The process of diversification can be studied at the phylogeographic level by attempting to identify the environmental features that promote and maintain population divergence. Here we investigate diversification in Rhinella granulosa, a Neotropical toad from northeastern Brazil, by testing a range of hypotheses that encompass different putative mechanisms reducing gene flow among populations. We sequenced single nucleotide polymorphisms and examined individual predictions related to the role of geographic barriers (rivers), ecological gradients, historical habitat stability, and spatial variation in climate seasonality, also known as the asynchrony of seasons hypothesis. This hypothesis postulates that temporal asynchrony of wet and dry seasons over short distances causes parapatric populations to become isolated by time. After determining genetic structure, inferring past distributions, ranking demographic models, and estimating the power of monthly climatic variables, our results identified two populations that are not associated with geographic barriers, biome gradients, or historical refugia. Instead, they are predicted by spatial variation in monthly rainfall and minimum temperature, consistent with the asynchrony of seasons hypothesis, supported also by our comparative framework using multiple matrix regression and linear mixed effects modeling. Due to the toad’s life history, climate likely mediates gene flow directly, with genetic differentiation being provoked by neutral mechanisms related to climate driven population isolation, and/or by natural selection against migrants from populations with different breeding times. The asynchrony of seasons hypothesis is seldom considered in phylogeographic studies, but our results indicate that it should be tested in systems where breeding is tightly coupled with climate.

通过尝试识别促进和维持种群分化的环境特征，可以在系统发育地理学水平上研究多样化的过程。在这里，我们通过测试一系列假设来研究颗粒莱氏蟾蜍（一种来自巴西东北部的新热带蟾蜍）的多样化，这些假设涵盖了减少种群间基因流动的不同假定机制。我们对单核苷酸多态性进行了测序，并检查了与地理障碍（河流）、生态梯度、历史栖息地稳定性和气候季节性空间变化（也称为季节异步假说）的作用相关的个体预测。该假设假设，短距离内干湿季节的时间不同步会导致近亲种群在时间上变得孤立。在确定遗传结构、推断过去的分布、对人口模型进行排名并估计每月气候变量的力量后，我们的结果确定了两个与地理障碍、生物群落梯度或历史避难所无关的种群。相反，它们是通过每月降雨量和最低温度的空间变化来预测的，与季节异步假说一致，也得到我们使用多元矩阵回归和线性混合效应模型的比较框架的支持。由于蟾蜍的生活史，气候可能直接介导基因流动，与气候驱动的种群隔离相关的中性机制和/或针对来自不同繁殖时间种群的移民的自然选择引发了遗传分化。系统发育地理学研究中很少考虑季节异步假说，但我们的结果表明，应该在育种与气候紧密结合的系统中对其进行测试。