Many organisms exhibit visually striking spotted or striped pigmentation patterns. The classical reaction-diffusion model predicts that such spatial patterns can form when a local autocatalytic feedback loop and a long-range inhibitory feedback loop interact. At its simplest, this self-organizing network only requires one self-activating activator that also activates a repressor, which inhibits the activator and diffuses to neighboring cells. However, we still know very little about the actual genes encoding the hypothetical activators and repressors underlying pigmentation systems, even less about the biophysical properties of these molecules, and virtually nothing about how modulation of the properties of these activators and inhibitors affects pattern evolution in nature. Through genetic analysis, transgenic experiments, and mathematical modeling, we have previously identified a pair of MYB proteins that constitute an activator-inhibitor, reaction-diffusion system underlying the formation of dispersed anthocyanin spots in monkeyflower petals. Here I present our most recent work on the genetic and developmental bases of pigment pattern evolution from dispersed spots to longitudinal stripes among species, through modulation of the prepattern of the R2R3-MYB activator and the biophysical properties of the R3-MYB inhibitor as well as two additional co-activators.