Limits to Plant Species' Ranges: A Unified Eco-Evolutionary Model and Molecular Mechanisms

The formation of a species' range generally involves dispersal and selection processes where dispersal enhancing species expansion is counterbalanced by selection that reduces mean population fitness at the edge of a species' range. The application of high-throughput sequencing has made it possible to identify the genetic basis for plant adaptation, which allows for assessing molecular mechanisms of genetic regulation at the edge. Here, we first review the theoretical advances in studying a species' range based on a unified eco-evolutionary model. Genetic loads generated by different evolutionary processes can be uniformly expressed in a general genetic component of regulation (r¯g). We then discuss the molecular basis of adaptation from recent empirical studies, including SNP-based detection of adaptation signatures, the regulation of gene expression, genomic structural variation, transposable elements, gene family expansion/contraction, and genome size. We also discuss the potential of applying estimates of molecular evolutionary rates (K_a/K_s) to infer plant adaptation in the edge populations by comparing the differences between the central and edge populations or between the central and edge communities. Finally, we discuss how plant mating systems regulate genetic loads in edge populations and alter the relative genetic loads at the gametophyte versus sporophyte phase. By linking the eco-evolutionary model with genomic evidence, this review explores several issues to incorporate genetic and ecological regulations and genomic variation into studies on a species' range. Addressing these issues can extend our understanding of the limits to a plant species' range.