Data from: The distinctive footprints of local hitchhiking in a varied environment and global hitchhiking in a subdivided population

Loci with higher levels of population differentiation than the neutral expectation are traditionally interpreted as evidence of ongoing selection that varies in space. This article emphasizes an alternative explanation which has been largely overlooked to date: in species subdivided into large subpopulations, enhanced differentiation can also be the signature left by the fixation of an unconditionally favourable mutation on its chromosomal neighbourhood. To discriminate among the two alternative scenarios one needs to investigate how genetic structure varies along the chromosomal region of the locus. Local hitchhiking is shown to generate a single sharp peak of differentiation centred on the adaptive polymorphism and the standard signature of a selective sweep only in those subpopulations where the allele is favoured. Global hitchhiking produces two domes of differentiation on either side of the fixed advantageous mutation and signatures of a selective sweep in every subpopulation, albeit of different magnitude. Investigating population differentiation around a locus that strongly differentiates two otherwise genetically similar populations of the marine mussel Mytilus edulis, plausible evidence for the global hitchhiking hypothesis has been obtained. Global hitchhiking is a neglected phenomenon that might prove to be important in species with large population sizes such as many marine invertebrates.

群体分化水平超出中性预期的基因座，传统上被解读为空间异质性持续选择的证据。本文着重探讨了一种长期被忽视的替代性解释：对于被划分为多个大亚种群的物种而言，群体分化程度的提升也可能是无条件有利突变在其染色体邻区发生固定后留下的特征印记。若要区分这两种情景，需探究遗传结构沿该基因座所在染色体区域的变化规律。研究表明，局部遗传搭车效应（local hitchhiking）仅会在携带该有利等位基因的亚种群中，形成以适应性多态性为中心的单一尖锐分化峰，以及选择性清除（selective sweep）的典型特征。全局遗传搭车效应（global hitchhiking）则会在已固定的有利突变两侧各形成一个分化隆起，并在所有亚种群中留下选择性清除的特征，只是不同亚种群的特征强度存在差异。通过对可使原本遗传相似的两个海洋贻贝（Mytilus edulis）种群产生显著分化的基因座周边开展群体分化分析，本研究获得了支持全局遗传搭车效应假说的可靠证据。全局遗传搭车效应是一种被长期忽视的现象，在种群规模庞大的物种（如多数海洋无脊椎动物）中或许具有重要意义。