explain how speciation may occur as a result of genetic isolation by: geographical separation (allopatric speciation), ecological and behavioural separation (sympatric speciation)

Evolution – Speciation through Genetic Isolation (Cambridge IGCSE/A‑Level Biology 9700)

1. Core Concepts

• Evolution: change in the genetic composition (allele frequencies) of a population over successive generations.
• Natural selection: differential survival and reproduction of individuals because of heritable variation in phenotype.

  1. Variation exists within the population.
  2. Variation influences survival or reproductive success.
  3. Variation is genetically inherited.

• Speciation: the process by which one species splits into two or more genetically distinct lineages that no longer inter‑breed in nature.

2. Inheritance – The Genetic Basis of Evolution (Syllabus 16)

• Alleles are transmitted according to Mendelian segregation and independent assortment (unless linked).
• Sex‑linked and autosomal traits can respond differently to selection because of their inheritance patterns.
• Mutations create new alleles – the raw material for evolution.

3. Population Genetics Fundamentals (AO1)

Allele frequencies change when the four mechanisms listed below act on a population.

3.1 Hardy–Weinberg Principle (AO2)

For a diploid, sexually reproducing population that meets the five assumptions (large size, random mating, no mutation, no migration, no selection), allele and genotype frequencies remain constant:

\[

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

\]

where p and q are the frequencies of two alleles at a locus. Deviations from the equilibrium indicate that one or more evolutionary forces are acting.

3.2 Inbreeding Coefficient (Genetic Drift)

In a small isolated population the probability that two alleles are identical by descent in generation t+1 is:

\[

F{t+1}= \frac{1}{2N} + \left(1-\frac{1}{2N}\right)Ft

\]

where N = effective population size and F_t = inbreeding coefficient in generation t. A high F increases homozygosity and can expose deleterious recessive alleles.

4. Evidence for Evolution (Syllabus 15)

5. Reproductive Isolation – Barriers to Gene Flow (AO1)

5.1 Pre‑zygotic barriers (prevent fertilisation)

1. Habitat isolation – different micro‑habitats within the same area.
2. Temporal isolation – breeding at different times (seasonal or daily).
3. Behavioural isolation – distinct courtship signals or mating rituals.
4. Mechanical isolation – incompatibility of reproductive structures.
5. Gametic isolation – sperm cannot fertilise the egg of another group.

5.2 Post‑zygotic barriers (reduce hybrid fitness)

1. Hybrid inviability – embryos die early.
2. Hybrid sterility – hybrids survive but are sterile (e.g., mules).
3. Hybrid breakdown – F₂ or later generations have reduced fitness.

6. Allopatric Speciation – Geographical Isolation (Syllabus 17)

6.1 How it occurs

1. A physical barrier (mountain range, river, ocean, glacier, human‑created habitat fragmentation) splits a once‑continuous population.
2. Gene flow is halted; each sub‑population evolves independently under mutation, drift, and natural selection.
3. Accumulated genetic differences eventually produce pre‑ or post‑zygotic barriers.

6.2 Classic Example – Galápagos Finches

• Storm‑driven colonisation of different islands created isolated founder populations.
• Each island offered a distinct food resource; selection favoured different beak shapes.
• Beak morphology, song patterns and mating preferences diverged, reducing inter‑island breeding.

6.3 Genetic Consequences

• Founder effects and bottlenecks intensify genetic drift; effective population size (Nₑ) often < 100 on small islands.
• Rapid increase in the inbreeding coefficient (F) leads to higher homozygosity.

7. Sympatric Speciation – Ecological & Behavioural Isolation (Syllabus 17)

7.1 How it occurs without a physical barrier

1. Individuals exploit different resources or micro‑habitats within the same geographic area (resource partitioning).
2. Assortative mating evolves – individuals preferentially mate with others that use the same resource or display the same trait.
3. Pre‑zygotic (habitat, temporal, behavioural) or post‑zygotic barriers develop, leading to reproductive isolation.

7.2 Example – Apple Maggot Fly (Rhagoletis pomonella)

• Original host: hawthorn fruit; a subset shifted to the introduced apple tree.
• Apples and hawthorns ripen at different times → temporal isolation.
• Flies mate on the host plant they emerged from → behavioural (habitat) isolation.
• Genetic analyses reveal divergence in mitochondrial DNA and host‑preference genes.

