Evolutionary and ecological trends in the Neotropical cycad genus Dioon (Zamiaceae): An example of success of evolutionary stasis

Abstract

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.