Speciation slowing down in widespread and long-living tree taxa: insights from the tropical timber tree genus Milicia (Moraceae)

The long generation time and large effective size of widespread forest tree species can result in slow evolutionary rate and incomplete lineage sorting, complicating species delimitation. We addressed this issue with the African timber tree genus Milicia that comprises two morphologically similar and often confounded species: M. excelsa, widespread from West to East Africa, and M. regia, endemic to West Africa. We combined information from nuclear microsatellites (nSSRs), nuclear and plastid DNA sequences, and morphological systematics to identify significant evolutionary units and infer their evolutionary and biogeographical history. We detected five geographically coherent genetic clusters using nSSRs and three levels of genetic differentiation. First, one West African cluster matched perfectly with the morphospecies M. regia, which formed a monophyletic clade at both DNA sequences. Second, a West African M. excelsa cluster formed a monophyletic group at plastid DNA and was more relat...

广布性林木物种的世代周期漫长且有效种群规模庞大，可能导致演化速率缓慢以及不完全谱系分选（incomplete lineage sorting），这会给物种界定工作带来极大挑战。我们以非洲用材树木属（Milicia）为研究对象，该属包含两个形态相似且常被混淆的物种：分别是从西非至东非广布的M. excelsa，以及仅分布于西非的M. regia。我们整合了核微卫星（nSSRs）、核DNA序列、质体DNA（plastid DNA）序列以及形态分类学数据，以识别重要的演化单元并推断其演化历史与生物地理历史。我们利用核微卫星（nSSRs）检测到5个地理上连贯一致的遗传聚类群，并识别出三个层级的遗传分化。其一，一个西非聚类群与形态种（morphospecies M. regia完全吻合，该类群在两种DNA序列分析中均形成单系支系（monophyletic clade）。其二，一个西非的M. excelsa聚类群在质体DNA分析中形成单系类群，且亲缘关系更为……