Data from: Testing mechanisms of Bergmann's rule: phenotypic but no genetic change in body size in three passerine bird populations

Bergmann's rule predicts a decrease in body size with increasing temperature and has much empirical support. Surprisingly, we know very little about whether 'Bergmann size clines' are due to a genetic response or is a consequence of phenotypic plasticity. Here we use data on body size (mass and tarsus length) from three long-term (1979-2008) study populations of great tits (Parus major), in which there has been a temperature increase, to examine mechanisms behind Bergmann's rule. We show that adult body mass decreased over the study period in all populations and that tarsus length increased in one population. Both body mass and tarsus length were heritable and under weak positive directional selection, predicting an increase, rather than decrease, in body mass. There was no support for micro-evolutionary change and thus the observed declines in body mass were a result of phenotypic plasticity. Interestingly, this plasticity was not in direct response to temperature changes but seemed to be due to changes in prey dynamics. Our results caution against interpreting recent phenotypic body size declines as an adaptive evolutionary response to temperature changes and highlight the importance of considering alternative environmental factors when testing size clines.

伯格曼法则（Bergmann's rule）预测体型会随温度升高而减小，且已有大量实证研究支持该假说。令人意外的是，学界对“伯格曼体型梯度（Bergmann size clines）”究竟源于遗传响应，还是表型可塑性（phenotypic plasticity）的结果，仍知之甚少。本研究采用三个长期（1979-2008年）大山雀（Parus major）研究种群的体型数据（体重与跗跖长度）——这些种群的栖息环境均出现了温度升高现象，以此探究伯格曼法则背后的作用机制。研究结果显示，所有种群的成体体重均在研究周期内呈下降趋势，而其中一个种群的跗跖长度有所增加。体重与跗跖长度均具有可遗传性，且受到微弱的正向定向选择，该选择理论上应促使体重增加，而非下降。未发现支持微进化变化（micro-evolutionary change）的相关证据，因此观测到的体重下降实则为表型可塑性的结果。有趣的是，这种可塑性并非直接响应温度变化，而是似乎源于猎物动态的改变。本研究结果警示，不应将近期观测到的体型表型下降直接解读为对温度变化的适应性进化响应，同时强调在检验体型梯度假说时，考虑其他潜在环境因素的重要性。