AeroLab Reading room

Biology / A2 level

Selection and evolution

Evolution changes inherited characteristics of populations over generations.

Learning objectives

The big idea

Evolution changes inherited characteristics of populations over generations. Mutation supplies new alleles; recombination reorganises them. Selection occurs when inherited differences affect reproductive success in a particular environment.

Allele frequencies can also change through genetic drift and migration. Reproductive isolation can contribute to speciation. Hardy–Weinberg calculations provide a comparison model with restrictive assumptions; a good numerical fit does not alone establish that every assumption holds.

Equations and relationships

p² + 2pq + q² = 1

Expected genotype frequencies for two alleles in the Hardy–Weinberg model.

Worked example

In a Hardy–Weinberg model, recessive allele frequency q=0.2. Find expected aa frequency.

Follow the solution

The expected homozygous recessive frequency is q²=0.2²=0.04, or 4%.

Investigate and explain

Using the worked example as supplied data, predict the effect of changing one condition. Draw a labelled model, record your reasoning and identify a variable that would need controlling. No human, animal or microbial experiment is required.

Related simplified PhET model. Explain what it represents and what the living system does differently.

Open the interactive investigation

Check your understanding

1. What changes through natural selection?

  1. Frequencies of heritable variants in a population
  2. Every individual’s genes because it needs to adapt
  3. The environment’s intentions
Check the answer and reasoning

Frequencies of heritable variants in a population

Differential survival and reproduction alter population composition across generations.

2. In a Hardy–Weinberg model, recessive allele frequency q=0.2. Find expected aa frequency.

  1. 0.2
  2. 0.04
  3. 0.4
Check the answer and reasoning

0.04

The expected homozygous recessive frequency is q²=0.2²=0.04, or 4%.

3. Does every evolutionary change improve adaptation?

  1. Yes
  2. Only in large populations
  3. No
Check the answer and reasoning

No

Drift, changing conditions and trade-offs mean allele-frequency change is not synonymous with progress toward perfection.