Function switching as a basis for bursts of rapid change during the evolution of pituitary growth hormone.

Pituitary growth hormone shows a pattern of molecular evolution in which occasional bursts of rapid change are imposed on a slow basal rate. It is suggested that these bursts of rapid evolution are a consequence of acquisition by this protein hormone of a secondary function, the importance of which varies. As the function of the hormone switches to accommodate the changes in role, its structure will also alter, adapting it to acquisition or loss of the secondary function. Several rounds of such "function switching" could give a substantial change in structure (the sum of several small changes) with little overall change in function. A similar process could underlie rapid bursts of evolution in other proteins.