LONG-TERM EFFECTS OF SELECTION BASED ON THE ANIMAL-MODEL BLUP IN A FINITE POPULATION

Monte Carlo simulation was used to assess the long-term effects of tru ncation selection within small populations using indices (I = omegaf m) combining mid-parent [f = (a(s) + a(d))/2] and Mendelian-sampling (m = a - f ) evaluations provided by an animal model BLUP (a = f + m). Phenotypic values of panmictic populations were generated for 30 disc rete generations. Assuming a purely additive polygenic model, heritabi lity (h2) values were 0.10, 0.25 or 0.50. Two population sizes were co nsidered: five males and 25 females selected out of 50 candidates of e ach sex (small populations, S) and 50 males and 250 females selected o ut of 500 candidates in each sex (large populations, L). Selection was carried out on the index defined above with omega = 1 (animal model B LUP), omega = 1/2, or omega = 0 (selection on within-family deviations ). Mass selection was also considered. Selection based on the animal m odel BLUP (omega = 1) maximized the cumulative genetic gain in L popul ations. In S populations, selection using omega = 1/2 and mass selecti on were more efficient than selection under an animal model (+ 3 to 7% and + 1 to + 4% respectively, depending on h2). Selection on within -family deviations always led to the lowest gains. In most cases, the variance of response to selection between replicates did not depend on the selection method. The within-replicate genetic variance and the a verage coefficient of inbreeding (F) were highly affected by selection with omega = 1 or 1/2, especially in populations of size S. As expect ed, selection based on within-family deviations was less detrimental i n that respect. The number of copies of founder neutral genes at a sep arate locus, and the probability vector of origin of the genes in refe rence to the founder animals, were also observed in addition to F valu es. The conclusion was that selection procedures placing less emphasis on family information might be interesting alternatives to selection based on animal model BLUP, especially for small populations with long -term selection objectives.