Climate adaptation and functional constraints drive pollen evolution in Apiales.

Pollen grains exhibit remarkable morphological diversity, shaped by selective pressures from environmental factors and mechanical constraints. Here, we investigate macroevolutionary patterns of pollen morphology in Apiales, an order of angiosperms with significant ecological and geographical diversity, to disentangle the roles of climate and functional constraints. We analyzed pollen morphology in 158 species of Apiales using morphometric and multivariate evolutionary approaches to evaluate the influence of climate and biomechanical constraints on traits such as pollen wall thickness, aperture structure, and overall grain shape, and to test for evidence of harmomegathy-related adaptation. Our results reveal three key findings. First, climate showed no significant effect on pollen size, challenging long-standing assumptions. Second, climate strongly influences pollen architecture, with drier, more seasonal climates being consistently associated with reduced apertures and thicker pollen walls. Finally, we detected an evolutionary lag, with changes in pollen wall thickness preceding aperture modifications, indicating that biomechanical constraints have shaped evolutionary trajectories. These results demonstrate that climate-driven adaptations in pollen architecture are mediated by functional constraints, consistent with a dynamic interaction between environmental selection and biomechanical properties of the pollen wall.