Genetic Diversity & Adaptation

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Genetic Diversity

Genetic diversity is the number of different alleles of genes in a population.
Individuals of the same species have very similar genomes, but differences in DNA base sequences exist between them; these differences are called genetic variation.
Mutation generates new alleles and contributes to genetic diversity or the size of the gene pool.
A new allele may be advantageous, disadvantageous, or have no apparent effect on phenotype because the genetic code is degenerate.
New alleles can remain hidden (not expressed) within a population for several generations before they contribute to phenotypic variation.
A population with a large gene pool or high genetic diversity has a strong ability to adapt to change.
A population with a small gene pool or very low genetic diversity is less able to adapt to environmental changes and can become vulnerable to extinction.
Cheetahs are an example of a species with a small gene pool; a large decline in numbers about 10,000 years ago left small, fragmented populations with inbreeding and low genetic variation.

Using Logarithms When Investigating Bacteria

Bacterial colonies can grow at rapid rates in culture, producing very large numbers of bacteria within hours.
Dealing with experimental data from large numbers of bacteria is difficult on traditional linear scales because there is a wide range of very small and very large numbers.
Logarithmic scales allow a wide range of values to be displayed on a single graph.
An order of magnitude means a tenfold (×10) change in quantity; on a base 10 logarithmic scale, each unit step represents one order of magnitude.
On a logarithmic scale, there are uneven intervals between values on one or more axes.
In an experiment with yeast cells, the number of cells increased rapidly; converting the numbers to logarithms allowed the wide range of cell numbers to fit easily onto the same scale.
The pH scale is logarithmic; the concentration of hydrogen ions varies massively between each pH level.

Natural Selection

Genetic variation exists within populations due to the presence of 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.
Natural selection can cause the frequency of alleles in a population to change over time.
The principles of natural selection: random mutation produces new alleles; some alleles benefit their possessor; the advantageous allele is passed on to the next generation; over several generations, the new allele increases in frequency.
In rabbits, brown fur is produced by a dominant allele and white fur by a recessive allele; foxes act as a selection pressure because white rabbits are less camouflaged and more likely to be seen by predators.
Brown rabbits have a selection advantage, so they are more likely to survive to reproductive age and pass on their alleles; over many generations, the frequency of alleles for brown fur increases and for white fur decreases.

Directional & Stabilising Selection

Selection pressures are environmental factors that affect the chance of survival of an organism.
Stabilising selection keeps allele frequencies relatively constant over generations; things stay as they are unless there is a change in the environment.
An example of stabilising selection is human birth weight: babies with very low or very high birth weight have higher mortality risks, while medium birth weight babies have the highest survival rates.
Directional selection produces a gradual change in allele frequencies over several generations; it usually happens when there is a change in environment/selection pressures or a new advantageous allele appears.
An example of directional selection is antibiotic resistance in bacteria due to the overuse of antibiotics; the presence of antibiotics is a selection pressure.
In directional selection, a mutation confers antibiotic resistance; bacteria with this mutation are more likely to survive and reproduce, while most without it die; over generations, the frequency of the beneficial allele increases.
Stabilising selection selects against extreme phenotypes and for intermediate phenotypes; directional selection favours one extreme phenotype, causing the mean trait value to change over time.

Environmental Adaptation

Natural selection results in species that are better adapted to their environment.
Certain alleles can produce features that make an organism better suited to its environment, giving greater chances of survival.
When new alleles arise by mutation, there is potential for relatively rapid change in a species if its environment changes.
Natural selection selects for favourable alleles that produce adaptations; individuals with these alleles are more likely to survive and produce more offspring, so the allele frequency increases.
Natural selection selects against unfavourable alleles; individuals with these alleles are less likely to survive and produce fewer offspring, so the allele frequency decreases.
Adaptations can be anatomical (structural features, e.g. white fur of a polar bear for camouflage, long limbs in desert animals for heat dissipation), physiological (internal biological processes, e.g. chemicals in mosquito saliva that stop blood clotting, lowering metabolism during hibernation), or behavioural (actions, e.g. cold-blooded reptiles basking in the sun, migration of birds).
Evolution is the change in adaptive features of a population over time as a result of natural selection.
If two populations of one species become isolated and so different in phenotype that they can no longer interbreed to produce fertile offspring, they have formed two new species (speciation).

