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ŠUMARSKI LIST 1-2/1966 str. 58     <-- 58 -->        PDF

heritable variabilities of the individuals within and between populations of the
same species come into being through change of the frequency of the genes
and of the population genotypes. Differentiation between the inhabitants of
various localities and between successive generations occurs especially through
the action of processes of dispersion, isolation and inbreeding (Falcone r
1960). Thus distinct populations are formed within a species, viz. races, which
distinguish themselves from the other races of the same species by one or more
heritable characters (S t e b b i n s 1950; Rohmeder and Schönbach 1959).

Genetic variation between populations of a species may be of the continuous
type (clinal, geographic races), or discontinuous type (local, ecological races,
ecotypes) (Lan gl et 1936, 1959; Dobzhansky 1951; Syrach La r sen
1956; Wrigh t 1962). A knowledge of the character of variation is of great
importance: in discontinuous variation it is necessary to know the ecological
limits of the races. In continuous variation it is possible to predict the characters
of some unknown provenance by their geographic position in relation to the
position of a provenance with known characters. The geographic races are
usually fairly distinguishable by their morphological features. In contrast, the
ecological races are usually separated only by thoir physiological characters
and are more difficult to distinguish from one another (Wrigh t 1962).

Races are described on the basis of results obtained from carefully-designed
comparative experiments in which the genetic differences are statistically
verified. The determination of the interspecific variability for the most important
characters of forest tree species is one of the most important needs of
improvement today. The determination of races is an important prerequisite
for the detection of a gene pool that deserves to be preserved (Ster n 1984).


A complete knowledge of the genetic composition of a population by
specification of all alleles or genotypes is impossible. Even under the assumption
that all loci are identifiable and that the number of existing loci is determined,
a complete catalogue of frequency of the allels of a definite population
would be such a large one that it would lose all purpose. The number of
potential diploid combinations on a genotypical level would be so great that
the preparation of a whole list of frequencies would be impossible. Thus the
gene pool and the system of genetic frequencies are usually described only in
relation to one or a number of loci with a limited number of alleles (L e r n e r
1958). Such a description is given on the basis of the dependence of alleles on
the phenotypic expression of the character to which they are genetically
related. Most economically important are of a characters as are most of metric
nature (Gustafsso n 1963). For these characters the population should be
described by expressions of mean values, variances, and, if necessary, other
statistical terms. The distribution of the phenotypical values of these characters
is usually continuous, without being subdivided into special classes, which is
typical of the characters based on the genetic differences in the individual loci.
Thus, if we show a continuous distribution of the phenotypic expression of a
metric character, we have also described a discrete genetic whole (L e r-
ne r 1958).