AQA A-level Biology: Genetic Diversity, Natural Selection and Required Practical 6 (7402) – Revision Notes

These revision notes condense AQA A-level Biology (7402) section 3.4.4, Genetic diversity and adaptation, with MS 2.5 and required practical 6, from the AQA AS and A-level Biology specification for first teaching 2015. Full explanations and worked examples are in the study guide. When you can recall everything here, try the practice questions. Course pages: AQA A-level Biology hub and the printable checklist. Use the free diagnostics to find your weak spots. Section 3.4.4 is topic 4, which is AS content as well as A-level content.

Key definitions

Term	Meaning
Allele	A different version of the same gene
Genetic diversity	The number of different alleles of genes in a population
Mutation	A change in the base sequence of DNA; random; the source of new alleles
Selection pressure	An environmental factor that affects survival or reproduction
Natural selection	Individuals with advantageous alleles survive and reproduce more, so the allele becomes more common over generations
Adaptation	A feature that increases an organism’s chance of survival and reproduction in its environment
Antibiotic resistance	The ability of bacteria to survive an antibiotic that would normally kill or stop them

Natural selection in five steps

1. Variation exists in the population because of different alleles.
2. A random mutation produces a new allele. Many mutations are harmful, but in some environments a new allele benefits its possessor.
3. A selection pressure acts. Carriers of the advantageous allele have increased reproductive success.
4. The advantageous allele is inherited by the next generation.
5. Over many generations the allele frequency increases.

Genetic diversity matters because a population with many alleles is more likely to contain individuals that survive a change in the environment.

Must-know distinctions

	Directional selection	Stabilising selection
Favoured	One extreme phenotype	Phenotypes near the mean
Selected against	The other extreme (and often the mean)	Both extremes
Mean	Shifts	Stays about the same
Range or spread	May stay similar	Narrows
Example	Antibiotic resistance in bacteria	Human birth weight

Mistake	Correct version
“Bacteria adapt to the antibiotic”	Resistant mutants were already present, survive and reproduce
“The antibiotic causes the mutation”	The antibiotic only selects; mutation is random
“Individuals evolve”	Populations evolve, as allele frequency changes
“Genetic diversity is the number of individuals”	It is the number of different alleles

Antibiotic resistance: method for an explanation

• Resistance allele arose by random mutation, before the antibiotic was used.
• The antibiotic is the selection pressure.
• Susceptible bacteria die; resistant bacteria survive and reproduce.
• The allele passes to daughter cells, so its frequency rises.
• Reproduction is fast, so the change happens in a short time.
• Link to practice: complete the prescribed course so fewer tolerant bacteria are left.

Worked reminder: a broth holds 5.0 × 10⁸ cells and 1 cell in 10⁶ carries a resistance allele. Resistant cells = 5.0 × 10⁸ × 10⁻⁶ = 500. After the antibiotic kills the susceptible cells, these 500 are almost all that is left to divide, so they found the next population.

Birth weight: method for an explanation

• Very low birth mass: heat loss, and organs may be less developed, so higher mortality.
• Very high birth mass: harder delivery, so higher mortality.
• Intermediate mass has the highest survival, so it is favoured.
• Say that the mean stays the same and the standard deviation or range decreases.
• If given mortality figures, compare an extreme with the lowest value as a ratio, for example 40 deaths against 5 deaths per 1000 is 8 times higher.
• If given standard deviations before and after, say the spread has fallen and quote both values.

Three types of adaptation

Type	Test	Example
Anatomical	Can you see or dissect the structure?	Streamlined body, thick fur, camouflage
Physiological	Is it a process or chemistry?	Producing a toxin-breaking enzyme, sweating
Behavioural	Is it an action?	Huddling, migration, display before mating

Adaptations in an unfamiliar organism: identify the feature, say which type it is, then link it to survival or reproduction, for example “thick fur reduces heat loss, so more individuals survive cold winters and reproduce”.

Adaptation and selection are major factors in evolution. Different environments favour different alleles, which contributes to the diversity of living organisms.

Interpreting selection data: checklist

1. Describe the overall trend first, with figures and units.
2. Use percentage points for changes in a percentage; use percentage change for changes in a quantity.
3. Decide the type of selection from what happens to the mean and range.
4. Explain with the selection chain, using the context of the question.
5. Note limitations: correlation is not proof of cause, and a small sample size reduces confidence.

