Convergent evolution in floral morphology in a plant ring species, the Caribbean Euphorbia tithymaloides

Premise of the Study: Ring species have long fascinated evolutionary biologists for their potential insights into lineage divergence and speciation across space. Few studies have investigated the potential for convergent or parallel evolution along the diverging fronts of ring species. We investigate a potential case of parallel floral variation in the Caribbean spurge Euphorbia tithymaloides, the only plant system with molecular support as a ring species. The terminal populations of each front, despite being the most divergent, exhibit such similar floral traits that were originally considered each other closest relative. Methods: We evaluate convergence in floral and leaf traits in relation to geography across 95 populations spanning the distribution of E. tithymaloides. We also re-analyze available genetic data (from previous phylogenetic analyses) in an explicitly spatial framework. Key Results: Floral morphology appears to have shifted in a convergent fashion along both geographic...研究前提：环物种（ring species）因其可为空间尺度下的谱系分化与物种形成研究提供独特视角，长期以来一直令进化生物学家着迷。目前鲜有研究探讨环物种分化前沿的趋同或平行演化潜力。本研究以红雀珊瑚（Euphorbia tithymaloides）为对象，探究其花部性状的潜在平行演化案例——该类群是目前唯一经分子数据验证为环物种的植物类群。两个分化前沿的末端种群虽为分化程度最高的类群，但其花部性状却高度相似，最初曾被认为是彼此的近缘类群。 方法：本研究针对覆盖红雀珊瑚全分布范围的95个种群，结合地理梯度特征，评估其花部与叶部性状的趋同演化情况。同时，本研究在明确的空间分析框架下，重新分析了既往系统发育研究中公开的遗传数据。 关键结果：花部形态似乎在两个地理维度上均以趋同方式发生了演化……