Date of this Version
Selection is the primary force for changing average genetic composition of herds, breeds, or species. Individual changes from one generation to the next associated with selection are usually small. In time, however, the change can be dramatic.
Selection is deciding which bulls and cows get to become parents and how many offspring we allow them to have. Both the will of man and the will of nature are directive forces in selection. Rate of progress from selection is determined by (1) average selection differential of parents for all traits under selection, (2) heritability of traits, (3) genetic correlations between traits, and (4) interval between generations of parents.
Selection differential is the difference in performance of selected sires and dams compared with the average of the unselected group from which they came.
Heritability is the fraction of the observed differences between animals caused by average genetic differences.
Genetic correlation is the average genetic association between traits.
Interval between generations is the average age of sires and dams when offspring are born (which in our herd was 4.4 years).