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

This guide covers AQA A-level Biology (7402) section 3.4.4, Genetic diversity and adaptation, as set out in the AQA AS and A-level Biology specification for first teaching 2015. It also covers required practical 6 and the mathematical skill MS 2.5 (logarithms). Section 3.4.4 sits in topic 4, which is AS content as well as A-level content. For quick recall use the revision notes, then test yourself with the practice questions. Course pages: AQA A-level Biology hub and the printable checklist. To find weak spots first, try the free diagnostics. Mutation, which supplies new alleles, is in the mutation and meiosis guide.

What the specification asks for

Statement in 3.4.4	What you must be able to do
Genetic diversity	Define it as the number of different alleles of genes in a population
Natural selection	Explain the principles in the evolution of populations, and why genetic diversity is needed
Directional selection	Explain it using antibiotic resistance in bacteria
Stabilising selection	Explain it using human birth weights
Adaptation	Distinguish anatomical, physiological and behavioural adaptations
Skills	Use unfamiliar information to explain how selection changes a population; interpret selection data; link adaptation and selection to the diversity of living organisms
MS 2.5	Use a logarithmic scale, for example for the growth of microorganisms
Required practical 6	Use aseptic techniques to investigate the effect of antimicrobial substances on microbial growth

Genetic diversity

A gene is a section of DNA at a particular position (locus) on a chromosome. Different versions of a gene are alleles. The specification defines genetic diversity as the number of different alleles of genes in a population. It counts how many kinds of allele exist, not how many individuals there are.

Take a gene for coat colour in a rodent. Population P has five alleles of the gene. Population Q has two. P has the greater genetic diversity for that gene, even if Q has more animals.

Genetic diversity is a factor enabling natural selection to occur. If every individual had the same alleles, they would all have the same chance of surviving a change such as a new disease. Selection would have nothing to act on. A population with many alleles is more likely to contain at least one individual able to survive and reproduce when conditions change.

New alleles arise by mutation. Mutation is random, which means it does not occur because the organism needs the change.

The principles of natural selection

The specification gives four linked steps. Learn them as a chain, because examiners reward the links between steps.

1. Random mutation can result in a new allele of a gene.
2. Many mutations are harmful. In certain environments, however, the new allele may benefit its possessor, leading to increased reproductive success.
3. The advantageous allele is inherited by members of the next generation.
4. Over many generations, the frequency of the new allele in the population increases.

Two points are often lost. First, individuals do not change their alleles in response to the environment; the population changes because carriers of the advantageous allele leave more offspring. Second, the environment does not cause the mutation. It only decides whether the allele is an advantage.

Writing an explanation. Use this order: variation exists (due to different alleles) → an environmental factor (the selection pressure) acts → carriers of the advantageous allele leave more offspring → they pass on the allele → its frequency rises in later generations.

Directional selection: antibiotic resistance

In directional selection one extreme phenotype has an advantage, so the population changes in one direction. Antibiotic resistance in bacteria is the specified example.

A resistant allele might code for an enzyme that breaks down the antibiotic. It arises by random mutation in a few cells, before the antibiotic is ever used. Presence of the antibiotic is the selection pressure.

Worked example 1: a culture meets an antibiotic. A broth culture contains 2.0 × 10⁹ cells. A resistance allele arises at a frequency of 1 cell in 10⁷.

1. Resistant cells = 2.0 × 10⁹ × 10⁻⁷ = 200.
2. Susceptible cells are almost all the rest. Suppose the antibiotic leaves only 1 in 10⁵ of them alive: 2.0 × 10⁹ × 10⁻⁵ = about 20 000 survivors.
3. All 200 resistant cells survive. Proportion resistant afterwards = 200 / (20 000 + 200) = 0.0099, so about 1%. Before treatment it was 0.00001%.
4. If the antibiotic stays present, only resistant cells divide. After 12 doublings the resistant population is 200 × 2¹² = 819 200 cells.

The proportion resistant rose from a tiny fraction to nearly all of the new population. Reproduction of survivors is what changes the population.

Why resistance spreads. Heavy antibiotic use means strong selection pressure. Failing to finish a prescribed course can leave the most tolerant bacteria alive. Bacteria such as MRSA are strongly favoured where antibiotics are in constant use.

Stabilising selection: human birth weight

In stabilising selection individuals near the mean have an advantage and both extremes are selected against. The range of phenotypes narrows and the mean stays about the same. Human birth weight is the specified example.

Very small babies lose heat quickly and are more likely to have underdeveloped organs. Very large babies are harder to deliver. Both extremes have higher mortality.

Worked example 2: reading mortality data. The invented figures below show deaths per 1000 live births.

Birth mass (kg)	below 2.0	2.0–2.5	2.5–3.0	3.0–3.5	3.5–4.0	4.0–4.5	above 4.5
Deaths per 1000	85	30	9	5	6	14	40

1. Mortality is lowest at 3.0–3.5 kg and rises on both sides: stabilising selection.
2. Comparing extremes with the lowest value: 85 / 5 = 17 times higher below 2.0 kg, and 40 / 5 = 8 times higher above 4.5 kg.
3. Before selection, birth mass has mean 3.3 kg and standard deviation 0.62 kg. Among survivors, the standard deviation is 0.50 kg. The range covered by the mean ± 2 standard deviations narrows from 2.06–4.54 kg (width 2.48) to 2.30–4.30 kg (width 2.00).
4. Percentage decrease in width = (2.48 − 2.00) / 2.48 × 100 = 19%. The mean has not changed, but the spread has fallen.

