As plants age, the morphology and physiology of the shoot changes—a phenomenon known as vegetative phase change. Often these changes are important in the life history of the plant and contribute to shifting strategies in growth, defense, and competition. The temporally regulated microRNAs miR156 and miR157 are known to control many aspects of vegetative phase change making them ideal for examining how this pathway contributes to the evolution of life history strategies. In this work, I examine this question using two genera of woody legumes. First, I use the New World Ant-Acacias (genus Vachellia) to examine if age-dependent changes in plant defenses evolve as a consequence of neutral constraints on development. I find that the miR156/157 pathway likely controls the age-dependent appearance of the swollen-thorn syndrome—a suite of traits important for interaction with mutualistic ants. This finding is in disagreement with the idea that the temporal regulation of the swollen-thorn syndrome has evolved as a consequence of selectively neutral processes such as ontogenetic drift and supports the idea that the timing of syndrome emergence is somehow adaptive. I discuss the possible agents of this selection and their relation to the miR156/157 pathway. I also investigate the frequency of life-history heterochrony and its mechanism using plants of the genus Acacia, which undergo a conspicuous shift in leaf morphology during their life cycle. This transition from a compound leaf to a simple-horizontally expanded leaf, known as a phyllode has previously been linked to the miR156/157 pathway. Using this morphological marker of vegetative iiiphase change I examine the timing of this transition in 147 species within a phylogenetic context. I find that the evolution of a prolonged or persistent juvenile phase has evolved independently at least 7 times in Acacia. For two of these events the rate and magnitude of miR156 decline is reduced supporting the idea that these species are neotenous. I use genome sequencing to characterize the MIR156/MIR157 gene families, and identify a gene with a mutation in a putative cis-regulatory element that may contribute to phenotypic differences between species.