Selection of flower color phenotype by contrasting pollinator visual perception in a hybrid zone between two...

Colorful traits in biology are often shaped by the perception biases of animals making choices based on color. In angiosperms, flower color diversity may reflect selection by pollinators with distinct visual systems, leading to divergence within or between species. Yet, how these visual biases influence continuous flower color variation-particularly in natural hybrid swarms-remains poorly understood.

We studied phenotypic selection in a natural hybrid zone between two Calceolaria species with markedly different flower colors, each primarily visited by pollinators with contrasting visual systems (birds vs. bees). We quantified each pollinator's contribution to quantity and quality of pollen deposition on stigmas. Selection mode and strength were estimated for (1) flower color as perceived by each pollinator (via relative photoreceptor stimulation) across three floral parts and (2) color and brightness differences among the three flower parts to assess the degree of visual differentiation among them. Using regression models, we disentangled binomial (whether a flower was visited) from count processes (pollination efficiency) when estimating fitness.

We detected linear selection on color with opposite signs across all flower parts, primarily driven by whether or not flowers were visited. While both pollinators generally favored high chromatic distances, only birds favored high differences in brightness between flower parts.

Birds and bees exerted selection in opposite directions on continuous flower color traits in terms of pollinator-specific perceived color variation. These findings underscore the role of visual biases in driving flower color divergence.