DIGITALNA ARHIVA ŠUMARSKOG LISTA
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| ŠUMARSKI LIST 1-2/1966 str. 24 <-- 24 --> PDF |
progress. In many cases there is a great difference between a number of cultivated forms and their wild ancestors. When selection concerns quantitative or continuously varying characters, mass selection can, under certain conditions, lead to substantial improvement, provided that these characters are inherited without manifesting dominance and overdominance in heterozygous loci, without epistasis between genes or different loci, and where environmental influence is only slight. On the above-mentioned assumptions, the response to selection, or the selection gain for sexually propagated plants, can be expected to be a product of the selection differential and the fraction of the total genetic variability of the phenotypic variability. More generally, the selection gains are a product of the selection differential (expressed either in absolute units of measurement or in units of standard deviation of the normal curve, see Falconer, 1960, and Williams, 1964), and the additive portion of the total genetic variability in relation to the phenotypic variability (see also Matthews , 1963 p. 107). Non-additive gene actions, such as dominance and epistatic interactions, and those between the effects of the genes and the environment, are responsible for a number of errors in the estimation of heritability, and can, under several circumstances, reduce the response to selection (see, e.g. Mather , 1955, and L e r n e r, 1958). Consequently, when quantitative characters are more or less controlled by genie interactions that deviate from simple additive effects, the phenotype may prove an unreliable indicator of the genotype. For instance, the dominance relations can only rarely be separated phenotypically. Moreover, one of the most important points is, to divide the phenotypic variance into its main component parts: the genotypic portion and the environmental fraction. As is well known, the magnitude of the nongenetic component is very large in forest tree populations, and particularly in uneven- aged forest and in forests growing under widely varying environmental conditions (see, e.g. Kiellander , 1956). Consequently, environmental influences must never be underestimated, but, at the same time, they are extremely difficult to assess and determine even in very well designed and conducted progeny trials in a natural environment. Despite the fact that the response to mass selection may vary in respect of different characters and in different tree species and populations, the forest tree populations contain a large reservoir, which is of additive genetic origin, and which will bring about improvement by selection. These variations in response occur because the relations between genotype and phenotype are weakened through complex environmental effects, and also on account of the weakening of the relations between the additive genetic fraction and the total genotypic portion of the phenotype due to non-additive gene interactions. Furthermore, natural selection has more or less efected the sorting out genotypes of forest tree species as cultivars, adapted to local ecological conditions, which are of great value to the breeder. Outbreeding species and populations always vary around the optimum, however, and, therefore, fitness can never be as high as in inbreeders. To summarize: 1) selection has led to great improvements in our domesticated animals and plants, 2) though selection cannot produce new genes, it can isolate cultivars or groups of individuals that are carriers of the desired |