Phylogenetic distancing. The most distant species were C. polylophus and M. aragonii ssp. eichlamii, which was taken as the intergeneric distance (100%). This distance was 31 times greater than that between the sister species C. apicicephalium and C. nizandensis. The distance between M. aragonii ssp. eichlamii and B. emoryi is 56.9%, while the most distant species within Cephalocereus lies in a 58.5% distance (C. polylophus/C. sanchezmejoradae). Other relevant comparisons are the distances among Neodawsonia species which barely reach 3.2% in C. apicicephalium/C. nizandensis, 4% in C. apicicephalium/C. totolapensis, and 5.4% in C. nizandensis/C. totolapensis. The intraspecific distances range from 4.3% in the disjunct C. euphorbioides to 9.1% in the widespread C. mezcalaensis, while the species C. columna-trajani and C. tetetzo, which present natural hybrids (Vite et al., 1996), lie in a 13.4% distance range. These results indicate not only the recentness of the Neodawsonia splitting, but the potential for other phylogenetically and geographically close species to produce genetic transferences or interchanges. Other examples are C. scoparius and C. nizandensis, separated barely 16 km away with a moderate phylogenetic distance from 25.9 to 28.8%, C. tetetzo and C. nudus, separated 25 km in a phylogenetic distance range from 15.9 to 20.4%, and C. macrocephalus and C. tetetzo, that make contact in one site and have phylogenetic distances from 11.5 to 13.1%. In this population of C. macrocephalus and C. tetetzo some putative hybrid individuals have been observed (Arias & Tapia, pers. obs.).
Up to date, several intergeneric hybrids have been reported in the tribe Echinocereeae, where Cephalocereus belongs, as ×Pachebergia (Backebergia × Pachycereus), ×Pacherocactus (Bergerocactus × Pachycereus), ×Myrtgerocactus (Bergerocactus × Myrtillocactus), ×Myrtillenocereus (Myrtillocactus × Stenocereus), and some interspecific hybrids are documented in the sister tribe Hylocereeae, between Hylocereus undatus, H. monacanthus, and H. megalanthus, where H. megalanthus probably speciate from same species parentals by autopolyploidy (Anderson, 2001; Arias & Terrazas, 2008; Plume et al., 2013; Granados-Aguilar et al., 2022). The placement of hybridizing species in the same genus or in separated genera based on their morphology have implications on what we suppose to occur at cellular and genetic levels in such hybridization event, whereas morphology alone can be an overestimated indicator of differentiation and reproductive isolation (Plume et al., 2013). The hybrid between C. columna-trajani and C. tetetzo was described initially as an intergeneric hybrid between Neobuxbaumia tetetzo and Cephalocereus columna-trajani, later recovered as sister species in a phylogenetic analysis, along with C. senilis, and reassigned altogether in the same genus (Cephalocereus) (Tapia et al., 2017). Other example is Backebergia militaris, which presented enough morphological characters to be assigned to a monotypic genus according to Bravo-Hollis (1953), but phylogenetic studies resolved its position close to Pachycereus pecten-aboriginum, with which hybridize to produce xPachebergia apicostata (Arias & Terrazas, 2008). The Cactaceae family, like other plant families, appears as a group inclined to hybridize at varying levels, since the same species, to interspecies, to intergeneric parentals, resulting in chaotic complexes as in wild opuntias, or in flashy cultivated varieties of Astrophytum (Granados-Aguilar et al., 2022).
Attending to a morphometric assessment, the study on Cephalocereus areoles and spines revealed simultaneous patterns of character variation and conservation (Tapia et al., 2016). In some species a unique spination pattern allowed the univocal distinction of individuals as part of one species, as in C. polylophus, C. euphorbioides, and C. parvispinus, which correspond to a phylogenetically differentiated group in the present study. Other individuals show similarities that allowed the distinction of species groups, particularly the groups C. mezcalaensis/C. nudus/C. scoparius and C. tetetzo/C. macrocephalus/C. fulviceps are of particular interest due to the closeness between the included species, both in geographic and phylogenetic distances, and the possibility of genetic transferences or interchanges to occur, which needs to be studied further in detail. It is not intended to point that hybridization is responsible for all the evolutionary changes, but reproductive processes, either continuous or occasional, need to be accounted and comprehensible analyzed to untangle the genetic or genomic contribution of parental organisms, either if they belong to the same or to different species.
