Evolutionary capacitance is the storage and release of variation, just as electric capacitors store and release charge. Living systems are robust to mutations. This means that living systems accumulate genetic variation without the variation having a phenotypic effect. But when the system is disturbed (perhaps by stress), robustness breaks down, and the variation has phenotypic effects and is subject to the full force of natural selection. An evolutionary capacitor is a molecular switch mechanism that can "toggle" genetic variation between hidden and revealed states. If some subset of newly revealed variation is adaptive, it becomes fixed by genetic assimilation. After that, the rest of variation, most of which is presumably deleterious, can be switched off, leaving the population with a newly evolved advantageous trait, but no long-term handicap. For evolutionary capacitance to increase evolvability in this way, the switching rate should not be faster than the timescale of genetic assimilation. This mechanism would allow for rapid adaptation to new environmental conditions. Switching rates may be a function of stress, making genetic variation more likely to affect the phenotype at times when it is most likely to be useful for adaptation. In addition, strongly deleterious variation may be purged while in a partially cryptic state, so cryptic variation that remains is more likely to be adaptive than random mutations are. Capacitance can help cross "valleys" in the fitness landscape, where a combination of two mutations would be beneficial, even though each is deleterious on its own. There is currently no consensus about the extent to which capacitance might contribute to evolution in natural populations. The possibility of evolutionary capacitance is considered to be part of the extended evolutionary synthesis. Switches that turn robustness to phenotypic rather than genetic variation on and off do not fit the capacitance analogy, as their presence does not cause variation to accumulate over time. They have instead been called phenotypic stabilizers. Enzyme promiscuity In addition to their native reaction, many enzymes perform side reactions. Similarly, binding proteins may spend some proportion of their time bound to off-target proteins. These reactions or interactions may be of no consequence to current fitness but under altered conditions, may provide the starting point for adaptive evolution. For example, several mutations in the antibiotic resistance gene B-lactamase introduce cefotaxime resistance but do not affect ampicillin resistance. In populations exposed only to ampicillin, such mutations may be present in a minority of