Selection for juvenile body weight in chickens: Standardizing for scaling

The influence of scaling on phenotypic responses to long-term selection was evaluated in lines of chickens selected for high (HWS) and low (LWS) 8-week body weight. The focus of the experiment was on phenotypic correlations between means and standard deviations, and unadjusted vs. standardized responses. Traits measured were body weight at 4 (BW4) and 8 wk (BW8). Correlations between means and standard deviations for BW8 (the selected trait) were positive and greater for BW4 in LWS than HWS. In both lines, scaling masked the degree more than the pattern of response; however, after standardization, it was greater in LWS than HWS. Selection changed the standard deviation units of juvenile BW with a 2-fold difference in BW8 for both HWS and LWS in which the latter reached a physiological limit. Standardization is a robust method to adjust for scaling, thus enhancing the interpretability of responses in long-term selection experiments. Because of changes in variances associated with those in phenotypic means, this method to correct for part-whole correlations was useful.