17.2.7—Selective breeding examples

Selective breeding examples show the same causal pattern in different contexts: humans choose a heritable target phenotype, reproduce selected organisms, and obtain a desired result over generations. The target is usually a useful product or function, but the outcome can bring biological trade-offs.

• Wheat and rice disease resistance: breeding for resistance to fungal or bacterial disease can reduce crop damage and increase yield. In wheat, introducing a resistance allele from another wheat species can take many generations and collaboration between breeders and researchers.
• Maize hybridisation: heavy inbreeding can increase homozygosity and the chance that harmful recessive alleles combine, causing inbreeding depression with weaker, less vigorous plants. Outbreeding can increase heterozygosity and hybrid vigour, giving taller, healthier plants and greater yield; farmers may also need uniform height and ripening time.
• Cattle milk yield: selecting high-yield females and breeding with high-yield lines can increase milk yield and economic benefit. However, selecting strongly for a production trait may neglect survival and health; the SME notes greater risk of ailments such as mastitis, milk fever and lameness in selectively bred cattle.

For an example, identify the human target, the selected heritable feature, the intended production or functional result, and the trade-off. Keep the direction of the claim narrow: disease resistance can protect yield, maize outbreeding can improve vigour and uniformity under the stated breeding design, and high milk yield can carry health costs. These are context-specific outcomes, not guarantees for every selectively bred population.

Target → cross-generation result → boundary:

• Crop disease resistance → less disease damage and higher yield → the resistance programme may take many generations.
• Maize hybridisation → heterozygosity, vigour and useful uniformity → breeding design must balance variation, homozygosity and harmful recessive alleles.
• Cattle milk yield → economic production gain → extreme focus on one production trait can neglect health and survival.

Artificial selection is human-directed and product-focused, unlike natural selection, where the environment determines which traits are better suited. This card uses only the stated SME examples; it does not add numerical yield changes, breed-specific claims or a general prediction that selective breeding always improves welfare.