Required Practical: Aseptic Techniques

Aseptic techniques ensure that microbes being investigated do not escape or become contaminated with unwanted, possibly pathogenic, microbes.
Sterilise Petri dishes and nutrient agar before use; disinfect work surfaces with disinfectant or alcohol before inoculation.
Work next to a lit Bunsen burner to create convection currents that stop contaminants falling onto the growth media.
Use flamed inoculating loops or sterile swabs; heat loops until red-hot before and after inoculation.
Keep the lid on Petri dishes when not in use, and lift the lid at an angle during inoculation to reduce contamination from airborne microorganisms.
Tape the Petri dish lids closed after plating to prevent escape of potentially harmful microorganisms.
Incubate plates at a safe temperature of 25 °C to prevent growth of pathogenic microorganisms that are more likely to grow at human body temperature.
Sterilise or dispose of all equipment after use.

Investigating Antimicrobial Substances

The disc diffusion experiment is used to test antibiotic effectiveness.
A colony-forming unit (CFU) is a live bacterial cell that can divide and form a colony on an agar plate.
Method: pre-soak paper discs in different antibiotic solutions of the same concentration; spread a sample of diluted bacterial broth onto a sterile agar plate to create a lawn; press discs evenly onto the agar; incubate overnight; examine results with the lid on; measure the clear areas around each disc.
Antibiotics diffuse outwards from each paper disc, forming a concentration gradient; the antibiotic is most concentrated at the disc.
If bacteria are vulnerable to an antibiotic, a clear area (zone of inhibition) appears around the disc; there are no bacteria present in this zone.
The clear area ends when the concentration of the antibiotic reaches a level at which the bacteria are no longer susceptible.
More effective antibiotics require a lower concentration to kill bacteria and produce larger clear zones; if bacteria are completely resistant, there is no clear zone.
The minimum inhibitory concentration (MIC) is the lowest concentration of a substance that will inhibit the growth of a microorganism.

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1.What is the definition of genetic diversity?
Easy
- AThe number of different alleles of genes in a population
- BThe total number of genes in a population
- CThe number of individuals in a population
- DThe number of different species in an ecosystem
2.What is meant by the term 'gene pool'?
Easy
- AThe total number of different alleles in a population
- BThe total number of genes in an individual
- CThe sum of all the alleles of all the genes in a population
- DThe number of individuals in a population
3.A population with a large gene pool has a strong ability to adapt to change. What happens to a population with a very low genetic diversity?
Easy
- AIt becomes more vulnerable to extinction
- BIt becomes more resistant to disease
- CIt evolves more rapidly
- DIt has a higher chance of survival
4.Which of the following are examples of aseptic techniques used in microbiology? (select all that apply)
Medium
- ASterilising Petri dishes
- BWorking next to a lit Bunsen burner
- CUsing flamed inoculating loops
- DIncubating plates at 37 °C
- ETaping Petri dish lids closed
5.Stabilising selection keeps allele frequencies relatively constant over generations.
Easy
True or false?
6.Directional selection favours extreme phenotypes at both ends of the range.
Easy
True or false?
7.Match the type of adaptation with its correct example.
Medium
- Anatomical
- Physiological
- Behavioural
- White fur of a polar bear
- Mosquitoes produce anti-clotting chemicals
- Cold-blooded reptiles bask in the sun
8.Place the following statements about natural selection into chronological order.
Medium
- Random mutation can produce new alleles of a gene
- A particular mutation gives a benefit to its possessor, leading to an increased chance of survival
- The advantageous allele is passed onto the next generation
- The new allele increases in frequency in the population

