Phylogeography of plants in Taiwan and the Ryukyu Archipelago

Taiwan and Ryukyus constitute an archipelago lying on the western rim of the Pacific Ocean. In contrast to volcanic islands that often arise sequentially, these continental islands emerged almost simultaneously as the Luzon arc collided with the Eurasian margin some 9 million years ago (Mya). Taiwan and Ryukyus attained their modern features and their current flora and fauna from the adjacent mainland and from tropical Asia only 5–6 Mya. Quaternary glaciation led to a drop in sea level of the South China Sea and a land bridge that connected the Taiwan‐Ryukyu Archipelago to the mainland, which allowed plants and animals to migrate across what was previously ocean. These islands provided refugia for northern species that migrated south during glacial periods, as indicated by unanticipated high levels of genetic diversity in island populations of plants like Cunninghamia and Pinus. For most insect‐pollinated species, allozymes and nuclear DNA markers indicate significant genetic differentiation between populations and between geographical regions of the archipelago. In contrast, organelle based DNA markers suggest a migrant‐pool model, where colonists are recruited from a random sample of source populations. Consistent with this model, low elevation species have high genetic heterogeneity within populations and low levels of genetic differentiation between populations. In contrast, colonization of alpine species appeared to follow a phalanx model due to the limited availability of high elevation habitats. Genetic differentiation was detected between fragmented populations of the alpine species. A scenario of stepwise colonization from the mainland to near and then distant islands remains to be tested, although several studies indicate no such pattern. These conflicting results challenge the ability to define clear conservation criteria for the rare plant species of the archipelago.