Genetic Diversity and Natural Selection — AQA A-level Biology

SPEC 3.4.4 • GENETIC INFORMATION — GENETIC DIVERSITY & ADAPTATION

Genetic Diversity & Adaptation

Natural selection, directional & stabilising selection, and how populations adapt to their environment — featuring Required Practical 6

Genetic Diversity — The Foundation

Before natural selection can happen, there must be variation. That variation comes from differences in alleles within a population.

Genetic Diversity

The number of different alleles of genes in a population.

Why It Matters

Genetic diversity is a factor enabling natural selection to occur. Without variation in alleles, there is nothing for selection to act on — all individuals would be equally affected by a selection pressure and no evolutionary change could happen.

Sources of New Alleles

New alleles arise by random mutation in DNA. Meiosis and random fertilisation then shuffle existing alleles into new combinations, but they don't create new alleles — only mutation does.

Natural Selection

The process by which individuals carrying alleles that give them a selective advantage are more likely to survive and have greater reproductive success, so they pass those alleles to the next generation. Over many generations, the frequency of the advantageous allele increases in the population.

Evolution

A change in allele frequency in a population over time.

Examiner Alert

⚠️ Evolution is NOT individual organisms changing. Individual bacteria do not "become" resistant — the allele for resistance was already present as a rare mutation. The antibiotic selects for existing variation; it does not cause the mutation. Write: "the frequency of the resistance allele increased" — never "the bacteria developed resistance".

Mechanism — Five Steps

Every natural selection answer must work through this sequence with named molecules and named selection pressures.

1Random mutation produces a new allele of a gene in one individual.

2Variation in phenotype results — the population now contains individuals with different versions of the gene.

3Selection pressure acts on the population (predation, disease, competition, climate, antibiotics).

4Individuals with the advantageous allele are more likely to survive and reproduce — differential reproductive success.

5The advantageous allele is inherited by the next generation. Over many generations, its frequency in the population increases.

Types of Selection

Selection pressures don't always favour the same type of phenotype. AQA requires you to know two types; a third is useful context.

Grey dashed = original distribution; purple solid = distribution after selection

Directional Selection

A type of natural selection that favours one extreme phenotype, causing the population mean to shift in one direction over generations.

When It Happens

Directional selection typically occurs when the environment changes (new predator, new pathogen, new climate, new antibiotic). The phenotype that was previously average no longer gives the highest reproductive success — one extreme now does.

Classic Example

Antibiotic resistance in bacteria — the AQA-named example.

Other Examples

Peppered moth industrial melanism; pesticide resistance in insects; warfarin resistance in rats.

Antibiotic Resistance — Step by Step

This is the AQA-specified example of directional selection. Learn the sequence with named molecules.

1A random mutation in a bacterium's DNA produces a new allele — for example, an allele coding for an enzyme such as β-lactamase that breaks down the antibiotic.

2An antibiotic is introduced (selection pressure). Non-resistant bacteria are killed.

3The resistant bacterium survives and reproduces by binary fission — approximately every 20 minutes under ideal conditions.

4The resistance allele is passed to every daughter cell. Within hours the frequency of the resistance allele in the population rises sharply.

5The resistance allele can also spread between species via horizontal gene transfer on plasmids (conjugation). Frequency of the resistance allele rises rapidly.

Examiner Alert

⚠️ Never write: "The bacteria got used to the antibiotic" or "bacteria developed resistance". The mutation arose randomly and independently of the antibiotic. The antibiotic provides the selection pressure; it does not cause the mutation.

Stabilising Selection

A type of natural selection that favours the intermediate phenotype, reducing variation in the population. The mean stays the same; the range narrows.

AQA-Named Example — Human Birth Weight

Very low birth weight (< 2.5 kg): high infant mortality — immature organs, poor thermoregulation, difficulty feeding.

Very high birth weight (> 4.5 kg): difficult delivery, increased risk of birth complications for both mother and baby.

Intermediate birth weight (~3.5 kg): highest survival rate. Alleles for intermediate birth weight pass on more often.

Result

Population mean unchanged. Variation around the mean is reduced. The environment is not changing, so selection acts to maintain the well-adapted intermediate phenotype.

Exam Tip

When asked whether a graph shows directional or stabilising selection: check the mean. If the mean has shifted → directional. If the mean is the same but the range is narrower → stabilising.

