Genetic relationships and ecological divergence in species and populations of Salix in Taiwan
Linking ecology with evolutionary biology is important to understanding how environments drive genetic divergence between and within species. Salix species that show clear phenotypic divergence into lowland riparian tree willows, middle to high elevation multi-stemmed shrubs and alpine dwarf shrubs can provide opportunities to study population and species divergence driven by ecological factors. Amplified fragment length polymorphism (AFLP) was used to quantify genetic variation of 185 individuals from nine populations of four Salix species that occur in Taiwan. Using variation partitioning by redundant analysis (RDA), we examined the effects between environmental and geographic isolation. Genome scan approaches were used to test for FST outliers potentially under selection. Because of dramatic environmental differences in growth habitats, we hypothesized that environmental heterogeneity played more important roles than geographic isolation in shaping genetic variation within and between Salixspecies.
We found that environments explain substantially larger proportion of genetic variation than geography for the total data. However, no genetic variation was explained by pure geography if only compared within and between species. Regional effects of environmental variables structured spatially explained more genetic variation than pure environments in most comparisons within and between species, indicating unmeasured environmental variables and past demographic histories played important roles in shaping population and species divergence. Annual mean temperature, aspect, and fraction of absorbed photosynthetically active radiation (fPAR) are the most important ecological factors shaping genetic variation within and between species based on forward selection analysis. Nevertheless, different combinations of environmental variables are found to be significantly correlated with genetic variation within and between species. We identified eight AFLP loci potentially evolved under selection within species using different outlier detection methods. These loci are found to be correlated with more than one environmental variable, suggesting local adaptation along environmental gradients at population level.