Rates of Genetic Changes in Populations of Farm Animals

Abstract

The means of many characteristics in populations of farm animals have already changed much in the few decades since moderately dependable records began to be kept. Some detailed examples of that are shown in Chapter 22 in "Genetics in the 20th Century". It is usually impossible to be at all sure how much these changes were caused by changes in the environment and how much by genetic changes in the populations of farm animals. Unquestionably, some environmental changes can cause large changes in the population means for some characteristics. Yet proof that improvements have occurred, in animal nutrition, in management and in control of diseases and parasites, does not prove that such improvements were responsible for any part of the generally upward trends in productivity per animal. Each such improvement in environment might possibly lead the owner to increase the number of animals per unit area, thus intensifying the competition and the opportunities for contagion and infestation, or otherwise to intensify his husbandry so that the individual animal may still face as grim a prospect environmentally as did its counterpart a half century or more ago.