explain, with examples, that phenotypic variation is due to genetic factors or environmental factors or a combination of genetic and environmental factors

Variation – Cambridge International AS & A Level Biology (9700)

Learning Objective

Explain, with examples, how phenotypic variation arises from genetic factors, environmental factors, or a combination of both. Extend this understanding to the sources of genetic variation, the role of natural selection, Hardy–Weinberg equilibrium, speciation mechanisms, and the quantitative methods used to analyse variation in populations.

1. Phenotypic Variation

• Phenotype: the observable characteristics of an organism (morphology, physiology, behaviour).
• Phenotypic variation: differences in phenotypes among individuals of the same species.
• Causes:

  • Differences in DNA sequence – genetic variation
  • Differences in the environment experienced during development – environmental variation
  • Interactions between genotype and environment – gene‑environment interaction

2. Genetic Factors – Sources of Genetic Variation

Key Points

• Genetic variation provides the raw material for evolution.
• Mutations create new alleles; recombination and gene flow reshuffle existing variation.
• Allele frequencies can change without selection (genetic drift).

3. Environmental Factors – Phenotypic Plasticity

Key Points

• Environmental variation does not alter the DNA sequence.
• Responses can be reversible (e.g., tanning) or irreversible (e.g., developmental changes).
• Plastic phenotypes may affect fitness and therefore be subject to selection.

4. Gene‑Environment Interactions (G × E)

• Most traits are polygenic; the genotype sets a potential range, the environment determines the actual expression.

Quantitative‑Genetics Concepts

• Heritability (h²) – proportion of phenotypic variance that is genetic.
• Reaction norm – graph of the phenotype expressed by a genotype across a range of environments.
• Example: Plotting plant height of two genotypes (G₁, G₂) against nitrogen level shows parallel or intersecting lines, indicating the nature of G × E.

5. Hardy–Weinberg Equilibrium

• Provides a null model for a non‑evolving population.
• Assumptions:

  • Large (effectively infinite) population size
  • Random mating
  • No mutation
  • No migration (gene flow)
  • No natural selection

• Allele‑frequency equations (single locus, two alleles):

  • p + q = 1 (p = frequency of allele A, q = frequency of allele a)
  • Genotype frequencies: p² (AA), 2pq (Aa), q² (aa)

• Example calculation:

  1. In a population of 200 beetles, 72 are homozygous recessive (aa).
  2. q² = 72/200 = 0.36 → q = √0.36 = 0.60.
  3. p = 1 – q = 0.40.
  4. Expected genotype frequencies: p² = 0.16 (AA), 2pq = 0.48 (Aa), q² = 0.36 (aa).
  5. Compare expected numbers (32 AA, 96 Aa, 72 aa) with observed numbers to test for deviation (χ² test).

• Deviations indicate that one or more Hardy–Weinberg assumptions are not met – i.e., evolution is occurring.

6. Natural Selection – How Variation Is Used

Link to Evolutionary Change

• Selection changes allele frequencies over generations – the core of evolution.
• Other evolutionary forces that modify variation:

  • Genetic drift – random changes, especially in small populations.
  • Gene flow – introduces or removes alleles.
  • Mutation – creates new alleles.

• Speciation often begins with disruptive selection combined with reduced gene flow (e.g., allopatric isolation, founder effect).

7. Speciation Mechanisms

• Allopatric speciation – geographic isolation splits a population; genetic drift and selection act independently (e.g., Galápagos finches on different islands).
• Peripatric speciation – a small peripheral isolate diverges rapidly (founder effect) – classic example: Hawaiian *Drosophila* radiation.
• Sympatric speciation – new species arise within the same geographic area, often via ecological niche partitioning or polyploidy (e.g., cichlid radiations in African rift lakes).
• Reinforcement – selection against hybrid offspring strengthens reproductive isolation (e.g., differences in mating calls of *Gryllus* crickets).

8. Analysing Phenotypic Variation in Populations

1. Calculate basic statistics:

   • Mean (μ) = Σx / N
   • Variance (σ²) = Σ(x − μ)² / (N − 1)
   • Standard deviation (σ) = √σ²

2. Construct and interpret:

   • Frequency tables
   • Histograms (shape indicates normal distribution, skewness, or multimodality)
   • Box‑and‑whisker plots (median, quartiles, outliers)

3. Determine the likely cause of variation:

   • Common‑garden experiments – raise genetically different individuals in the same environment.
   • Twin studies or pedigree analysis – estimate heritability.
   • Reaction‑norm experiments – plot phenotype of the same genotype across environmental gradients.

9. Connections to Other Syllabus Topics

• DNA & the molecule of heredity – mutations alter the DNA sequence, creating new alleles.
• Protein synthesis – altered coding sequences can change enzyme activity, leading to phenotypic effects.
• Inheritance – Mendelian and non‑Mendelian patterns explain how genetic variation is transmitted.
• Population genetics – Hardy–Weinberg equilibrium provides a baseline for detecting selection, drift, or gene flow.

10. Summary Table – Sources of Variation and Their Evolutionary Significance

Glossary

Suggested Diagrams (for classroom use)

• Flowchart: DNA → Gene → Protein → Phenotype → Fitness → Natural Selection → Evolution
• Reaction‑norm graph showing two genotypes across an environmental gradient (parallel vs. intersecting lines).
• Histogram illustrating directional, stabilising, and disruptive selection.
• Hardy–Weinberg diagram with arrows indicating the effects of mutation, gene flow, drift, and selection.
• Map of allopatric vs. sympatric speciation scenarios.