Floral adaptation and modular trait evolution associated with a shift from bee to hummingbird pollination.

Abstract

Pollination shifts result in floral divergence, yet how such shifts influence variation and correlation among floral traits is unclear. We examine whether a transition from bee to hummingbird pollination in sister species of Neotropical spiral gingers - Costus kuntzei, with ancestral bee pollination, and Costus wilsonii, with derived hummingbird pollination - drives phenotypic variation, functional integration and morphological modularity. Using field measurements and observations, we assess pollinator assemblages and pollination syndrome divergence by quantifying a suite of floral traits related to shape, size, colour, scent and nectar reward. We then test whether the shift from bee to hummingbird pollination has led to differences in floral phenotypic variation and integration, and we use geometric morphometric analyses to evaluate modularity hypotheses based on floral function and genetic or developmental constraints. We find that C. wilsonii exhibits changes in, and loss of, multiple floral traits, along with extreme specialization on a single hermit hummingbird species. In contrast, C. kuntzei is visited by a more species-rich pollinator assemblage of euglossine bees and a tabanid fly. Both plant species show similar patterns of floral variation, integration and modularity, with reduced floral variation and modularity patterns concentrated in traits involved in pollinator fit and pollen transfer. The findings suggest that phenotypic patterns reflect functional traits shaped by divergent pollinator-mediated adaptation. This floral adaptation has resulted in stable patterns of trait variation and correlation, consistent with selection for efficient pollination and pollinator fit in both species.