THE EFFICIENCY OF THREE METHODS OF SELECTION

THE practical value of a plant or animal is almost always affected by several traits. Hence deciding which are the most valuable individuals to select for parents of the next generation forces the breeder to consider several different characteristics. These are not likely all to be equally important or all to be independent of each other. There are many ways of selecting for several different things but these will not often be equally efficient. The most efficient method is that which results in the maximum genetic improvement per unit of time and effort expended. In planning some of their experimental work, the authors had occasion to compare the theoretical consequences of three fairly simple ways of selecting for net merit, considered as a compound of several different characteristics. While the results are not completely generalized, they are striking enough and seem to be of enough general validity to deserve presenting to plant and animal breeders. The three methods are designated as the "tandem" method, the "total score" method and the method of "independent culling levels." The "tandem method" is to select for one trait at a time until that is improved ; then for a second trait; later for a third, etc.; until finally each has been improved to the desired level. The "total score method" is to select for all the traits simultaneously by using some index of net merit constructed by adding into one figure the credits and penalties given each animal according to the degree of its superiority or inferiority in each trait. In the method of "independent culling levels" a certain level of merit is established for each trait, and all individuals below that level are discarded, regardless of the superiority or inferiority of their other traits. In order to select most efficiently, the following things need to be known for each trait which influences practical value: (1) The amount by which each unit of variation in it actually raises or lowers an individual's practical value (relative economic value of the trait) ; (2) the extent to which observed variations in it represent differences in genetic constitution (its heritability) ; and (3) the genetic and environmental correlations of each trait with the others. The method of total score is most efficient, while the tandem method is the least efficient of the three. The reasons for that and the extent of it will appear in what follows.