You are viewing a free preview of this lesson.
Subscribe to unlock all 10 lessons in this course and every other course on LearningBro.
Natural selection and genetic drift are the two main mechanisms that change allele frequencies in populations over time. While natural selection is a directional force driven by differential fitness, genetic drift is a random process most powerful in small populations. Understanding both is crucial for the Edexcel A-Level Biology specification and for appreciating how evolution operates at the population level.
Natural selection is the process by which organisms with traits that increase their fitness are more likely to survive, reproduce, and pass those traits to the next generation. In genetic terms:
Exam precision: Natural selection does not act on genes directly — it acts on phenotypes. However, because phenotypes are influenced by genotypes, selection indirectly changes allele frequencies.
There are three main types of selection, distinguished by which phenotypes are favoured:
flowchart TD
A["Types of Natural Selection"] --> B["Stabilising Selection"]
A --> C["Directional Selection"]
A --> D["Disruptive Selection"]
B --> B1["Favours intermediate phenotype<br/>Reduces variation<br/>Example: human birth weight"]
C --> C1["Favours one extreme phenotype<br/>Shifts population mean<br/>Example: antibiotic resistance"]
D --> D1["Favours both extreme phenotypes<br/>Increases variation<br/>Example: beak size in finches"]
Stabilising selection favours the intermediate phenotype and selects against both extremes. It reduces phenotypic variation in the population and is the most common type of selection in stable environments.
Classic example — Human birth weight:
Directional selection favours one extreme phenotype over the other. It shifts the population mean in one direction and is typically seen when the environment changes.
Classic example — Antibiotic resistance in bacteria:
Another example — Peppered moth (Biston betularia):
Disruptive selection favours both extreme phenotypes and selects against the intermediate. It increases variation and can lead to speciation if gene flow between the two groups is interrupted.
Example — Beak size in African seedcracker finches:
| Feature | Stabilising | Directional | Disruptive |
|---|---|---|---|
| Phenotype favoured | Intermediate | One extreme | Both extremes |
| Effect on mean | No change | Shifts in one direction | May split into two peaks |
| Effect on variation | Decreases | Decreases (shifts) | Increases |
| Typical environment | Stable | Changing | Variable with distinct niches |
| Evolutionary outcome | Maintenance of status quo | Adaptation to new conditions | Potential speciation |
Genetic drift is the random change in allele frequencies in a population due to chance events in reproduction. It is NOT driven by fitness differences — it is purely stochastic.
In a large population, chance deviations in allele frequency tend to cancel out. In a small population, a few individuals failing to reproduce can significantly shift allele frequencies. Imagine flipping a coin: in 1,000 flips, you expect close to 50% heads; in 10 flips, getting 70% heads is quite likely.
The founder effect occurs when a small group of individuals colonises a new area, carrying only a subset of the alleles from the original population. The new population has reduced genetic diversity and may have very different allele frequencies.
Examples:
A population bottleneck occurs when a population is drastically reduced in size by a catastrophic event (disease, natural disaster, hunting). The surviving individuals carry only a fraction of the original genetic diversity. Even if the population recovers in numbers, the genetic diversity remains low for many generations.
Examples:
flowchart LR
A["Large, diverse population"] -->|"Catastrophic event"| B["Bottleneck: small surviving population"]
B -->|"Population recovers in numbers"| C["Large population but LOW genetic diversity"]
D["Large, diverse population"] -->|"Small group colonises new area"| E["Founder population: small, low diversity"]
E -->|"Population grows"| F["Large population but different allele frequencies"]
| Feature | Natural Selection | Genetic Drift |
|---|---|---|
| Mechanism | Differential reproductive success based on fitness | Random sampling of alleles |
| Direction | Non-random — favours advantageous alleles | Random — any allele can increase or decrease |
| Effect on variation | Can increase or decrease, depending on type | Always decreases (fixes or loses alleles) |
| Population size | Operates in all populations | Strongest in small populations |
| Adaptive? | Yes — leads to adaptation | No — changes are random |
| Predictability | Outcome is predictable (fitter alleles increase) | Outcome is unpredictable |
Gene flow (migration) is the movement of alleles between populations through the migration of individuals or their gametes (e.g. pollen). Gene flow:
Gene flow is important because it prevents populations from diverging too rapidly and maintains genetic connectivity between subpopulations.
