SPEC 3.7.3 - GENETICS, POPULATIONS, EVOLUTION & ECOSYSTEMS
Evolution & Speciation
Natural selection, genetic drift, and the mechanisms by which new species form — the engine of biological diversity
Key Definitions
Key Definition
Evolution — a change in allele frequency in a population over time.
Key Definition
Genetic drift — a random change in allele frequency due to chance events, especially significant in small populations.
Key Definition
Allopatric speciation — the formation of new species following geographic isolation of populations, which prevents gene flow.
Sympatric speciation — the formation of new species within the same geographic area, without geographic isolation (e.g. via polyploidy or assortative mating).
Reproductive isolation — the inability of members of two populations to interbreed and produce fertile offspring; the final barrier confirming speciation.
Natural Selection — Mechanism
Natural selection acts on the variation present in a population to change allele frequencies over generations. The mechanism requires four conditions:
- 1Variation exists within the population — individuals differ in phenotype due to differences in alleles (arising from mutation).
- 2Heritable — the variation has a genetic basis and can be passed to offspring.
- 3Differential survival and reproduction — some variants are better suited to the environment; they survive longer and reproduce more.
- 4Allele frequency changes — favoured alleles become more common in the next generation; unfavoured alleles decrease in frequency.
The Outcome
Over many generations, allele frequencies shift in the direction that increases fitness in the current environment. This is adaptive evolution — natural selection is the only mechanism of evolution that consistently increases fitness.
Types of Natural Selection
TypeWhat is selectedEffect on distributionExample
DirectionalOne extreme phenotype has highest fitnessMean shifts toward the favoured extreme; distribution narrows at the disfavoured endAntibiotic resistance; industrial melanism in peppered moths
StabilisingThe mean (intermediate) phenotype has highest fitness; extremes selected againstMean stays the same; variance decreases (narrower distribution)Human birth weight — very low and very high birth weights have higher infant mortality
DisruptiveBoth extremes have higher fitness than the intermediate phenotypeDistribution splits into two distinct peaks; intermediate phenotype decreasesBeak size in some bird populations where small and large seeds predominate but intermediate-sized seeds are rare
Examiner Alert
Stabilising selection does not increase the mean — it reduces variance around an existing mean. Directional selection changes the mean. Students commonly confuse these. State clearly what happens to both mean and variance for each type.
Genetic Drift
Key Definition
Genetic drift — a random, chance-driven change in allele frequency. Unlike natural selection, it is not adaptive; it operates independently of fitness.
Why Small Populations?
In a large population, random effects average out across many individuals. In a small population, the chance survival or death of just one or two individuals can dramatically alter allele frequencies. A rare allele may be lost entirely by chance — even if it was neutral or beneficial.
Consequences of Drift
Allele frequencies change unpredictably
Alleles can become fixed (frequency = 1) or lost (frequency = 0) by chance
Genetic diversity is reduced
No consistent direction — different from selection
Key Distinction
Natural selection is directional and adaptive — it consistently favours alleles that increase fitness. Genetic drift is random and non-adaptive — it can increase or decrease any allele regardless of its effect on fitness. In large populations, selection dominates; in small populations, drift can overpower selection.
Bottleneck & Founder Effects
Bottleneck Effect
A population is severely reduced by a catastrophic event (disease, natural disaster, hunting). The survivors are a small, random subset of the original population. Their allele frequencies may differ greatly from the original population — some alleles may have been lost entirely. The reduced population then expands from this limited genetic base.
Example: Northern elephant seals were hunted almost to extinction in the 1890s; the surviving population shows very low genetic diversity compared to other seal species.
Founder Effect
A small group breaks away from a larger population and establishes a new population in a new area. The founders carry only a subset of the original alleles. Allele frequencies in the new population may be very different from the source population, and rare alleles in the source may become common — or absent.
Example: Amish populations show elevated frequencies of certain rare genetic conditions that were carried by the original small founding group.
Common Confusion
Both effects are examples of genetic drift — they both produce random changes in allele frequency due to small population size, not selection. The bottleneck effect involves an existing population shrinking; the founder effect involves a new population being established from a small sample.
Allopatric Speciation — Step by Step
Allopatric speciation occurs when a population is divided by a geographic barrier, preventing gene flow between the two groups.
- 1Geographic isolation: a barrier (mountain range, sea, river, glacier) divides one population into two, with no migration between them.
