few dozen migrants are exchanged per generation. This provides little information to determine whether migration can ensure demographic coupling. The resulting difficulties in delineating conservation units for the management of commercially exploited marine fish species are well illustrated in the case of the European sea bass (Dicentrarchus labrax). Previous attempts to assess connectivity patterns in the northeast Atlantic have been hampered by a lack of spatial genetic structure. In contrast, mark‐recapture data suggested low migration rates between regional spawning areas. Here, we show how a spatial gradient of introgressed Mediterranean ancestry across the northeast Atlantic reflects cryptic patterns of genetic and demographic connectivity. Using a 1K SNP chip data set in 827 individuals sampled from Portugal to the North Sea, we found null overall genetic differentiation across the northeast Atlantic. We however detected a subtle latitudinal admixture gradient originating at the edge of the contact zone with the Mediterranean sea bass lineage. Two significant breaks in the ancestry gradient at the tip of Galicia and northern Brittany indicated barriers to effective dispersal between demographically distinct units. Moreover, a northward expansion signal in Irish and North Seas was revealed by the surfing of rare Mediterranean alleles at the edge of the species range. Our results show that introgressed ancestry gradients offer a powerful alternative to assess genetic and demographic connectivity when the neutral migration‐drift balance is not informative.

中文翻译：

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渐渗祖先的空间梯度揭示了高基因流海鱼中的隐秘连通性模式。

评估高基因流物种的种群之间的遗传连通性有时不足以评估人口统计学的连通性。每代交换几十个移民后，遗传分化迅速变为零。这提供了很少的信息来确定迁移是否可以确保人口统计的耦合。在欧洲鲈鱼（Dicentrarchus labrax）的案例中很好地说明了在划定保护单位以管理商业开发的海洋鱼类物种时所造成的困难。）。缺乏空间遗传结构阻碍了先前评估东北大西洋连通性模式的尝试。相反，标记回收数据表明区域产卵区域之间的迁移率较低。在这里，我们展示了横跨东北大西洋的地中海血统的空间梯度如何反映出遗传和人口连通性的隐秘模式。使用从葡萄牙到北海的827个样本中的1K SNP芯片数据集，我们发现整个东北大西洋没有总体遗传分化。但是，我们检测到了轻微的纬度混合梯度，该梯度起源于与地中海鲈鱼世系的接触区的边缘。在加利西亚和布列塔尼北部，祖先梯度发生了两次重大突破，这表明在人口统计学上不同的单位之间进行有效分散的障碍。此外，由于在物种范围边缘的稀有地中海等位基因的冲浪，揭示了爱尔兰和北海向北扩张的信号。我们的结果表明，当中性迁移漂移平衡不能提供足够的信息时，渗入的祖先梯度提供了一种强大的替代方法来评估遗传和人口的连通性。