Practice question

Bulk breeding creates a heterogeneous population after hybridization and allows natural selective forces to act over multiple selfing generations without intensive artificial culling. Seeds from F2-F5 are harvested in bulk and sown without pedigree tracking, exposing the population to prevailing biotic and abiotic stresses in target environment. Plants susceptible to disease, drought, lodging, or winter kill fail to contribute seed, while better adapted, disease-tolerant types increase in frequency through differential survival and reproductive success. This mimics Darwinian selection acting on naturally occurring variation, gradually eliminating poorly adapted alleles while retaining broad variability. Human intervention remains minimal, conserving labor and permitting large populations to evolve under local conditions. Artificial selection is imposed only in later generations F6-F7 when homozygosity is high, to extract superior pure lines from naturally enriched pool that have already survived environmental filters. Population genetics models show allele frequency change under natural selection equals s pq, where selection coefficient s from local stress eliminates maladapted genotypes while preserving balanced polymorphisms important for stability.