Lives in the balance: Why do we hold onto potentially harmful, disease-causing mutations?

The persistence of heritable disease raises an evolutionary paradox. When DNA goes awry, and a harmful mutation sprouts up that affects survival, why aren’t these simply purged from a population over generations? Authors of a new report have performed the first systematic test of a counteracting evolutionary force that may work to maintain disease mutations at a higher frequency than expected. This force, called balancing selection, can lead to the maintenance of mutations over long evolutionary times.