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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