When did bumblebees reach South America? Unexpectedly old montane species may be explained by Mexican stopover (Hymenoptera: Apidae)

A problem for understanding bumblebee biogeography is that if bumblebees dispersed from Asia through North America to South America, if they are poor at long-distance dispersal with establishment over sea, and if the land bridge between North and South America was not established until c. 3 Ma BP, then there is an apparent conflict with the divergence among currently endemic South American lineages having been dated as early as 15–17 Ma. Using the first complete phylogenetic trees for all known and accepted extant species of the groups involved, we show how this conflict could be resolved. We suggest that characterizing bumblebees as being associated generally with temperate flower-rich meadows conflates divergent habitat specializations between two early lineages, associated with northern lowland grasslands and with southern montane grasslands respectively, which may have driven divergences in behaviour and in biogeographic processes. First, for most of the lowland grassland group of bumblebees, estimated dates of divergence are consistent with dispersal to South America via the land-bridge corridor that opened at c. 3 Ma, followed by extant endemic lineages diverging in situ within South America. In contrast, for the second group that occupies montane grassland habitats (and for a few montane lineages of the ‘lowland’ group), we suggest that dispersal to South America at c. 3 Ma could be consistent with older divergence for currently endemic species if: (1) many of the extant South American lineages had already diverged outside the region before 3 Ma in neighbouring Mesoamerica; and (2) they had been constrained within the high mountains there, dispersing southwards into South America only once the isthmus corridor had become established; and (3) some of those ancestral montane lineages had become extirpated from Mesoamerica during subsequent warm climatic fluctuations. This interpretation re-emphasizes that biogeographic studies need to consider habitat-specific dispersal models that change through time.

当前关于熊蜂（bumblebee）生物地理学（biogeography）的研究存在一项核心难题：若熊蜂从亚洲经北美扩散至南美，且其跨海定殖的长距离扩散（long-distance dispersal）能力较弱，同时南北美之间的陆桥（land bridge）直至距今约3 Ma BP尚未形成，则该情景与当前已测年的南美特有支系分化时间（可早至15~17 Ma）存在显著冲突。本研究依托首个覆盖所有已知、公认的现存相关类群的完整系统发育树（phylogenetic trees），阐明了该矛盾的可行解决路径。我们提出，将熊蜂笼统界定为与温带富花草地相关联的类群，实则混淆了两个早期分化支系间迥异的生境特化（habitat specialization）特征——二者分别对应北方低地草原与南方山地草原，其分化或推动了行为与生物地理学过程的分异。其一，多数熊蜂低地草原类群的估算分化时间，与经由距今约3 Ma形成的陆桥廊道扩散至南美、随后南美现存特有支系在原地发生分化的过程完全契合。与之相对，对于占据山地草原生境的第二类群（以及部分‘低地类群’的山地支系），若满足以下三项条件，则其在距今约3 Ma扩散至南美与当前特有类群的古老分化时间并不矛盾：（1）多数现存南美支系已于3 Ma前在邻近的中美洲（Mesoamerica）完成分化；（2）这些支系当时局限于当地高海拔山地，直至巴拿马地峡廊道形成后才向南扩散进入南美；（3）部分祖先山地支系在后续的暖期气候波动中从当地中美洲绝灭。该研究结论再次强调，生物地理学研究需纳入随时间动态变化的生境特异性扩散模型。