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）预测生物体型会随环境温度升高而减小，且已有诸多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）”究竟源于遗传适应性响应，还是表型可塑性的结果，仍知之甚少。本研究以1979-2008年长期监测的三个大山雀（Parus major）种群的体型数据（体重与跗跖长度）为材料，结合研究区域的温度升高趋势，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在研究周期内均呈下降趋势，而其中一个种群的跗跖长度有所增加。体重和跗跖长度均具有可遗传性，且受到微弱的正向定向选择，理论上应促使体重增加而非降低。本研究未发现微进化改变的相关证据，因此观测到的体重下降实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。本研究结果告诫人们切勿将近期观测到的体型下降简单解读为对温度变化的适应性进化响应，同时凸显了在检验体型梯度假说时，考虑其他潜在环境因素的重要性。