Adaptive radiation of a lineage into a range of organisms with different niches underpins the evolution of life’s diversity. Although the role of the environment in shaping adaptive radiation is well established, theory predicts that the evolvability and niche of the founding ancestor are also of importance. Direct demonstration of a causal link requires resolving the independent effects of these additional factors. Here, we accomplish this using experimental bacterial populations and demonstrate how the dynamics of adaptive radiation are constrained by the niche of the founder. We manipulated the propensity of the founder to undergo adaptive radiation and resolved the underlying causal changes in both its evolvability and niche. Evolvability did not change, but the propensity for adaptive radiation was altered by changes in the position and breadth of the niche of the founder. These observations provide direct empirical evidence for a link between the niche of organisms and their propensity for adaptive radiation. This general mechanism may have rendered the evolutionary dynamics of extant adaptive radiations dependent on chance events that determined their founding ancestors.