Morphometric and geographical variation in the Ceratozamia mexicana Brongn. (Zamiaceae) complex: evolutionary and taxonomic implications

Abstract

The Cycadales contain 11 nonproblematic genera and approximately 331 species where, in a number of cases, delimitation and circumscription is not so clear. Ceratozamia, the most diverse cycad genus in Mexico, clearly illustrates this situation because a large percentage (close to 70%) of the 28 currently valid species in the genus was recently described on the basis of inconspicuous differences. We evaluated population-level morphological variation (i.e. phenotype) in related plants of Ceratozamia that share geological and ecological units, seeking a refined taxonomic assessment of their populations in an explicitly evolutionary context. The populations/species of interest included the type species Ceratozamia mexicana, as well as the taxa Ceratozamia brevifrons, Ceratozamia decumbens, and Ceratozamia morettii. Data analysis was performed using uni- and multivariate techniques on morphological variability, which showed statistically significant differences. The characters ‘leaflet width’ and ‘leaflet shape’ supported diagnostic distinctions between groups of populations, and implied a classificatory rearrangement with nomenclatural consequences. This research is an initial step in the detailed, statistics-oriented study of phenotypic variation in the Ceratozamia populations of central Veracruz in Mexico that will allow the construction of evolutionary hypotheses in the context of phylogeography and historical demography.