SELECTION and QUANTITATIVE CHARACTERS IN FIELD CROPS

Summary 1. It is vital that there should be no decrease in the rate of improvement of crop plants. In drafting the background to the inheritance of quantitative characters it is suggested that greater understanding of genetic mechanisms is needed rather than further increase in biometrical complexity. Selection is a very powerful force; it could be used to conserve genetic variation whilst effecting useful change. 2. Various breeding techniques, examined in a framework of directional, stabilizing and disruptive selection, are found to be tied to certain crops. Breeders of self‐fertilizing plants handle quantitative variation differently from those working with outbreeders. 3. Physiological analysis and partitioning of characters have not in themselves made large contributions to solving problems caused by complex traits, but both have been informative in conjunction with biometrical studies. In some circumstances more use could profitably be made of selection indices. 4. Heterozygote advantage and genetic balance play major parts in stabilizing selection. Modal selection can sometimes lead to increased fitness, conforming with the concept of genetic homeostasis. 5. Disruptive selection has apparently operated during the evolution of crop plants. To be beneficial to breeders it should be multidirectional; intercrossing of peripheral lines might then form ‘springboard’ populations allowing further advance from selection plateaux. 6. As the boundaries of cultivation are extended so the need will increase for plant populations with inherent stability of performance. Few crops are yet amenable to the production of F 1 hybrid seed, so polygenes should be manipulated to form balanced, heterozygous adaptive complexes. Systems of selection must be refined to deal effectively with the heterogeneous populations from which such complexes are likely to arise. 7. Studies on composite crosses and related projects tie in well with other work on selective forces in adaptation. New allopolyploids offer hope in some plant groups, but induced mutation is of restricted value. 8. Cryptic variation and selection can be important in many breeding methods, and should be considered when dealing with quantitative aspects. 9. Changes in locality can reveal startling amounts of unsuspected variability: such ‘environmental segregation’ could be harnessed, for example, by alternation of selection centres when breeding for general adaptability. 10. Persistence of variation, which may occur through selection favouring hetero‐zygotes despite strict inbreeding, is also considered in terms of heterozygous gene complexes. Tightness of linkage is the key to the durability and behaviour of super‐genes, and analysis of the simpler ones could lead to better understanding of the vastly complicated systems controlling fitness and productivity.