Practice question

Natural selection predominates during early generations of bulk method, making artificial selection ineffective because heterozygosity buffers phenotypic expression and heritability of yield is low. Plants in F2-F4 still segregate, genotype-by-environment interaction confounds single plant performance, and competitive ability may not correlate with grain yield in pure stands. Breeders therefore postpone intensive selection until F5-F7, when inbreeding coefficient approaches 0.9375 and plants become largely homozygous. At this stage environmental fitness differences have already filtered maladapted types, family uniformity improves, and additive genetic variance for agronomic traits becomes reliably observable without dominance noise. Selecting individual plants or spikes in advanced generations yields lines that breed true, have stable performance across locations, and retain adaptation achieved via earlier natural selection. Early artificial culling would defeat the purpose of allowing nature to shape population structure and would risk eliminating valuable recombinants. Heritability estimates increase from F2 to F6 because dominance variance declines, additive variance remains, and environmental variance can be reduced by replicated plot testing and spatial adjustment, improving selection precision in advanced generations.