Mass selection is more effective in:
Effectiveness hinges on population structure and genetic variance. Cross-pollinated species such as maize, pearl millet, and sunflower maintain high heterozygosity, large additive genetic variance, and expose extensive segregation each generation due to allogamy and random mating mediated by wind or insects. Mass selection in such highly variable base populations can shift allele frequencies rapidly because selected individuals outcross, recombining favorable alleles through pollen mixing. In self-pollinated wheat or rice, landraces already consist of homozygous pure lines; selecting best phenotypes merely picks best existing line without creating new recombination, and variability rapidly depletes after few cycles. Moreover, self-pollinated crops suffer from inbreeding depression less relevant, but mass selection cannot break linkage blocks efficiently. Recurrent phenotypic mass selection therefore became foundation for open-pollinated variety improvement, leveraging pollination biology to remix genes each cycle and sustain response. Open-pollinated crops have high outcrossing rate maintained by self-incompatibility, male sterility, or protandry, ensuring gene flow each cycle and providing continuous recombination to assemble favorable epistatic combinations that mass selection can exploit effectively for population improvement.
Ref: Frey KJ. Iowa State – Mass selection in open-pollinated crops. Hallauer & Miranda Quantitative Genetics in Maize