describe the principles of selective breeding (artificial selection)

Natural and Artificial Selection

1. Introduction

Selection is the process by which certain traits become more common in a population over generations. In biology two main types are recognised:

• Natural selection – driven by environmental pressures.
• Artificial (selective) breeding – driven by human choice.

2. Principles of Selective Breeding (Artificial Selection)

Selective breeding is the intentional mating of individuals that possess desirable traits in order to increase the frequency of those traits in subsequent generations.

2.1 Key Concepts

• Phenotype – the observable characteristics of an organism.
• Genotype – the genetic makeup that underlies the phenotype.
• Heritability (h²) – the proportion of phenotypic variation that is genetic. It is expressed as a value between 0 and 1.
• Selection differential (S) – the difference between the mean phenotype of the selected parents and the mean phenotype of the whole population.
• Response to selection (R) – the change in the mean phenotype of the offspring generation. It is given by the breeder’s equation:

  \$R = h^{2}\,S\$

2.2 Steps in a Selective Breeding Programme

1. Define the breeding objective – e.g., higher milk yield, disease resistance, specific flower colour.
2. Assess the existing population – measure phenotypic variation and estimate heritability.
3. Select parent individuals that best express the desired trait(s).
4. Control mating – arrange crosses to combine favourable alleles and avoid inbreeding depression.
5. Evaluate offspring – record trait values, calculate \$S\$ and \$R\$, and decide whether to continue, modify, or terminate the programme.
6. Repeat the cycle over multiple generations to achieve cumulative improvement.

2.3 Types of Artificial Selection

• Directional selection – favouring extreme values of a trait (e.g., larger fruit size).
• Stabilising selection – maintaining an intermediate optimum (e.g., uniform egg size in poultry).
• Disruptive selection – favouring both extremes and reducing the intermediate form (rare in breeding but can occur in ornamental plants).

3. Comparison: Natural vs. Artificial Selection

4. Case Studies

4.1 Dairy Cattle – Increasing Milk Yield

Through successive generations, breeders selected cows with the highest recorded milk output. Heritability of milk yield is relatively high (\$h^{2}\approx0.30\$), allowing a substantial response to selection. Over 50 years, average milk yield increased by more than 200 %.

4.2 Corn (Maize) – The Green Revolution

Hybridisation and selection for traits such as short stalks, uniform kernels, and disease resistance produced varieties that could support higher planting densities and gave yields up to three times those of traditional landraces.

4.3 Domestic Dogs – Breed Diversity

Selection for behavioural and morphological traits has produced over 400 recognised breeds. Many traits (e.g., brachycephalic skulls) are maintained despite associated health problems, illustrating a trade‑off between human preference and animal welfare.

5. Advantages and Limitations of Artificial Selection

• Advantages

  • Accelerates improvement of economically important traits.
  • Allows control over genetic composition of populations.
  • Facilitates the development of new varieties adapted to specific environments.

• Limitations

  • Reduced genetic diversity can increase susceptibility to disease.
  • Unintended consequences (e.g., reduced fertility, health issues).
  • Requires accurate measurement of traits and knowledge of heritability.

6. Summary

Artificial selection is a powerful tool that harnesses the principles of genetics to shape organisms for human benefit. By understanding the relationship between genotype, phenotype, heritability, and selection differential, breeders can predict and direct evolutionary change. However, responsible breeding must balance desired improvements with the maintenance of genetic health.