The Mexican transition zone originated from the overlap of the Nearctic and the Neotropical biotas. By analysing the genetic variation within the cycad genus Dioon, a relict Neotropical group, Gutiérrez-Ortega et al. test the association betweenthe diversification of Dioon with the biogeographic provinciality of the Mexican transition zone.
The variation of two chloroplast DNA fragments and genome-wide single nucleotide polymorphisms reveals four Dioon lineages that are geographically structured. The reconstruction of the expansion of Dioon suggests that Dioon migrated along the mountain chains of Mexico. While dispersing towards their current distribution, each geographic region provided disparate ecological conditions that facilitated divergence and speciation during the Miocene period, around 15 million years ago.