According to paragraph 5, by comparing the enzymes from two related groups of fishes on opposite sides of the Isthmus, Graves found evidence that A. there were slight genetic divergences between the two groups B. the Atlantic group of fishes were descended from the Pacific group of fishes C. the temperature of water on either side of the Isthmus had changed D. genetic changes in the Atlantic group of fishes were more rapid and frequent than in the Pacific group of fishes