outline the following examples of selective breeding: the introduction of disease resistance to varieties of wheat and rice, inbreeding and hybridisation to produce vigorous, uniform varieties of maize, improving the milk yield of dairy cattle

Selection & Evolution – Cambridge International AS & A Level Biology (9700)

1. Key Terminology (required by the syllabus)

• Gene pool – total set of alleles present in a population.
• Genotype – the genetic constitution of an individual (the alleles it carries).
• Phenotype – observable characteristics produced by the genotype and the environment.
• Fitness – relative ability of an organism to survive and reproduce in its environment.
• Selective pressure – any biotic or abiotic factor that influences survival or reproductive success.
• Allele frequency – proportion of a particular allele among all alleles at a locus in the gene pool.
• Natural selection – differential survival and reproduction that changes allele frequencies.
• Artificial (selective) breeding – intentional mating by humans to obtain desired traits.
• Heterosis (hybrid vigour) – increased performance of a hybrid compared with its parents.
• Estimated Breeding Value (EBV) – statistical prediction of an animal’s genetic merit for a trait.
• Marker‑assisted selection (MAS) – use of DNA markers linked to desirable genes to speed up breeding.
• Genomic selection – prediction of breeding value using genome‑wide SNP data.
• Hardy–Weinberg equilibrium (HWE) – a null model describing allele‑ and genotype‑frequencies in a non‑evolving population.

2. Natural Selection

Natural selection acts on existing variation. The three‑step process is:

1. Variation – individuals differ in genotype and phenotype.
2. Fitness differences – some variants survive or reproduce better under the prevailing selective pressure.
3. Change in allele frequencies – the gene pool shifts toward the favoured alleles.

2.1. Example – Peppered Moth (Biston betularia)

• Industrial soot darkened tree bark → dark (melanic) form had higher camouflage.
• Dark‑allele frequency rose from ≈2 % (pre‑industrial) to >95 % in polluted areas.
• Air‑quality improvement restored the advantage of the light form, reversing the trend.

2.2. Example – Antibiotic Resistance in Bacteria

• Random mutations generate resistant genotypes.
• Antibiotic use creates strong selective pressure favouring resistant bacteria.
• Allele frequency of resistance can increase dramatically within a few generations.

2.3. Quantitative Activity – Calculating Change in Allele Frequency

In a population of 200 beetles, allele A (conferring resistance) has an initial frequency of 0.30. After one generation, 80 % of A carriers survive, whereas only 50 % of a carriers survive. Calculate the new frequency of A.

1. Genotype numbers (Hardy–Weinberg): AA=0.09×200=18, Aa=0.42×200=84, aa=0.49×200=98.
2. Survivors: AA=18×0.80=14.4, Aa=84×0.80=67.2, aa=98×0.50=49.
3. Total surviving alleles = 2(14.4+67.2)+2(49)=302.4.
4. Number of A alleles = 2(14.4)+67.2=96.
5. New frequency of A = 96 / 302.4 ≈ 0.317 (increase from 0.30).

3. Artificial Selection (Selective Breeding)

Artificial selection uses the same genetic principles as natural selection but with explicit human goals (e.g., disease resistance, higher yield, product quality). The general workflow is:

1. Identify the trait of interest and its genetic basis.
2. Source donor material (wild relatives, landraces, elite lines).
3. Apply appropriate breeding technique(s) – cross‑breeding, back‑crossing, inbreeding, hybridisation, MAS, genomic selection.
4. Select and test progeny over several generations.
5. Release the improved variety or line for commercial use.

3.1. Disease‑Resistance Breeding in Wheat and Rice

Wheat – Stem Rust (Puccinia graminis f. sp. tritici)

• Target genes: Sr genes (e.g., Sr31, Sr24, Sr36, Sr50).
• Source of resistance: Wild relatives such as Aegilops tauschii and Triticum monococcum.
• Method:

  1. Cross donor (carrying Sr) with an elite cultivar.
  2. Back‑cross repeatedly to recover the elite background while retaining the Sr allele.
  3. Use MAS to track the Sr allele each generation.

• Outcome: Varieties such as ‘Kavkaz’, ‘Mace’ and ‘Mikado’ possess durable resistance to the Ug99 race group.

Rice – Blast Disease (Magnaporthe oryzae)

• Target genes: Pi genes (Pi9, Pi2, Pi1, Pi54 …).
• Source of resistance: Landraces and the wild species Oryza rufipogon.
• Method – Gene pyramiding:

  1. Identify several Pi genes that give complementary resistance spectra.
  2. Cross donor lines each carrying a different Pi gene.
  3. Apply MAS to combine (pyramid) three or more Pi genes into a single cultivar.

• Outcome: Cultivars such as ‘IR64‑Pi9’ and ‘Nipponbare‑Pi2‑Pi9‑Pi54’ display broad‑spectrum, long‑lasting blast resistance.

3.2. Inbreeding and Hybridisation in Maize (Zea mays)

Maize breeding exploits the contrast between inbreeding (to obtain uniform, homozygous lines) and hybridisation (to obtain heterosis).

1. Inbreeding (self‑pollination or sib‑mating)

   • 5–6 generations of selfing produce inbred lines >99 % homozygous.
   • Uniformity allows precise measurement of agronomic traits.
   • Inbreeding depression (reduced vigor) is an inevitable side‑effect.

