Data from: Environmental harshness is positively correlated with intraspecific divergence in mammals and birds

Life on Earth is conspicuously more diverse in the tropics. Although this intriguing geographical pattern has been linked to many biotic and abiotic factors, their relative importance and potential interactions are still poorly understood. The way in which latitudinal changes in ecological conditions influence evolutionary processes is particularly controversial, as there is evidence for both a positive and a negative latitudinal gradient in speciation rates. Here, we identify and address some methodological issues (how patterns are analysed and how latitude is quantified) that could lead to such conflicting results. To address these issues, we assemble a comprehensive data set of the environmental correlates of latitude (including climate, net primary productivity and habitat heterogeneity) and combine it with biological, historical and molecular data to explore global patterns in recent divergence events (subspeciation). Surprisingly, we find that the harsher conditions that typify temperate habitats (lower primary productivity, decreased rainfall and more variable and unpredictable temperatures) are positively correlated with greater subspecies richness in terrestrial mammals and birds. Thus, our findings indicate that intraspecific divergence is greater in regions with lower biodiversity, a pattern that is robust to both sampling variation and latitudinal biases in taxonomic knowledge. We discuss possible causal mechanisms for the link between environmental harshness and subspecies richness (faster rates of evolution, greater likelihood of range discontinuities and more opportunities for divergence) and conclude that this pattern supports recent indications that latitudinal gradients of diversity are maintained by simultaneously higher potentials for both speciation and extinction in temperate than tropical regions.

地球上的生物多样性在热带地区显著更高。尽管这一引人关注的地理分布格局已被关联至诸多生物与非生物因素，但各因素的相对重要性及潜在交互作用仍尚未被充分阐明。生态条件随纬度变化的模式如何影响进化过程，这一问题尤其存在争议，因为现有研究在物种形成速率的纬度梯度方向上同时存在正向和负向的证据。本研究识别并解决了可能导致上述矛盾结果的若干方法学问题（包括格局分析方式与纬度量化方法）。为解决上述问题，我们构建了一套涵盖纬度相关环境因子（包括气候、净初级生产力（net primary productivity）与生境异质性（habitat heterogeneity））的综合数据集，并将其与生物学、历史及分子数据相结合，以探究近期分化事件——亚种形成（subspeciation）的全球分布格局。令人意外的是，我们发现温带生境所典型具备的严苛环境条件（更低的初级生产力、更少的降雨量以及更不稳定且难以预测的温度），与陆生哺乳动物和鸟类的亚种丰富度呈正相关关系。因此，我们的研究结果表明，生物多样性更低的区域其种内分化程度更高，这一格局不受采样变异与分类学认知的纬度偏差影响，表现出较强的稳健性。我们探讨了环境严苛性与亚种丰富度之间关联的潜在因果机制（包括更快的进化速率、种群分布间断的更高概率，以及更多的分化机会），并最终得出结论：该格局支持近期的研究结论，即温带地区相较于热带地区，同时具备更高的物种形成与灭绝潜能，这正是维持生物多样性纬度梯度的原因。