As Goethe first posited in 1790, the plant shoot is composed of only two vegetative organ types: leaf and stem. This seemingly simple body plan has been reiterated and modified over the course of evolution to construct an amazing amount of morphological diversity. Much of this diversity in plant form is dictated by the location, frequency, and timing of branch outgrowth. Branch development also contributes to variation in shoot architecture within species, where this variation has the potential to influence fitness. For instance, the development of branches from axillary meristems affects leaf placement and light interception. Axillary meristem fate (quiescent, vegetative, or floral) may limit the number of subsequent meristems available for sexual and asexual reproduction and plays a key role in life-history trade offs. The developmental dynamics of shoot architecture can be subject to selection, making shoot architecture an excellent system for studying morphological and functional divergence of natural populations.

I use a comparative approach to investigate the evolution and development (evo-devo) shoot architecture. Typically, evo-devo studies seek to understand the molecular and developmental basis of morphological change. These studies have focused primarily on macroevolutionary problems by comparing fixed developmental patterns across broad phylogenetic distances, often between model organisms such as Arabidopsis and Antirrhinum (snapdragon). These broad-scale comparative studies have identified key gene networks that generate morphological differences between major clades. Presumably, changes in these developmental pathways also are responsible for population level variation, but whether they actually are - and how these changes manifest - is less clear. In light of this, a recent NSF workshop on Frontiers in Evolutionary Biology recognized the need to integrate development and microevolution. One approach to this problem is to re-evaluate the same gene networks that produce morphological differences among clades, but at the population level where developmental variation arises and is subject to evolutionary processes. Limited work in this area has demonstrated that intraspecific differences in gene expression can have major consequences for morphological development (Fondon & Garner, 2004; Shapiro et al., 2004; Moczek, 2006; Ehrenreich et al., 2007). These studies serve to highlight the importance and paucity of microevolutionary developmental research, which is especially lacking in plants where population level differences in developmental gene expression are rarely quantified. I address this problem by examining the molecular developmental basis for intraspecific variation of shoot architecture in two populations of Mimulus guttatus (monkeyflower). My dissertation will determine whether gene networks responsible for generating differences in shoot architecture in broadly divergent taxa are also responsible for population level variation in M. guttatus.