Global biogeography and diversification of palms sheds light on the evolution of tropical lineages. I. Historical biogeography

AIM: Palms (Arecaceae/Palmae) are a model group for evolutionary studies in the tropics. Family-wide data on taxonomy, phylogenetics and distribution are now available, but a general framework of palm evolution is still lacking. The overall aim of this study, published in two companion papers, is to seek evolutionary explanations for the geographical distribution of palm lineages and species diversity patterns at global and regional levels. In this first paper we undertake a detailed analysis of palm biogeography for all major lineages in a global context, comparing our results to the fossil record, molecular dating studies and previously established biogeographical hypotheses for the family. LOCATION: Global. METHODS: A dated phylogeny of all palm genera and distributional data were used to infer ancestral areas. A global model of area evolution for tropical lineages was formulated and ancestral areas were reconstructed using a maximum likelihood approach under the dispersal–extinction–cladogenesis model. RESULTS: The crown node divergence of palms and divergences of all five subfamilies occurred before the end of the Cretaceous within Laurasian regions. Early range expansions into South America took place in the stem lineages of subfamilies Arecoideae and Ceroxyloideae, and into Africa and South America in Cretaceous lineages within subfamily Calamoideae. The largest subfamily Arecoideae underwent its early diversification history in South America and has been an important source of lineages expanding into other regions, notably in the major Indo-Pacific tribe Areceae. In contrast, the history of subfamily Coryphoideae was primarily Laurasian, supporting earlier boreotropical hypotheses for the group. Dispersals across Wallace's Line are prevalent in major groups of Arecoideae, Calamoideae and Coryphoideae, both before and after the Miocene geological evolution of the Malesian Archipelago. MAIN CONCLUSIONS: We present the first robust biogeographical hypothesis for the evolution of palms in space and time. Although palms show evidence of dispersal limitation, our findings highlight the role of long-distance dispersal events in the establishment of major biogeographical patterns of palm clades. This is consistent with the growing evidence for long-distance dispersal as a major mechanism underpinning the distribution of tropical lineages.

研究目标：棕榈（Arecaceae/Palmae）是热带演化研究的经典模式类群。目前学界已获取该科全类群的分类学、系统发育学与分布数据，但棕榈科演化的通用研究框架仍付阙如。本研究分为两篇姊妹论文发表，其核心目标是在全球及区域尺度上，阐释棕榈类群的地理分布与物种多样性格局的演化成因。在本系列首篇论文中，我们于全球尺度下对所有主要棕榈类群的生物地理学特征展开细致剖析，并将研究结果与该科的化石记录、分子定年研究及已发表的该科生物地理学假说进行比对。

研究区域：全球范围。

研究方法：我们利用所有棕榈属的定年系统发育树与分布数据推断祖先分布区；构建了热带类群分布区演化的全球模型，并基于扩散-灭绝-分支（dispersal–extinction–cladogenesis, DEC）模型，采用最大似然法重建各分支的祖先分布区。

研究结果：棕榈科冠群节点的分化以及全部5个亚科的分化事件，均发生于白垩纪末期之前的劳亚古陆（Laurasian）区域。槟榔亚科（Arecoideae）与蜡椰亚科（Ceroxyloideae）的茎类群早期便完成了向南美洲的分布区扩张；而省藤亚科（Calamoideae）内的白垩纪类群则同时向非洲与南美洲拓展分布范围。作为最大亚科的槟榔亚科（Arecoideae），其早期多样化进程发生于南美洲，且是向其他区域扩散类群的重要来源，尤其是印太地区主要族槟榔族（Areceae）。与之相对，蒲葵亚科（Coryphoideae）的演化历史主要局限于劳亚古陆，这为该类群早期提出的北热带（boreotropical）假说提供了支撑。在马来群岛（Malesian Archipelago）中新世地质演化的前后时期，槟榔亚科、省藤亚科与蒲葵亚科的主要类群均普遍发生了跨越华莱士线（Wallace's Line）的扩散事件。

主要结论：本研究首次提出了棕榈科在时空维度下演化的可靠生物地理学假说。尽管棕榈类群存在扩散限制的相关证据，但本研究结果凸显了长距离扩散事件在塑造棕榈支系主要生物地理格局过程中的关键作用。这与日益增多的研究证据相契合——长距离扩散是支撑热带类群地理分布的核心机制。