Directional vs Stabilising — Compare

FeatureDirectionalStabilising

Phenotype favouredOne extremeIntermediate

Effect on meanMean shifts towards favoured extremeMean unchanged

Effect on variationVariation may stay similar or reduceVariation reduced

When it occursEnvironment is changingEnvironment is stable

AQA exampleAntibiotic resistance in bacteriaHuman birth weight

Evolutionary outcomePopulation adapts to new conditionsPopulation maintains existing adaptation

Synoptic Link

Connect this to Topic 7.4 (Evolution & Speciation). Directional selection in geographically isolated populations is the engine of allopatric speciation. When two populations are subject to different directional pressures, their allele frequencies diverge — eventually producing reproductive isolation and new species.

Adaptations — Three Types

Adaptation

A heritable characteristic that gives an organism a selective advantage — it increases the chance of surviving and reproducing successfully in its particular environment.

Natural selection produces species that are better adapted to their environment. Adaptations fall into three categories.

Anatomical (Structural)

Physical features of the body. Examples: streamlined body in dolphins (reduces drag when swimming); thick fur of Arctic fox (insulation); long neck of giraffe (reaches high foliage).

Physiological (Biochemical)

How the body works at a cellular or biochemical level. Examples: high-affinity haemoglobin in llamas at altitude (more efficient O₂ loading in thin air); antifreeze proteins in Antarctic fish; venom production in snakes.

Behavioural

Actions the organism performs. Examples: migration of wildebeest to follow rainfall; hibernation in hedgehogs; courtship displays in birds of paradise.

Examiner Alert

⚠️ Adaptations are HERITABLE. A bodybuilder's muscles are not an adaptation — they were acquired in one lifetime and cannot be passed on. If a characteristic is not encoded by alleles, it is not an adaptation.

Required Practical 6 — Aseptic Technique

Required Practical 6

Title: Use of aseptic techniques to investigate the effect of antimicrobial substances on microbial growth.

IV: Type or concentration of antimicrobial substance (e.g. different antibiotics; different concentrations of garlic extract)

DV: Diameter (or area) of the clear zone of inhibition around each disc, measured in mm with a ruler.

Control variables:

Volume of bacterial culture spread on plate — so the starting density is the same on every plate.

Bacterial species and strain — different species have different susceptibilities.

Incubation temperature and time — affects growth rate; standardly 25 °C for 24–48 h for school microbiology (not 37 °C, to minimise risk of growing human pathogens).

Volume / concentration of antimicrobial on each disc — otherwise you're comparing dose, not substance.

Results: Larger zone of inhibition = more effective antimicrobial. Measure diameter of each zone; calculate area using A = πr². Compare means across repeats.

RP6 — Aseptic Technique Steps

1Sterilise work area with disinfectant; wash hands; work near a Bunsen flame (convection currents carry airborne microbes upwards, away from plates).

2Flame the neck of the broth bottle before and after pouring — creates convection current preventing airborne contaminants entering.

3Pour sterile molten agar into a sterile Petri dish; allow to set. Spread bacterial culture evenly using a sterile spreader.

4Place sterile paper discs soaked in different antimicrobials onto the agar using flamed forceps. Include a control disc soaked in sterile water.

5Tape lid (do not seal fully — allows O₂ but prevents lid falling off); incubate upside down at 25 °C for 24–48 hours; measure zones of inhibition.

Why incubate at 25 °C, not 37 °C?

37 °C is human body temperature — it selects for potential human pathogens. 25 °C allows most school-safe microbes to grow but reduces risk of culturing anything dangerous to students.

Reading a Zone of Inhibition

The diameter of the clear zone around each disc indicates how effective the antimicrobial is.

Interpreting Results

Bacteria inside the clear zone have been killed — only bacteria at the edge of the zone (where antibiotic concentration drops below the minimum inhibitory concentration) remain. Any colonies growing within what should be a clear zone may carry a resistance allele. Isolating and re-testing such colonies is one way to provide evidence of a pre-existing resistance allele in the population.

DEEP Answer — Antibiotic Resistance

How to structure a 6-mark extended answer on directional selection:

DDefine: Natural selection is the process by which individuals carrying alleles that give a selective advantage have greater reproductive success, so they pass those alleles to the next generation and the allele frequency in the population changes over time (this is evolution).

EExplain mechanism: A random mutation in one bacterium produces a new allele coding for an enzyme (e.g. β-lactamase) that breaks down the antibiotic. When the antibiotic is applied, non-resistant bacteria die; the resistant bacterium survives and reproduces by binary fission. The resistance allele passes to every daughter cell.

EEvidence: Methicillin-resistant Staphylococcus aureus (MRSA) emerged after widespread methicillin use in hospitals from the 1960s; resistance allele frequency rose from near zero to the dominant form within decades.

PPractical/Quantitative: In RP6, zone-of-inhibition diameter (mm) quantifies antibiotic effectiveness against a bacterial strain. If bacteria sampled from a strain already exposed to an antibiotic show a smaller zone than a non-exposed strain, this supports a higher resistance allele frequency. Bacterial counts in cultures often span many orders of magnitude, so log scales are commonly used when plotting population growth.

