A floral formula describes the numbers of units that comprise the four main whorls of a flower. The floral formula of a plant is thought to be one of the most constant of taxonomic characters given that it is most likely maintained by reproductive organ canalization. Breakdown or instability of developmental canalization, however, brought about by environmental stress, inbreeding, or extensive hybridization, can occasionally lead to inconstancy of floral form and variation in the numbers of units within a whorl. To determine the extent to which natural populations vary in levels of developmental instability, meristic variation was assessed in multiple populations of several annual Phlox species growing in northern Colorado. The production of flowers with five sepals, five petals, five anthers, and a tricarpellate gynoecium with three stigmatic lobes (a 5:5:5:3 floral formula) is highly conserved in the Phlox genus and is also prevalent among many members of the Polemoniaceae family. It is hypothesized that variation in numbers of anthers and carpels could result in a change in reproductive fitness due to variation in total pollen production and available ovules. Previous studies suggest that it is rare that entire plants will deviate from the normal floral formula, but rather single flowers or subsets of flowers within an individual will exhibit aberrant numbers. Therefore, if a plant possesses greater instability and produces an abundance of abnormal flowers that have greater than average numbers of floral parts, that plant has the capacity to make a greater genetic contribution to the next generation. Additionally, in order to determine the levels of heredity and generational instability, greenhouse pollinations were performed on flowers with normal and abnormal formulas in all relevant combinations and the progeny scored for floral formula and relative fitness.

Key words: developmental instability, floral formula, meristic variation, Phlox, Polemoniaceae