Evidence for selectively constrained 3D flower shape evolution in a Late Miocene clade of Malagasy Bulbophyllum orchids

SUMMARY Questions concerning the evolution of complex biological structures are central to the field of evolutionary biology. Yet, still little is known about the modes and temporal dynamics of three-dimensional (3D) flower shape evolution across the history of clades. Here, we combine high resolution X-ray computed tomography with 3D geometric morphometrics and phylogenetic comparative methods to test models of whole-flower shape evolution in the orchid family, using an early Late Miocene clade (c. 50 spp.) of Malagasy Bulbophyllum as model system. Based on landmark data of 38 species, our high-dimensional model fitting decisively rejects a purely neutral mode of evolution, suggesting instead that flower shapes evolved towards a primary adaptive optimum. Only a small number of recently evolved species/lineages attained alternative shape optima, resulting in an increased rate of phenotypic evolution. Our findings provide evidence of constrained 3D flower shape evolution in a small-sized clade of tropical orchids, resulting in low rates of phenotypic evolution and uncoupled trait?diversification rates. We hypothesize that this deep imprint of evolutionary constraint on highly complex floral structures might reflect long-term (directional and/or stabilizing) selection exerted by the group?s main pollinators (flies).