Short term selection experiments, which may be of less than ten generations, with production traits forming the selection objective, should estimate genetic parameters in the base population and genetic covariances with traits that are difficult to measure, after sufficient differences between selection lines have been achieved. Knowledge of genetic and phenotypic relationships between and within sets of production and non-production traits, for example physiological predictors of genetic merit, is required for the evaluation 
of alternative selection strategies. Information on relationships between traits may provide insight into the biological mechanisms involved in generating differences between selection lines. Selection experiments whose selection objective is a non-production trait are generally used for physiological studies, to establish genetically differing lines of animals, when the rate of genetic response in the selection objective is not of prime importance. To complement short term experiments, the long term selection experiments should be 
able to test for non-linearity of response, which includes selection limits, and to test theories on population size, such as predicted rates of inbreeding. Laboratory animals form appropriate populations for the examination of quantitative genetic theory using long term selection experiments, while short term selection experiments with meat, milk, egg or fibre producing animals should provide information that can be used directly in commercial breeding programmes. This paper briefly reviews pig selection experiments and some 
of the results obtained, to discuss aspects of selection experiments that should be considered for the continuation of existing experiments or before future pig selection experiments are established.

N. D Cameron

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 19. Selection and quantitative genetics; growth; reproduction; lactation; fish; fiber; meat., , 41–48, 1994
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