Image above: Rapid and sustained evolution of breeds. (A) Purebred St. Bernard skulls from ≈1850 (Top), 1921 (Middle), and 1967 (Bottom). (B) Purebred bull terrier skulls from 1931 (Top), 1950 (Middle), and 1976 (Bottom) (24). (C) Purebred Newfoundland skulls from 1926 (Top), 1964 (Middle), and 1971 (Bottom). Source:http://www.pnas.org/content/101/52/18058/F3.expansion.html