outline the theory of evolution as a process leading to the formation of new species from pre-existing species over time, as a result of changes to gene pools from generation to generation

Evolution – Cambridge International AS & A Level Biology (9700) – Topic 17

1. Definition

Evolution is the change in the genetic composition of a population’s gene pool over successive generations, ultimately leading to the formation of new species from pre‑existing ones.

2. Gene Pool and Genetic Variation

  • Gene pool: the total set of alleles present in all individuals of a population.

  • Allele frequency (p, q, …): proportion of a particular allele among all alleles at a locus.

  • Sources of genetic variation:

    • Mutation – permanent changes in DNA sequence:

      • Point mutations (base‑substitutions)

      • Insertions / deletions (indels)

      • Chromosomal rearrangements (inversions, translocations)

      • Whole‑genome duplication (polyploidy)

      • Causes: replication errors, UV radiation, chemicals, transposable elements

    • Sexual recombination – independent assortment and crossing‑over during meiosis, producing new allele combinations.

    • Gene flow – migration of individuals or gametes between populations.

    • Polyploidy – duplication of the entire set of chromosomes; a major driver of sympatric speciation in plants.

3. Mechanisms that Alter Allele Frequencies

MechanismKey Features (Process)Typical Effect on the Gene Pool
Natural Selection

  • Differential survival & reproductive success.

  • Fitness (w) = expected number of offspring that survive to reproduce.

  • Selection pressures may be biotic (predation, competition) or abiotic (temperature, pH).

  • Types of selection:

    • Directional – favours one extreme phenotype (e.g., larger beaks in finches during drought).

    • Stabilising – favours intermediate phenotypes, reducing extremes (e.g., birth weight in humans).

    • Disruptive – favours both extremes, disadvantaging the intermediate (e.g., seed size in a habitat with two distinct dispersal agents).

  • Artificial selection – intentional breeding for desired traits (e.g., wheat yield).

Increase in frequency of alleles that raise fitness; decrease of deleterious alleles.
Genetic Drift

  • Random sampling of gametes each generation.

  • Bottleneck effect – severe, temporary reduction in population size.

  • Founder effect – new population established by a few individuals.

  • Effective population size (Ne) – the number of breeding individuals that determines the magnitude of drift; variance in allele frequency per generation ≈ ⁄(2Ne).

Allele frequencies fluctuate randomly; rare alleles may be lost or become fixed, especially when Ne is small.
MutationSpontaneous changes in DNA; can be neutral, beneficial or harmful. Provides the raw material for evolution.Introduces new alleles (novel genetic variation).
Gene Flow (Migration)

  • Movement of individuals or gametes between populations.

  • Change in allele frequency: Δp = m (pm – p), where m = proportion of immigrants each generation, pm = allele frequency in migrants.

  • Isolation‑by‑distance – gene flow declines as geographic distance increases, producing a gradual cline in allele frequencies.

Populations become genetically more similar; can introduce advantageous alleles or counteract divergence.

4. Quantitative Tools

4.1 Hardy–Weinberg Equilibrium

Provides a null model for a non‑evolving population. For a locus with two alleles A (frequency p) and a (frequency q):

p² + 2pq + q² = 1

Genotype frequencies:

  • AA = p²

  • Aa = 2pq

  • aa = q²

Example (exam style): In a population of 200 beetles, 72 are homozygous recessive (aa). Determine p, q and the expected numbers of AA and Aa if the population were in Hardy–Weinberg equilibrium.

  1. q² = 72 ⁄ 200 = 0.36 → q = √0.36 = 0.60

  2. p = 1 – q = 0.40

  3. AA (p²) = 0.40² = 0.16 → 0.16 × 200 = 32 individuals

  4. Aa (2pq) = 2 × 0.40 × 0.60 = 0.48 → 0.48 × 200 = 96 individuals

4.2 Selection Coefficient (s)

Quantifies the fitness disadvantage of a genotype relative to the most fit genotype (fitness = 1).

Fitness of a less‑fit genotype = 1 – s.

Example: In a moth population, dark‑coloured genotype has fitness = 1, light‑coloured genotype has fitness = 0.85. The selection coefficient s = 0.15, indicating a 15 % disadvantage.

4.3 Migration Rate (m) – Numerical Example

Suppose a population of 500 individuals receives 10 % immigrants each generation (m = 0.10). If the allele frequency of a beneficial allele in the resident population is p = 0.30 and in the immigrants it is pm = 0.70, the change in allele frequency is:

Δp = 0.10 (0.70 – 0.30) = 0.04

Thus the allele frequency in the next generation becomes p′ = p + Δp = 0.34.

4.4 Effective Population Size (Ne)

Variance in allele frequency due to drift per generation:

Var(p) ≈ pq ⁄ (2Ne)

When Ne is small, drift can overwhelm selection, leading to rapid fixation or loss of alleles.

5. Speciation – Formation of New Species

Speciation occurs when reproductive isolation prevents gene flow between two groups, allowing independent evolutionary trajectories.

5.1 Allopatric Speciation

  • Geographic barrier (mountain range, river, island) splits a population.

  • Independent mutation, drift, and natural selection cause divergence.

  • Example: Darwin’s finches on different Galápagos islands.

5.2 Peripatric (Founder) Speciation

  • A small peripheral population becomes isolated.

  • Strong founder effect + drift + different selective pressures accelerate divergence.

  • Example: Island dwarfism in rodent species.

5.3 Parapatric Speciation

  • Adjacent populations experience contrasting environments.

  • Limited gene flow; hybrids have reduced fitness (reinforcement) leading to complete isolation.

  • Example: Grass species occupying a moisture gradient on a hillside.

5.4 Sympatric Speciation

  • New species arise within the same geographic area.

  • Key mechanisms:

    • Polyploidy – whole‑genome duplication (especially common in flowering plants).

    • Ecological niche differentiation – e.g., insects shifting to a novel host plant.

    • Sexual selection – strong mate preferences for particular traits.

  • Examples: Polyploid wheat varieties; cichlid fish radiations in African Rift lakes.

6. Evidence Supporting Evolution

  • Fossil record – chronological succession and transitional forms (e.g., Archaeopteryx linking dinosaurs and birds).

  • Comparative anatomy – homologous structures such as the pentadactyl limb in vertebrates.

  • Embryology – conserved early developmental stages (pharyngeal arches in fish, amphibians, and mammals).

  • Molecular biology – DNA and protein sequence similarities; human‑chimpanzee genome similarity ≈ 98 %.

  • Observed micro‑evolution – real‑time examples:

    • Industrial melanism in the peppered moth (Biston betularia).

    • Beak‑size changes in Darwin’s finches during drought.

    • Antibiotic‑resistant bacteria (e.g., MRSA).

    • Lactase persistence in human populations with a long history of dairy farming.

7. Summary

Evolution is a continuous process driven by changes in allele frequencies within a gene pool. The main mechanisms—natural selection (directional, stabilising, disruptive), genetic drift (influenced by effective population size), mutation, recombination, gene flow, and polyploidy—act together over many generations. Quantitative tools such as the Hardy–Weinberg model, selection coefficients, migration rates, and calculations of Ne allow us to detect and measure evolutionary change. When reproductive isolation is achieved, divergent populations become distinct species, giving rise to the biodiversity observed today.

Suggested diagram: Flowchart showing how mutation, recombination, gene flow and drift generate genetic variation; how natural selection (directional, stabilising, disruptive) and artificial selection act on fitness; and how prolonged isolation (allopatric, peripatric, parapatric, sympatric) leads to speciation.