Here we will review recent progress in research on phylogenetic relationships, evolution, ecology, and speciation in alpine willows. Despite a long history of research on willows, the evolutionary and ecological factors for this species richness are poorly known. Many species dominate subalpine and alpine types of vegetation. The genus Salix (willows), with 33 species, represents the most diverse genus of woody plants in the European Alps. Introgression and/or transgressive segregation may have resulted in novel genotypes that are able to expand the ecological spectrum of either parent.Heredity advance online publication, 16 March 2016 doi:10.1038/hdy.2016.14. One factor contributing to hybrid establishment in sympatry with their parents is their ability to occupy more extreme niches than either parental species within a mosaic-like pattern of microhabitats on the forefield. Our results show that willow hybrids originated after glacier retreat and have established persistent populations within a few decades.
Remarkably, the hybrids exhibited a higher tolerance to cold temperatures, nutrient-poor and acidic soils than either parent. The parental species showed significant differences in niche characteristics for temperature, soil pH, nutrients and moisture. Both hybrid populations comprised F1 and later generation hybrids (F2 and backcrosses), confirming hybrid fertility. Allelic structure of hybrids confirms the assumed parentage and in situ origin of the crosses on two independent sites within the last decades. We also studied niche differentiation between the hybrids and the parental species based on indicator values, soil pH and water retention potential measurements. We analyzed the composition of two hybrid populations between Salix purpurea and Salix helvetica with nine microsatellite markers by using Bayesian methods (structure and NewHybrids), and simulations. Here we studied hybrid populations that have recently formed and established on glacier forefields of the European Alps owing to secondary contact of a lowland colonizer with a subalpine species. Little attention has been paid to the evolutionary consequences of the colonizing dynamics and succession processes following glacier retreat.