THE MOLECULAR EVOLUTION OF VERTEBRATE GROWTH-HORMONES - A PATTERN OF NEAR-STASIS INTERRUPTED BY SUSTAINED BURSTS OF RAPID CHANGE

It has been demonstrated previously that in mammals the evolution of p ituitary growth hormone shows an unusual pattern, with an underlying s low rate and at least two sustained bursts of rapid evolution (in the artiodactyls and primates), during which the rate increased at least 2 5-fold. It is demonstrated here that a similar pattern applies for gro wth hormone evolution throughout the vertebrates, with a basal rate si milar to that seen in mammals, but bursts of rapid evolution in the am phibia and the elasmobranchs, and several bursts in the teleosts. The placental growth-hormone-like proteins of primates show a similar patt ern. It is argued that the bursts of evolution seen for growth hormone are a consequence of selection and that this may reflect changes in t he functions of the hormone additional to its basic growth-promoting a ctions.