Phenotypic responses of chickens to long-term, bidirectional selection for juvenile body weight—Historical perspective

A long-term selection experiment for high (HWS) and low (LWS) BW at 8 wk of age (BW8) was conducted in White Plymouth Rock chickens. Over 54 generations of selection, responses to bidirectional selec- tion were profound. Increase in BW8 in line HWS was linear, and there was a significant quadratic response in line LWS for BW at both 4 and 8 wk of age. Although there is no indication that line HWS has come close to approaching a selection limit in more than 50 genera- tions, selection limits occurred in line LWS chickens at generation 48 for females and generation 50 for males. Evidence also exists that one or more beneficial muta- tions have occurred in line HWS, aiding in progressive increases in BW8 over generations. Analyses of ratios of BW at 4 wk of age with those at 8 wk of age (ratio 4/8) revealed that LWS females grew proportionately faster through 4 wk of age than LWS males or HWS chickens. Comparisons of the selected lines with contemporary lines in which selection had been relaxed (discontinued) indicated that, in line HWS, the relaxed lines generally regressed toward original (preselection) values, suggest- ing that the linear response to single-trait selection was at least partially due to continued genetic variance. In LWS chickens, a series of plateaus in selection re- sponse occurred, but relaxed contemporary lines still regressed toward preselection values for BW8. In spite of the length of this selection experiment (54 genera- tions), genetic variance and beneficial mutations have allowed continued, linear response to selection for in- creased BW8. Response to selection for decreased BW8 has been tempered by physiological barriers that have decreased survival of young chicks or the ability of fe- males to reproduce. These findings are discussed in a historical perspective.