Plus de questions

9.Which of the following statements about genetic diversity are correct? (select all that apply)
Medium
- AGenetic diversity is the number of different alleles of genes in a population.
- BGenetic diversity is the total number of individuals in a population.
- CGenetic diversity arises from differences in DNA base sequences between individuals of the same species.
- DA population with low genetic diversity has a strong ability to adapt to environmental change.
- EMutation generates new alleles and so contributes to genetic diversity.
10.Which of the following are examples of aseptic techniques that should be used when investigating the effect of antimicrobial substances on microbial growth? (select all that apply)
Medium
- AWorking next to a lit Bunsen burner to create convection currents.
- BIncubating the plates at 37 °C to encourage rapid bacterial growth.
- CPassing an inoculating loop through a flame until it is red-hot before and after use.
- DLifting the lid of the Petri dish at an angle during inoculation.
- ELeaving the Petri dish lid off during incubation to allow air exchange.
11.Match each type of adaptation with its correct example.
Medium
- Anatomical
- Physiological
- Behavioural
- Mosquitoes produce chemicals that stop blood clotting when they bite.
- Cold-blooded reptiles bask in the sun to absorb heat.
- The white fur of a polar bear provides camouflage in the snow.
12.Bacteria are grown in a liquid broth culture. The number of viable cells per mm³ and the pH of the broth are both measured over 12 hours. When plotting the data, the number of viable cells per mm³ is plotted on a logarithmic scale, whereas pH is plotted on a linear scale. Why is this?
Hard
- AThe number of viable cells changes by several orders of magnitude, so a logarithmic scale allows a wide range of values to be displayed on a single graph.
- BThe number of viable cells is always a negative value, so a logarithmic scale is needed to make it positive.
- CThe pH of the broth changes by a factor of ten each hour, so it must be plotted on a linear scale.
- DThe number of viable cells changes by only a small amount, so a logarithmic scale makes the change easier to see.
13.In a disc diffusion experiment, a bacterial strain is highly resistant to the antibiotic on a particular disc. What would you expect to see around that disc?
Hard
- AA very large clear zone, because the bacteria are resistant to the antibiotic.
- BNo clear zone, because the bacteria are not inhibited by the antibiotic.
- CA clear zone of exactly the same size as for a non-resistant strain.
- DA clear zone that is larger than for a non-resistant strain but smaller than for a susceptible strain.
14.Directional selection favours one extreme phenotype and causes the mean trait value in the population to change over time.
Easy
True or false?
15.Stabilising selection selects against extreme phenotypes and maintains intermediate phenotypes, keeping allele frequencies relatively constant over generations.
Medium
True or false?
16.A student plots the number of viable bacteria per mm3 of broth on a logarithmic scale and the pH of the broth on a linear scale. Why is a logarithmic scale used for the bacterial cell numbers?
Medium
- AThe bacterial numbers span a very wide range of values, so a log scale allows them all to be displayed on one graph
- BThe bacterial numbers are always whole numbers, which cannot be plotted on a linear scale
- CA log scale makes the pH values easier to compare with the bacterial numbers
- DA log scale is needed because the number of bacteria decreases over time
17.During a disc diffusion experiment, a bacterial strain is completely resistant to the antibiotic on a particular paper disc. What will be seen around that disc after incubation?
Medium
- ANo clear zone, because the bacteria can grow right up to the disc
- BA very large clear zone, because the bacteria are unaffected by the antibiotic
- CA clear zone the same size as those around the other discs
- DA clear zone that is exactly half the size of the largest zone
