Speciation in the bee-pollinated Mimulus lewisii and bird-pollinated M. cardinalis group is hypothesized to have occured through divergence in floral form that led to a change in breeding system. This effectively reproductively isolates the species. Previous studies have provided extensive genetic, phylogenetic and natural selection data which allow for a rare opportunity to now study and interpret ontogenetic changes as sources of evolutionary novelties in floral form. Three populations of M. cardinalis and four populations of M. lewisii(representing both described races) were studied from initiation of floral apex to anthesis using SEM and light microscopy. Allometric analyses were conducted on data derived from floral organs. Sympatric populations of the species from Yosemite National Park were compared. Calyces of M. lewisii initiate later than those of M. cardinalis relative to the inner whorls, and sepals are taller and more acute. Relative times of initiation of petals, sepals and pistil are similar in both species. Petal shapes differ between species throughout development. Corolla aperture shape becomes dorso-ventrally narrow during development of M. lewisii, and laterally narrow in M. cardinalis. In both species, the adaxial filaments curve adaxially as they elongate. In M. lewisii, the abaxial stamens curve away from the median while those of M. cardinalis are directed adaxially. Allometric analyses reveal that differences in corolla length dimensions between species are controlled by alterations in rate of growth, while differences in stamen and pistil dimensions are controlled by alterations in timing of termination of growth relative to corolla growth.

Key words: allometry, bee/bird pollination, evolution, Mimulus, ontogeny, Scrophulariaceae