DIGITALNA ARHIVA ŠUMARSKOG LISTA
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| ŠUMARSKI LIST 1-2/1966 str. 63 <-- 63 --> PDF |
and the practical foresters´ knowledge of forests (Schreine r 1950). After summing up the data of investigations on the variability of populations, one must select: (1) the representatives of geographic races; (2) the representatives of ecologic races. The best representatives of these races should be selected by their genetically related and economically important characters. If the available data do not offer a satisfactory insight into the existence of races and their separation, we should select: (1) the best populations and individuals grown in normal sites for this species; (2) the best populations and individuals grown under the conditions of ecological extremes. In practice, one must usually combine choice of representatives of known races with selection of representatives on the basis of forest inventory data. Wrigh t (27) is of the opinion that different geographic races are formed at distances greater than 200 km and that local, ecologic races arise in site extremes and under conditions of isolation. Further studies of the natural forest resources will yield better knowledge of races, as well as completing the selection of material. Valuable data will also be obtained from studies of earlier selections. Discussion about the problem how to select should be conducted in two directions: (1) to determine the number of population samples to select within a species; (2) to determine the number of genotypes to select within a population. The number of population samples of a species which will be selected for preservation as the gene pool of natural forests of the species depends on the genetic differentiation of populations, i.e. on the existence of races and their number. The genetic diferentiation of the populations of a species depends on the size of the area of distribution (in the geographical and altitudinal sense), the uniformity or variability of its sites, and the spatial isolation of the habitat. The greater the area of distribution, the more varied will be the sites. The greater the intermediate range of individual growing areas, the greater will be the genetic differentiation of populations, and, of course, the greater will be the number of necessary samples of the gene pool to be preserved. This general rule can also serve as a guide when deciding how many population samples are needed for those species whose races have not yet been thoroughly investigated. Theoretically, it is necessary to preserve samples of the genetic material of all races of a species. Practically, the limits will be set at the outset by economic considerations. In any case, choice should begin with the most characteristic and significant populations. Then it should gradually go more deeply into details. The problem of the number of genotypes of a population to select in order to preserve to a satisfactory extent its gene pool is of particular concern. We have already discussed one aspect of this problem: considering the heterozygous character of most genotypes of forest tree populations, we may conclude that the variability of the gene pool to be selected is may times greater than the number of selected genotypes, and that the genetic material |