18.In a disc diffusion experiment, antibiotic X produces a clear zone of radius 8 mm and antibiotic Y produces a clear zone of radius 4 mm. Using Area = π × r², what is the ratio of the area of the zone for X to the area of the zone for Y?
Medium
- A4:1
- B2:1
- C8:1
- D16:1
19.A clear zone of inhibition around a paper disc has a radius of 6 mm. Using Area = π × r² and π = 3.14, what is the area of the zone of inhibition?
Medium
- A113.04 mm²
- B37.68 mm²
- C18.84 mm²
- D452.16 mm²
20.A bacterial broth is diluted to a concentration of 1 × 10⁸ CFU mm⁻³. What does one colony-forming unit (CFU) represent?
Medium
- AA live bacterial cell that can divide to form a colony on an agar plate
- BA dead bacterial cell that cannot grow on agar
- CA single antibiotic molecule that inhibits bacterial growth
- DA clear zone of inhibition on an agar plate
21.In a disc diffusion experiment, the clear zone around a disc ends at a certain distance from the disc. Why does the clear zone end at this point?
Hard
- AThe antibiotic concentration has fallen to a level at which the bacteria are no longer inhibited
- BThe antibiotic has stopped diffusing outwards from the disc
- CThe bacteria inside the zone have all mutated to become resistant
- DThe agar has absorbed all of the antibiotic from the disc
22.In a disc diffusion experiment, two antibiotics are tested against the same bacterial strain. Antibiotic P produces a clear zone of diameter 20 mm; antibiotic Q produces a clear zone of diameter 10 mm. Which statement is correct?
Easy
- AP is more effective than Q because it produces a larger clear zone at the same concentration
- BQ is more effective than P because it produces a smaller clear zone
- CP and Q are equally effective because both produce a clear zone
- DQ is more effective because a smaller zone means the bacteria are more sensitive
23.A multodisc containing several different antibiotics is placed on a lawn of bacteria. Which statement correctly describes the effect of antibiotic concentration on the size of the zone of inhibition?
Medium
- AA more effective antibiotic diffuses outwards and inhibits growth at lower concentrations, producing a larger clear zone
- BA less effective antibiotic produces a larger clear zone because it diffuses further
- CThe size of the clear zone depends only on the incubation temperature, not on the antibiotic
- DAll antibiotics produce clear zones of the same size regardless of concentration
24.Which of the following statements about logarithmic scales when investigating bacteria are correct? (select all that apply)
Hard
- AEach unit step on a log₁₀ scale represents a tenfold (order of magnitude) change
- BA logarithmic scale has equal intervals between all values on the axis
- CA logarithmic scale allows a wide range of bacterial numbers to be displayed on a single graph
- DThe pH scale is an example of a logarithmic scale
- ELogarithmic scales can only be used for values below 1000
25.A student investigates the effect of antibiotics on bacterial growth using a disc diffusion method. Which of the following are valid reasons why the paper discs are evenly distributed and not touching the edge of the plate or each other? (select all that apply)
Medium
- ASo that the clear zones around each disc do not overlap, allowing each antibiotic to be measured separately
- BSo that the antibiotic from one disc does not affect the zone of inhibition of another disc
- CSo that the bacteria cannot grow between the discs
- DSo that the discs do not fall off the plate during incubation
- ESo that the results for each antibiotic can be compared fairly
26.In a disc diffusion experiment, the clear zone around a paper disc contains no bacteria.
Easy
True or false?
27.A base-10 logarithmic scale has equal intervals between the numbers on the axis.
Easy
True or false?
28.If a bacterial strain is completely resistant to an antibiotic on a disc, a clear zone will still form around that disc.
Medium
True or false?
29.Match each type of adaptation with the correct example.
Medium
- Anatomical adaptation
- Physiological adaptation
- Behavioural adaptation
