Genomic admixture and species delimitation in forest trees

The study of natural hybrid zones can provide insights into the interplay of gene flow and divergent selection in the maintenance of species barriers between hybridizing species. In tree species, porous genomes and incomplete barriers to reproduction may allow neutral and selectively advantageous loci to freely cross species barriers, whereas loci under divergent selection or linked to those under selection will be retained, contributing to reproductive isolation and the maintenance of species barriers. Tree species are characterized by their long-generation times, outcrossing mating systems, and effective seed and pollen dispersal. Several of these features have certainly influenced the genetic structure and the patterns of hybridization we see nowadays. Hybrid zones of tree species may present some distinctive features in relation to hybrid zones in other taxa, such as the widespread occurrence of exogenous selection and environmental-dependent hybrid zones, their ancient nature, the asymmetry of introgression, and the permeability of tree genomes. This chapter aims to review the recent literature in hybridization and hybrid zones studies in forest tree species, and to discuss the implications of these studies for the study of evolution.