Toona ciliata and Toona sinensis belong to the Toona genus of the Meliaceae family and are important timber species in China. T. ciliata is an endangered species at level II due to overcutting and a low rate of natural regeneration. T. sinensis was cultivated as an economic and nutritious tree for more than 2000 years. The two species differ in flower and leaf morphological traits, reproductive systems, and range size of natural distribution. To reveal the potential molecular basis of these divergences, we examined the similarities and differences in their whole genome sequences. Results indicate that T. ciliata had a higher number of expanded gene families than T. sinensis. The whole genome duplication (WGD) occurred before their speciation. The long-terminal repeats (LTRs) insertion was earlier in the T. ciliata genome (3.2985 ± 2.5007 Mya) than in the T. sinensis genome (3.1516 ± 2.2097 Mya). Twenty-five gene families in the T. ciliata genome were detected to be under positive selection compared with background branches of ten different land species. The T. ciliata genome was highly collinear with the T. sinensis genome, but had low collinearity with the genomes of more distant species. These genomic and evolutionary divergences are potentially associated with the differences between T. ciliata and T. sinensis in terms of their reproductive systems and ecological adaptation.

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 (${\overline{r}}_g$). 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.