4B - Investigating selection

Question 1

What do different types of natural selection lead to?

Answer 1

Different allele frequency patterns.

Question 2

What are types of natural selection?

Answer 2

Stabilising selection and directional selection.

Question 3

What type of selection does antibiotic resistance show?

Answer 3

Directional selection.

Question 4

What is directional selection?

Answer 4

Where individuals with alleles for characteristics of an extreme type are more likely to survive and reproduce.

Question 5

Why might directional selection occur?

Answer 5

In response to an environmental change.

Question 6

Explain how antibiotic resistance is an example of directional selection

Answer 6

Some individuals in a population have alleles that give them resistance to an antibiotic.

The population is exposed to the antibiotic, killing bacteria without the resistant allele.

The resistant bacteria survive and reproduce without competition, passing on the allele that gives antibiotic resistance to their offspring.

After some time, most organisms in the population will carry the antibiotic resistance allele.

Question 7

What type of selection does human birth weight show?

Answer 7

Stabilising selection.

Question 8

What is stabilising selection?

Answer 8

Where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce.

Question 9

When does stabilising selection occur?

Answer 9

When the environment isn’t changing, and it reduces the range of possible characteristics.

Question 10

Explain how human birth weight is an example of stabilising selection

Answer 10

Humans have a range of birth weights.

Vet small babies are less likely to survive - partly because they find it hard to maintain their body temperature.

Giving birth to large babies can be difficult, so large babies are less likely to survive too.

Conditions are most favourable for medium-sized babies - so weight of human babies tends to shift towards the middle of the range.

Question 11

On a graph, what type of distribution do traits influenced by stabilising selection show?

Answer 11

A normal distribution.

Question 12

What does it mean if characteristics are influenced by polygenes?

Answer 12

They are more likely to be influenced by the environment than ones determined by a single gene.

Question 13

What does the effect of the environment on polygenes mean happens?

Answer 13

Produces individuals in a population that vary about the mean so when you plot this variation on a graph, you get a normal distribution curve.

Question 14

Describe a practical to investigate the effects of antibiotics on bacterial growth

Answer 14

Use a sterile pipette to transfer the bacteria from the liquid broth (a mixture of distilled water, bacterial culture and nutrients) to an agar plate. Spread the bacteria over the plate using a sterile lawn spreader.

Use sterile forceps to place the paper discs soaked with different antibiotics spaced apart on the plate. Make sure you add a negative control disc soaked only in sterile water.

Lightly tape a lid on, invert, and incubate the plate at about 25 degrees celsius for 48h. This allows the bacteria to grow (forming a ‘lawn’). Anywhere the bacteria cannot grow can be seen as a clear patch in the lawn of bacteria - an inhibition zone.

The larger the inhibition zone, the more bacteria were inhibited from growing and so the more effective the antibiotic.

A similar technique can be used to test the effects of antiseptics or disinfectants on microbial growth.

Question 15

What techniques should be used to prevent contamination of microbial cultures?

Answer 15

Aseptic techniques.

Question 16

When investigating the effects of antibiotics on bacterial growth, what aseptic techniques should you use?

Answer 16

• Regularly disinfect work surfaces.
• Don’t put any utensils on the work surface.
• Contaminated utensils should be put in a beaker of disinfectant.
• Use sterile equipment and discard safely after use.
• Work near a bunsen flame as hot air rises so any microbes in the air should be drawn away from the culture.
• Minimise the time spent with the lid off of the agar plate.
• Briefly flame the neck of the glass container of broth just after it is opened and just before it is closed.
• Wash hands thoroughly before and after handling cultures.

Question 17

Why should aseptic techniques always be used when investigating the effects of antibiotics on bacterial growth?

Answer 17

To prevent contamination of cultures by unwanted microorganisms. This is important as contamination can affect the growth of the microorganism that you are working with. It is also important to avoid contamination with disease-causing microbes that could make you ill.