The limits to artificial selection for body weight in the mouse I. The Limits Attained in Earlier Experiments

Abstract

1. The results of some selection experiments for body weight in the mouse, conducted in the past in this laboratory, have been examined from the point of view of the limits ultimately reached.2. The limits that are apparently attained do not necessarily remain stable over prolonged periods of time; two large lines showed marked decreases despite continued selection for high body weight.3. Selection for high body weight reached a limit in the region of 30 g. at 6 weeks of age; small mice reached their limit at around 12 g.4. The time taken to reach the limit may vary from ten to thirty generations, even for this one trait.5. The total response for unidirectional selection was between two and six times the phenotypic standard deviation, or three to twelve times the additive genetic standard deviation.6. Consideration of the half-life of the selection responses excluded the likelihood of the chance fixation of alleles unfavourable to the direction of selection.7. The loci contributing to the response could each have an effect amounting to anything from one-half to one phenotypic standard deviation in the base population.8. This indicated that up to twenty loci had contributed to the response.9. The intensity of selection practised was close to the optimum for obtaining the maximum total response.10. The rule of parsimony would indicate the exhaustion of the additive genetic variance as an adequate explanation of the limits attained.