Table 1 gives the number of offspring that were tested each generation and their average BEC, 4 hours consumption in g/kg, age, and body weight (at the time of DID testing). The mean BEC and consumption values for the selected parents of each generation also are provided. The selection differentials (S) were calculated from the mean BECs of the mice selected to breed the next generation minus the mean of the population from which they were selected. The number of breeder pairs (families) varied slightly across generations because some pairs were infertile or offspring were too few or died before testing. Figure 2 shows the realized response to selection R, the change in BEC from the S0 foundation mean value (e.g., S2–S1), plotted against the cumulative S. The regression of R on cumulative S gave an estimate of heritability from the slope of the regression line (h2 = .09). Response to selection has not slowed, suggesting that additive genetic variability remains in the population (see Supplement 1).
