PHENOTYPIC SELECTION CAN BE BETTER THAN SELECTION FOR BREEDING VALUE

Monte Carlo computer simulations were carried out in order to compare unrestricted phenotypic and restricted combined-index selection, appli ed over one generation of single-pair mating, for a wide range of fami ly sizes and heritabilities, and for two sizes of breeding populations . The simulations demonstrate that when restrictions are imposed to co ntrol reduction of diversity to equivalent population size (status num ber), there are situations where phenotypic selection will achieve gre ater gain than that achievable by combined-index selection. On average , unrestricted phenotypic selection was found to be similar to or bett er than restricted combined-index selection at the sam status number f or heritabilities higher than 0.2. For lower heritabilities, phenotypi c selection was less efficient. Selection in larger families resulted in greater gain, but at the expense of genetic diversity. Unrestricted phenotypic selection often proved to be a more efficient method, pres erving more genetic diversity per unit gain. Restricting the number of selections per family is an efficient method to balance gain and dive rsity after combined-index selection. The size of the breeding populat ion appeared to have a minor effect on gain, but can affect the rate a t which overall genetic diversity is eroded.