describe the principles of selective breeding (artificial selection)

Selection and Evolution (Cambridge International AS & A Level Biology – Topic 17)

1. Introduction to Selection

• Selection – a change in the frequency of traits (or the underlying alleles) in a population over generations.
• Two broad categories:
  • Natural selection – driven by environmental pressures that affect survival and reproductive success.
  • Artificial (selective) breeding – driven by human choice to enhance traits of economic or aesthetic value.

2. Genetic Basis of Variation

Understanding selection requires a clear link between DNA, genotype and phenotype.

• DNA – the molecule of heredity; carries genes that encode proteins and regulatory elements.
• Alleles – alternative forms of a gene; diploid organisms carry two alleles per locus.
• Genotype → Phenotype – the genotype determines the biochemical pathways that give rise to observable traits.
• Mutation – the ultimate source of new alleles.
  • Types: point mutation, insertion, deletion, duplication, chromosomal rearrangement.
  • Typical mutation rate in eukaryotes ≈ 10⁻⁸ mutations per base per generation.
  • Example: a single‑base substitution in the β‑globin gene (HBB) creates the sickle‑cell allele (HbS).
• Phenotypic variance (V_P) – the total variation in a trait within a population.
  • V_P = V_G + V_E where V_G is genetic variance and V_E is environmental variance.
• Heritability (h²) – proportion of phenotypic variance that is genetic: $$h^{2}= \frac{V_{G}}{V_{P}}$$

3. Natural Selection

3.1 Mechanism (variation → differential fitness → change in allele frequency)

1. Variation – individuals differ because of genetic differences (mutation, recombination).
2. Differential survival & reproduction (fitness) – traits that increase an organism’s ability to survive and reproduce are favoured.
3. Change in allele frequency – alleles linked to higher fitness increase in the next generation.

3.2 Types of Natural Selection

• Directional selection – favours one extreme phenotype (e.g., larger beaks in finches when hard seeds dominate).
• Stabilising selection – favours intermediate phenotypes, reducing extremes (e.g., optimal clutch size in many birds).
• Disruptive selection – favours both extremes, reducing the intermediate form (e.g., colour morphs of the peppered moth in a heterogeneous environment).

3.3 Quantitative description – population genetics

The change in allele frequency (Δp) caused by selection can be expressed as:

$$\Delta p = \frac{p\,q\,s}{\bar{w}}$$

• p = frequency of the favoured allele, q = 1 – p.
• s = selection coefficient (the proportional reduction in fitness of the less‑favoured genotype; s = 0 means no selection, s = 1 means complete elimination).
• \bar{w} = mean fitness of the whole population (average of the fitness values of all genotypes).

3.4 Worked example – directional selection on beak size

In a finch population the allele B (large beak) has frequency p = 0.40; the alternative allele b (small beak) has q = 0.60. Fitness values are:

Genotype	Frequency (before selection)	Fitness (w)
BB	p² = 0.16	1.00
Bb	2pq = 0.48	1 – s = 0.90
bb	q² = 0.36	1 – 2s = 0.80

Assume s = 0.10. Mean fitness:

$$\bar{w}= (0.16)(1.00)+(0.48)(0.90)+(0.36)(0.80)=0.16+0.432+0.288=0.880$$

Change in allele frequency:

$$\Delta p = \frac{(0.40)(0.60)(0.10)}{0.880}= \frac{0.024}{0.880}=0.0273$$

After one generation the frequency of B becomes p′ = p + Δp = 0.427, illustrating how a modest selection coefficient can shift allele frequencies noticeably in a few generations.

3.5 Real‑world examples

• Industrial melanism in the peppered moth (Biston betularia).
• Antibiotic resistance in bacteria – rapid natural selection under drug pressure.
• Beak‑size evolution in Galápagos finches during drought periods.

4. Other Evolutionary Mechanisms

Mechanism	Cause	Effect on allele frequencies	Typical outcome
Genetic drift	Random sampling error in small populations (founder effect, bottleneck)	Allele frequencies change unpredictably; can lead to fixation or loss	Reduced genetic variation; may oppose selection
Gene flow (migration)	Movement of individuals or gametes between populations	Alleles are added to or removed from a population	Homogenises populations; can introduce new variation
Mutation	Errors in DNA replication or damage	Creates new alleles (usually at low frequency)	Source of raw genetic material for evolution
Speciation	Reproductive isolation (allopatric, sympatric, peripatric, parapatric)	Populations diverge in allele frequencies until interbreeding fails	Formation of new species

4.1 Genetic Drift – numerical illustration

Consider a population reduced to 10 individuals (5 diploid organisms) that originally carried allele A at frequency 0.5 (5 copies of A, 5 copies of a). By chance only 2 copies of A survive the bottleneck. New frequency:

$$p' = \frac{2}{10}=0.20$$

The allele has dropped from 0.5 to 0.20 purely by chance – a classic bottleneck effect.

