Adaptation across environmental gradients has been demonstrated in numerous systems with extensive dispersal, despite high gene flow and consequently low genetic structure. The speed and mechanisms by which such adaptation occurs remain poorly resolved, but are critical to understanding species spread and persistence in a changing world. Here, we investigate these mechanisms in the European green crab Carcinus maenas, a globally distributed invader. We focus on a northwestern Pacific population that spread across >12 degrees of latitude in 10 years from a single source, following its introduction <35 years ago. Using six locations spanning >1500 km, we examine genetic structure using 9376 single nucleotide polymorphisms (SNPs). We find high connectivity among five locations, with significant structure between these locations and an enclosed lagoon with limited connectivity to the coast. Among the five highly connected locations, the only structure observed was a cline driven by a handful of SNPs strongly associated with latitude and winter temperature. These SNPs are almost exclusively found in a large cluster of genes in strong linkage disequilibrium that was previously identified as a candidate for cold tolerance adaptation in this species. This region may represent a balanced polymorphism that evolved to promote rapid adaptation in variable environments despite high gene flow, and which now contributes to successful invasion and spread in a novel environment. This research suggests an answer to the paradox of genetically depauperate yet successful invaders: populations may be able to adapt via a few variants of large effect despite low overall diversity.

中文翻译：

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尽管基因流高且遗传多样性低，但平衡的多态性促进了入侵螃蟹的快速选择

尽管基因流动高，遗传结构低，但在许多广泛分布的系统中已经证明了跨环境梯度的适应。这种适应发生的速度和机制仍然很难解决，但对于理解物种在不断变化的世界中的传播和持久性至关重要。在这里，我们研究了欧洲绿蟹Carcinus maenas中的这些机制，全球分布的入侵者。我们专注于西北太平洋人口，该人口在 10 年内从单一来源传播到 >12 度纬度，继 <35 年前引入之后。使用跨越> 1500 km的六个位置，我们使用9376个单核苷酸多态性（SNP）检查遗传结构。我们发现五个地点之间的高度连通性，这些地点之间的结构显着，以及与海岸的连通性有限的封闭泻湖。在五个高度连接的位置中，观察到的唯一结构是由少数与纬度和冬季温度密切相关的 SNP 驱动的斜线。这些单核苷酸多态性几乎完全存在于强连锁不平衡中的一大群基因中，这些基因先前被确定为该物种耐冷适应的候选者。该区域可能代表一种平衡的多态性，尽管基因流量很高，但它进化为促进在可变环境中的快速适应，现在有助于在新环境中成功入侵和传播。这项研究提出了一个解决基因退化但成功的入侵者悖论的答案：尽管总体多样性较低，但种群可能能够通过一些大效应的变体来适应。