"Fitness" no longer responds to selection because natural selection in the past has raised it to the maximum possible level. All resources available from theenvironment areexploited maximally. Lifetime reproduction, in practice measured as total number of young weaned, or total weight weaned, over the lifetime, is the trait closest to fitness.
The paper discusses how lifetime reproduction is inherited. The everpresent natural selection, and past selection, constrain what can now be done. Quantitative genetics as usual ly taught has overlooked the importance, in practice, of this constraint on lifetime reproduction. The possible goals of animal improvement are 8lso constrained greatly by this fact. The paper seeks to make animal breeders sware of these limiting constraints.

R. G Beilharz

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume XI. Genetics of reproduction, lactation, growth, adaptation, disease, and parasite resistance., , 49–54, 1986
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