A species’ range is a physical representation of the limits of evolution as it epitomizes a species’ evolutionary niche in a specific period of time. Range limits are dynamic barriers affected by the abiotic and biotic environment, population dynamics, and, least well understood, genetic mechanisms. There are currently four hypotheses that best account for the genetic mechanisms behind the formation of range limits. 1) An increase in deleterious mutation accumulation as a result of low density at the edges of ranges relative to the center, making edge populations more susceptible to genetic drift and the fixation of deleterious mutations. 2) Higher levels of genetic swamping where because edge populations are also frequently locally adapted, gene flow from the center to the edge of a population can create maladaptive hybrids at the edge. 3) A high variance in fitness in extreme environments which present the opportunity for natural selection to act but also may allow fitness to aggressively decline to zero. And 4) the idea that the alleles for adapting beyond an edge exist in standing genetic diversity but are not in the right location and are therefore dispersal limited. We are testing each of these hypotheses using an outdoor common garden experiment with Arabidopsis thaliana on Stanford University’s campus in Central California. To disentangle the genetic and ecological components of range limit formation, we used a sophisticated irrigation system that simulated 14 regimes of drought stress, varying in frequency and abundance, simultaneously. We selected ecotypes from the 1001 Genomes Project (1001genomes.org/) that fit the criteria of the four range limits by screening for Ka/Ks ratio, polygenic score for survival in drought conditions, distance to edge of A. thaliana’s range, and fitness in previous drought experiments. In total, we planted 25,920 plants on the same day in November 2021 and monitored them throughout their entire life cycle. We measured fitness via survival and seeds produced, along with phenological variables including first day of flowering, day of death, and lifetime duration, as well as ecologically-relevant phenotypes such as stomata, trichome indices, growth rate, and ∂C13. This experiment will allow us to elucidate which hypothesis or combination of hypotheses contributes the most to the formation of range limits. It is essential that we understand the mechanism behind range limit formation as climate change is altering ecosystems at an alarming rate, and the capabilities of a population to respond may be the only way to avoid extinction.