Flower shape, color and size are extensively studied to both identify and classify different angiosperm taxa. The availability of well-supported molecular phylogenies produced using complex models of sequence evolution, coupled with an understanding of the genes that regulate morphological form in model organisms, and new methods to infer gene expression patterns in diverse species now allow us to understand the genetic basis of morphological differences among closely related species. Studies in Plantaginaceae, Gesneriaceae, Fabaceae and Brassicaceae show the importance of CYCLOIDEA (CYC), RADIALIS (RAD) and DIVARICATA (DIV) in regulating flower shape, but also show divergence in gene function within flowering plants. Previous studies in the zygomorphic model species Antirrhinum majus (snapdragon) have shown that AmCYC is expressed in the adaxial (dorsal) petals of flowers where it activates AmRAD . This expression of AmRAD within adaxial petals represses AmDIV expression causing AmDIV to be restricted to abaxial (ventral) and lateral petals. Like Antirrhinum , traditional Diplacus flowers have distinct dorsal, ventral and lateral petal identities. However, within the clade actinomorphic flowers have evolved independently on two occasions: once in D. pictus and once in D. mohaviensis. mRNA reveal DIV expression to be conserved between D. pictus and snapdragon, whereas CYC and RAD expression, and presumably function, differ between the two species. DpCYC is expressed in a narrow portion on the upper lip of abaxial petals, whereas DpRAD is expressed within both lateral and abaxial petals. D. pictus flowers are characterized by a novel upturned abaxial petal which may be linked to localized CYC expression along the upper surface of the structure. This study sheds new light on the mechanisms regulating flower shape in an endemic Californian monkey flower and shows the importance of testing hypotheses from model species such as Arabidopsis and snapdragon in non-model taxa such as D. pictus to undercover the true variety of mechanisms driving morphological evolution.