DIGITALNA ARHIVA ŠUMARSKOG LISTA
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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). SELECTION OF CHARACTERS FOR THE DETECTION OF THE GENE FOOL 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). |