Are organisms free to reach their adaptive optima or constrained by hard-wired developmental programs? Recent evidence suggests that the arrangement of stomata on abaxial (lower) and adaxial (upper) leaf surfaces may be an important adaptation in plants, but stomatal traits on each surface likely share developmental pathways that could hamper evolution. I reviewed the quantitative genetics of stomatal density to look for loci that (1) affected ab- or adaxial density independently or (2) pleiotropically affected stomatal density on both surfaces. I also used a macroevolutionary approach to test for independent versus correlated evolution of stomatal traits (density, size, and pore index) on each surface. For this, I compiled anatomical and phylogenetic data from the literature on >1000 amphistomatous species. Naturally occurring and laboratory-induced genetic variation alters stomatal density on one surface without affecting the other, indicating that development does not strongly constrain the spectrum of available mutations. Over macroevolutionary time, stomatal pore index and density evolved independently on each surface, whereas stomatal size was constrained by correlated evolution. Genetics and phylogenetics demonstrate mostly independent evolution of stomatal function on each leaf surface, facilitating largely unfettered access to fitness optima.