2. Hybridisation (crossing two inbred lines)

   • Cross a maternal line (e.g., SSS) with a paternal line (e.g., RRS) → F₁ hybrid.
   • F₁ shows heterosis: higher grain yield, greater stress tolerance, uniform plant height.
   • Hybrid seed is produced commercially by a detasselling system and sold as “single‑seed‑type” (SST) seed.

Typical Results

• Hybrid maize yields are 15–25 % greater than the best open‑pollinated varieties.
• Hybrid vigour is most evident in quantitative (polygenic) traits such as yield and drought tolerance.

3.3. Improving Milk Yield in Dairy Cattle

• Target traits: Daily milk volume, fat % and protein %, lactation length, udder health.
• Measurement: Automated milking systems record individual yields; test‑day milk composition is analysed chemically.
• Selection tools:

  1. Phenotypic selection – choose bulls and cows with the highest recorded performance.
  2. Estimated Breeding Values (EBVs) – statistical models combine pedigree, performance and environmental data to predict genetic merit.
  3. Genomic selection – DNA chips screen thousands of SNP markers; the genomic EBV (gEBV) predicts performance before the animal has produced milk.

• Result: Modern Holstein‑Friesian cows routinely produce >10 000 L of milk per lactation – roughly three times the yield of early‑20th‑century dairy breeds.

4. Speciation

Speciation is the process by which reproductive isolation leads to the formation of new species.

• Allopatric speciation – geographic isolation (mountains, rivers) splits a population; divergent evolution and genetic drift eventually produce reproductive barriers.
• Sympatric speciation – new species arise within the same area, often through polyploidy (common in plants) or strong disruptive selection (e.g., host‑plant shifts in insects).
• Polyploidy – whole‑genome duplication creates instant reproductive isolation, especially in wheat (hexaploid Triticum aestivum).

Diagram suggestion

Side‑by‑side schematic showing (i) a population split by a barrier → allopatric speciation, and (ii) a single population undergoing polyploidy → sympatric speciation.

5. Population‑Genetic Forces

Besides natural selection, four additional forces can change allele frequencies.

5.1. Hardy–Weinberg Equilibrium (HWE)

HWE provides the null expectation for a non‑evolving population:

• Allele frequencies: p + q = 1.
• Genotype frequencies: p² (AA), 2pq (Aa), q² (aa).
• Assumptions: large population, random mating, no selection, mutation, migration, or drift.

Any deviation indicates that one or more evolutionary forces are acting.

6. Evidence for Evolution

• Fossil record – shows chronological succession of forms and transitional fossils (e.g., Tiktaalik).
• Comparative anatomy – homologous structures (tetrapod fore‑limbs) reveal common ancestry; analogous structures (bird wings vs. insect wings) illustrate convergent evolution.
• Embryology – early developmental stages of vertebrates share pharyngeal arches and tail buds.
• Molecular phylogenetics – DNA and protein sequence comparisons generate phylogenetic trees that match the fossil and anatomical data.
• Biogeography – distribution patterns (e.g., marsupials in Australia, lemurs in Madagascar) reflect historical isolation and speciation.

7. Links to Topic 18 – Biodiversity & Conservation

• Human‑imposed selective pressures (intensive agriculture, pesticide use) can reduce genetic diversity in crops and wild relatives.
• Loss of habitat and climate change alter natural selective pressures, accelerating extinction risk.
• Conservation strategies (gene banks, in‑situ reserves) aim to preserve the gene pool needed for future breeding and adaptation.
• Examples: the use of wild wheat relatives from the Fertile Crescent to introduce new rust‑resistance genes; maintaining heirloom rice varieties for Pi‑gene diversity.

8. Genetic Technology – Modern Applications of Selection Principles

• DNA fingerprinting & microsatellites – assess genetic diversity, parentage, and traceability in breeding programmes.
• CRISPR/Cas9 genome editing – precise insertion or knockout of target genes (e.g., editing the wheat TaEDR1 gene to confer powdery‑mildew resistance).
• Genetically modified (GM) crops – transgenes for insect resistance (Bt‑cotton) or herbicide tolerance (Roundup‑Ready soy) illustrate artificial selection at the molecular level.
• Genomic selection platforms – combine high‑density SNP chips with statistical models to predict breeding values early in the life cycle.

9. Comparison of Breeding Strategies (Key Examples)

10. Cross‑Topic Links (Syllabus Integration)

• DNA → Gene → Trait → Selection: Sr and Pi resistance genes are DNA sequences that encode R‑proteins; these proteins give the phenotype (disease resistance) that artificial selection exploits.
• Biochemical pathways: R‑gene mediated resistance involves MAP‑kinase cascades and production of phytoalexins.
• Organisms & their environment: Farming practices, climate, and pathogen pressure constitute selective pressures that shape both natural and artificial evolution.
• Observation & experiment: Breeding programmes rely on controlled experiments (field trials, inoculation tests, test‑crosses) to evaluate fitness of selected genotypes.
• Population genetics tools: HWE calculations, selection coefficients, and drift simulations are used in A‑Level exam questions.

11. Suggested Diagrams for Classroom Use

1. Flowchart of wheat stem‑rust resistance breeding – from gene discovery in a wild relative to commercial release.
2. Maize breeding cycle – inbreeding → test‑cross → hybrid seed production (detasselling illustration).
3. Schematic of dairy cattle selection – pedigree tree, EBV calculation, integration of genomic markers.
4. Hardy–Weinberg diagram showing expected vs. observed genotype frequencies when selection is applied.
5. Allopatric vs. sympatric speciation diagram (including polyploidy in wheat).