Directional versus stabilising. In directional selection the mean shifts. In stabilising selection the mean stays and the spread shrinks. Use “mean” and “range” or “standard deviation” in your answer.

Adaptation

Natural selection results in species that are better adapted to their environment. Adaptations may be:

Type	What it is	Examples
Anatomical	A structural feature of the body	Thick fur or blubber; streamlined body; camouflage pattern
Physiological	A process inside the body	Antifreeze proteins in the blood of some polar fish; production of enzymes that break down a toxin; sweating
Behavioural	A way the organism acts	Huddling in groups to save heat; courtship displays; moving to shade in the heat

Give the single best label for the feature in the question. Organisms do not “try” to adapt. Adaptations exist because past individuals with those alleles reproduced more.

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

Using unfamiliar information

Questions often describe an organism you have not studied. Apply the same chain.

Worked example 3. A weed shows resistance to a herbicide. Resistant plants were 3% of a field population in the first year of spraying and 70% five years later. Explain the change.

• Random mutation produced a resistance allele in a few plants; it was present before spraying.
• Herbicide was the selection pressure. Susceptible plants died or produced few seeds.
• Resistant plants survived, reproduced more and passed the allele on in seeds.
• Over the generations the proportion of resistant plants rose from 3% to 70%, so the allele frequency rose.

Interpreting selection data

Worked example 4. Percentage of bacterial isolates resistant to an antibiotic in a hospital (invented data):

Year	1	2	3	4	5	6
Resistant (%)	3	7	15	31	52	70

1. Year-to-year rises are 4, 8, 16, 21 and 18 percentage points. The rise gets faster, then slows.
2. The resistant share went up each year, so the allele frequency is increasing: directional selection.
3. Susceptible isolates fell from 97% to 30%.
4. Data alone show correlation. They do not show that antibiotic use caused the change; you would need data on antibiotic use.

MS 2.5: logarithmic scales

Bacterial numbers cover many orders of magnitude, for example 10³ to 10⁹ cells per cm³. On a logarithmic scale each equal step on the axis is a tenfold change. A log₁₀ value is the power of ten: log₁₀ 1000 = 3, and log₁₀ (3.2 × 10⁶) = 6.505.

Worked example 5. A culture doubles every 30 minutes. Counts per cm³ are:

Time (h)	0	1	2	3	4	5
Count	5.0 × 10⁴	2.0 × 10⁵	8.0 × 10⁵	3.2 × 10⁶	1.28 × 10⁷	5.12 × 10⁷
log₁₀ count	4.699	5.301	5.903	6.505	7.107	7.709

1. During exponential growth log₁₀ count rises by the same amount each hour: 0.602.
2. A doubling adds log₁₀ 2 = 0.301 to the log value. So 0.602 per hour means two doublings per hour.
3. Generation time = 1 h / 2 = 30 minutes.

A straight rising line on a log graph shows exponential growth.

Required practical 6: aseptic technique and antimicrobial substances

Aim: investigate the effect of antimicrobial substances (antibiotics, antiseptics or disinfectants) on microbial growth. Aseptic technique prevents contamination of the culture by unwanted microbes, and protects you from harmful ones.

Typical method (check the version your teacher gives you).

1. Disinfect the bench and wash hands. Work near a Bunsen flame so rising air carries microbes away.
2. Flame the inoculating loop until red hot, cool it without touching anything, and transfer a small amount of broth culture.
3. Lift the lid of the sterile agar plate only as far as necessary. Spread the bacteria evenly with a sterile spreader to give a lawn.
4. Using sterile forceps, place paper discs soaked in each antimicrobial substance on the lawn. Place one disc soaked in sterile water as a control.
5. Seal the lid with tape, but not all the way round, and label it on the base. Invert the plate.
6. Incubate at a temperature your teacher specifies, kept lower than body temperature in school to reduce the chance of growing pathogens.
7. Measure the diameter of each clear zone, where bacteria have not grown. Dispose of plates as your school’s risk assessment requires.

Worked example 6. Mean clear-zone diameters over three plates (disc excluded):

Substance	Diameters (mm)	Mean (mm)	Area (mm²)
Antibiotic X	18, 20, 19	19	284
Antibiotic Y	12, 11, 13	12	113
Disinfectant Z	22, 21, 23	22	380
Sterile water	0, 0, 0	0	0

• Area = πr², where r = diameter / 2. For X, area = π × 9.5² = 284 mm².
• X’s zone is 284 / 113 = 2.5 times the area of Y’s, though the diameter is only 1.6 times larger. Compare areas, since the zone is a circle.
• The control shows the disc and water alone do not stop growth.
• A larger zone suggests stronger inhibition, but different substances diffuse at different rates, so be cautious.

Variables to control: volume and concentration on each disc, agar depth, lawn density, incubation time and temperature. Repeats give a mean and show anomalies.

Common errors

• Defining genetic diversity as variation between individuals instead of the number of different alleles.
• Writing that bacteria “become resistant” or “adapt” to the antibiotic, rather than that resistant mutants survive and reproduce.
• Describing stabilising selection without saying the mean stays the same and the range falls.
• Calling a log scale’s equal steps “equal differences” instead of equal multiples.

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.