Phylogenetic and geographic diversification/differentiation as an evolutionary avenue in the genus Cephalocereus (Cactaceae) Evolutionary Avenue in Cephalocereus

Abstract Phylogenetic and geographic distances were estimated to produce a combined Distancing Index as a measure of historical reproductive isolation in the genus Cephalocereus. Geographic and climatic barriers were inferred from potential and observed distributions. Distances were extracted from a Bayesian tree for seven chloroplast regions and 26 terminals, and from the geolocation dataset for the exact same sampling. Distance matrices were relativized, and a Mantel test was applied to identify deviations between phylogenetic and geographic distances. Heatmaps and scatterplots were implemented to visualize combined trends. Two basal clades show patterns of differentiation or complete isolation; the first includes C. scoparius, C. apicicephalium, C. nizandensis, and C. totolapensis, and the second includes C. parvispinus, C. polylophus, and C. euphorbioides. The species C. fulviceps, and C. sanchezmejoradae appear in a differentiated grade as sisters of a well-defined clade that includes C. mezcalaensis, C. macrocephalus, C. tetetzo, C. senilis, C. columna-trajani, C. multiareolatus, and C. nudus, where geographic or phylogenetic distances lie below the mean, indicating a diversification process in absence of hard barriers. At the generic level, separation is related to climatic factors as temperature and moisture, while factors as the altitude could be determinants of separation at the species level. The steady accumulation of variants may lead to opposed evolutionary outcomes: differentiation or diversification, in isolated and non-isolated lineages, respectively. More studies are needed on how genetic variation is transferred or interchanged between and among lineages, and how morphological differentiation of diverging lineages account for reproductive isolation.