explain that natural selection occurs because populations have the capacity to produce many offspring that compete for resources; in the ‘struggle for existence’, individuals that are best adapted are most likely to survive to reproduce and pass on t

Natural and Artificial Selection (Cambridge International AS & A Level Biology 9700 – Topic 17)

Learning Objective

Explain that natural selection occurs because populations can produce many offspring that compete for limited resources; in the “struggle for existence”, individuals best adapted to their environment are most likely to survive, reproduce and pass their alleles to the next generation.

1. Sources of Genetic Variation

• Mutation – random changes in DNA (point mutations, insertions, deletions, chromosomal rearrangements such as translocations, inversions).
• Genetic recombination – independent assortment of chromosomes and crossing‑over during meiosis generate new allele combinations.
• Gene flow (migration) – movement of individuals or gametes between populations introduces alleles that were absent locally.

2. Types of Natural Selection

3. Genetic Drift

• Definition: Random changes in allele frequencies that are most pronounced in small populations.
• Bottleneck effect – a sharp, temporary reduction in population size (e.g., a natural disaster) that reduces genetic diversity.
• Founder effect – a new population established by a few individuals carrying only a subset of the original genetic variation (e.g., colonisation of an isolated island).

4. Gene Flow

• Definition: Transfer of alleles between populations by migration of individuals or gametes.
• Quantitative illustration: If p is the allele frequency in the resident population, p_m the frequency in migrants, and m the proportion of migrants each generation, the new allele frequency after migration is
  

  p′ = (1 – m) p + m p_m

• Gene flow can introduce advantageous alleles or dilute locally‑adapted ones, counteracting genetic drift.

5. Speciation

Formation of new species when reproductive isolation prevents gene flow between diverging populations.

• Allopatric speciation – geographic separation (e.g., island colonisation, mountain ranges).
• Sympatric speciation – reproductive isolation without physical barriers (e.g., polyploidy in flowering plants, host‑shift in phytophagous insects).

6. Evidence for Evolution

7. Natural Selection – Process Overview

1. Genetic variation exists in the population (see Section 1).
2. Populations produce more offspring than can survive (over‑production).
3. Individuals enter a struggle for existence – competition for food, shelter, mates, etc.
4. Environmental pressures (predation, climate, disease, competition) act as selective agents.
5. Individuals possessing advantageous traits have higher survival and reproductive success.
6. These individuals pass their alleles to the next generation, increasing the frequency of favourable alleles.

8. Quantitative Treatment of Selection

Hardy–Weinberg baseline

In a large, randomly mating population with no evolutionary forces:

\$p^{2}+2pq+q^{2}=1\$

where p = frequency of the dominant allele, q = frequency of the recessive allele.

Relative fitness (w) and selection coefficient (s)

• Relative fitness of a genotype: w = 1 – s (0 < s < 1).
• For a favourable allele A (genotypes AA, Aa, aa) the change in allele frequency per generation can be expressed as:
  

  \$\Delta p = \frac{p(1-p)s}{\bar w}\$

  where \(\bar w\) is the mean fitness of the population.

Example calculation – directional selection

Assume genotype fitnesses: w_AA=1.0, w_Aa=0.9, w_aa=0.7. With an initial p = 0.4:

1. Hardy–Weinberg genotype frequencies:
   

   AA = p² = 0.16, Aa = 2pq = 0.48, aa = q² = 0.36.

2. Weighted frequencies (multiply by fitness):
   

   AA = 0.16 × 1.0 = 0.16
   

   Aa = 0.48 × 0.9 = 0.432
   

   aa = 0.36 × 0.7 = 0.252.

3. Mean fitness:
   

   \(\bar w = 0.16 + 0.432 + 0.252 = 0.844\).

4. New allele frequency:
   

   \(p' = \dfrac{p^{2}w{AA}+pq w{Aa}}{\bar w}

   = \dfrac{0.16 + 0.24}{0.844}

   \approx 0.476.\)

9. Illustrative Examples of Natural Selection

• Industrial melanism in the peppered moth – directional selection driven by soot‑covered trees.
• Antibiotic resistance in bacteria – rapid directional selection under drug pressure.
• Beak‑size changes in Darwin’s finches during drought – shift between directional and stabilising selection.
• Sickle‑cell heterozygote advantage – balancing selection in malaria‑endemic regions.
• Camouflage colouration in the peppered moth and snowshoe hare – seasonal disruptive selection.

10. Artificial Selection

Humans deliberately choose breeding individuals based on traits of interest, applying the same principles as natural selection but with a different selective agent.

• Selective breeding of dogs for size, temperament, coat colour.
• Crop improvement – high‑yield wheat, disease‑resistant rice, dwarf maize.
• Laboratory selection of fruit flies for longer lifespan or altered behaviour.

11. Comparison of Natural and Artificial Selection

12. Summary

Natural selection is a cornerstone of evolution. It requires:

• Genetic variation generated by mutation (including chromosomal rearrangements), recombination and gene flow.
• Over‑production of offspring leading to competition for limited resources.
• Environmental pressures that create a “struggle for existence”.
• Differential survival and reproductive success of the best‑adapted individuals.
• Resulting changes in allele frequencies, which can be described mathematically (Hardy–Weinberg equilibrium, selection coefficient, relative fitness).
• Artificial selection applies the same mechanisms but replaces natural selective agents with human choice, allowing rapid development of desired traits.