- White fur of a polar bear providing camouflage
- Mosquitoes producing chemicals that stop blood clotting
- Cold-blooded reptiles basking in the sun to absorb heat
30.Match each term about selection with its correct description.
Medium
- Directional selection
- Stabilising selection
- Selection pressure
- Favours one extreme phenotype, causing the mean trait value to change over time
- Selects against extreme phenotypes and maintains intermediate phenotypes
- An environmental factor that affects the chance of survival of an organism
31.Place the following statements about evolution by natural selection into chronological order, from earliest (1) to latest (4).
Medium
- Random mutation can produce new alleles of a gene
- A particular mutation gives a benefit to its possessor, leading to an increased chance of survival
- The advantageous allele is passed onto the next generation
- The new allele increases in frequency in the population
32.Place the following steps of the disc diffusion method into the correct order, from first (1) to last (5).
Medium
- Pre-soak paper discs in the different antibiotic solutions
- Spread a sample of diluted bacterial broth onto the surface of a sterile agar plate to create a lawn
- Lightly press the paper discs onto the surface of the agar, evenly distributed
- Keep the agar plate in the incubator overnight
- Measure the clear areas around each paper disc to compare antibiotic effectiveness
33.Using Table 1, state and explain which drug was the most effective against all species of bacteria.
Hard
- ADrug C, because it has the lowest overall MIC values across the four species
- BDrug A, because it has the lowest MIC against E. coli
- CDrug D, because it has the highest MIC values, showing it is strongest
- DDrug B, because it has the same MIC as drug A against S. pneumoniae
34.According to Table 1, drug A is more effective against Enterococcus faecalis than drug B.
Medium
True or false?
35.In a disc diffusion experiment, paper discs are pre-soaked in different antibiotic solutions. Why must all the antibiotic solutions be of the same concentration?
Medium
- ASo that the effects of the different antibiotics can be compared fairly
- BSo that the antibiotics diffuse out of the discs at exactly the same rate
- CSo that the bacteria cannot become resistant to any of the antibiotics
- DSo that the clear zones produced are all the same size
36.Which statement correctly explains why a Bunsen burner is lit on the bench during inoculation of an agar plate?
Medium
- AIt sterilises the inoculating loop before and after use
- BIt creates convection currents that prevent airborne contaminants from falling onto the growth media
- CIt warms the agar so that bacteria can grow more quickly
- DIt kills any bacteria already present in the agar
37.A student measures the clear zone around an antibiotic disc and records a radius of 7 mm. Using Area = π × r² and π = 3.14, what is the area of the zone of inhibition?
Medium
- A153.86 mm²
- B43.96 mm²
- C21.98 mm²
- D615.44 mm²
38.Two antibiotics are tested against the same bacterium. Antibiotic R gives a clear zone of radius 3 mm; antibiotic S gives a clear zone of radius 6 mm. How many times larger is the area of S's zone than R's zone?
Hard
- A2 times
- B4 times
- C6 times
- D9 times
39.Which of the following measures help to maintain aseptic technique during a disc diffusion experiment? (select all that apply)
Medium
- AFlame the inoculating loop until it is red-hot before and after transferring the culture
- BLift the Petri dish lid at an angle rather than removing it completely
- CIncubate the plates at 25 °C rather than 37 °C
- DTape the Petri dish lid closed after plating
- ELeave the Petri dish lid off during incubation to allow air circulation
40.A clear zone of inhibition contains no bacteria because the antibiotic concentration is high enough to prevent bacterial growth there.
Easy
True or false?
41.What is the term for the lowest concentration of a substance that will inhibit the growth of a microorganism?
Easy