- 2No gene flow: alleles can no longer move between populations. The two gene pools become separated.
- 3Different mutations: new mutations arise independently in each population, creating different alleles in each gene pool.
- 4Different selection pressures: each population faces different environmental conditions; natural selection favours different alleles in each. Allele frequencies diverge.
- 5Reproductive isolation develops: over many generations, the populations become so genetically different that if they were reunited, they could no longer interbreed to produce fertile offspring — speciation is complete.
Key Point — Reproductive Isolation Confirms Speciation
Geographic isolation alone does not create new species — it only prevents gene flow. Speciation is only confirmed when reproductive isolation develops, meaning the populations can no longer interbreed even if the geographic barrier is removed.
Sympatric Speciation
Key Definition
Sympatric speciation — speciation that occurs within the same geographic area, without a physical barrier to gene flow. Reproductive isolation arises from other mechanisms.
Polyploidy (plants)
A cell division error produces an organism with extra sets of chromosomes. Polyploid individuals cannot interbreed with the original diploid population (gametes are incompatible) but can self-fertilise or breed with other polyploids — instant reproductive isolation without geographic separation.
Example: Many crop plants (wheat, cotton) are natural polyploids that arose sympatrically.
Assortative Mating
Individuals preferentially mate with others that share a similar phenotype. Over generations, this reduces gene flow between phenotypic groups within the same population, eventually leading to reproductive isolation without geographic separation.
Example: Some fish species show colour-based mate choice that leads to divergence within the same lake.
Examiner Alert
Sympatric speciation is less common than allopatric but is well-documented in plants via polyploidy. AQA requires you to know both mechanisms. The key feature of sympatric speciation is that reproductive isolation arises without geographic separation.
Reproductive Isolation Mechanisms
Reproductive isolation prevents gene flow between populations, allowing allele frequencies to diverge independently. Once present, it confirms speciation.
MechanismDescriptionExample
TemporalPopulations breed at different times (different seasons, times of day)Two plant species that flower in spring vs autumn in the same habitat
BehaviouralDifferent courtship displays, calls, or mating signals prevent recognitionBird species with different songs; frogs with different mating calls
MechanicalIncompatible reproductive structures prevent mating or pollinationFlowers with different structures that fit different pollinators; incompatible genitalia
GameticGametes are biochemically incompatible — sperm cannot fertilise eggs from the other populationSea urchins: gamete recognition proteins differ between species
Hybrid sterilityMating occurs and produces hybrid offspring, but hybrids are sterileMule (horse × donkey) — viable but sterile; liger (lion × tiger)
Exam Tip
AQA questions often describe a scenario and ask you to identify the type of reproductive isolation. Focus on when the barrier acts: before mating (temporal, behavioural, mechanical), at fertilisation (gametic), or after fertilisation (hybrid sterility).
Selection vs Drift — How to Distinguish
FeatureNatural SelectionGenetic Drift
DirectionConsistent and directional — favoured alleles always increaseRandom — no consistent direction across generations
Population sizeActs in all populations regardless of sizeStrongest in small populations; negligible in large populations
Fitness linkChanges linked to survival/reproduction advantageChanges unrelated to fitness; purely by chance
OutcomeAdaptive — increases mean fitness in the current environmentNon-adaptive — may reduce or have no effect on fitness
RateProportional to selection coefficient and allele frequencyInversely proportional to population size
Evidence Question Strategy
If asked to evaluate whether a change is due to selection or drift: selection is supported by consistent directional change across generations + evidence that the favoured phenotype has higher survival/reproduction. Drift is supported by small population size + non-directional, unpredictable fluctuations.
Examiner Traps
Trap 1 — "Evolution" means visible change
Evolution is defined as a change in allele frequency — not a change in phenotype, body plan, or complexity. A population can evolve without any visible change if recessive allele frequencies shift.
Trap 2 — Geographic isolation = new species
Geographic isolation alone does NOT create new species. It creates the conditions for divergence. Speciation only occurs when reproductive isolation develops. Without reproductive isolation, the populations can still interbreed when the barrier is removed.
Trap 3 — Stabilising selection increases the mean
Stabilising selection maintains the existing mean — it does not shift it. It eliminates extreme variants on both sides, reducing variance. Only directional selection shifts the mean.
