Exploring sub-individual variability: role of ontogeny, abiotic environment and seed-dispersing birds

Within-individual trait variation - otherwise known as sub-individual variation - is an important component of phenotypic variation, with both a genetic and epigenetic basis. We explore its adaptive value and the effects of ontogeny and the environment on sub-individual variability. We conducted a field study to analyse the effects of tree age, soil pH, soil water content and soil nutrients on sub-individual variability in fruit size of hawthorn (Crataegus monogyna) in three sites in northwest Spain. Additionally, we examined how bird-mediated selection influences average and sub-individual variation in fruit size. Results show that average and sub-individual variations in fruit size were related to fitness affecting seed dispersal. Older trees produced larger fruits, but tree age did not affect sub-individual variation in fruit size. Abiotic environmental factors differently affected sub-individual variation and average fruit size. Seed-dispersing birds exerted correlated selection on average and variation in fruit size, favouring trees with larger and less variable fruit size at one site. Our work suggests that the fruit size variation within individual trees, the sub-individual variation, is modified by abiotic environmental factors and, additionally, is an adaptive trait that responds to natural selection.