Deep coevolutionary history between plants and their pollinator mutualists and herbivorous antagonists is hypothesized to be a principal mechanism that shapes angiosperm floral displays and foliar defenses. My dissertation is dedicated to understanding general patterns organizing the evolution of this phenotypic diversity and testing the ecological consequences of evolutionary shifts in reproductive and defensive traits in space and time. In chapter 1, through a meta-analysis, I discovered the impact of study design, plant life-history, and mating system as general predictors of the outcome of pollinator-mediated plant-plant interactions. In chapter 2, with an original molecular phylogeny of temperate North American Coreopsideae (Asteraceae), I found that plant mating system transitions showed correlated evolution with multivariate floral display traits. Increased degree of selfing was associated with stronger negative correlations between floral advertisements, and relaxed reward signaling honesty. Chapters 3 and 4 tested how convergent floral display loss at the species level can impact interactions between closely related species in contemporary communities, ultimately impacting species co-distributions and microevolution. I zoomed in on two representative Coreopisdeae, Bidens frondosa and B. cernua. The former represents one of three instances of visual display loss in the clade, and the later has a conspicuous sunflower-like display. I detected strong facilitative interactions through pollinators and found that such facilitation between plants of contrasting floral form explained regional species co-occurrence patterns for not just B. frondosa and B. cernua, but Bidens spp. more generally. I also tested the impact of congener co-occurrence on floral trait evolution. The presence of these florally contrasting congeners intensified selection for larger display in the facilitated B. frondosa, but led to selection for smaller display in the facilitator B. cernua. In chapter 5, I shifted focus to plant-herbivore interactions, and discovered that specific molecular substitutions in monarch butterflies, previously thought to be a general resistance mechanism shared by other herbivores, likely reflect fine-grained host adaptation. In sum, my research has advanced our understanding of the evolution of floral displays within and between species, the broad-scale impacts of plant-plant interactions on species co-occurrence and phenotypic selection, and the evolutionary forces that shape adaptive defensive strategies.