Trap 4 — Genetic drift is a form of selection
Genetic drift is NOT natural selection. It is a random process independent of fitness. An allele can drift to fixation even if it is slightly harmful. Do not confuse drift with "weak selection".
DEEP Chain — Extended Response
"A population of rabbits on an island is separated from a mainland population by rising sea levels. Describe and explain how this could lead to the formation of a new species over thousands of generations."
DEEP Answer Framework
D — Define
"Allopatric speciation is the formation of new species following geographic isolation. The sea barrier prevents gene flow between island and mainland populations, separating their gene pools."
E — Explain the Mechanism
"On each side of the barrier, different mutations arise randomly. The island and mainland environments differ, so natural selection favours different alleles in each population. Over thousands of generations, allele frequencies diverge. Different alleles accumulate and genetic differences build up."
E — Evidence / Example
"For example, if the island has different vegetation, alleles affecting coat colour or digestive enzymes that suit island food may be selected for on the island but not on the mainland. Genetic drift may also alter allele frequencies further, especially if the island population started small."
P — Practical / Evaluation
"Eventually, if the barrier were removed and the populations reunited, they could no longer interbreed — reproductive isolation (e.g. incompatible courtship behaviour or gametes) has developed. This confirms speciation. The process is called allopatric speciation because isolation was geographic."
Synoptic Links
Link to Hardy-Weinberg (7.03)
Evolution = change in allele frequency. Hardy-Weinberg shows what allele frequencies look like when NO evolution occurs. When natural selection, genetic drift, or gene flow act, Hardy-Weinberg equilibrium breaks down — this is the mathematical link between the two lessons.
Link to Mutation (3.4 / 3.7.1)
Mutation is the ultimate source of new alleles — without mutation, there would be no variation for natural selection or drift to act upon. All allelic variation ultimately traces back to mutation events.
Link to Meiosis & Variation (3.5)
Crossing over and independent assortment during meiosis generate phenotypic variation by shuffling allele combinations. This variation is the raw material on which selection acts in each generation.
Link to Antibiotic Resistance (4.5)
Antibiotic resistance is the classic A-level example of directional selection. Random mutation produces a resistant allele; selection pressure (antibiotic treatment) kills non-resistant bacteria; resistant individuals reproduce; resistant allele frequency rises rapidly. Evolution in real time.
Practice Question
A population of finches on a mainland is blown by a storm to a remote island. The island has very different vegetation, and only seeds of a specific size are available. Describe and explain:
(a) Why the initial island population may show reduced genetic diversity compared to the mainland. [2 marks]
(b) How natural selection could lead to a change in beak size in the island population over many generations. [4 marks]
(c) Under what conditions would the island and mainland populations eventually be classified as separate species? [2 marks]
a
Founder effect — the small group of finches that colonised the island carries only a subset of the mainland's alleles. Some alleles present in the mainland population will be absent in the founders, reducing genetic diversity.
b
Natural variation exists in beak size due to different alleles. Finches with beak sizes better suited to the available seeds survive longer and reproduce more. They pass their alleles on to offspring. Over many generations, the allele(s) associated with the favourable beak size increase in frequency — this is directional selection.
c
The two populations are separate species when reproductive isolation has developed — they can no longer interbreed to produce fertile offspring, even if geographically reunited. Geographic isolation alone is not sufficient.
Lesson Summary
Core Content — Spec 3.7.3
Evolution = change in allele frequency over time
Natural selection : variation → differential survival/reproduction → allele frequency change
Directional : mean shifts; Stabilising : variance decreases, mean stable; Disruptive : two peaks develop
Genetic drift : random allele frequency change; most significant in small populations
Bottleneck : population severely reduced; Founder effect : small group colonises new area
Allopatric speciation : geographic isolation → no gene flow → divergence → reproductive isolation
Sympatric speciation : no geographic barrier — e.g. polyploidy, assortative mating
Reproductive isolation confirms speciation: temporal, behavioural, mechanical, gametic, hybrid sterility
Selection vs Drift Summary
Selection: directional, adaptive, fitness-linked, acts in all populations.
Drift: random, non-adaptive, independent of fitness, strongest in small populations.
Speciation Pathway
Isolation → no gene flow → independent mutation + selection → allele frequency divergence → reproductive isolation = new species.
Geographic isolation does not alone create new species.
Next: Key Definitions