Cycads represent an example of the success of evolutionary stasis. Despite their early origin, they survived multiple events of mass extinction and diversified in modern tropical ecosystems during the Cenozoic without major changes in their morphology. What factors have allowed their persistence and diversification despite their conservative nature? We reviewed documentation on the micro- and macro-evolutionary processes involved in the diversification of the Neotropical genus Dioon. Dioon comprises 18 species from varied habitats in Mexico and Honduras, and serves as a model to understand the patterns of cycad diversification. Here, we synthesize evidence reached from different fields, especially biogeography, phylogenetics, population ecology, and speciation, to propose a mechanism that can explain the current patterns of biodiversity in Dioon. At the macroevolutionary scale, a Paleogene origin for Dioon is more likely than an alternative hypothesis of a Neogene origin. Dioon lineages have dispersed along with the expansion of tropical forests throughout main mountain chains. Subsequently, climate change, and particularly aridification, promoted the fragmentation of the tropical forests, allowing the main Dioon clades to evolve in isolation at distinct biogeographic regimes. At the microevolutionary scale, low seed dispersal capability, stochastic demographic processes, and niche conservatism restrict the lineages to isolate themselves at narrow habitats, promoting local adaptation in populations. Local adaptation seems to be a process achieved through many generations under stabilizing selection. Altogether, these processes shaped the diversification in Dioon. This review attempts to stimulate further research on cycads and other biological groups that have diversified despite their apparent evolutionary stasis.

中文翻译：

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新热带苏铁属 Dioon（Zamiaceae）的进化和生态趋势：进化停滞成功的例子

苏铁植物代表了进化停滞的成功例子。尽管它们起源较早，但它们在多次大规模灭绝事件中幸存下来，并在新生代的现代热带生态系统中实现了多样化，形态上没有发生重大变化。哪些因素让他们在保守的情况下得以坚持和多元化？我们回顾了新热带狄翁属多样化所涉及的微观和宏观进化过程的文献。Dioon包括来自墨西哥和洪都拉斯不同栖息地的 18 个物种，可作为了解苏铁多样化模式的模型。在这里，我们综合了来自不同领域的证据，特别是生物地理学、系统发育学、种群生态学和物种形成，提出了一种可以解释狄翁当前生物多样性模式的机制。在宏观进化尺度上，狄翁的古近纪起源比新近纪起源的替代假设更有可能。随着热带森林在主要山脉中的扩张，狄翁谱系也随之分散。随后，气候变化，特别是干旱化，促进了热带森林的破碎化，使主要的狄翁分支在不同的生物地理状况下孤立地进化。在微观进化尺度上，低种子传播能力、随机的人口统计过程和生态位保守主义限制了谱系在狭窄的栖息地中孤立自己，促进了种群的局部适应。局部适应似乎是在稳定选择下经过许多代而实现的过程。总而言之，这些过程塑造了迪翁的多元化。这篇综述试图促进对苏铁类和其他生物类群的进一步研究，这些生物类群尽管明显处于进化停滞状态，但仍然实现了多样化。