B5. The Effects of Selection

Question 1

Types of selection - 1. Directional selection (2 points and its example)

Answer 1

Directional selection is where individuals with alleles for characteristics of an extreme type are more likely to survive and reproduce.
This could be in response to an environmental change.

Some individuals in a bacterial population have alleles that give them
resistance to an antibiotic. The population is exposed to the antibiotic,
killing bacteria without the resistance 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 2

Types of selection - 2. Stabilising selection (2 points and its example)

Answer 2

Stabilising selection is where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce.
It occurs when the environment isn’t changing, and it reduces the range of possible characteristics.

Humans have a range of birth weights. Very small babies are less likely to survive - they have a high surface area to volume ratio, which means they find it hard to maintain their body temperature. This puts pressure on their respiratory and cardiac systems, which can be fatal.
Very large babies are less likely to survive too. Giving birth to large babies can be difficult because their large size makes it harder for them to fit through the mother’s pelvis. This can lead to complications for both mother and child.
Conditions are most favourable for medium-sized babies- so the weight of human babies tends to shift towards the middle of the range

Question 3

A population of rabbits has varying fur length. Longer fur helps to keep the rabbits warmer. The graph below shows how the average fur length of the rabbits changed over a period of six years, which had particularly cold winters. The bars span the difference between the shortest and longest fur lengths recorded.

Describe what the data shows:

Suggest a possible cause:

Answer 3

Describe what the data shows:
Over the first two years the average fur length is about 21 mm. However, the average length gradually increases from 21 mm to 24 mm. This shows directional selection.

Suggest a possible cause:
The rabbits with the longer fur are more likely to survive the cold winters than the short-furred rabbits. This makes them more likely to reproduce and, when they do, they pass on the allele for longer fur to the next generation. Over time, the allele for longer fur becomes more common in the population and the average fur length of the rabbits increases.