Is there spatial variation in phenotypic selection on floral traits in a generalist plant-pollinator system?

Abstract

The selective role of pollinators on the floral phenotype has been identified as the main force behind angiosperm diversification. However, in generalized plant-pollinator interactions this association may not be that evident, since pollinator assemblages vary in composition among populations, possibly influencing the direction and intensity of floral trait selection. In this study we determine whether there was spatial variation in pollinator assemblages and the selective forces they exert in ten populations of Alstroemeria ligtu var. simsii. We characterized the whole pollinator assemblage in ten populations, quantified four floral attraction traits and determined potential selection targets in each of the study populations. Our results revealed that populations differed in the composition of their pollinator assemblages, and the pollinators with the highest visitation rates also differed among populations. Using phenotypic selection analysis we detected significant differentials and selection gradients only in two of the ten populations, for corolla tube length and ratio of the nectar guide. The spatial variation in selection showed that linear selection acting upon these traits differed significantly among the studied populations. Our study indicates that selection can be detected in generalized plant-pollinator systems and brings new evidence of phenotypic selection variation in space for this endemic plant species. Future studies are needed to determine whether the selective patterns described here are consistent over time and whether they produce evolutionary change in the populations under study.

The selective role of pollinators on the floral phenotype has been identified as the main force behind angiosperm diversification. However, in generalized plant-pollinator interactions this association may not be that evident, since pollinator assemblages vary in composition among populations, possibly influencing the direction and intensity of floral trait selection. In this study we determine whether there was spatial variation in pollinator assemblages and the selective forces they exert in ten populations of Alstroemeria ligtu var. simsii. We characterized the whole pollinator assemblage in ten populations, quantified four floral attraction traits and determined potential selection targets in each of the study populations. Our results revealed that populations differed in the composition of their pollinator assemblages, and the pollinators with the highest visitation rates also differed among populations. Using phenotypic selection analysis we detected significant differentials and selection gradients only in two of the ten populations, for corolla tube length and ratio of the nectar guide. The spatial variation in selection showed that linear selection acting upon these traits differed significantly among the studied populations. Our study indicates that selection can be detected in generalized plant-pollinator systems and brings new evidence of phenotypic selection variation in space for this endemic plant species. Future studies are needed to determine whether the selective patterns described here are consistent over time and whether they produce evolutionary change in the populations under study.