Genotypes

Genotypes with good agricultural fitness will be retained in the population.

Bulk selection promotes intergenotypic competition. By allowing natural selection to operate on early generations, the gene frequencies in the population at each generation will depend upon:

The genetic potential of a genotype for productivity.

The competitive ability of the genotype.

The effect of the environment on the expression of a genotype.

The proportions and kinds of genotypes advanced to the next generation. The effects of these factors may change from one generation to the next. More importantly, it is possible that desirable genotypes may be out-competed by more aggressive undesirable genotypes. For example, tall plants may smother short desirable plants. It is not possible to predict which F2 plant’s progeny will be represented in the next generation, nor predict the genetic variability for each character in any generation. The role of natural selection in bulk breeding is not incontrovertible. It is presumed to play a role in genetic shifts in favor of good competitive types, largely due to high fecundity of competitive types. Such an impact is not hard to accept when traits that confer advantage through resistance to biotic and abiotic stresses are considered. For example, if the bulk population were subjected to various environments. If two genotypes are in competition, their survival depends on the number of seed produced by each genotype as well as the number of seeds produced by their progeny. Using the natural relationship developed by W. Allard for illustration, the survival of an inferior genotype may be calculated as: An ¼ a [1] Sn 1 where An is the proportion of inferior genotypes, n is the generation, a is the initial proportion of the inferior genotype, and S is the selection index. Given two genotypes, A and B, in equal proportions in a mixture, and of survival capacities A¼1, B¼0.9, the proportion of the inferior genotype in F5 would be: A5 ¼ ð0:5Þ [1] ð0:9Þ5 1 ¼ 0:3645 ðor 36:45%Þ This means the inferior genotype would decrease from 50 to 36.45% by F5. Conversely, the proportion of the superior genotype would increase to 63.55%. As previously indicated, the bulk selection method promotes intergenotypic competition, it is important to point out that the outcome is not always desirable because a more aggressive inferior genotype may outcompete a superior but poor competitor. In a classic study by C.A. Suneson, an equal mixture of four barley cultivars was followed. After more than five generations, the cultivar Atlas was represented by 88.1%, Club Mariot by 11%, Hero by 1%, while Vaughn was completely eliminated. However, in pure stands, Vaughn out-yielded Atlas. It may also be said that if the genotypes whose frequency in the population increased over generations are the ones of agronomic value, then the competition in bulking is advantageous to plant breeding. The effect of natural selection in bulk population can be positive or negative, and varies according to the traits of interest, the environment under which the population is growing, and the degree of intergenotypic competition. If there is no competition between plants, genotype frequencies would not be changed significantly. Also, the role of natural selection in genetic shifts would be less important when the duration of the period is less as is the case in bulk breeding. This is so because natural selection acts on the heterozygotes in the early generations. However, the goal of bulk breeding is to develop pure lines. By the time this is released, the breeding program would have ended, giving natural selection on time to act on the pure lines.