The role of frugivores in the evolution of fruit in Ficus (Moraceae)

We attempt to resolve a central controversy of plant-animal interactions: whether seed dispersers are an important factor in shaping the evolution of fruit traits. By focusing on a genus (Ficus) with extremely variable fruit traits, we could study fruit variation using a phylogeny to control for the evolutionary non-independence of fig species. We studied six characters that were hypothesized to most likely influence a frugivore's ability to find and access fruits: color, odor, size, softness, exposure away from foliage, and location on the tree. This is the first study on the role of seed dispersers in fruit evolution to combine quantitative measures of color and odor in a detailed phylogenetic comparative analysis. We took a closer look at fruit color because it is probably the first trait mediating fruit-frugivore interactions and because it has historically been mismeasured or ignored in studies of fruit trait evolution. We asked: (1) Can frugivores discriminate between different colors? (2) Have fruit colors of plants that share dispersers converged to a common signal? (3) Do fruit characters co-occur in non-random patterns among species of Ficus ? (4) If they do, are the patterns most likely the result of phylogeny or selective pressure by different types of frugivores?Results show that color can be used as a signal by frugivores because it can be discriminated by birds and primates, two of the most common frugivores worldwide. Fruit color has converged in distantly related species, which can be predicted under selective pressure by frugivores. However, we cannot discard the role of microbes as a selective force that determines the amount of the pigment anthocyanin, which has antifungal properties. Color, especially brightness and contrast against the background, have evolved in correlation with fruit odor, size, softness, and exposure away from leaves. Results show that the correlation among these traits cannot be explained solely by phylogeny: a large portion of the variation in fruit traits is adaptive. The most likely explanation for the correlated evolution of six of the traits studied here is selection by frugivores. This pattern holds for figs in one geographic region and worldwide, and that it holds both within and between fig species. Taken together, this evidence argues for the prevalence of seed dispersers over phylogeny as the main driver of the evolution of fruit traits in figs.