4.2 Gene Flow – diagram description & migration rate

Imagine two neighbouring populations, Pop 1 (size N₁) and Pop 2 (size N₂). Each generation a proportion m of individuals in Pop 1 are replaced by migrants from Pop 2.

• If m = 0.05, 5 % of Pop 1’s gene pool each generation originates from Pop 2.
• The change in allele frequency in Pop 1 can be approximated by: $$\Delta p = m(p_{2} - p_{1})$$ where p₁ and p₂ are the allele frequencies in the two populations.

Figure suggestion: two circles representing the populations with arrows indicating migrants and the value of m noted.

4.3 Speciation – reproductive isolation mechanisms

• Allopatric speciation – geographic separation; e.g., Darwin’s finches on different Galápagos islands.
• Sympatric speciation – reproductive isolation without physical barrier; e.g., cichlid fish in African Rift lakes diversifying by colour‑based mate choice.
• Peripatric speciation – a small peripheral population becomes isolated; e.g., island dwarf elephants derived from mainland ancestors.
• Parapatric speciation – adjacent populations experience divergent selection across a gradient; e.g., grass species that adapt to wet vs. dry soils along a riverbank.

5. Artificial Selection (Selective Breeding)

5.1 Core Principles

• Humans deliberately choose parents that exhibit a desired phenotype.
• Because the trait is heritable, favourable alleles increase in frequency in the next generation.
• The quantitative response is predicted by the breeder’s equation: $$R = h^{2}\,S$$ where R = response to selection (change in mean phenotype of offspring), S = selection differential (difference between the mean phenotype of selected parents and the whole population).

5.2 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, estimate heritability (h²) and calculate the current population mean.
3. Select parent individuals that best express the target trait(s).
4. Control mating – arrange crosses to combine favourable alleles, monitor inbreeding coefficients, and maintain an adequate effective population size (N_e).
5. Produce and evaluate offspring – record trait values, compute S and R, and decide whether to continue, modify, or stop the programme.
6. Repeat the cycle over successive generations to achieve cumulative improvement.

5.3 Types of Artificial Selection

• Directional selection – e.g., selecting for larger fruit size in tomatoes.
• Stabilising selection – e.g., maintaining uniform egg size in commercial poultry.
• Disruptive selection – occasionally used in ornamental horticulture to develop two contrasting colour morphs.

5.4 Case Studies (Artificial)

• Dairy cattle – milk yield: Heritability of milk yield ≈ 0.30. Over 50 years, selective breeding raised average yield by > 200 %.
• Maize (Zea mays) – Green Revolution: Hybridisation and selection for short stalks, uniform kernels and disease resistance tripled yields compared with traditional landraces.
• Domestic dogs – breed diversity: Selection for morphology and behaviour has produced > 400 breeds; some traits (e.g., brachycephaly) persist despite health drawbacks, illustrating trade‑offs.

6. Comparison: Natural vs. Artificial Selection

Aspect	Natural Selection	Artificial Selection
Driving force	Environmental pressures (predation, climate, competition)	Human preferences, economic or aesthetic goals
Speed of change	Usually gradual, over many generations	Can be rapid when a strong selection differential is applied
Genetic diversity	Often maintained or increased by varied selective pressures	May decline due to repeated use of a few elite individuals (risk of inbreeding depression)
Scope of traits	Traits that enhance survival and reproductive success	Any heritable trait valued by humans, even if it reduces fitness in the wild
Typical examples	Camouflage, antibiotic resistance, beak size in finches	Dog breeds, high‑yield wheat, dairy cattle with high milk output

7. Integration of Concepts

• Both natural and artificial selection act on genetic variation generated by mutation and reshaped by gene flow and genetic drift.
• Population‑genetic equations (e.g., Δp = p q s/ \bar{w}) describe natural selection; the breeder’s equation (R = h² S) is a practical tool for artificial selection.
• A solid grasp of the DNA → genotype → phenotype pathway is essential for predicting responses to any selective pressure.

8. Advantages and Limitations of Artificial Selection

• Advantages
  • Accelerates improvement of economically important traits.
  • Enables precise control over the genetic composition of populations.
  • Facilitates development of varieties adapted to specific environments (e.g., drought‑tolerant crops).
• Limitations
  • Reduced genetic diversity can increase vulnerability to disease or environmental change.
  • Unintended health problems may arise (e.g., reduced fertility, skeletal disorders).
  • Accurate measurement of traits and reliable estimates of heritability are required.

9. Summary

Selection—whether natural, driven by the environment, or artificial, driven by human choice—acts on genetic variation that originates from DNA mutations. Natural selection changes allele frequencies according to fitness differences and can be described with population‑genetic formulas. Artificial selection exploits the breeder’s equation to predict phenotypic improvement. Mastery of these concepts, together with an understanding of genetic drift, gene flow, mutation, and speciation, provides a complete picture of evolution as required by the Cambridge A‑Level syllabus.