Interspecific Variation in Fruit Shape: Allometry, Phylogeny, and Adaptation to Dispersal Agents

Abstract

Investigations on fruit and fruiting characteristics of animal—dispersed, fleshy—fruited plants have been generally interpreted in terms of adaptations by plants to dispersal agents. Most often, these studies did not formulate non—adaptive, alternative null hypotheses, and the potential influence of phylogenetic effects on observed patterns was not assessed either. This paper presents an analysis of interspecific variation in fruit shape (as assessed by length and width) among vertebrate—dispersed plants of the Iberian Peninsula. Tests of predictions from both adaptive (to dispersal agents) and null (based on allometry) hypotheses are presented and the influence of phylogenetic effects is also accounted for. Interspecific variation in fruit shape was unrelated to seed dispersal mode ("bird dispersed" vs. "bird plus mammal dispersed"), and predictions from adaptive hypotheses were not supported. Variation in fruit shape did not depart significantly from that predicted by the allometry—based, null hypothesis. Deviations of individual species from the predicted allometric relationship were unrelated to dispersal mode, and originated from genus— and species—specific variation in fruit shape. There exists a considerable influence of phylogeny on fruit shape variation, and nearly half of total variance is attributable to variation among genera within families. After accounting for phylogenetic effects, the null hypothesis still held within taxonomic categories above the species level. Observed constancy in the relative variation of fruit length and width, despite variation in dispersal mode and morphological type, is interpreted in terms of shared morphogenetic and physical constraints independent of dispersal. The implications of substantial phylogenetic effects on interspecific patterns of variation in fruit and fruiting traits are discussed.