17.2.6—Selective breeding principles

Artificial selection, or selective breeding, is directional human choice: people select organisms with a desirable phenotype and breed them so that the trait becomes more common or more strongly expressed over many generations. It depends on heritable variation, even when the alleles underlying the phenotype are not fully known.

Use this repeated chain:

1. Identify phenotypic variation in the population.
2. Select individuals showing the desired phenotype.
3. Choose a suitable breeding partner showing the desired phenotype; the SME method cautions against closely related partners.
4. Breed the selected individuals.
5. Let offspring mature, test the target phenotype, and select the strongest performers for further breeding.
6. Repeat the selection and breeding process over generations, while recognising biological limits to how extreme a trait can become.

• Artificial selection: humans set the breeding target and repeatedly choose parents with the desired phenotype.
• Natural selection: environmental pressures affect which phenotypes survive and reproduce; there is no deliberate human breeding target.
• In both cases, a lasting population change requires heritable differences and differential reproductive success, but the selecting agent and direction of selection differ.

Selecting for one desired phenotype can also increase genetically linked traits, some of which may harm health. Avoiding closely related breeding partners is an explicit control in the SME method; this card does not claim a fixed amount of genetic-diversity loss or a guaranteed outcome. Artificial selection is therefore a trade-off between the target trait, biological limits and possible linked effects, not a promise of unlimited improvement.

The selection acts on observed phenotypes, not necessarily on a known genotype. This card covers the general process and its contrast with natural selection; it does not add particular breed examples, Hardy–Weinberg calculations or unsupported genetic detail.