When organisms adapt to new environments, are the genetic mechanisms that underlie evolutionary trajectories predictable or stochastic? Recently, genomic studies have provided tantalizing clues that the genetic mechanisms that underlie evolutionary trajectories may be more constrained and deterministic than previously thought. Theoretical work has suggested that pleiotropy, or the influence of a single gene on multiple traits, might be an important deterministic factor during adaptive evolution. Specifically, high levels of pleiotropy are predicted to decrease the frequency with which a locus is used over the course of evolution because it is much more likely that mutations in that locus will have negative fitness consequences. Despite these theoretical predictions, however, we lack explicit tests of how pleiotropy contributes to evolutionary predictability. The objective of the PLEVOCON project is to test whether pleiotropy is a source of evolutionary constraint that underlies the predictability of evolutionary responses. The threespine stickleback (Gasterosteus aculeatus) is an ideal system in which to test whether pleiotropy is a source of evolutionary constraint. Crucially, these small fish have independently and repeatedly adapted to diverse freshwater habitats in the northern hemisphere since the retreat of the glaciers 12,000 years ago. Stickleback living in similar habitats have evolved similar phenotypes, providing an opportunity to ask whether the same genes underlie adaptation to these habitats, and whether genes that are repeatedly used for adaptation have lower levels of pleiotropy. I will utilize the existing wealth of genetic and genomic resources in this system and develop innovative statistical approaches to carry out the first genome-wide test of whether pleiotropy is a source of constraint in evolutionary responses.