7.3 Polyploidy – Rapid Sympatric Speciation in Plants

Whole‑genome duplication creates an individual with double the chromosome number. Because meiosis in a diploid (2x) × tetraploid (4x) cross produces unbalanced gametes, the two groups become reproductively isolated instantly.

\[

\text{Gamete chromosome number: } n{\text{diploid}} = x,\; n{\text{tetraploid}} = 2x

\]

• Common in angiosperms (e.g., wheat, *Spartina* hybrids).
• Often followed by ecological differentiation, reinforcing isolation.

8. Speciation in Plants – Beyond Polyploidy (Syllabus 18)

• Hybridisation followed by chromosome doubling (allopolyploidy) – e.g., *Tragopogon* species in the UK.
• Breakdown of self‑incompatibility – leads to autonomous selfing and reproductive isolation within a population.
• Ecological divergence – different soil types, pollinator syndromes or flowering times can drive isolation.

9. Classification, Biodiversity & Conservation (Syllabus 18)

• Speciation underpins the definition of species (biological, morphological and phylogenetic concepts).
• Understanding speciation helps identify biodiversity hotspots where rapid diversification occurs.
• Conservation strategies often aim to preserve genetic connectivity; fragmentation can unintentionally promote allopatric speciation and loss of genetic diversity.

10. Molecular Tools in Speciation Research (Syllabus 19)

• DNA barcoding – uses a short, standardised gene region (e.g., COI) to identify cryptic species.
• Single‑nucleotide polymorphism (SNP) genotyping – detects fine‑scale genetic structure between incipient species.
• Whole‑genome sequencing – reveals patterns of divergence, introgression and the genomic architecture of reproductive barriers.
• CRISPR/Cas9 – experimental manipulation of candidate speciation genes (e.g., colour‑pattern loci in butterflies).

11. Practical Skills – Investigating Genetic Isolation (AO3)

Suggested activity: “Simulating the effect of habitat fragmentation on gene flow”.

1. Use a spreadsheet (or free software such as PopG) to model two sub‑populations of 200 individuals each.
2. Assign initial allele frequencies for a neutral locus (e.g., A/a = 0.5).
3. Run simulations for 50 generations under three scenarios:

   • Full gene flow (5 % migrants each generation).
   • Reduced gene flow (1 % migrants).
   • No gene flow (complete barrier).

4. Record allele frequencies each generation, calculate F_ST and plot the results.
5. Analyse how reduced migration accelerates divergence and discuss the implications for allopatric speciation.

12. Comparison of Allopatric and Sympatric Speciation

13. Summary of Key Points (AO1‑AO3)

• Evolution = change in allele frequencies; natural selection requires variation, differential fitness and heritability.
• Mutation, gene flow, genetic drift and non‑random mating are the four mechanisms that alter gene pools.
• Genetic isolation stops gene flow, allowing independent evolution and the buildup of reproductive barriers.
• Allopatric speciation: physical separation → drift & selection → pre‑/post‑zygotic barriers (e.g., Galápagos finches).
• Sympatric speciation: ecological or behavioural divergence within the same area → assortative mating → isolation (e.g., apple maggot fly, polyploid plants).
• Evidence comes from fossils, comparative anatomy, molecular data and natural examples such as ring species.
• Understanding mechanisms and evidence enables students to analyse novel scenarios (AO2) and evaluate the relative importance of drift vs. selection (AO3).

14. Sample Examination Questions (AO1‑AO3)

1. Explain how a river can lead to allopatric speciation in a population of freshwater fish. In your answer, discuss the roles of genetic drift, natural selection and the possible development of pre‑zygotic barriers.
2. Describe the process by which the apple maggot fly (*Rhagoletis pomonella*) illustrates sympatric speciation. Highlight the importance of host preference, temporal isolation and any genetic evidence that supports divergence.
3. Compare and contrast the genetic consequences of a small isolated population that experiences a founder effect with those of a polyploid event in a plant species. In your comparison, evaluate which mechanism is likely to produce reproductive isolation more rapidly and why.
4. Using the concept of ring species, explain how continuous gene flow can still result in two end populations that are reproductively isolated. Provide a specific example and discuss the implications for defining species.
5. Outline a practical investigation that could be used to demonstrate the effect of reduced gene flow on allele frequencies in a simulated population. Include the variables you would control, the data you would record and how you would analyse the results.