The evolution of floral traits associated with specialized plant-pollinator interactions (pollination syndrome) enables divergence in reproductive strategies and is key for the diversification of flowering plants. Pollination syndromes are widely observed in flowering plants with a prime example in the Mimulus genus where hummingbird-pollinated M. cardinalis flowers are larger, red in color, with exserted stamens and pistils whereas self-pollinated M. parishii have small, inconspicuous, pale pink flowers with minimal anther-stigma separation. Although the variation of some pollination syndrome traits such as flower color has been relatively well understood from a genetic perspective, very little is known about the genetic mechanisms underlying the variation of more polygenic traits such as flower size, stamen length, and pistil length. Here we report the identification of a quantitative trait locus (QTL) controlling pistil length variation between the hummingbird-pollinated M. cardinalis and self-pollinated M. parishii. Introgression of this QTL from M. cardinalis to M. parishii resulted in a near-isogenic line with increased pistil length (3 mm longer than the wild-type M. parishii). Fine-scale recombination mapping using the near-isogenic line narrowed the causal gene down to a small genomic region. Functional characterization of candidate genes in this interval is underway. These results suggest that even polygenic, dimensional traits such as pistil length can be genetically dissected through the construction of near-isogenic lines, recombination mapping, and subsequent transgenic characterization, an approach that will enable us to systematically study the pollination syndrome evolution in the monkeyflower system, gene by gene, mutation by mutation.