Natural selection is the only evolutionary mechanism that produces adaptation — genetic drift, gene flow, and mutation change allele frequencies but do not direct change towards better-adapted phenotypes.
For evolution by natural selection to occur, three conditions must be met:
Natural selection changes allele frequencies through differential reproductive success and is the only mechanism that leads to adaptation. It operates in three modes: stabilising, directional, and disruptive. Genetic drift is the random change in allele frequencies due to chance, most significant in small populations, and includes the founder effect and bottleneck effect. Together, natural selection, genetic drift, gene flow, and mutation drive the evolution of populations.
Natural selection and genetic drift are the two principal microevolutionary forces — the mechanisms that change allele frequencies in finite populations from one generation to the next. The conceptual divide is clean: natural selection is directional and predictable from fitness differences; genetic drift is random and matters when N is small. Both operate on the raw material of genetic variation generated by mutation (lessons 1–2), meiotic recombination — independent assortment and crossing-over (lesson 3) — and sexual reproduction (random fusion of gametes), and both feed forward into the Hardy–Weinberg goodness-of-fit framework of lesson 10. Selection is the only force that produces adaptation: differential survival driven by fitness differences in a particular environment yields differential reproduction, allele-frequency change, and — over generations — phenotypic match between organism and habitat. Drift, by contrast, is adaptively neutral sampling noise: in a population of finite size N, only a finite number of gametes contribute to the next generation, so allele frequencies wobble even in the absence of selection, and the wobble is large when N is small. The candidate's task at A-Level is to distinguish the two mechanisms by their fingerprints, name the three modes of selection (stabilising, directional, disruptive) with named exemplars, recognise sexual selection as a special case of natural selection, explain bottleneck and founder effects as drift in action, and link the whole framework to speciation through reproductive isolation.
This material sits in Edexcel 9BI0 Topic 8 (Grey Matter — Coordination, Response and Gene Technology) within the evolution-and-classification strand that runs alongside the inheritance and Hardy–Weinberg content. Required knowledge covers: the sources of genetic variation — random mutation as the ultimate source of new alleles (lessons 1–2), meiotic crossing-over and independent assortment as the source of new combinations (lesson 3), and random fusion of gametes at fertilisation; selection pressures (predation, disease, climate, competition for food and mates) acting differentially on phenotypes; the three modes of natural selection — directional (e.g. industrial melanism in Biston betularia; antibiotic resistance), stabilising (e.g. human birth weight) and disruptive (e.g. African seed-cracker finch beak size); sexual selection as a sub-form of natural selection driven by mate choice or intra-sexual competition (e.g. peacock tail, bird-of-paradise display); the causal chain differential survival → differential reproduction → allele-frequency change → adaptation over generations; genetic drift as random allele-frequency change that becomes large in finite populations, formalised through effective population size (N_e); the bottleneck effect (e.g. cheetah, northern elephant seal) and founder effect (e.g. Pingelapese achromatopsia, Afrikaner porphyria) as paradigm drift case studies; the distinguishing fingerprints of selection vs drift; the neutral theory at the level of theory attribution — most molecular variation is selectively neutral and changes by drift; and speciation by allopatric (geographic isolation) vs sympatric (e.g. polyploidy in plants, sympatric divergence in Lake Victoria cichlids) routes through pre-zygotic (temporal, behavioural, mechanical) and post-zygotic (hybrid inviability, hybrid sterility) reproductive isolation mechanisms. Synoptic links run back to lesson 1 (mutation as the ultimate variation source), lesson 3 (meiosis as the immediate variation source) and forwards to lesson 10 (Hardy–Weinberg quantifies selection and drift signals). Refer to the official Pearson Edexcel 9BI0 specification document for exact wording.
