Results are presented that describes a selection experiment showing the fundamental changes in growth and fitness associated with a shift towards higher growth rates in four environments. The experiment uses Tribolium castaneum to model a selection and mating scheme frequently occurring in commercial populations of livestock where there is an exchange of elite male germplasm to enhance performance in other populations under different environmental conditions. Objectives were to estimate direct response to selection for increased pupa weight in four environments and correlated response in family size. Estimates of genetic parameters are reported for the base population, combining all data across all lines and 23 generations of selection, and within each line.;Sufficient protein in the diet rather than a deficiency of protein was identified as a major contributing factor influencing phenotypic, genetic, and environmental changes across generations. Relative humidity created only minor changes in mean pupa weight between lines on the same diet. Animal models that failed to property account for males used across environments seriously underestimated the additive genetic variance in the population. The optimum environment, 80% relative humidity (RH) and 5% yeast-fortified whole wheat flour diet, and the poorest environment, 67% RH and a diet of flour alone, set maximum and minimum limits on estimates of phenotypic and additive genetic variance. A large ratio of additive genetic variance to phenotypic variance in some environments than others was clear evidence of genotype by environment interaction.;The experiment draws attention to the fact that undesirable correlated responses in reproductive success are frequently associated with selection for growth. Correlated responses in reproductive success can no longer be ignored, or left unmeasured in populations under intense selection for growth.