4.4Genetics, Variation and Interdependence
Genetic Diversity and Adaptation — AQA A Level Biology 7402 Revision Notes & Topical Questions
10 min read 17 parts Lesson 21 of 46 Unit quiz
Genetic Diversity, Logarithmic Scales and Natural Selection — AQA A LEVEL BIOLOGY 7402 notes: Explain genetic diversity, interpret logarithmic scales and describe how natural selection works.Organisms of the same species have very similar genomes, but any two individuals (even tw…
- genetic variation
- selection pressures
- Worked example 1
- Worked example 2
- Worked example 3
- Variation exists within a population owing to random mutation
Section 1 of 17Genetic diversity
Organisms of the same species have very similar genomes, but any two individuals (even twins) have differences in DNA base sequence.
If a population has a small gene pool or very low genetic diversity it is much less able to adapt to changes in the environment and may become vulnerable to extinction. Cheetahs are an example: about 10 000 years ago their numbers declined greatly, leaving small, fragmented populations with no mixing between them and large amounts of inbreeding.
Section 2 of 17Using logarithms with bacteria
Bacterial colonies can grow rapidly, producing very large numbers within hours.
- or the hydrogen ion concentration at each pH level.
- (it varies massively between levels)
- but you may need to interpret results using logarithmic scales or explain the benefit of one.
Section 3 of 17Natural selection
Genetic variation exists within populations due to different alleles. There is differential reproductive success between organisms with different alleles of the same gene. Under certain environmental conditions, individuals with certain alleles have an increased chance of survival and reproduction. The sequence is:
The principles are always the same; you only need to edit the details to the species in the question.
Section 4 of 17Selection pressures
Environmental factors that affect the chance of survival are selection pressures. For example, high competition for food between lions when prey is scarce selects for faster, more powerful lions that are better hunters. These pressures can change allele frequencies in a population by natural selection, as described in the next lesson.
Section 5 of 17Worked examples
Explain why a population with low genetic diversity is vulnerable to extinction. [2]2 marks
It is unlikely to contain alleles that suit a changed environment or new disease, so few individuals survive and reproduce.
Explain the benefit of plotting bacterial counts on a logarithmic scale. [2]2 marks
The numbers cover a very large range, and a log scale lets them be shown clearly on a single graph.
Explain how natural selection could lead to antibiotic resistance. [4]4 marks
A random mutation gives resistance to an antibiotic. When the antibiotic is used it kills sensitive bacteria; the resistant bacterium survives and reproduces, passing on the allele. Resistance becomes more common.
Section 6 of 17Stabilising and directional selection
Stabilising selection selects against the extreme phenotypes (for example very high and very low birth weights) and for the intermediate phenotypes (medium birth weight); it reduces variation and the mean stays the same. Directional selection is natural selection that produces a gradual change in allele frequencies over several generations.
- and antibiotic resistance for directional selection) are named in the specification.
Section 7 of 17Adaptation to the environment
Natural selection results in species better adapted to their environment. Certain alleles produce features that make an organism better suited to its environment. When new alleles arise by mutation, there is potential for relatively rapid change if the environment changes. Types of adaptation:
- such as mosquitoes producing chemicals that stop blood clotting so they can feed more easily.
Section 8 of 17Speciation
If two populations of one species are isolated from each other and become so different in phenotype that they can no longer interbreed to produce fertile offspring, they have formed two new species.
Section 9 of 17Required practical: aseptic techniques
When investigating the effect of antimicrobial substances on microbial growth, aseptic techniques must be used to ensure the microbes being investigated do not escape and are not contaminated by other, possibly pathogenic, microbes.
Section 10 of 17Worked examples
Distinguish stabilising from directional selection. [2]2 marks
Stabilising selection favours the average phenotype and removes the extremes; directional selection favours one extreme, shifting the mean.
Explain why incubation is at 25 °C rather than 37 °C in school laboratories. [2]2 marks
37 °C is the temperature at which human pathogens grow best; a lower temperature reduces this risk.
Explain how speciation can occur. [3]3 marks
Populations isolated geographically experience different selection pressures. Allele frequencies diverge and eventually they cannot interbreed to produce fertile offspring.
Section 11 of 17Disc diffusion method
- The antibiotic solutions must be at the same concentration so that their effects can be compared.
- Include a control disc soaked in sterile water.
The larger the clear zone, the more effective the antibiotic against that bacterium, because it prevented bacterial growth further from the disc. The control disc should show no clear zone.
Section 12 of 17Interpreting the plate
After incubation, bacteria grow as a cloudy lawn everywhere except around discs releasing effective antibiotic. Compare the areas of the clear zones to rank effectiveness. Source of error: antibiotic diffusion rates differ with molecular size, so zone size is not a perfect comparison.
Section 13 of 17Minimum inhibitory concentration (MIC)
When antibiotics treat bacterial infections the dosage is carefully controlled.
Section 14 of 17Safety and reliability
Wear gloves and eye protection, disinfect the bench before and after, and dispose of plates by autoclaving.
Section 15 of 17Worked examples
Explain why the discs must be soaked in antibiotics of the same concentration. [2]2 marks
So the only variable is the type of antibiotic, and so results can be compared fairly.
A clear zone has a diameter of 16 mm. Calculate its area. [2]2 marks
r = 8 mm; area = π × 82 = 201 mm2.
Define minimum inhibitory concentration. [1]1 mark
The lowest concentration of an antibiotic that prevents visible growth of bacteria.
Section 16 of 17Key terms
| Term | Meaning |
|---|---|
| Genetic variation | Differences in DNA base sequence between individuals |
| Gene pool | All alleles in a population |
| Selection pressure | Environmental factor that affects survival |
| Allele | Version of a gene |
| Logarithmic scale | Scale in which each interval is a multiple of the last |
| Term | Meaning |
|---|---|
| Stabilising selection | Selection favouring intermediate phenotypes |
| Directional selection | Selection favouring one extreme phenotype |
| Speciation | Formation of new species |
| Aseptic technique | Method preventing contamination |
| Adaptation | Feature that increases survival and reproduction |
| Term | Meaning |
|---|---|
| Lawn | Continuous layer of bacterial growth on agar |
| Clear zone | Region without growth around an antibiotic disc |
| MIC | Minimum inhibitory concentration |
| Nutrient agar | Jelly containing nutrients for microbial growth |
Section 17 of 17Exam technique
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