Prolactin stimulates mammary gland growth and development in mammals and regulates salt and water balance in freshwater fish. Many scientists think that this wide range of functions indicates that prolactin
A) is a nonspecific hormone.
B) interacts with many different receptor molecules.
C) is derived from two separate sources.
D) is an evolutionarily conserved hormone.
E) has a unique mechanism for eliciting its effects.
Answer: D