Selection

SELECTION (from Latin selectio—choice), a form of breeding organisms by man which leads to hereditary changes in a desired direction [with incorrect selection technique or with 'unconscious selection' (unconscious selection according to Darwin) undesirable changes may also occur]. Any improvement (better adaptation to human needs) of selection-breeding organisms proceeds along one of two paths—either a change in their phenotype by changing external conditions (feeding, housing, fertilization, crop rotation, etc.), or a change in their genotype by means of: replacement by a more productive breed, hybridization (which zootechnicians incorrectly call metization), or selection. Breed replacement and hybridization are forms of using the results of selection. Selection in the national economy gives less rapid but significantly more stable results than changing external conditions. The substantial effect of selection is achieved only after 5–10 generations of breeding, whereas changing conditions gives the main results in the same generation, but these results completely disappear after one, often two generations when the previous conditions return. Any breeding of a group of organisms leads to a certain reorganization of their genes. The task of the breeder is to conduct breeding in such a way as to increase the concentration of useful genes in the selected population. Modern selection, based on genetic data, carries this task to completion, requiring the complete elimination of other allelomorphs of these genes, i.e., the creation of a homozygous group of individuals satisfying the set goal. Such a group will not change during further breeding without selection. Homozygosity is achieved by inbreeding (see). Homozygosity is not required for plants reproducing vegetatively (e.g., potatoes or cotton reproduced by rhizomes); their hereditary structure does not change during reproduction, so for them heterozygosity is, on the contrary, advantageous, allowing the use of a constantly acting heterosis, i.e., a favorable result of crossing two varieties in the first generation. Every form of selection includes two moments—selection among organisms (i.e., leaving only those most satisfying the selection tasks and rejecting the rest) and some system of crossings (the latter in more primitive forms of selection is not controlled and is often disorderly). Selection requires objective and accurate accounting of the traits of an individual. For such accounting, in a number of cases special techniques must be applied, up to installations reproducing droughts in the laboratory, or infecting organisms with a certain infection. However, evaluating an organism by its phenotype is insufficient; for selection, an evaluation of the genotype is required. Therefore, evaluation by the phenotype of offspring and rejection of whole groups is applied. Evaluating by traits which it is desirable to improve and assigning them primary importance, it is nevertheless necessary to consider the properties of the organism comprehensively. The more initial individuals are taken and the more intense the selection, the better the result. The time required for selection is expressed by the number of generations: the fewer generations needed per generation, the faster selection is conducted. Selection methods depend on: 1) biology of reproduction (vegetative or sexual, self-fertilization or cross-fertilization, how many offspring an organism produces, fertilization and reproduction once in life or several times, etc.); 2) goals of selection—what traits need to be improved; 3) degree of genetic study of the organism; and 4) method of economic use of the organism (whether it must be killed for evaluation, whether a trait can be assessed from one organism or many are needed, etc.). Analytical selection is called selection within one breed, synthetic selection is the combination of the properties of two or many breeds into one. At the beginning of selection, its task must be clearly formulated. Based on the task and taking into account the world's availability of diverse forms of a given organism, the initial material for selection is chosen. Its choice ensures success by half, since selection cannot create new genes, but only recombines them. Therefore the aphorism is correct: 'To create a breed, one must first obtain it in hand.' After studying the material of the initial breeds, individuals most satisfying the selection tasks are chosen from it—founder individuals. Selection itself may proceed once, i.e., within one generation, or repeatedly, in a number of generations. Selection may be individual or mass. The most perfect method of selection is the pure-line method (see) applied for self-pollinating plants (Swedish method), first applied in 1893 at the Swedish Selection Station (it should be noted that it began to be applied even before the development of the science of pure lines). The number of initial individuals is taken to be significant, up to several tens of thousands. Culling is carried out annually based on the evaluation of the entire offspring of the initial individual; seeds of the retained lines are sown whole, not mixed with each other. After sufficient reproduction of the lines, their agricultural evaluation begins. After 6–8 generations from the start of work, as a result of culling, only 2–3 of the best lines remain from all lines, which are ready and reproduced selection varieties. For plants reproducing by cross-pollination but allowing forced self-pollination, the latter is applied as the most intimate inbreeding (see), leading to homozygosity most quickly (among animals such a method can be applied at present only to bees, by inseminating the queen with her parthenogenetically developed son). After 4–5 generations of self-pollination with the rejection of whole lines, lines practically homozygous are obtained. Since their viability is somewhat reduced as a result of inbreeding, they are crossed with each other in pairs. This crossing is performed between the retained lines in all combinations and is called diallelic. The best combinations of lines are retained and bred with the application of inbreeding for a number of generations, after which selection ends. The next method, applied to non-self-pollinating plants and to all animals, is the method of blood lines. Founders are selected in pairs and the offspring of individual pairs are crossed with each other in a number of generations often in combination with crossing parents with children by intimate inbreeding (brothers with sisters), with the application of the rejection of whole lines. This method also leads to practical homozygosity, but requires a greater number of generations for this than self-fertilization. The resulting lines can be used directly or, with a noticeable reduction in viability as a result of inbreeding, after diallelic crossings and repetition of inbreeding, as in the previous method. Besides the individual selection methods listed, there is a number of mass selection methods, in which inbreeding is not applied and therefore constant breeds are not obtained. The most perfect of them, widely applied in animal husbandry, is mass selection with progeny testing. Breeders are evaluated precisely, no special system of crossings is applied. This form of selection can significantly increase the concentration of useful genes. Less perfect is continuous mass selection with evaluation by phenotype. It can give noticeable results in the first generations, especially in organisms not previously subjected to selection and having great hereditary diversity, but they are not stable and are quickly lost. The most imperfect method of selection, although the fastest, is single mass selection by phenotype. Its results are felt only in one or two generations, and this method is now abandoned. Finally, organisms bred without special selection are still subject to unconscious selection to a greater or lesser degree, for example, leaving more hardworking horses in the farm for direct use is also selection, since they produce offspring. Our task is the complete elimination of unconscious selection, since it often leads to negative results; for example, the use of the largest animals from the herd for meat leads to its diminution. An important problem of selection is the use of the obtained varieties and breeds. This especially manifests the advantages of planned economy, allowing one season to replace one variety with a better one in the whole region. The main selection work is conducted by selection stations. In the USSR their network is widely developed. Selection of the majority of plants is united in the All-Union Institute of Plant Growing, which stands in its work ahead of all selection institutes in the world. Selection of animals is conducted by trusts and sectoral institutes. The scale of socialist economy has allowed selection for a number of cultures to be raised to an unprecedented height.