Mimetic colour pattern evolution in the highly polymorphic Bombus trifasciatus (Hymenoptera: Apidae) species complex and its comimics

Müllerian mimetic systems have uncovered some of the dynamic processes by which natural selection can drive the radiation of convergent and divergent phenotypes. We examined evolution involving Müllerian mimicry in bumble bees by documenting the distribution and evolution of colour patterns amongst three colour-polymorphic lineages -Bombus trifasciatus Smith, Bombus haemorrhoidalis Smith, and Bombus breviceps Smith - that mimic each other across ~14 colour groups in South-East Asia. Using mitochondrial DNA sequence data, we estimated relationships within each lineage to infer the processes that gave rise to the colour diversity and develop hypotheses on species recognition. We expanded on our assessment of species delineation in the B.trifasciatus lineage using three nuclear gene fragments and morphometrics. Comparison of colour patterns amongst georeferenced specimens showed considerable variation in the degree and geographical range of mimicry amongst mimicry groups. Phylogenetic estimates show high rates of colour pattern evolution, with colour variation often exceeding variation within the fast-evolving mitochondrial genes. The molecular data, and to some degree the morphometric data, support unique histories for several taxa recognized previously within the B.trifasciatus lineage, which may include several species. Early vicariant events within the B.trifasciatus lineage are likely to have occurred ~2.2Mya in the mountains of south-west China.