Genetic and Ecological Divergence of <i>Pinus durangensis</i> and <i>Pinus martinezii</i>: Two Endemic Lineages with Contrasting Evolutionary Histories

Premise of research. Population isolation and geographical fragmentation favor local adaptation by increasing the phenotypic, genetic, and ecological differentiation of species and the origin of new evolutionary lineages. In this work, we determined the effect of geographic differentiation and environmental heterogeneity on the genetic structure of the populations of two closely related Pinus lineages. The aim was to clarify their taxonomic status and reconstruct the evolutionary scenarios that promoted their possible divergence. Methodology. We examined the genetic variability, ecological differentiation, and genetic and geographic isolation of Pinus durangensis populations and the Pinus martinezii synonym using chloroplast microsatellites in 307 individuals from 20 natural populations. The environmental effect was explored with ecological niche modeling, the niche similarity index I, and the Palmer drought severity index. Pivotal results. We found population substructuring and the presence of a genetic barrier that indicates the presence of two distinct groups: a northern group corresponding to P. durangensis and a southern group representing P. martinezii. The ecological niche differentiation supports this pattern in which neither of the two species predicts the ecological niche of the other. Therefore, clear patterns of isolation by distance and environment are presented. Conclusions. The results support the hypothesis that P. durangensis and P. martinezii are two independent lineages historically isolated by geographical and environmental barriers and both individuals are associated with the emergence of the Trans-Mexican Volcanic Belt and the glacial events of the Pleistocene.