Coexisting plants that share pollinators can compete through interspecific pollen transfer. A long-standing idea holds that divergence in floral morphology may reduce this competition by placing pollen on different regions of the pollinator's bodies. However, surprisingly little empirical support for this idea exists. Burmeistera is a diverse neotropical genus that exhibits wide interspecific variation in the degree to which the reproductive parts are exserted outside the corolla. Coexisting Burmeistera share bats as their primary pollinators, and the degree of exsertion determines the site of pollen deposition on the bats' heads. Here we study the mechanism, process and pattern of floral character displacement for assemblages of coexisting Burmeistera. Flight cage experiments with bats and pairs of Burmeistera species demonstrate that the greater the divergence in exsertion length, the less pollen transferred interspecifically. Null model analyses of exsertion lengths for 19 species of Burmeistera across 18 sites (each containing two to four species) demonstrate that observed assemblage structure is significantly overdispersed relative to what would be expected by chance. Local evolution, rather than ecological sorting, appears to be the primary process driving this pattern of overdispersion because local adaptation of the nine widespread species accounts for a large portion of the observed pattern. Taken together, results of this study provide strong support for the idea that competition through interspecific pollen transfer can drive character displacement in plants.

Aim We review several aspects of the structure of regional and local assemblages of nectar-feeding birds and bats and their relationships with food plants to determine the extent to which evolutionary convergence has or has not occurred in the New and Old World tropics.Location Our review is pantropical in extent and also includes the subtropics of South Africa and eastern Australia. Within the tropics, it deals mostly with lowland forest habitats.Methods An extensive literature review was conducted to compile data bases on the regional and local species richness of nectar-feeding birds and bats, pollinator sizes, morphology, and diets. Coefficients of variation (CVs) were used to quantify the morphospace occupied by the various families of pollinators. The extent to which plants have become evolutionarily specialized for vertebrate pollination was explored using several criteria: number and diversity of growth forms of plant families providing food for all the considered pollinator families; the most common flower morphologies visited by all the considered pollinator families; and the number of plant families that contain genera with both bird-and bat-specialized species.Results Vertebrate pollinator assemblages in the New World tropics differ from those in the Old World in terms of their greater species richness, the greater morphological diversity of their most specialized taxa, and the greater degree of taxonomic and ecological diversity and morphological specialization of their food plants. Within the Old World tropics, Africa contains more specialized nectarfeeding birds than Asia and Australasia; Old World nectar-feeding bats are everywhere less specialized than their New World counterparts.Main conclusions We propose that two factors -phylogenetic history and spatio-temporal predictability (STP) of flower resources -largely account for hemispheric and regional differences in the structure of vertebrate pollinator assemblages. Greater resource diversity and resource STP in the New World have favoured the radiation of small, hovering nectar-feeding birds and bats into a variety of relatively specialized feeding niches. In contrast, reduced resource diversity and STP in aseasonal parts of Asia as well as in Australasia have favoured the evolution of larger, non-hovering birds and bats with relatively generalized feeding niches. Tropical Africa more closely resembles the Neotropics than Southeast Asia and Australasia in terms of resource STP and in the niche structure of its nectar-feeding birds but not its flower-visiting bats.

It has been proposed frequently, from Darwin's time onwards, that specialized pollination increases speciation rates and thus the diversity of plant species (i.e. clade species richness). We suggest here that the correlation between clade species richness and floral specialization is real, but that clade species richness is frequently the cause, not the result of floral specialization. We urge a broader, variance-partitioning perspective for assessing the causes of this correlation by suggesting four models of how the diversityspecialization correlation might come about: (1) floral specialization promotes initial reproductive isolation (''Initial-RI'' model), (2) floral specialization promotes reinforcement of reproductive isolation upon secondary contact (''Reinforcement'' model), (3) floral specialization reduces the extinction rate by promoting tighter species packing (''Extinction'' model), (4) floral specialization is the result of high clade species richness, which increases the number of related species in communities, and thus selects for floral character displacement (''Character-Displacement'' model). These hypotheses are evaluated by comparing the relationships between species richness, speciation mechanisms, and pollination precision, accuracy, and specialization in the broader literature and, more specifically, in four study systems: Dalechampia (Euphorbiaceae), Collinsia (Plantaginaceae), Burmeistera (Campanulaceae), and Stylidium (Stylidiaceae). These systems provide stronger support for the character-displacement hypothesis, wherein local species diversity drives the evolution of specialized pollination. Although the two reproductive-isolation hypotheses may hold for plants like orchids, with extremely precise pollination systems, the reproductive character-displacement hypothesis seems likely to be more important for plant groups with less precise pollination systems.

One classic explanation for the remarkable diversity of flower colors across angiosperms involves evolutionary shifts among different types of pollinators with different color preferences. However, the pollinator shift model fails to account for the many examples of color variation within clades that share the same pollination system. An alternate explanation is the competition model, which suggests that color divergence evolves in response to interspecific competition for pollinators, as a means to decrease interspecific pollinator movements. This model predicts color overdispersion within communities relative to null assemblages. Here, we combine morphometric analyses, field surveys, and models of pollinator vision with a species-level phylogeny to test the competition model in the primarily hummingbird-pollinated clade Iochrominae (Solanaceae). Results show that flower color as perceived by pollinators is significantly overdispersed within sites. This pattern is not simply due to phylogenetic history: phylogenetic community structure does not deviate from random expectations, and flower color lacks phylogenetic signal. Moreover, taxa that occur in sympatry occupy a significantly larger volume of color space than those in allopatry, supporting the hypothesis that competition in sympatry drove the evolution of novel colors. We suggest that competition among close relatives may commonly underlie floral divergence, especially in species-rich habitats where congeners frequently co-occur. K E Y W O R D S :Color vision, interspecific pollen transfer, phenotypic community structure, phylogenetic community structure, phylogenetic signal, reproductive character displacement, signal evolution.