White Plymouth Rock Chickens (2 of 2) (IMAGE) University of Oxford Caption A selective mating approach within the population that started in 1957 has resulted in an over tenfold difference in the size of the chickens. Credit Virginia Tech/ John McCormick Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.