[[alternative]]Geographic Model of Speciation in a Continental Island: Divergence Genetics of Hwameis (Garrulax canorus and G. taewanus)

[[abstract]]The origins of endemic species on remote oceanic islands have been well explained by a strict allopatric speciation model as advocated by Mayr (1963). A strictly allopatric speciation model indicates that populations are isolated by a geographic barrier, and gene flow would cease completely after the barrier was formed. However, for species inhibiting continental islands, their speciation processes might be more complicated. In continental island systems, geographic barriers might not always exist during glacial episodes. These islands may periodically be connected to the adjacent continent at the lowstand of sea level during glacial periods. Therefore, the speciation mode for continental islands biota might not completely agree with the strictly allopatric speciation mode. In this study, we adopted an “isolation with migration” model to estimate divergence times, migration rates after splitting, and the effective population sizes of a pair of sister species, Hwamei (Garrulax canorus) and Taiwan Hwamei (G. taewanus) which are distributed on the Asian mainland and in Taiwan, respectively. We sequenced three nuclear introns and seven anonymous nuclear loci (with a total length of 3958 bp) to reveal that two hwamei species have diverged 3.3 million years ago which is earlier than the time estimated from mtDNA data (about 1.5 million years ago). Our results provided an unequivocal evidence of gene flow from G. canorus to G. taewanus (mGt = 0.88、2NGtmGt = 0.12) since their divergence. However, migration rate in the opposite direction was relatively low (mGc = 0.06、2NGcmGc = 0.048). We proposed two possible scenarios of speciation processes to explain the divergence with migration of two hwamei species. First, divergence of the two hwamei species was initiated allopatrically by the development of oceanic or other habitat barriers. However, gene flow periodically occurred when land bridges were exposed. Second, the divergence of the two hwamei species might be initiated parapatrically due to local selection or high levels of genetic structure between the populations. In conclusion, our results suggest that the divergence between hwameis does not fit a strictly allopatric speciation mode. Our results provided a good example to reject the strictly allopatric speciation paradigm and shed a new light on the origin of continental island biota.