42.The human genome contains approximately 3 billion base pairs. If genetic variation between humans is about 0.1%, calculate the number of base pairs that are likely to differ between one human and another.
Medium
43.Complete the sentence about genetic diversity.
Easy
A population with a large gene pool has a strong ability to adapt to change, whereas a population with a small gene pool is vulnerable to ____.
44.The human genome contains approximately 3 billion (3 000 000 000) base pairs. If the genetic variation between humans is about 0.1%, calculate the number of base pairs that are likely to differ between one human and another.
Medium
45.A clear zone of inhibition around a paper disc has a diameter of 14 mm. Using Area = π × r² and π = 3.14, calculate the area of the clear zone in mm². (Give your answer to 1 decimal place.)
Medium
46.A bacterial broth culture contains 1 × 10⁸ CFU mm⁻³. A student spreads 0.1 mm³ of this broth onto an agar plate. Calculate the number of colony-forming units spread onto the plate. Give your answer in standard form.
Medium
47.A clear zone of inhibition has an area of 200.96 mm². Using Area = π × r² and π = 3.14, calculate the radius of the clear zone in mm.
Medium
48.Complete the sentence about logarithmic scales.
Medium
On a base-10 logarithmic scale, each unit step represents one ____ of magnitude, which is a tenfold (×10) change in quantity.
49.Complete the sentence about the minimum inhibitory concentration.
Medium
The minimum inhibitory concentration (MIC) is the ____ concentration of a substance that will inhibit the growth of a microorganism.
50.In a disc diffusion experiment, what name is given to the clear area around a paper disc where no bacteria have grown?
Medium
51.State the term used to describe a live bacterial cell that can divide and form a colony on an agar plate.
Medium
52.Using Table 1, calculate how many times greater the MIC of drug D is compared with drug C against Campylobacter enteritis.
Medium
53.Explain why a catastrophic event that causes a sharp decline in numbers of a population is dangerous for the future of a species, even when the species is not totally wiped out.
Medium
54.Explain why Scenario B leads to a lower frequency of the new allele in the population than Scenario A.
Hard
55.Explain why the mutation experienced in Scenario C will not remain in the population.
Hard
56.After 30 days, all of the bacteria developed resistance against Penicillin but not against drug Y. Explain this in terms of natural selection.
Hard
57.Explain why a population with a very low genetic diversity is at greater risk of extinction than a population with high genetic diversity.
Hard
58.The human genome contains approximately 3 billion base pairs. If genetic variation between humans is about 0.1%, calculate the number of base pairs that are likely to differ between one human and another.
Medium
- A2700000 base pairs
- B3600000 base pairs
- C3000000 base pairs
- D3300000 base pairs
59.The human genome contains approximately 3 billion (3 000 000 000) base pairs. If the genetic variation between humans is about 0.1%, calculate the number of base pairs that are likely to differ between one human and another.
Medium
- A3300000 base pairs
- B3600000 base pairs
- C3000000 base pairs
- D2700000 base pairs
60.A clear zone of inhibition around a paper disc has a diameter of 14 mm. Using Area = π × r² and π = 3.14, calculate the area of the clear zone in mm². (Give your answer to 1 decimal place.)
Medium
- A169.3 mm²
- B153.9 mm²
- C184.7 mm²
- D138.5 mm²
61.A bacterial broth culture contains 1 × 10⁸ CFU mm⁻³. A student spreads 0.1 mm³ of this broth onto an agar plate. Calculate the number of colony-forming units spread onto the plate. Give your answer in standard form.
Medium
- A12000000 CFU
- B9000000 CFU
- C10000000 CFU
- D11000000 CFU
62.A clear zone of inhibition has an area of 200.96 mm². Using Area = π × r² and π = 3.14, calculate the radius of the clear zone in mm.
Medium
- A10 mm
- B9 mm
- C7 mm
- D8 mm
63.Using Table 1, calculate how many times greater the MIC of drug D is compared with drug C against Campylobacter enteritis.
Medium
- A7.1
- B9.1
- C8.1
- D10.1

