Investigating Selection 4B

Question 1

What type of natural selection does antibiotic resistance show

Answer 1

Directional

Question 2

What are the two types of natural selection

Answer 2

Stabilising and directional

Question 3

Explain directional selection in terms of antibiotic resistance

Answer 3

some individuals in a population have an allele which gives them antibiotic resistance
Population is exposed to the antibiotic killing off the ones without the resistance
Resistant bacteria survive and reproduce, without competition passing on the allele
The allele frequency therefore increases

Question 4

Explain stabilising selection in terms of human birth weight

Answer 4

Humans have a range of birth weights
Very small babies - less likely to survive as they find it hard to maintain their body temperature
Giving birth to large babies can be difficult therefore they are less likely to survive
Conditions are most favourable for medium sized babies , so the weight of human babies tends to shift towards the middle of the range

Question 5

When does directional selection occur

Answer 5

In response to an environmental change when the mean moves away from the original

Question 6

Examples of directional selection

Answer 6

antibiotic resistant bacteria or moth colour

Question 7

When does stabilising selection occur

Answer 7

when the environment is not changing and the mean stays in the middle

Question 8

Examples of stabilising selection

Answer 8

egg number in birds or birth weight in humans

Question 9

Describe the test for effects of antibiotics using agar plates

Answer 9

1. Bacteria used usually grown in liquid broth
2. Use a sterile pipette to transfer the bacteria from the broth to an agar plate and spread the bacteria over the plate using a sterile plastic spreader
3. use sterile forceps to place disks soaked in different antibiotics spaced apart. Add a negative control disks soaked in distilled water
4. tape a lid on lightly and incubate at 25 degrees C for 48 hours allowing the bacteria to grow forming a lawn anywhere the bacteria can’t grown is shown as a clear patch called an inhibition zone
5. Size of inhibition zone shows how well an antibiotic works larger the zone the more effective the antibiotic.
6. Similar technique can be used to test the effectiveness of antiseptics and other disinfectants on microbial growth

Question 10

Name the aseptic techniques

Answer 10

Regularly disinfect work surfaces to minimise contamination
Don’t put utensils on work surfaces
Contaminated utensils should be placed in disinfectant
Use sterile equipment and discard after use
Work near a Bunsen burner as heat rises so any microbes in the air should be drawn away from your culture
Minimise the time spent with the lid of the agar plate to reduce the chance of airborne microorganisms contaminating it
Briefly flame the neck of the glass container of broth just before it’s opened and just before it’s closed this causes air to move out of the container preventing any unwanted objects from falling in
You should also take steps to protect yourself, wash your hands thoroughly before and after handling cultures