Estimation of heritability and genetic trend in populations at a physiological limit.

Limits on physiological processes, though perhaps unknown, must exist. The reported simulations evaluate the effect of a physiological limit on the estimation of genetic parameters and genetic progress. Simulation experiments reveal no change in the estimate of heritability, even for limits as restrictive as 1.5 phenotypic standard deviations above the population mean. However, estimates of additive genetic and environmental variance shrink as limits on performance increase in severity. Simulated physiological limits do not affect the rate of genetic progress. However, absolute measures of estimated genetic change are less than those predicted by response equations.