A Historical Misstep: Niche Shift to Specialisation Is Pushing Insular Ginger Towards an Evolutionary Dead End

Niche specialisation is a double-edged sword as it aids species in adapting to a particular environment but makes them more susceptible to environmental change, which may result in species extinction. Although it has long been debated whether niche specialisation necessarily falls into an 'evolutionary dead end', empirical evidence from a population genetics perspective remains scant, especially when comparing both ecological generalists and specialists simultaneously. In this study, we scrutinised two Taiwan endemic gingers (Zingiber pleiostachyum and Z. shuanglongense) to evaluate how their contrasting patterns in niche breadth evolution have shaped their evolutionary trajectories. We utilised a genome-wide sequencing approach to investigate the demographic histories of each species, assess their maladaptation to future climate change, and estimate their mutational loads. Our results revealed distinct demographic histories between these two gingers. Z. shuanglongense, as the specialist, despite an initial increase during the Last Glacial Maximum (~22 Kya), has been subjected to a long-term decrease in effective population size (Ne), while Z. pleiostachyum is on the contrary increasing, leading to a significantly larger current Ne. Furthermore, ecological specialists are much more vulnerable to future climate change and exhibit greater drift-associated deleterious mutations compared to generalists, directly affecting species' fitness. This study strongly supports the idea that the transition in niche breadth towards specialisation will push Z. shuanglongense perilously close to extinction and also sheds light on species conservation within limited migratory space.