Animal-pollinated flowers are complex structures that require a precise configuration of floral organs for proper function. Plants pollinated by highly specialized pollinators are expected to exhibit strong integration among functionally related floral organs. Flowers, however, often participate in multiple functions and consequently may be modular, that is, composed of independent suites of characters that are each tightly integrated. Nicotiana alata and N. forgetiana (Solanaceae) are two closely related species of wild tobacco with floral morphologies that appear to correspond to their predominant pollinators, hawkmoths and hummingbirds respectively.

To investigate phenotypic integration and modularity in N. alata and N. forgetiana flowers, I examined: (1) architectural and temporal variation in phenotypic integration of flowers in the two species and their artificial F3 hybrids, (2) the genetic basis of phenotypic integration and floral modularity in the two species and their artificial F4 hybrids, (3) allometric variation in geometric morphometric shape variables in the flowers of the two species. These complementary approaches identified two independent suites of highly correlated floral traits and confirm their identity as modules. The limb module includes measures of the corolla limb while the length module includes measures of the corolla tube, gynoecium, androecium length. Several lines of evidence support the conclusion that the modules are similar in both species, persist despite multiple generations of recombination in artificial hybrids, and therefore have an underlying genetic basis. This modular genetic architecture has likely maintained correlations among functionally related characters during floral evolution in these species while allowing for independent evolution of characters involved in separate functions, such as pollinator attraction and pollen transfer. Intra-floral modularity may have been critical for rapid specialization of these species to different pollinators.