Adaptation is central to population persistence in the face of environmental change, yet we seldom precisely understand the origin and spread of adaptive variation in natural populations. Snowshoe hares (Lepus americanus) along the Pacific Northwest coast have evolved brown winter camouflage through positive selection on recessive variation at the Agouti pigmentation gene introgressed from black-tailed jackrabbits (Lepus californicus). Here, we combine new and published whole-genome and exome sequences with targeted genotyping of Agouti to investigate the evolutionary history of local seasonal camouflage adaptation in the Pacific Northwest. We find evidence of significantly elevated inbreeding and mutational load in coastal winter-brown hares, consistent with a recent range expansion into temperate coastal environments that incurred indirect fitness costs. The genome-wide distribution of introgression tract lengths supports a pulse of hybridization near the end of the last glacial maximum, which may have facilitated range expansion via introgression of winter-brown camouflage variation. However, signatures of a selective sweep at Agouti indicate a much more recent spread of winter-brown camouflage. Through simulations, we show that the delay between the hybrid origin and subsequent selective sweep of the recessive winter-brown allele can be largely attributed to the limits of natural selection imposed by simple allelic dominance. We argue that while hybridization during periods of environmental change may provide a critical reservoir of adaptive variation at range edges, the probability and pace of local adaptation will strongly depend on population demography and the genetic architecture of introgressed variation.

中文翻译：

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雪鞋野兔局部适应性季节性伪装的起源和传播。

适应是面对环境变化的人口持久性的核心，但是我们很少准确地了解自然种群中适应性变化的起源和传播。西北太平洋沿岸的雪兔（Lepus americanus）通过从黑尾长耳兔（Lepus californicus）渗入的Agouti色素沉着基因的隐性变异上进行积极选择，进化出了棕色冬季伪装。在这里，我们结合新发布的全基因组，并用的靶向基因分型外显子组序列刺鼠调查西北太平洋局部季节性伪装适应的演变历史。我们发现有证据表明沿海冬棕野兔的近交繁殖和突变负荷显着增加，这与最近进入温带沿海环境的范围扩展产生了间接适应成本。全基因组的渐渗道长度分布支持最后一个冰期最大值末尾的杂交脉冲，这可能通过冬棕迷彩变异的渐渗促进了范围扩展。然而，在阿古蒂的选择性扫荡的签名表示最近出现了冬季棕色迷彩。通过模拟，我们表明，杂种起源与隐性冬季棕色等位基因随后的选择性扫掠之间的延迟很大程度上可以归因于简单等位基因优势对自然选择的限制。我们认为，尽管环境变化时期的杂交可能会在范围边缘提供重要的适应性变异库，但局部适应的可能性和步伐将在很大程度上取决于人口人口统计学和渗入变异的遗传结构。