Fitness surfaces and local thermal adaptation in Drosophila along a latitudinal gradient

Local adaptation is commonly cited to explain species distribution, but how fitness varies along continuous geographical gradients is not well understood. Here we combine thermal biology and life-history theory to demonstrate that Drosophila populations along a 2,500 km latitudinal cline are adapted to local conditions. We measured how heat tolerance and viability rate across 8 populations vary with temperature in the laboratory, and then simulated their expected cumulative Darwinian fitness employing high-resolution temperature data from their 8 collection sites. Simulations indicate a trade-off between annual survival and cumulative viability, as both mortality and the recruitment of new flies are predicted to increase in warmer regions. Importantly, populations are locally adapted and exhibit the optimal combination of both traits to maximize fitness where they live. In conclusion, our method is able to reconstruct fitness surfaces employing empirical life-history estimates and reconstructs peaks representing locally adapted populations, allowing to study geographic adaptation in silico.

局域适应常被用以解释物种分布规律，但适合度沿连续地理梯度的变化机制尚未得到充分阐释。本研究结合热生物学与生活史理论，证实沿2500公里纬度梯度分布的果蝇（Drosophila）种群均已发生局域适应。我们在实验室中测定了8个果蝇种群的耐热性与存活率随温度的变化规律，并利用8个采样点的高分辨率温度数据，模拟了它们的预期累积达尔文适合度（Darwinian fitness）。模拟结果显示，年度存活与累积存活率之间存在进化权衡：在温度更高的区域，果蝇死亡率与新个体补充率均会升高。值得注意的是，各果蝇种群均已发生局域适应，并展现出与当地环境匹配的最优性状组合，以最大化其在栖息环境中的适合度。综上，本研究方法可通过经验性生活史参数估算重建适合度曲面，并还原代表局域适应种群的适合度峰值，从而实现对地理适应的in silico研究。