Natural and Artificial Selection – A‑Level Biology (9700)
Learning Objective (AO1)
Explain why natural selection operates: populations produce many offspring that compete for limited resources; in the “struggle for existence”, individuals that are best adapted are most likely to survive, reproduce and pass their alleles to the next generation.
1. Core Concepts of Selection & Evolution
- Variation – Heritable differences between individuals arise from:
- DNA mutations (point, frameshift, chromosomal rearrangements)
- Genetic recombination (independent assortment, crossing‑over during meiosis)
- Gene flow (migration, hybridisation between populations)
- Over‑production of offspring – Most species produce far more progeny than can be supported by the environment.
- Struggle for existence – Competition for food, water, shelter, mates, and avoidance of predators or disease.
- Survival of the fittest – “Fit” = individuals whose phenotypic traits increase the probability of surviving to reproductive age.
- Reproductive success – Fit individuals leave more offspring, thereby increasing the frequency of the advantageous alleles in the gene pool.
2. Step‑by‑Step Process of Natural Selection
- Source of genetic variation – Mutations, recombination and gene flow create a pool of different alleles.
- Population growth beyond resource capacity – More individuals are born than can be sustained.
- Struggle for existence – Individuals compete for limited resources; some die or fail to reproduce.
- Differential survival & reproduction – Those possessing advantageous traits survive longer and produce more viable offspring.
- Change in allele frequencies – The genotypes that contribute the most offspring increase in frequency in the next generation.
- Adaptation over many generations – The population becomes better suited to its environment.
Note: Step 5 can be quantified using the allele‑frequency equation (see Section 3). The numerator represents the total contribution of each genotype to the next generation, weighted by its relative fitness, while the denominator ( \(\bar w\) ) normalises the values to give a proportion.
3. Quantitative Modelling of Selection
3.1 Allele‑frequency change under selection
For a diploid locus with two alleles, A and a, the frequency of A after one generation of selection is:
\[
p' = \frac{p^{2}w{AA}+pq\,w{Aa}}{\bar w}
\]
where:
- p = initial frequency of allele A
- q = 1-p = frequency of allele a
- w{AA}, w{Aa}, w_{aa} = relative fitnesses of the three genotypes
- \bar w = p^{2}w{AA}+2pq\,w{Aa}+q^{2}w_{aa} = mean fitness of the population
3.2 Selection coefficient (s) and dominance coefficient (h)
Fitness values are often expressed as:
\[
\begin{aligned}
w_{AA} &= 1 \\
w_{Aa} &= 1 - h\,s \\
w_{aa} &= 1 - s
\end{aligned}
\]
where s (0 < s ≤ 1) measures the disadvantage of the homozygous recessive genotype, and h (0 ≤ h ≤ 1) indicates the degree of dominance of the allele A.
3.3 Hardy–Weinberg equilibrium (baseline for no selection)
\[
p^{2}+2pq+q^{2}=1
\]
Deviations from this ratio indicate that forces such as selection, mutation, migration or genetic drift are acting.
3.4 Worked Example – Selection on a beetle population
- Initial frequencies: p = 0.60, q = 0.40.
- Fitnesses: w{AA}=1.00, w{Aa}=0.90, w_{aa}=0.60.
- Mean fitness:
\[
\bar w = (0.60)^{2}(1.00)+2(0.60)(0.40)(0.90)+(0.40)^{2}(0.60)=0.84
\]
- New allele frequency:
\[
p' = \frac{(0.60)^{2}(1.00)+(0.60)(0.40)(0.90)}{0.84}=0.66
\]
The advantageous A allele has increased from 0.60 to 0.66 in one generation.
4. Illustrative Natural‑Selection Case Studies
- Industrial melanism in the peppered moth (Biston betularia) – Dark (melanic) forms increased in polluted areas because they were less visible to birds.
- Antibiotic resistance in bacteria – Over‑use of antibiotics creates a strong selective pressure; resistant mutants survive and proliferate.
