Macromeria viridiflora (Boraginaceae), a perennial species that occurs as isolated populations on the Sky Islands of southwestern North America, displays geographic variation in floral morphology that may be related to pollinator variation. Analysis of morphometric data from eight sites across the range of the species shows significant among-population variation in 19 out of 20 vegetative and floral traits measured. Flower size variation is particularly strong and follows a latitudinal cline, with flowers being much larger in the southern part of the range and smaller in the northern part of the range. Observations also indicate differences in floral visitors between northern and southern populations. While flowers in all populations were visited by hummingbirds, the large-bodied hummingbirds visiting plants in the southern regions are not present in the northern regions, where flowers are visited by hummingbirds with nearly half the body size and much shorter bills. This difference in body size of pollinators mirrors the geographic variation in flower size in M. viridiflora, suggesting that pollinator-mediated selection may be acting upon the species. Floral variation will also be placed into a phylogenetic context to determine the historical direction of change in flower size.

Key words: geographic variation, Macromeria viridiflora, morphometrics, pollinator-mediated selection, sky islands