Phenotypic and genetic variability in quantitative traits are ubiquitous in natural populations and in populations of livestock under artificial selection. Models to explain the level of genetic variation usually focus on the balance between its gain by mutation and loss by stabilising or directional selection and drift (see e.g. Falconer and Mackay, 1996; Bürger, 2000). The models do not explain, however, why the heritability of growth rate is typically c.25% in many species, even in highly selected modern broiler populations (Koerhuis and Thompson, 1997), or why the coefficient of variation (CV) of body size is typically c.10%, even in long-term selected mouse lines (Bünger et al., 2001). As natural selection on fitness-associated traits such as litter size is presumably directional, it can account for their low heritability but not for their high variability and genetic CVs (‘evolvability’, Houle, 1992) typically as high as for other traits such as body size less clearly associated with fitness. 

William G Hill

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 2002. Session 19, , 19.02, 2002
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