- Darwin’s finches (Galápagos) – Beak‑size variation correlates with seed availability; drought years favour larger beaks.
- Sickle‑cell trait in humans – Heterozygotes (HbAS) have a selective advantage in malaria‑endemic regions.
5. Artificial Selection
Humans deliberately choose individuals with desirable traits to breed, thereby applying a directed selection pressure.
- Selection agent: breeder, farmer, or researcher.
- Typical outcomes: rapid fixation of traits, often at the cost of reduced genetic diversity.
- Examples: domestic dogs, high‑yield wheat varieties, laboratory Drosophila lines selected for eye colour.
6. Natural vs. Artificial Selection – Comparison
| Aspect | Natural Selection | Artificial Selection |
|---|
| Driving force | Environmental pressures (predation, climate, competition, disease) | Human preferences, economic or research goals |
| Selection agent | Survival & reproductive success in nature | Human breeders or scientists |
| Time scale | Typically long (thousands–millions of years) | Often short (a few generations to decades) |
| Genetic diversity | Maintained by mutation, gene flow, and balancing selection | Can be severely reduced by strong, directional selection |
| Typical outcomes | Adaptations that increase fitness in a specific environment | Traits that may be maladaptive in the wild (e.g., exaggerated size, loss of defence mechanisms) |
7. Links to Other A‑Level Topics
- Inheritance (Topic 15) – Genotype ↔ phenotype relationships, dominance, co‑dominance, and polygenic traits explain how selected traits are transmitted.
- Genetic Technology (Topic 18) – PCR, gel electrophoresis and CRISPR are used to identify alleles under selection and to create novel variants.
- Classification & Biodiversity (Topic 16) – Phylogenetic trees illustrate evolutionary relationships shaped by natural selection.
- Conservation (Topic 19) – Understanding selection helps predict how populations will respond to habitat loss, climate change, and introduced species.
8. Practical Skills (AO2/AO3)
Practical Tip – Investigating Selection in Drosophila
- Set up two populations: one exposed to a temperature stress, the other kept at optimal temperature.
- Record survival rates and count the number of offspring produced over three generations.
- Analyse data to calculate changes in allele frequency using the equation in Section 3.1.
- Discuss sources of error (e.g., uncontrolled humidity, counting bias) and how they affect the reliability of conclusions.
9. Checklist – Coverage of the Cambridge 9700 Syllabus (Topic 17: Selection & Evolution)
| Syllabus Item | Status |
|---|
| Sources of variation (mutation, recombination, gene flow) | ✓ Detailed |
| Over‑production & competition | ✓ Included |
| Natural selection process (step‑by‑step) | ✓ Presented |
| Mathematical description (fitness, allele‑frequency change, selection coefficient, Hardy–Weinberg) | ✓ Comprehensive |
| Case studies (peppered moth, antibiotic resistance, finches, sickle‑cell) | ✓ Provided |
| Artificial selection (definition, outcomes, examples) | ✓ Covered |
| Comparison natural vs. artificial selection | ✓ Table |
| Links to inheritance, genetic technology, classification, conservation | ✓ Highlighted |
| Practical skills – design, analyse, evaluate a selection experiment | ✓ Drosophila activity |
| Key terminology (allele, genotype, phenotype, fitness, HW equilibrium) | ✓ Used throughout |
10. Key Take‑aways
- Populations produce more offspring than the environment can support, creating a struggle for existence.
- Heritable variation provides the raw material for selection.
- Individuals best suited to their environment survive longer and reproduce more, shifting allele frequencies over generations.
- Natural selection is an unguided, environment‑driven process; artificial selection is a human‑directed analogue.
- Quantitative tools (fitness values, selection coefficients, Hardy–Weinberg) allow predictions of evolutionary change and are essential for exam questions.
Suggested Diagram
Flowchart illustrating the natural‑selection cycle:
Variation → Over‑production → Struggle for existence → Differential survival & reproduction → Change in allele frequency → Adaptation
Include a small inset showing a genotype‑fitness table and a simple phylogenetic tree to link evolution with classification.