Data from: Evolution of viviparity: a phylogenetic test of the cold-climate hypothesis in Phrynosomatid lizards

The evolution of viviparity is a key life-history transition in vertebrates, but the selective forces favoring its evolution are not fully understood. With >100 origins of viviparity, squamate reptiles (lizards and snakes) are ideal for addressing this issue. Some evidence from field and laboratory studies supports the “cold-climate” hypothesis, wherein viviparity provides an advantage in cold environments by allowing mothers to maintain higher temperatures for developing embryos. Surprisingly, the cold-climate hypothesis has not been tested using both climatic data and phylogenetic comparative methods. Here, we investigate the evolution of viviparity in the lizard family Phrynosomatidae using GIS-based environmental data, an extensive phylogeny (117 species), and recently developed comparative methods. We find significant relationships between viviparity and lower temperatures during the warmest (egg-laying) season, strongly supporting the cold-climate hypothesis. Remarkably, we also find that viviparity tends to evolve more frequently at tropical latitudes, despite its association with cooler climates. Our results help explain this and two related patterns that seemingly contradict the cold-climate hypothesis: the presence of viviparous species restricted to low-elevation tropical regions and the paucity of viviparous species at high latitudes. Finally, we examine whether viviparous taxa may be at higher risk of extinction from anthropogenic climate change.

胎生（viviparity）的演化是脊椎动物关键的生活史转型事件，但推动其演化的选择压力尚未得到完全阐明。有鳞目爬行动物（squamate reptiles，即蜥蜴与蛇类）的胎生起源已超过100次，是探究该科学问题的理想类群。现有野外与实验室研究的部分证据支持“寒冷气候假说”：胎生可使母体为发育中的胚胎维持更高体温，从而在寒冷环境中获得演化优势。令人意外的是，此前尚未有研究结合气候数据与系统发育比较方法（phylogenetic comparative methods）对该假说开展检验。本研究针对角蜥科（Phrynosomatidae）蜥蜴的胎生演化展开探究，采用了基于地理信息系统（GIS）的环境数据、涵盖117个物种的大规模系统发育树，以及新近开发的比较分析方法。研究发现，胎生与最热产卵季的较低气温存在显著关联，有力支持了寒冷气候假说。值得注意的是，研究还发现尽管胎生与较冷气候存在关联，但胎生在热带纬度地区的演化频率反而更高。本研究结果解释了两种看似与寒冷气候假说相悖的相关模式：一是胎生物种仅局限分布于低海拔热带区域，二是高纬度地区胎生物种较为稀少。最后，本研究还探讨了胎生类群是否面临更高的由人为气候变化引发的灭绝风险。