Remarkably high rate of molecular evolution of ruminant placental lactogens

The sequences of ovine and bovine placental lactogens (based on published cDNA sequences) are remarkably different, indicating a very rapid rate of evolution. Analysis of the cDNA sequences indicates that the rate of nonsynonymous substitution in these proteins is considerably greater than the rate of synonymous substitution. This is an unusual situation, which suggests that the observed rapid rate of evolution is due to incorporation of adaptive rather than neutral mutations.