Instituto de Biología, Universidad Nacional Autónoma de México, CDMX 04510, Mexico.
NatureServe, Western Regional Office, 1680 38th St., Suite 120, Boulder, Colorado, 80301, USA.
Am J Bot. 2019 Jul;106(7):1032-1045. doi: 10.1002/ajb2.1318. Epub 2019 Jul 7.
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 investigated 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 they were originally considered each other's closest relative.
We evaluated convergence in floral and leaf traits in relation to geography across 95 populations spanning the distribution of E. tithymaloides. We also reanalyzed available genetic data (from previous phylogenetic analyses) in an explicitly spatial framework.
Floral morphology appears to have shifted in a convergent fashion along both geographic fronts of E. tithymaloides, resulting in shorter and more compact inflorescences in Antillean populations compared to the typical elongate "slipper-like" cyathia characteristic of the area of origin. Patterns of spatial genetic variation were more consistent with a two-fronted invasion of the Caribbean than with a simpler model of isolation-by-distance.
Floral divergence in E. tithymaloides is consistent with convergent evolution along the two fronts of a ring species. We outline several (not mutually exclusive) mechanisms that could be driving patterns in morphology, including shifts toward generalized pollination with reduced reliance on hummingbirds, shifts in floral structure closely matching available hummingbird bill traits, and shifts toward increased selfing.
环种长期以来一直令进化生物学家着迷，因为它们有可能深入了解谱系在空间上的分歧和物种形成。很少有研究调查过在环种的分歧前沿沿线发生趋同或平行进化的潜力。我们研究了加勒比海大戟属植物 Euphorbia tithymaloides 中潜在的平行花部变异，这是唯一具有分子支持的作为环种的植物系统。尽管每个前沿的末端种群是最具分歧的，但它们表现出如此相似的花部特征，以至于它们最初被认为是彼此最接近的亲缘关系。
我们评估了跨越分布在 Euphorbia tithymaloides 中的 95 个种群的地理分布的花部和叶部特征的趋同。我们还在明确的空间框架中重新分析了可用的遗传数据（来自先前的系统发育分析）。
花部形态似乎沿着 E. tithymaloides 的两个地理前沿以趋同的方式发生了变化，导致安的列斯群岛种群的花序变得更短更紧凑，与起源地典型的拉长的“拖鞋状”聚伞花序相比。空间遗传变异模式更符合加勒比地区的两前沿入侵，而不是更简单的隔离距离模型。
E. tithymaloides 的花部分歧与环种的两个前沿的趋同进化是一致的。我们概述了几种（非互斥）可能驱动形态模式的机制，包括向依赖蜂鸟减少的广义传粉的转变，与可用蜂鸟喙特征密切匹配的花部结构的转变，以及向增加自交的转变。
