Since the advent of agriculture over 12,000 years ago, humans have been a powerful evolutionary force shaping the phenotypes of plants. While domestication selection is often considered a unique form of plant evolution, understanding how crops differ from their wild progenitors can shed light on basic evolutionary processes, and help us link plant traits to ecological outcomes. Thus far, the study of domestication has focused on changes in plant-level trait means. For example, crop plants often exhibit shifts in the average size of their seeds, sugar content of their fruits, or toxin concentration of their leaves. However, this focus on trait averages overlooks an important component of the plant phenotype: variability in traits within plants, among plant reiterated organs. While this variability is often considered noise around a plant-level mean, trait variances are critical for biotic interactions, and can evolve together with or independently of trait means. In this study, we asked how levels of within-plant, among-leaf trait variability have changed with crop domestication. We predicted that domestication selection should favor homogeneity of plant organs, and that this should decrease within-plant variability. We studied this using 12 crop species, their wild relatives, and landraces. Specifically, we explored whether variability of nutritive traits, resistance traits, and/or ‘leaf quality’ (e.g. herbivore performance) has shifted with domestication. For one plant species (alfalfa; Medicago sativa), we assessed trait variability within and among leaf age classes, to understand the role of leaf ontogeny in shaping patterns of within-plant variability between wild and domestic plants.
Results/Conclusions
We found frequent shifts in levels of within-plant variability with domestication. In contrast with our predictions, we found that trait variability often increased with domestication. This sometimes occurred in concert with shifts in the mean level of a trait, but more often occurred independently. Leaf ontogeny was an important driver of within-plant, among-leaf trait variability: for some traits, variability shifted among, but not within leaf age classes; for others, differences in variability between wild and domestic plants only manifested in leaves of particular ages. We conclude that within-plant variability is a key component of the ‘domestication syndrome’ and of plant trait evolution more generally, and discuss how our findings inform understanding of ecological processes and biotic interactions (e.g. herbivory) in crop systems.