4B- Investigating Selection

Question 1

Natural selection alters allele frequency. What are the two types of selection that affect allele frequency?

Answer 1

Stabilising and directional.

Question 2

What is directional selection?

Answer 2

Directional selection is where alleles from extreme types are more likely to survive and reproduce.

Eg. Bacteria evolving antibiotic resistance.

Those in the population who don’t have the allele but are exposed to the bacteria die.

The resistant bacteria is free to reproduce without competition, passing the alleles to the offspring, leading to a larger population with antibiotic resistance.

Question 3

What is stabilising selection?

Answer 3

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.

Eg. Human birth rate.

Small babies less likely to survive as they can’t regulate body heat. Larger babies can get stuck. So medium side babies are most likely to survive so the population becomes middle sized.