Data from: Winter wren populations show adaptation to local climate

Most studies of evolutionary responses to climate change have focused on phenological responses to warming, and provide only weak evidence for evolutionary adaptation. This could be because phenological changes are more weakly linked to fitness than more direct mechanisms of climate change impacts, such as selective mortality during extreme weather events which have immediate fitness consequences for the individuals involved. Studies examining these other mechanisms may be more likely to show evidence for evolutionary adaptation. To test this, we quantify regional population responses of a small resident passerine (winter wren Troglodytes troglodytes) to a measure of winter severity (number of frost days). Annual population growth rate was consistently negatively correlated with this measure, but the point at which different populations achieved stability (λ = 1) varied across regions and was closely correlated with the historic average number of frost days, providing strong evidence for local adaptation. Despite this, regional variation in abundance remained negatively related to the regional mean number of winter frost days, potentially as a result of a time-lag in the rate of evolutionary response to climate change. As expected from Bergmann's rule, individual wrens were heavier in colder regions, suggesting that local adaptation may be mediated through body size. However, there was no evidence for selective mortality of small individuals in cold years, with annual variation in mean body size uncorrelated with the number of winter frost days, so the extent to which local adaptation occurs through changes in body size, or another mechanism remains uncertain.当前绝大多数针对气候变化进化响应的研究，均聚焦于气候变暖引发的物候响应，仅能为进化适应提供有限的佐证。出现这一情况的原因可能在于：相较于气候变化影响的更直接作用路径（如极端天气事件期间的选择死亡，该过程会对涉及个体的适合度产生即时影响），物候变化与个体适合度的关联强度更低。针对这类其他作用机制开展的研究，或许更易捕捉到进化适应的相关证据。为验证这一假说，我们针对一种小型留居雀形目鸟类——冬鹪鹩（Troglodytes troglodytes），以霜冻日数作为冬季严酷程度的量化指标，分析其区域种群的响应特征。种群年增长率与该指标始终呈显著负相关，但不同种群达到种群稳定状态（λ=1）的临界阈值存在区域差异，且与历史平均霜冻日数密切相关，这为本地适应提供了强有力的佐证。尽管如此，区域种群丰度的空间差异仍与区域平均冬季霜冻日数呈负相关，这一现象可能源于气候变化的进化响应速率存在时间滞后效应。按照伯格曼法则（Bergmann's rule）的理论预期，较寒冷区域的冬鹪鹩个体体重更大，这表明本地适应或可通过体型介导实现。但研究并未发现寒冷年份中体型较小个体存在选择死亡的证据，个体平均体型的年际变化与冬季霜冻日数并无关联，因此，本地适应在多大程度上通过体型变化或其他机制实现，目前仍有待明确。