Intergenotypic Competition Between Tall and Dwarf Wheats. II. In Hybrid Bulks 1

Intergenotypic competition was studied in populations developed from a hybrid of two wheats ( Triticum aestivum L. em. Thell.), ‘D6301’ (short‐statured) and ‘Ramona 50’ (standard‐height). The F 2 through F 6 generations were grown in the field in two replicate subpopulations without artificial selection. Heights of individual plants, taken from the F 3 , F 4 , F 5 , and F 6 bulk populations, were studied in the same environment. Mean height increased and frequency of short‐statured plants decreased markedly from F 3 to F 6 . Height of 62 random lines isolated from each of the F 2 , F 4 , and F 6 generations showed a decrease in phenotypic variance from F 2 to F 6 . Grain yields of the F 2 , F 4 , and F 6 bulk populations and the means of 62 random lines (in F 3 , F 5 , and F 7 ) indicated a significant increase from the F 4 to F 6 and from the F 5 to F 7 generations. At F 7 the yield of the bulk population equalled the yield of the lower‐yielding parent, Ramona 50. Variance in yield among lines increased from F 3 to F 7 , but extremely short genotypes did not appear in the F 7 . Extremely tall genotypes yielded less than those of intermediate height. Thus, in the bulk population directional and stabilizing selection affected both yield and plant height. Some lines in the F 7 had higher yields than either parent. Thus, in spite of natural selection against the agronomically desirable short plants, the bulk populations were useful as source materials in selection for increased grain yield.