The sequences of ovine and bovine placental lactogens (based on publis
hed cDNA sequences) are remarkably different, indicating a very rapid
rate of evolution. Analysis of the cDNA sequences indicates that the r
ate of nonsynonymous substitution in these proteins is considerably gr
eater than the rate of synonymous substitution. This is an unusual sit
uation, which suggests that the observed rapid rate of evolution is du
e to incorporation of adaptive rather than neutral mutations.