Diversity, Classification and Variation- Investigating Selection

Question 1

What are the two types of natural selection?

Answer 1

Stabilising selection and directional selection

Question 2

What is directional selection?

Answer 2

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

Question 3

What is an example of directional selection?

Answer 3

Bacteria evolving antibiotic resistance

Question 4

What is the process of directional selection?

Answer 4

1. Some individuals in a population have alleles that give them resistance to an antibiotic
2. The population is exposed to the antibiotic and bacteria that doesn’t have the resistant allele is killed
3. The resistant bacteria survive and reproduce without competition so they pass on the allele that gives antibiotic resistance to their offspring
4. After some time, most organisms in the population will carry the antibiotic resistance allele

Question 5

What is stabilising selection?

Answer 5

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

Question 6

When does stabilising selection occur?

Answer 6

When the environment isn’t changing

Question 7

When does directional selection occur?

Answer 7

In response to a change in the environment

Question 8

What is an example of stabilising selection?

Answer 8

Human birth weight

Question 9

What is the process of stabilising selection?

Answer 9

1. Humans have a range of birth weights
2. Very small babies are less likely to survive because they find it hard to maintain their body temperature
3. Giving birth to large babies can be difficult, so large babies are also less likely to survive
4. Conditions are more favourable for medium-sized babies so weight of human babies tends to shift towards the middle of the range

Question 10

How do you test the effects of antibiotics using agar plates?

Answer 10

1. Use a sterile pipette to transfer the bacteria from the liquid broth to an agar plate and spread the bacteria using a sterile spreader
2. Use sterile forceps to place paper discs soaked with different antibiotics spaced apart on the plate- add a negative control disc soaked in sterile water
3. Lightly tape on a lid and incubate the plate at about 25 degrees C for 48 hours which allows the bacteria to grow- anywhere bacteria can’t grow can be seen as an inhibition zone
4. Size of the inhibition zone is how well antibiotic works- larger zone means more bacteria were inhibited from growing

Question 11

How do you use aseptic techniques to prevent contamination of microbial cultures?

Answer 11

• Regularly disinfect work surfaces
• Contaminated utensils should be placed 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 culture
• Minimise time spent with lid off the agar plate
• Flame the neck of the glass container of broth