Geographical variation in the bill–flower fit in a plant–pollinator interaction in western Mexico

Few studies have examined geographical variation in traits involved in biotic interactions, although such variation is likely key in the evolution of interactions. The fit between hummingbird bills and the flowers they feed on suggests mutual selection pressures. We tested for geographical variation in the pollinator–flower fit in the Mexican hermit Phaethornis mexicanus and Heliconia spp. flowers by analyzing the morphospaces of the interacting species and the ways they vary. To analyze the geographical variation in the morphology of P . mexicanus and Heliconia spp. flowers, we grouped samples into three operational geographic units (OGUs). Within each OGU, we assessed pollinator–flower specificity by analyzing floral resource preference, and morphological data using correlations and principal component analysis. Our data on floral preferences and morphology indicate geographical variation in the P . mexicanus – Heliconia interaction in all of the OGUs. Heliconia was the preferred floral resource (83% of the samples), suggesting a preference for these flowers as a food source. We also detected morphological overlap between the bill of P . mexicanus and the corollas of both Heliconia species, at least in the two westernmost OGUs. That P . mexicanus morphology tracked variation in the different Heliconia spp. across the hummingbird's range suggests that the specificity of the hummingbird–plant interaction exerts a significant selection pressure on hummingbird morphology. The morphological overlap observed in the two northernmost OGUs is probably due to the evolution of a close biotic interaction, which is reflected in the high correlation values in the morphological characters involved. Abstract in Spanish is available with online material.