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#selection methods

3 public questions tagged with this topic.

Plant Breeding 3

A disadvantage of mass selection is:

Retaining many parental plants without progeny testing yields genetically heterogeneous population where individual plants differ in maturity, height, and quality, reducing market acceptability and certification standards. Seed lot lacks uniformity required for mechanized harvesting, DUS testing, and uniform product quality demanded by modern industry. Phenotypic selection confounds environment with genotype, so inferior heterozygous plants with favorable micro-environment may be selected, limiting genetic advance, especially for low heritability traits like yield where h² below 0.2. Without controlled pollination, pollen from undesirable off-types contaminates selected individuals, diluting gains through random mating with poor types. Variability also complicates distinctness, uniformity, stability testing for variety release. In self-pollinated crops, mass selection quickly plateaus, and response to selection declines after few cycles because additive variance exhausts without deliberate recombination. Hence breeders shift to pedigree or progeny-based methods for uniformity and precise fixation. Statistical approaches like family-based selection, half-sib progeny testing, and use of augmented designs can partially overcome uniformity issues by incorporating pedigree information and spatial correction, but add complexity and cost compared to simple phenotypic mass selection in pure line crops.

Ref: Allard RW. Chapter 8 – Limitations of mass selection. Poehlman 1995: uniformity issues

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Plant Breeding 3

A major advantage of mass selection is:

Because mass selection avoids intensive inbreeding and retains many selected parents bulked together, resulting population preserves substantial genetic diversity and heterozygosity buffer. Broad base of alleles confers buffering against diverse environmental stresses, disease races, and climatic fluctuations, granting stable performance across years and locations. Unlike pure lines that may fail in off-type environments, mass-selected varieties contain mixture of genotypes complementary in stress tolerance, maintaining yield stability due to population buffering mechanism described by Allard. Historically, landraces developed by farmer mass selection in centers of diversity exhibit wide adaptation and resilience to marginal soils. Method also conserves rare minor genes and polygenic complexes underlying local adaptation. Low cost, no need for controlled pollination, and maintenance of variability make it attractive for low-input traditional systems and for initial improvement before advanced pedigree programs requiring purity. Heterogeneous populations also exhibit compensatory growth where different genotypes fill gaps, maintain canopy closure, reduce weed invasion, and provide insurance against complete crop failure under unpredictable monsoon variability common in traditional farming systems and marginal environments.

Ref: Harlan JR. Crops and Man – landrace adaptation; Ceccarelli S. Participatory breeding for wide adaptation

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Plant Breeding 3

Mass selection is based primarily on:

Mass selection operates directly on visible phenotypic values of individual plants in a population. Breeder walks field, visually scores traits such as plant height, earliness, seed color, or disease-free appearance, and harvests seeds from best looking individuals to constitute next generation bulk. Underlying genotype is not tested through progeny rows, molecular markers, or combining ability assays. Success depends on high heritability where phenotype mirrors additive genetic value, correlation between observable character and breeding value, and low environmental noise. It is simplest breeding method practiced since domestication, requiring no controlled crosses or pedigree books. However, it cannot distinguish homozygous PP from heterozygous Pp if dominance conceals recessive allele, and is confounded by macro-environmental variation across field blocks. Modification with stratification or selection of many individuals and progeny testing improves its genetic gain for low heritability characters. Heritability and selection intensity determine response to mass selection; high heritability traits like plant height respond quickly, whereas low heritability yield traits require stratified mass selection or honeycomb designs to reduce environmental variance and improve accuracy.

Ref: Gardner CO. Mass selection genetics; Acquaah Chapter 10 – Phenotypic selection basis

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