One of the universal truths of evolution is that it occurs unequally across the tree of life: whereas some traits and lineages can remain inert (or nearly so) over millions of years, others appear to achieve evolutionary overdrive. My research centers around exploring the mechanisms underlying this phenomenon from single species to entire lineages. These explorations range from the evolutionary physiology of lizards and amphibians to the comparative biomechanics of fishes and stomatopods. Despite the different systems and approaches, an emergent theme from these inquiries is that ‘constraint’ serves as a dual-edged sword for evolutionary diversity. Behavioral constraint, for example, can result in both faster or slower evolution, depending on the ecological context. Mechanical constraint can enhance the rate of evolution, but the magnitude of this phenomenon varies across contexts. Evolution is a noisy process, but the dual role of “constraint” as both a motor and brake for diversity is a widespread signal.