Geographic barriers and splitting events. The effects of the climatic and geographic barriers between and among Cephalocereus species are consistent with a hypothesis of reproductive isolation. The clade formed by C. scoparius, C. apicicephalium, C. nizandensis, and C. totolapensis, which inhabit a region between the Tehuantepec Isthmus and the adjacent part of the Sierra Madre del Sur, represents an early divergence from the Marshallocereus/Cephalocereus ancestor, just followed by the Bergerocactus ancestor splitting. The species C. scoparius has a disjunct distribution occupying also a portion in the center of the Veracruzan province, caused probably by a relatively recent long distance dispersal event, denoted by a phylogenetic distance of 8.8%. The clade formed by C. euphorbioides, C. polylophus, and C. parvispinus becomes the next splitting event coupled with the dispersal northward into the Sierra Madre del Sur and then followed by the crossing of the Transmexican Volcanic Belt barrier. C. parvispinus remained in the Sierra Madre del Sur, probably ecologically isolated, and C. polylophus and C. euphorbioides ancestor colonized the Sierra Madre Oriental and the Veracruzan provinces. The phylogenetic distance between C. polylophus and C. euphorbioides is 20.3%, indicating an early divergence event between these species after their dispersal from the southern regions.
On the counterpart, species non-isolated by hard or absolute barriers, show shorter distances either phylogenetic, geographic or both. This is the case of the clade formed by C. mezcalaensis, C. multiareolatus, C. nudus, C. macrocephalus, C. tetetzo, C. columna-trajani, and C. senilis. The only species from this clade that dispersed across the Transmexican Volcanic Belt is C. senilis. The phylogenetic distances among species of this clade ranges from 7.7% in the sister species C. columna-trajani/C. senilis, to 21.9% in C. senilis/C. nudus, while the geographic distances range from zero up to ~ 1’000 km. The species C. fulviceps and C. sanchezmejoradae have phylogenetic distances higher than 28% from the species belonging to the previous clade, despite the absence of a hard barrier, indicating the possible existence of ecological ancient barriers instead.
Absolute geographic distances. Considering the size of Cephalocereus individuals and the type of pollinators (moths and bats), the distance for effective pollination ranges from meters up to several kilometers. The seeds display patterns of local dispersion led by fruit dehiscence and gravity action, or long-distance dispersal by migrating bats and birds, also up to several kilometers, and longer than in pollination (Valiente-Banuet et al., 1997; González-Terrazas et al., 2016). Phylogenetically differentiated clades and/or species are separated from other species or clades by barriers at least 80 kilometers wide or in altitudinal ranges of several hundreds of meters. In the absence of barriers, some species can occupy land stripes up to 800 km in length, like C. nudus which inhabits along the Pacific coastline in southern Mexico, from Jalisco and Colima to Oaxaca. In this case, the occupation of a broad region might had occurred gradually, pollination may be restricted to each population and seed dispersal limited to close populations. Given this structuration, it is likely that the most distant populations located in Colima and Jalisco are currently undergoing a differentiation process from same species populations in Oaxaca (> 600 km apart). Other species with disjunct distributions (C. euphorbioides and C. scoparius), show moderately to low phylogenetic distancing indicating recent dispersal events around 300 or 400 km away from their original distributions, probably driven by migrating birds or bats, and have become genetically isolated relatively few generations ago.
Other component not-explicitly analyzed here is the populational genome, which act as a repository of evolutionary variance, where the genetic variants are gained and lost. Populational genome is involved in a third evolutionary process, the anagenesis (Emerson & Patiño, 2018), which is related to a slow and steady evolutionary phase where the main changes correspond to gene recombination and allelic fluctuation (Schnable et al., 1998), due to both determined and stochastic processes, as gene drift and selective factors, and require biparental and more variable molecular markers to be analyzed.