The relative role of genetic adaptation and phenotypic plasticity is of fundamental importance in evolutionary ecology [M. J. West-Eberhard, Proc. Natl. Acad. Sci. U.S.A. 102 (suppl. 1), 6543–6549 (2005)]. European eels have a complex life cycle, including transitions between life stages across ecological conditions in the Sargasso Sea, where spawning occurs, and those in brackish and freshwater bodies from northern Europe to northern Africa. Whether continental eel populations consist of locally adapted and genetically distinct populations or comprise a single panmictic population has received conflicting support. Here we use whole-genome sequencing and show that European eels belong to one panmictic population. A complete lack of geographical genetic differentiation is demonstrated. We postulate that this is possible because the most critical life stages—spawning and embryonic development—take place under near-identical conditions in the Sargasso Sea. We further show that within-generation selection, which has recently been proposed as a mechanism for genetic adaptation in eels, can only marginally change allele frequencies between cohorts of eels from different geographic regions. Our results strongly indicate plasticity as the predominant mechanism for how eels respond to diverse environmental conditions during postlarval stages, ultimately solving a long-standing question for a classically enigmatic species.

遗传适应和表型可塑性的相对作用在进化生态学中至关重要 [MJ West-Eberhard, Proc.国家。阿卡德。科学。美国102（增刊 1），6543–6549（2005）]。欧洲鳗鱼有一个复杂的生命周期，包括产卵的马尾藻海以及从北欧到北非的咸水和淡水水体中不同生态条件下生命阶段之间的转变。大陆鳗鱼种群是由适应当地环境且遗传上不同的种群组成，还是由单一的恐慌种群组成，这一点得到了相互矛盾的支持。在这里，我们使用全基因组测序表明欧洲鳗鱼属于一种恐慌种群。证明完全缺乏地理遗传分化。我们假设这是可能的，因为最关键的生命阶段——产卵和胚胎发育——发生在马尾藻海几乎相同的条件下。我们进一步表明，最近被提出作为鳗鱼遗传适应机制的代内选择只能略微改变来自不同地理区域的鳗鱼群体之间的等位基因频率。我们的结果强烈表明，可塑性是鳗鱼在幼体后期如何应对不同环境条件的主要机制，最终解决了一个经典神秘物种的长期存在的问题。