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 resulting in an impressive amount of morphological variation among species. Much of this diversity in plant form is dictated by the location, frequency, and timing of branch outgrowth (Halle et al., 1978; Bell, 1991; Barthelemy & Caraglio, 2007). Branch development also contributes to variation in shoot architecture within species, where this variation has the potential to influence fitness. The development of branches from axillary meristems affects leaf placement and light interception (Bell, 2008). Meristem fate (quiescent, vegetative, or floral) may limit the number of subsequent meristems available for sexual (Geber, 1990; Lortie & Aarssen, 1997; Lortie & Aarssen, 2000a) and asexual (Watson, 1984) reproduction and plays a key role in life-history trade offs (Geber, 1990). Finally, the developmental dynamics of shoot architecture can affect fitness and are subject to selection (Lortie & Aarssen, 1997; Lortie & Aarssen, 2000b), 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) of shoot architecture. Modern evo-devo studies seek to understand the molecular and developmental basis of morphological novelty and diversification as well as to uncover conserved developmental mechanisms and deep homologies. Such evo-devo studies have focused primarily on macroevolutionary problems by comparing developmental patterns across broad phylogenetic distances, usually focusing on model organisms such as Arabidopsis, Pisum, and Solanum (e.g. Blein et al., 2008). Broad-scale comparative studies have identified key gene networks responsible for morphological similarities and differences between major clades (reviewed in Carroll, 2008). However, whether the key genes and gene networks identified in model organisms are responsible for morphological variation at the population level is less clear. In order to fully understand the evolution of morphology and development, we need to examine these molecular developmental pathways within and among populations, where genetic divergence, adaptation, and speciation occur. A recent NSF workshop on Frontiers in Evolutionary Biology (2005) emphasized the need to integrate development and microevolution. One approach to this integration is to examine genes and gene networks that are well characterized in model taxa, but in the context of natural variation at the population level (Fondon & Garner, 2004; Shapiro et al., 2004; Moczek, 2006; Ehrenreich et al., 2007). Studies such as these four serve to highlight the importance and paucity of microevolutionary developmental research, which is especially lacking for plants. I examine the molecular developmental basis for intraspecific variation of shoot architecture in two populations of Mimulus guttatus (monkeyflower). Specifically, I will determine whether conserved gene networks responsible for patterning shoot architecture in broadly divergent model taxa also are responsible for natural morphological variation at the population level in M. guttatus.