大陸性島嶼生物之地理種化模式：以畫眉為例

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

李菁紋(Jing-Wen Li)

指導教授 ： 李壽先

國立臺灣師範大學/理學院/生命科學研究所/碩士(2005年)

摘要

Mayr (1963)提出的異域種化模式是指兩族群因地理障礙的產生，造成族群分化且分化後將不再有基因交流(gene flow)，海洋性島嶼的生物起源便被廣泛認為是異域種化(allopatric speciation)的經典例子。然而，大陸性島嶼物種的種化過程可能較為複雜；過去冰河週期時，大陸性島嶼與鄰近大陸因海平面的上升下降，經歷週期性的相連，此連結為原本分隔的族群提供了基因交流的機會。因此，Mayr(1963)提出的異域種化模式可能並不完全適用於解釋大陸性島嶼物種的起源。本研究以畫眉(Garrulax canorus)及其姊妹種台灣畫眉(G. taewanus)為例子，採用“isolation with migration model”估算兩物種的分化時間、分化後的基因交流頻率、兩物種的有效族群量及祖先族群量。畫眉分佈於東亞大陸東南部及中南半島北部，而台灣畫眉則僅分佈於台灣。本研究選取10個核DNA片段，其中包含3個核基因內含子(nuclear intron)和7個基因體中位置未知的核片段（共3958bp），分別對兩種畫眉進行定序及族群遺傳參數的估算。結果顯示，兩種畫眉於3.3百萬年前開始分化，此分化時間明顯的早於依據粒線體DNA序列所推估出來的分化時間(1.5百萬年)。基因交流頻率方面，畫眉播遷至台灣的基因交流頻率明顯不等於零(mGt = 0.88、2NGtmGt = 0.12)，而反方向的播遷則明顯較低(mGc = 0.06、2NGcmGc = 0.048)。因此兩種畫眉在分化後有不對稱的基因交流現象。依據結果，我們提出兩種可能種化的過程：第一，兩種畫眉是在異域（如海水屏障或棲地不相連）的情況下開始分化，而在陸地相連時兩族群發生基因交流。第二，在並域(parapatry)的狀況下，兩族群可能因天擇壓力不同或族群高度結構化，造成分化。無論是上述何種機制，皆顯示畫眉種化過程不符合傳統的異域種化模式。此研究提供了一個檢測異域種化的範例且對大陸性島嶼的物種起源機制提出了新的觀點。

關鍵字

種化 ； 大陸性島嶼 ； 畫眉

並列摘要

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.

並列關鍵字

speciation ； continental island ； isolation with migration