Explaining change in an unfamiliar population

Use this skeleton and put the organism’s details into each line.

Step	Wording to include
Variation	“Different alleles were already present because of random mutation”
Pressure	Name the factor in the question
Survival	“Individuals with the allele were more likely to survive and reproduce”
Inheritance	“They passed the allele to their offspring”
Result	“The allele frequency increased over many generations”

MS 2.5: logarithms

Idea	Rule
Log scale	Each equal step is a tenfold increase
log₁₀ of a power of ten	log₁₀ 10ⁿ = n
Doubling	Adds 0.301 to log₁₀
Exponential growth	Gives a straight line on a log axis
Generation time	0.301 divided by the slope (log₁₀ units per hour) gives hours

Method for a generation time: take two points in the straight-line part, find the change in log₁₀ count, divide by the time difference to get the slope, then divide 0.301 by it.

Worked reminder: a culture rises from 4.0 × 10⁴ to 4.096 × 10⁷ in 5 h. The ratio is 1024 = 2¹⁰, so there are 10 doublings in 5 h, giving a generation time of 30 minutes.

Required practical 6 at a glance

Aim: use aseptic techniques to investigate the effect of antimicrobial substances on microbial growth.

Precaution	Reason
Disinfect bench, wash hands	Remove unwanted microbes
Work near a flame	Rising air carries microbes away from the culture
Flame the loop, cool it before use	Sterilises it without killing the culture on contact
Open the plate lid as little as possible	Limits contamination from the air
Tape the lid but not fully round	Stops lid slipping and contamination, while air can still enter
Invert the plate	Stops condensation dripping on to the agar
Incubate at a lower temperature than body temperature	Reduces the chance of growing harmful pathogens

Analysis steps. Measure clear-zone diameters, find the mean for each substance, calculate area = π × (diameter / 2)², and compare the results with the control.

Controls and variables. A disc with sterile water is the control. Keep disc size, volume, concentration, agar depth, lawn density, incubation time and temperature the same.

Reliability. Repeat plates, calculate the mean and look for anomalies. A larger zone suggests stronger inhibition, but substances diffuse at different rates, so state this as a limitation.

Quick self-test

1. Define genetic diversity.
2. Population A has seven alleles of a gene and population B has three. Which has the greater genetic diversity?
3. What is log₁₀ (1 × 10⁸)?
4. Bacteria divide every 20 minutes. Starting from one cell, how many cells are there after 2 hours, assuming no deaths?
5. State what happens to the mean and the range in stabilising selection.
6. A clear zone has a diameter of 16 mm. Calculate its area to 3 significant figures.
7. The percentage of resistant isolates rises from 12% to 30%. State the rise.
8. Is the ability to hibernate anatomical, physiological or behavioural? Give a reason.
9. A log₁₀ count rises by 0.903 in 1 hour. Calculate the generation time in minutes.
10. Why is the plate incubated upside down?

Answers

1. The number of different alleles of genes in a population.
2. Population A.
4. 6 divisions in 2 h, so 2⁶ = 64 cells.
5. The mean stays about the same; the range narrows.
6. π × 8² = 201 mm².
7. 18 percentage points.
8. Hibernation is mainly behavioural, because it is an action chosen by the organism; the body changes underlying it are physiological. Either label can be argued if the reasoning is clear.
9. 0.301 / 0.903 = 0.333 h = 20 minutes.
10. Condensation does not drip on to the agar and disturb colonies.

Where marks are usually lost

• Defining genetic diversity as the number of individuals or “variety” without alleles.
• Saying bacteria “mutate to become resistant” because of the antibiotic.
• Not stating that the resistance allele was inherited and its frequency increased.
• Forgetting that stabilising selection leaves the mean unchanged.
• Quoting percentage when the question needs percentage points.
• Comparing clear-zone diameters when areas are needed.
• Omitting the control or the controlled variables in the practical.
• Treating log scale steps as equal additions.
• Giving a general answer that does not use the organism in the question.

Official syllabus

This page follows the AQA AS and A-level Biology specification (7401 and 7402), for first teaching 2015, published by AQA, section 3.4.4 and required practical 6.