Genetic Consequences of Anagenetic Speciation in Endemic Angiosperms of Ullung Island, Korea

摘要

Two major modes of speciation exist in the evolution of endemic plants of oceanic islands: cladogenesis and anagenesis. The former is where an immigrant population becomes established and then disperses into different ecological zones on the same, orneighboring, island. Over time and in isolation, these different lineages diverge and adapt to their new environments, such that eventually they are regarded as distinct species. These cladogenetic radiations have been chronicled in considerable detail in Hawaii, Galapagos, and the Canary Islands. Anagenesis is a process of transformation of species. After establishment, an immigrant population grows in size but does not split into divergent lines due to alack of ecological opportunity. Gene flow is maintained among subpopulations, and genetic variation accumulates due to mutation and recombination. The highest known level of anagenesis in the endemic flora of any oceanic island occurs in Ullung Island, Korea. This small island lies137 km east of the Korean Peninsula. It is of volcanic origin, with no known connections to the mainland. Here, ca. 33 endemic species occur that represent transformational evolution from original immigrants during the past not more than1.8 million years. A number of studies in this island have been done on isozymic genetic variation in natural populations and more recently also using AFLP and SSR molecular markers. The results from isozymes show a pattern of very limited genetic variation within these endemic species. More comprehensive AFLP and SSR markers, however, reveal much higher levels of variation in some species, even to the extent of approximating that seen in progenitor populations on the continent. The studies in Ullung Island show that anagenetic species canbe expected to harbor high levels of genetic variation similar to that of the progenitor population. The individuals in subpopulations on the island appear to be held together genetically by gene flow such that no geographic partitioning of the geneticvariation occurs over the landscape.