Notes de révision

## Genetic Diversity - **Genetic diversity** = the number of different alleles of genes in a population. - **Genetic variation** = small differences in DNA base sequences between individuals of the same species. - **Mutation** generates new alleles and contributes to genetic diversity / the size of the gene pool. - New alleles may be advantageous, disadvantageous, or have no apparent effect on phenotype (the genetic code is degenerate). - New alleles can remain hidden (not expressed) for several generations before contributing to phenotypic variation. - A large gene pool / high genetic diversity gives a **strong ability to adapt to change**; a small gene pool / low genetic diversity makes a population **vulnerable to extinction**. - Cheetahs: small gene pool due to a large decline ~10,000 years ago, leaving small fragmented populations; no mixing between populations and large amounts of inbreeding occurred. ## Using Logarithms When Investigating Bacteria - Bacterial colonies can grow at rapid rates, producing very large numbers within hours. - **Logarithmic scales** allow a wide range of values (very small to very large) to be displayed on a single graph. - An **order of magnitude** means a tenfold (×10) change in quantity. - On a log₁₀ scale, each unit step represents one order of magnitude. - Log scales have **uneven intervals** between values on one or more axes. - The pH scale is logarithmic; hydrogen ion concentration varies massively between each pH level. ## Natural Selection - **Principles:** 1. Random mutation can produce new alleles of a gene. 2. Under certain environmental conditions, the new allele may benefit its possessor, leading to an increased chance of survival and increased reproductive success. 3. The advantageous allele is passed on to the next generation through reproductive success. 4. Over several generations, the new allele will increase in frequency in the population. - **Fitness** = an organism's ability to survive and pass on its alleles to offspring. - **Selection pressure** = an environmental factor that affects the chance of survival of an organism. - Rabbits: brown fur (dominant) camouflages better than white fur (recessive homozygous); foxes act as a selection pressure; brown rabbits survive and reproduce more, so over many generations the frequency of brown fur alleles increases and white fur alleles decreases. ## Directional & Stabilising Selection - **Stabilising selection** keeps allele frequencies relatively constant over generations; things stay as they are unless there is a change in the environment. It selects against extreme phenotypes and for intermediate phenotypes. - Example: human birth weights — very low and very high birth weights have higher mortality risk; medium birth weight has the highest survival rate. - **Directional selection** produces a gradual change in allele frequencies over several generations; it favours one extreme phenotype, causing the mean trait value to change over time. - Example: antibiotic-resistant bacterial strains becoming more common due to overuse of antibiotics. ## Environmental Adaption - Natural selection results in species that are better adapted to their environment. - **Adaptation** can be anatomical, physiological, or behavioural: - **Anatomical** — structural features (e.g. white fur of polar bear for camouflage; long limbs in desert animals for heat dissipation). - **Physiological** — internal biological processes (e.g. mosquitos produce chemicals that stop blood clotting; lowering metabolism during hibernation). - **Behavioural** — actions or behaviours (e.g. cold-blooded reptiles bask in the sun; migration of birds to warmer climates). - **Evolution** = the change in adaptive features of a population over time as a result of natural selection. - If the environment is static, selection pressures do not change and evolution will not occur. - If two populations of one species are isolated and become so different in phenotype that they can no longer interbreed to produce fertile offspring, they have formed two new species (speciation). ## Required Practical: Aseptic Techniques - Aseptic techniques ensure microbes don't escape or become contaminated with unwanted, possibly pathogenic, microbes. - **Examples:** - Sterilise Petri dishes (heating to high temperature to kill microorganisms). - Sterilise nutrient agar before pouring into sterile Petri dishes. - Disinfect work surfaces with disinfectant or alcohol before inoculation. - Work next to a lit Bunsen burner (convection currents stop contaminants falling onto growth media). - Use flamed inoculating loops or sterile swabs (heat until red-hot before and after inoculation). - Keep lid on Petri dishes when not in use; lift lid at an angle during inoculation. - Tape Petri dish lids closed after plating. - Incubate plates at a safe temperature of 25 °C (prevents growth of pathogenic microorganisms). - Sterilise or dispose of all equipment after use. - **Disc diffusion experiment:** - **Colony-forming unit (CFU):** a live bacterial cell that can divide and form a colony on an agar plate. - Method: pre-soak paper discs in antibiotic solutions (same concentration); create a lawn of bacteria; press discs evenly onto agar; incubate overnight; measure clear areas (zones of inhibition). - **Zone of inhibition area = π × r²** - More effective antibiotics produce larger clear zones; if bacteria are completely resistant, there is no clear zone. - **Minimum inhibitory concentration (MIC)** = the lowest concentration of a substance that will inhibit the growth of a microorganism.

Genetic diversity: The number of different alleles of genes in a population.
Genetic variation: The small differences in DNA base sequences between individual organisms within a species population.
Gene pool: The total number of alleles of all the genes in a population.
Mutation: A random change in the DNA base sequence that can produce new alleles and contributes to genetic diversity or the size of the gene pool.
Fitness: An organism's ability to survive and pass on its alleles to offspring.
Selection pressure: An environmental factor that affects the chance of survival of an organism.
Natural selection: The process by which organisms with advantageous alleles have an increased chance of survival and reproduction, causing allele frequencies in a population to change over time.
Stabilising selection: Natural selection that keeps allele frequencies relatively constant over generations, selecting against extreme phenotypes and for intermediate phenotypes.
Directional selection: Natural selection that produces a gradual change in allele frequencies over several generations, favouring one extreme phenotype.
Adaptation: A feature that enables an organism to survive in the conditions in which it normally lives; can be anatomical, physiological, or behavioural.
Evolution: The change in adaptive features of a population over time as a result of natural selection.
Speciation: The formation of new species from pre-existing species over time as a result of accumulated genetic differences.
Aseptic technique: Practical measures taken to ensure microbes being investigated don't escape or become contaminated with another unwanted, possibly pathogenic, microbe.
Colony-forming unit (CFU): A live bacterial cell that can divide and form a colony on an agar plate.
Zone of inhibition: The clear area around a paper disc in a disc diffusion experiment where no bacteria are present; area = π × r².
Minimum inhibitory concentration (MIC): The lowest concentration of a substance that will inhibit the growth of a microorganism.
Order of magnitude: A tenfold (×10) change in quantity.

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