Phylogenetic constraints, conservatism, and convergence shape three-dimensional variation in flower morphology of a tropical orchid radiation.

Despite advances in phylogenetic comparative methods, challenges remain to distinguish between various macroevolutionary patterns of phenotypic variation (e.g., conservatism, convergence) and to infer their underlying proximate (genetic, developmental) or ultimate (selective versus neutral) causes. Furthermore, especially for complex morphological structures, little is known about the extent to which rates of trait and species diversification are coupled. To address these issues, we explored the modes and temporal dynamics of high-dimensional (3D) flower shape evolution in Malagasy Bulbophyllum orchids (crown age: c. 12.7 Ma), representing the largest dataset of its kind in an angiosperm lineage (111 spp.). We identified three major flower shape types, which likely evolved under the influence of pollinator-mediated selection around a "primary" (lineage-wide) adaptive optimum over time. Moreover, we uncovered multiple instances of convergence and, in a recently derived subclade, a notable trend of shape "reversion" towards an earlier rather than more recent ancestral state. We conclude that the evolution of 3D flower shapes in orchids can be strongly influenced by phylogenetic constraint and conservatism while having no significant influence on species diversification. Hence, 3D flower shape (as potential pre-zygotic reproductive isolation trait) may play less a role in explaining the diversification dynamics of orchids than generally assumed.