The cycads are generally perceived as a very ancient lineage. In addition, high extinction rates and very long generation times may preclude the dynamics of genetic variation. More than 20 of the known and putative species of the Mexican cycad genus Ceratozamia were examined for variation at the molecular level in non-coding regions from the chloroplast and nuclear genomes. Cladistic analyses of single and combined sequences of ITS and trnL-F non-coding region supported three main clades within Ceratozamia. The rate of change of these genomes is considered fast and appropriate to recover variation at the level of genera. However, only 33 nucleotide positions were informative out of 2184, among the species. Such low level of variation suggests that, despite the genus being considered very old, the species appear very young as inferred by the amount of sequence divergence. The three clades, the pattern of low divergence, and restricted distribution of most species has biogeographic implications. First, it suggests a probable ancestral geographic area for Ceratozamia in Southeast Mexico, already known as a biodiversity "hot spot". Second, the time of speciation within the genus appears to be associated to the post-Pleistocene spread of floristic communities from proposed Pleistocene tropical refugia of S. E. Mexico that date back to or even beyond Miocene times.

Key words: cycads, gymnosperm, nuclear and chloroplast DNA, phylogeny