Question (8 marks):
(a) The peppered moth (Biston betularia) exists in two forms — a pale (typica) form and a dark melanic (carbonaria) form, the difference controlled at a single locus with the melanic allele dominant. In a pre-industrial woodland in 1830, the population frequency of the dark form is 2%, with pale moths cryptically matched to lichen-covered bark. By 1895, in heavily industrialised regions where soot has killed lichens and blackened bark, the dark-form frequency has risen to 95%. Using the language of natural selection, explain — in terms of phenotype, fitness, allele frequency and the type of selection — how this allele-frequency shift has occurred. (5)
(b) After Clean Air Acts in the 1950s–1960s, lichens recolonised industrial woodlands and bark lightened. By 2000, dark-form frequency in those woodlands had fallen to ~10%. Identify the type of selection now operating and predict, with reasoning, whether the carbonaria allele is likely to be eliminated entirely. (2)
(c) Suggest one reason why a small post-bottleneck population recovering from collapse might fail to track an environmental change of this kind, even when the appropriate allele is still present. (1)
Solution with mark scheme:
(a) M1 (AO1.2) — variation and heritability. A pre-existing genetic polymorphism at the melanism locus provides phenotypic variation; the carbonaria and typica forms differ in wing colour and the difference is heritable (controlled by a single autosomal locus, carbonaria dominant). M1 (AO2.1) — selection pressure. The post-industrial environment imposes a predation pressure — birds visually predate moths resting on bark, so camouflage (crypsis) determines survival probability.
A1 (AO2.1) — fitness differential. On lichen-dark, sooty bark, dark moths are cryptic while pale moths are conspicuous, so dark moths have higher survival probability and therefore higher reproductive success (Darwinian fitness). A1 (AO2.2) — allele-frequency consequence. Dark survivors transmit the dominant melanic allele to a larger fraction of the next generation; over many generations, the frequency of the melanic allele rises from ~1–2% to high values (95% phenotype frequency implies most individuals carry at least one carbonaria allele).
A1 (AO3.2) — name the mode. This is directional selection — one extreme of the phenotypic distribution (dark) is favoured, the population mean shifts towards that extreme, and the phenotype frequency moves monotonically in one direction across generations.
(b) M1 (AO3.2) — type of selection. With light bark restored, the selection pressure has reversed: pale moths are now the cryptic phenotype and dark moths are conspicuous. This is again directional selection, but in the opposite direction to the 1830–1895 episode. A1 (AO3.1) — fate of the allele. The carbonaria allele is unlikely to be eliminated entirely: as a dominant allele its homozygotes are the most exposed, but rare heterozygotes (carriers) escape the worst of the directional pressure and gene flow from neighbouring (still-polluted or mixed) woodlands continually reintroduces the allele. A residual frequency of a few per cent is the typical post-recovery equilibrium.
(c) M1 (AO3.1) — drift overrides selection at small N. In a small population, genetic drift can dominate over weak selection: the appropriate (favoured) allele can be lost by chance sampling before selection has time to raise its frequency, especially if its starting frequency is low. The smaller the effective population size (N_e), the more likely drift is to override directional selection.
Total: 8 marks (M4 A4).
Question (6 marks): The Pingelapese population on Pingelap atoll (Pacific Ocean) was reduced to about 20 survivors by a typhoon in the late 18th century. One survivor was a heterozygous carrier of a recessive allele for achromatopsia (complete colour blindness). The current population is a few thousand and ~5–10% are affected by achromatopsia, compared with ~0.003% worldwide. Explain, in terms of bottleneck, founder effect, genetic drift and allele frequency, why the disease frequency is so much higher than the global average, and identify one piece of evidence that would distinguish drift from selection as the cause.
Mark scheme decomposition by AO:
Subscribe to continue reading
Get full access to this lesson and all 10 lessons in this course.