Speciation along a latitudinal gradient: The origin of the Neotropical cycad sister pair - (Zamiaceae).

Latitude is correlated with environmental components that determine the distribution of biodiversity. In combination with geographic factors, latitude-associated environmental variables are expected to influence speciation, but empirical evidence on how those factors interplay is scarce. We evaluated the genetic and environmental variation among populations in the pair of sister species , two cycads distributed along a latitudinal environmental gradient in northwestern Mexico, to reveal their demographic histories and the environmental factors involved in their divergence. Using genome-wide loci data, we determined the species delimitation, estimated the gene flow, and compared multiple demographic scenarios of divergence. Also, we estimated the variation of climatic variables among populations and used ecological niche models to test niche overlap between species. The effect of geographic and environmental variables on the genetic variation among populations was evaluated using linear models. Our results suggest the existence of a widespread ancestral population that split into the two species ~829 ky ago. The geographic delimitation along the environmental gradient occurs in the absence of major geographic barriers, near the 28th parallel north, where a zonation of environmental seasonality exists. The northern species, , occurs in more seasonal environments but retains the same niche of the southern species, . The genetic variation throughout populations cannot be solely explained by stochastic processes; the latitudinal-associated seasonality has been an additive factor that strengthened the species divergence. This study represents an example of how speciation can be achieved by the effect of the latitude-associated factors on the genetic divergence among populations.