Genetic Drift and Speciation Revision Notes | A Level AQA Biology

1. Why Populations Change — and Why It Matters

Picture two populations of the same bird species separated by a mountain range. Over thousands of generations, they look different, behave differently, and eventually cannot breed with each other at all. A new species has formed — not through any grand plan, but through the quiet accumulation of genetic change. This lesson unpacks exactly how that happens.

Two distinct mechanisms drive changes in allele frequency: natural selection, which is directional and driven by the environment, and genetic drift, which is random and driven purely by chance. Understanding both — and knowing when each dominates — is central to explaining speciation at A Level.

2. Genetic Drift — When Chance Shapes a Gene Pool

In any population, allele frequencies can shift from one generation to the next because of which individuals happen to survive and reproduce. This random fluctuation is called genetic drift. Crucially, it has nothing to do with whether an allele is advantageous or harmful — it is pure chance.

Here is the key insight: in a large population, random chance events average out. If one individual carrying a rare allele dies by accident, thousands of other copies of that allele still exist in the population. The frequency barely shifts. In a small population, however, losing even one individual can remove a significant proportion of a rare allele from the gene pool entirely.

Think of it like flipping a coin. Flip it 1000 times and you will get close to 50% heads. Flip it only 4 times and you might get 3 heads or even 4 — a dramatic deviation from 50%. Small sample size amplifies the effect of chance. A small population is the biological equivalent of that four-flip experiment.

• In a large population: random deaths and reproductive failures have negligible effect on allele frequency — the law of large numbers buffers against chance.
• In a small population: a single chance event (a storm, a predator attack, a disease outbreak) can dramatically alter allele frequencies in one generation.
• Over time, genetic drift in small populations can cause alleles to become fixed (frequency = 1.0) or lost (frequency = 0) entirely, reducing genetic variation.
• Genetic drift can cause neutral or even slightly disadvantageous alleles to become fixed — something natural selection alone would not produce.

Two specific scenarios produce the small populations where drift is most powerful. The bottleneck effect occurs when a catastrophic event drastically reduces population size, leaving a small, genetically unrepresentative survivor group. The founder effect occurs when a small number of individuals colonise a new area, carrying only a subset of the original gene pool with them. Both scenarios are classic triggers for rapid genetic drift.

Genetic drift is important not just as a standalone mechanism but because it interacts with isolation during speciation — a connection we will build on throughout this lesson. Small, isolated populations are precisely where drift operates most powerfully, accelerating divergence from the parent population.

3. Natural Selection, Isolation, and Changing Allele Frequencies

While genetic drift is random, natural selection is directional. When two populations of the same species experience different environments, different alleles confer survival and reproductive advantages in each location. Over generations, the allele frequencies in the two populations diverge — and so do their phenotypes.

The mechanism is straightforward: individuals with alleles that produce advantageous phenotypes in their local environment are more likely to survive, reproduce, and pass those alleles to offspring. Alleles producing less favourable phenotypes become rarer. Repeat this across hundreds of generations and the two populations can look, behave, and function very differently — even though they started from the same ancestral gene pool.

For this divergence to proceed without being reversed, the two populations must be isolated from one another. Without isolation, gene flow — the movement of alleles between populations through interbreeding — would constantly mix the two gene pools back together, preventing divergence. Isolation is therefore the essential precondition for speciation.

It is worth being precise about what changes during this process. Natural selection and genetic drift both alter allele frequencies within a population's gene pool. As allele frequencies shift, the frequencies of associated phenotypes change too. The population's characteristics — its typical appearance, physiology, and behaviour — gradually diverge from those of the isolated sister population.

4. From Divergence to New Species — The Gene Pool Model

Diverging allele frequencies are the mechanism; a new species is the outcome. The critical question is: at what point have two populations diverged enough to count as separate species?

The biological species concept provides the answer used at A Level. Two populations belong to different species when their members can no longer interbreed to produce fertile offspring. This inability to interbreed is the defining criterion — not appearance, not geography, not behaviour alone.

The sequence of events leading to a new species can be summarised as a chain: reproductive separation prevents gene flow, allowing each population's gene pool to accumulate independent changes through natural selection and genetic drift. Over time, these genetic differences become so extensive that members of the two populations can no longer interbreed and produce fertile offspring. At that point, speciation is complete and a new species has arisen from the existing one.

Worked example: Tracing the Route from One Species to Two

Describe the sequence of events by which one ancestral species can give rise to two distinct species.

1. A single population with a shared gene pool and the ability to interbreed freely. — All individuals belong to one species; gene flow maintains similar allele frequencies across the population.
2. A barrier (geographical, ecological, or behavioural) divides the population into two isolated groups. — Gene flow between the two groups is reduced or eliminated. Each group now evolves independently.
3. Natural selection favours different alleles in each environment; genetic drift causes random allele frequency changes, especially if populations are small. — The gene pools of the two populations diverge. Allele frequencies, and therefore phenotype frequencies, become increasingly different.
4. Genetic differences accumulate to the point where, if members of the two populations meet, they cannot interbreed to produce fertile offspring. — This may be due to differences in mating behaviour, incompatible reproductive anatomy, hybrid inviability, or hybrid sterility.
5. The two populations are now reproductively isolated regardless of whether the original barrier persists. — A new species has arisen from the existing ancestral species. The two gene pools are now permanently separate.

Answer: Reproductive separation prevents gene flow, allowing independent accumulation of genetic differences through natural selection and genetic drift. When these differences are sufficient to prevent interbreeding and production of fertile offspring, speciation is complete and a new species has arisen from the existing one.

5. Allopatric and Sympatric Speciation — Two Routes to the Same Outcome

Speciation can occur via two broad routes, distinguished by whether or not a geographical barrier is involved. Both routes lead to the same outcome — reproductive isolation and the formation of a new species — but the mechanisms of isolation differ significantly.

Allopatric speciation (from the Greek for 'different land') occurs when a physical geographical barrier separates a population into two groups. The barrier prevents gene flow directly by making movement between the two groups impossible or extremely rare. Classic examples include a mountain range rising between two populations, a river changing course, or a small group colonising an island. Each isolated population then evolves independently under its own selection pressures and genetic drift.

Sympatric speciation (from the Greek for 'same land') occurs within the same geographical area. Here, reproductive isolation arises without a physical barrier. Instead, isolation may result from differences in mating season timing, preference for different microhabitats within the same area, polyploidy (particularly in plants, where chromosome number changes prevent interbreeding with the parent species), or behavioural differences in mate choice. Because the populations share the same space, sympatric speciation requires a stronger reproductive barrier to prevent gene flow.

A common point of confusion is whether allopatric speciation requires the geographical barrier to remain in place permanently. It does not. Once the gene pools have diverged sufficiently to prevent interbreeding, the barrier can be removed and the two species will remain distinct — they are reproductively isolated by their genetics, not by geography. The barrier is the trigger, not the permanent mechanism.

Both allopatric and sympatric speciation ultimately produce the same result: two gene pools that accumulate independent differences until interbreeding and production of fertile offspring is no longer possible. The route differs; the destination is the same.

6. Bringing It Together — Exam Technique and Common Mistakes

Speciation questions appear regularly in AQA A Level papers and tend to reward students who can construct a precise, sequenced argument rather than a vague description. The marking schemes are looking for specific biological language and logical cause-and-effect chains.

When a question asks you to explain how speciation occurs, the examiner wants a chain of reasoning: isolation prevents gene flow, which allows allele frequencies to diverge through natural selection and/or genetic drift, which eventually leads to reproductive incompatibility. Each arrow in that chain needs to be made explicit — do not assume the examiner will fill in the gaps.