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Correlated evolution of fruit and leaf size in bird-dispersed plants: species-level variance in fruit traits explained a bit further?



The astounding morphological diversity exhibited by the fruits of vertebrate-dispersed plants has been traditionally interpreted as the adaptive outcome of divergent selective pressures exerted on plants by the broad array of frugivorous animals involved in seed dispersal. Although the selective capacity of frugivores provides support to this interpretation, recent studies have challenged it by documenting a strong phylogenetic component associated to interspecific variation in most fruit characteristics. Size-related fruit traits provide a conspicuous exception to this pattern, because they exhibit considerable variation at the between-species level which is largely independent of phylogeny and is correlated with consumption by differently-sized dispersal agents. Substantial species-level variance in size-related traits may reflect genuine disperser-driven diversification, but may also be partly influenced by correlated evolution of fruit size with the size of other plant structures. This latter possibility is tested here for bird-dispersed plants of the Iberian Peninsula using phylogenetically independent contrasts. Results demonstrate the existence of correlated evolution of fruit and leaf size at the species level. As all the plant taxa considered have their fruits eaten, and seeds dispersed, by the same relatively reduced set of frugivorous bird species, results suggest that a significant fraction of the variation in fruit size represented in the species sample may be explained as an indirect consequence of variation in leaf size, rather than being associated with adaptive divergence related to seed dispersal agents.