| Summary: | In the first part of this thesis the effects of natural selection on two contrasting synthetic populations, of known initial genetical composition, after successive generations of mating in isolated, competitive conditions, are investigated for a number of quantitatively inherited characters. Predictions of the magnitude and direction of the changes expected are made from a knowledge of the genetical architecture of the traits, from the relative fitnesses of the genotypes of the initial population and from estimates of the breedirgsystem operating in successive generations of the population. The accuracy of these predictions, and the validity of, the underlying genetical theory and assumptions, are tested by monitoring the observed changes occurring in the two experimental, synthetic populations. Sensitivity to macro-environmental variation between seasons may be a major determinant of the competitive ability of the competing genotypes, so in the second part the measurement and prediction of sensitivity and its determinants arestudied. Joint selection is performed for both extremes of mean performance and sensitivity to macro-environmental differences in several seasons, using family selection, and in a number of environments within the same, season, using lines derived by single seed descent. Indirect selection, as a practical alternative to direct selection, through the correlation between sensitivity and pattern of growth, as summarised by the quadratic component of the growth curve and the ratio of height in the middle of the season to final height, is investigated.
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