Practice Question 1

Q1. A population of insects was sprayed with a new pesticide. Over ten generations, the proportion of insects surviving the pesticide increased from 0.2% to 68%.

(a) Name the type of selection shown. [1 mark]

(b) Explain how this change occurred. [4 marks]

(a)

Directional selection.

(b)

(1) A random mutation produced a new allele conferring pesticide resistance — the allele was present before the pesticide was applied. (2) When the pesticide was applied it acted as a selection pressure — non-resistant insects died. (3) Resistant insects survived and reproduced, passing the resistance allele to their offspring. (4) Over generations the frequency of the resistance allele increased in the population.

Key words:

random mutation, allele, selection pressure, survive, reproduce, frequency.

Practice Question 2

Q2. A student placed four discs soaked in different antibiotics onto an agar plate seeded with E. coli. After 48 hours at 25 °C, the zones of inhibition were measured.

(a) State two control variables the student should have kept constant. [2 marks]

(b) Explain why the plate was incubated at 25 °C rather than 37 °C. [2 marks]

(c) One disc produced no zone of inhibition. Give two possible biological explanations. [2 marks]

(a)

Any two of: volume of bacterial culture spread on the plate; volume / concentration of antibiotic on each disc; incubation time; bacterial species / strain. Each with a reason (e.g. "so the starting bacterial density is the same").

(b)

37 °C is human body temperature and would select for potential human pathogens; 25 °C reduces the risk of culturing organisms harmful to students while still allowing growth.

(c)

Any two of: the bacteria carried a resistance allele to this antibiotic; the antibiotic concentration on the disc was too low to have an effect; the antibiotic did not diffuse properly through the agar.

Practice Question 3

Q3. In humans, the distribution of birth weights has remained almost unchanged for thousands of years, even though medicine has allowed more very-low and very-high birth-weight babies to survive.

(a) Name the type of selection that has historically acted on human birth weight. [1 mark]

(b) Explain how this selection maintains the distribution. [3 marks]

(c) Suggest how modern medical care might change this selection pattern over many generations. [2 marks]

(a)

Stabilising selection.

(b)

(1) Babies with very low birth weight have reduced survival due to immature organs / poor thermoregulation. (2) Babies with very high birth weight have reduced survival due to difficult delivery. (3) Babies of intermediate birth weight have highest survival, so alleles for intermediate birth weight are passed on more often — the mean stays the same and variation is reduced.

(c)

Modern medicine (e.g. incubators, Caesarean sections) removes the selection pressure against extreme birth weights. Over many generations the variation in birth weight may increase because alleles producing very low or very high weights are no longer selected against.

Practice Question 4 — Synoptic

Q4. Two populations of the same beetle species were separated by a new river 5,000 years ago. One population lives on light sandy ground; the other on dark volcanic soil. Today the two populations differ in shell colour: the sandy-ground population is pale; the volcanic-soil population is dark. Beetles from the two populations can no longer interbreed to produce fertile offspring.

(a) Explain how directional selection produced the colour difference between the two populations. [4 marks]

(b) State what this example illustrates about the role of natural selection in the evolution of new species. [2 marks]

(a)

(1) Random mutations produced a range of shell colour alleles in each population. (2) On sandy ground, pale beetles were better camouflaged from predators — they survived and reproduced more. (3) On volcanic soil, dark beetles were better camouflaged — they survived and reproduced more. (4) Over generations the frequency of pale alleles increased in the sandy population and the frequency of dark alleles increased in the volcanic population.

(b)

(1) Different selection pressures on reproductively isolated populations cause allele frequencies to diverge. (2) Over time this can lead to reproductive isolation and the formation of a new species (allopatric speciation — linked to Topic 7.4).

Summary

Must-Know Definitions

Genetic diversity — number of different alleles of genes in a population.

Natural selection — individuals with alleles giving a selective advantage have greater reproductive success and pass those alleles on, so allele frequency in the population changes over time.

Evolution — a change in allele frequency in a population over time.

Adaptation — a heritable characteristic giving a selective advantage in a particular environment.

Directional

One extreme favoured → mean shifts. Antibiotic resistance.

Stabilising

Intermediate favoured → variation reduced. Birth weight.

Adaptations — Three Types

Anatomical — structural (streamlined body, thick fur).

Physiological — biochemical (high-affinity Hb at altitude).

Behavioural — actions (migration, hibernation).

Top Exam Tip

Whenever you explain natural selection: always include the words random mutation, allele, selection pressure, survive and reproduce, and frequency. Missing any one of these usually costs a mark.