Divergent selection for growth in Japanese quail under split and complete nutritional environments. 4. Genetic and correlated responses from generations 12 to 20.

Divergent selection during Generations 12 to 20 for high (H) and low (L) 4-wk BW under split-(SD) and complete-(CD) diet environments resulted in continued progress in both upward and downward directions. Following 20 generations of selection, there were 4.8- and 3.6-fold differences between BW of H and L lines in the SD and CD environments, respectively. Regression coefficients of mean 4-wk BW on generation of 1.88 and 1.58 g in the H lines and -1.50 and -1.37 g in the L lines indicated symmetrical selection responses. Realized heritabilities for 4-wk BW ranged from .2 to .4; selection differentials were significantly (P less than .01) larger in the H lines than in the L lines. Directional changes in 2-wk and adult BW and in egg weight across generations were similar to that of the selection trait (4-wk BW). Selection resulted in decreased hatchability and egg production and an increase in the age of sexual maturity. However, these changes were smaller than corresponding changes observed in earlier generations (0 to 11). Egg size of lines selected in the SD environment continued to be larger than egg size of corresponding lines in the CD environment. Considerable genetic variation for BW remains under both SD and CD environments following 20 generations of divergent selection. Smaller gains resulting from lower heritabilities agree with theoretical expectations of decreasing responses accompanying continuous selection.